Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap

[ExtraThiccBoi]

Hi,
just fixed this amp, changed all caps, some resistors for metal films for lower noise, better therm stability and audiophoolery.
I have a specific strange issue with a hiss that goes up as i turn up the volume potentiometer, but there is no active stage before the volume control.
There is a preamp with tone controls (tied in feedback loop) after the volume control, then it goes to the main amplifier board. I have tested the main amplifier board separately and that has very low noise floor, doesnt put out any of this mystery hiss. The tone control is quite a complicated circuit for me (beginner here). From what i understand, as i turn the volume control up, i raise the line impedance from 0 (shorted to ground, no audio signal) to whatever the value is (the pot is 100k log stereo). This introduces noise to the line. Could this be caused by a noisy transistor on the tone control board? I have couple of those 2SC2320 and 2SA999 transistors with matching beta (salvaged from the phono amplifier of this integrated amp). Anyone got any ideas on what it could be? I don't have a scope around unfortunately
Thanks for any help

[mzacharias]

Does it do this with shorted inputs?

[xwarp]

Both channels?

[shakalnokturn]

I'd answer the above questions first...

If it affects a single channel and still has as much or almost as much hiss with selected input grounded (line not phono input), I'd be going straight to the differential pair on the tone control board.

Sometimes it is also worth putting the cover back on and testing with lights off...

[ExtraThiccBoi]

Does it do this with shorted inputs?

Will test, i have my source connected all the time.

Both channels?

Yep both channels.

I'd answer the above questions first...

If it affects a single channel and still has as much or almost as much hiss with selected input grounded (line not phono input), I'd be going straight to the differential pair on the tone control board.

Sometimes it is also worth putting the cover back on and testing with lights off...

Ill see if it hisses with shorted inputs
But one thing i tried. If i disconnect the volume pot from the tone board input, the hiss gets super intense, that first transistor has 220K and 390K resistors to ground, so when it sees that, it hisses even more.
I have not changed those to metal film nor the 470R ones connecting the differential pair.

[LateLesley]

It could be that you actually introduced the noise just changing components, and using noisy ones, or putting capacitors in the wrong way around. I used to think it wasn't possible, but learned a LOT watching Mr Carlson's Lab on youtube. He's very methodical, and almost OCD about things, but he makes you think about stuff you''d normally not think about. So I think he's worth a watch, you can learn a lot from him.

https://youtu.be/uGXQc-zanWg?t=6310

[ExtraThiccBoi]

It could be that you actually introduced the noise just changing components, and using noisy ones, or putting capacitors in the wrong way around. I used to think it wasn't possible, but learned a LOT watching Mr Carlson's Lab on youtube. He's very methodical, and almost OCD about things, but he makes you think about stuff you''d normally not think about. So I think he's worth a watch, you can learn a lot from him.

https://youtu.be/uGXQc-zanWg?t=6310

I specifically watched the capacitor video and also the super probe, but i don't have a scope.
I made sure to mark foil ends on all film caps i replaced, i used a headphone amplifier to see when the hum is stronger.
I watch his channel, i like his OCD approach   
I can tell the hiss is a little small bit quieter (but still annoying) than before changing the film caps and carbon resistors.

[LateLesley]

Yeah, I like his OCD approach too, you learn so much from it.

What I was going to suggest, is why not get one of those cheap scope kits, DSO138?? It's not a real scope, it's only single channel, and compared to a real scope, its a toy. But compared to NO scope, it's surprisingly capable. It'll do audio stuff at least, and an inexpensive way to get some form of visual of what is going on. Is it accurate? No. But it is enough to give you an idea of stuff going on. I'm basically saying, if you have no scope, it's a cheap way into having something,and it isn't too big.

I got one of the kit DSO138's for about £15 I think it was, complete with a case, and actually had a lot of fun putting it together, and it gave me something to see signals.

I know the feeling of not having a scope. I just scored a cheap one last night on our favourite auction site, but as a for parts and not working. It's from a house clearance, VERY dirty, and apparently no power. I thought it would be a good project one to strip apart, clean everything, and change caps while its in bits, and test all the circuitry. I'm not expecting to have it running quickly, and be learning LOTS here testing and troubleshooting it, especially safe ways to check the high voltage parts. (I think from the service manual that there is almost 2KV (!!!) in parts of it, excluding the HT of course, which I know to be wary around.)

Anyway, I hope you find the source of your noise, and can quieten it down. Good luck and happy hunting.

[magic]

The noise is obviously Johnson noise of the Rs or current noise of the Qs producing voltage across the Rs. I'm not really familiar with audio amps, particularly vintage, but the volume pot and the input resistors do seem rather high value to me, compared to the stuff I have seen in newer audio designs.

Given that both channels are identical and the noise is probably insignificant compared to normal signal (isn't it?), I would guess the amp came that way from the factory.
I doubt that swapping the Qs for identical ones harvested from the same unit is going to change much.

I would start with disconnecting the volume pot and checking just how low resistance is needed from preamp input to ground to make it shut up. Then multiply that by 4 and see if it produces a reasonable value for a replacement volume pot. If so, problem solved.

Also, use this as an opportunity to compare carbon and metal film and see that they probably don't make much practical difference.

[ExtraThiccBoi]

The noise is obviously Johnson noise of the Rs or current noise of the Qs producing voltage across the Rs. I'm not really familiar with audio amps, particularly vintage, but the volume pot and the input resistors do seem rather high value to me, compared to the stuff I have seen in newer audio designs.

Given that both channels are identical and the noise is probably insignificant compared to normal signal (isn't it?), I would guess the amp came that way from the factory.
I doubt that swapping the Qs for identical ones harvested from the same unit is going to change much.

I would start with disconnecting the volume pot and checking just how low resistance is needed from preamp input to ground to make it shut up. Then multiply that by 4 and see if it produces a reasonable value for a replacement volume pot. If so, problem solved.

Also, use this as an opportunity to compare carbon and metal film and see that they probably don't make much practical difference.

By resistance to "make it shut up" you mean the max resistance producing tolerable noise floor?
As you said, if i feed it from my DAC which is quite a high output source, i have to move the knob just the slightest bit to get enough volume, so the noise is very low. But if i was to feed it from something like a phono preamp or another low output device, i would most likely go into annoying noise range.
I have the amp apart again, so i'm going off my memory here, i have one mm probe on the preamp gnd, one on the left channel, measuring the pot resistance as i turn the knob.
At the positions when i get loud enough volume, it's between 6 and 18K.
I think i could go with a 50K pot, those alps direct fit in replacements cost eyewatering 10 euros a piece though.
There are 4.7K resistors in parallel to gnd with the volume pot, i don't know if i would have to change their value as well.
Or maybe changing just the input resistors on the base of Q501 & Q502 would help?
The preamp sees 67K as max resistance when the pot is all the way up.

[magic]

By resistance to "make it shut up" you mean the max resistance producing tolerable noise floor?

Exactly.

I think i could go with a 50K pot, those alps direct fit in replacements cost eyewatering 10 euros a piece though.

50k would only give 6dB noise reduction I think, possibly not enough.

There are 4.7K resistors in parallel to gnd with the volume pot, i don't know if i would have to change their value as well.

I have no idea what they are. How are they connected to the pot?

The preamp sees 67K as max resistance when the pot is all the way up.

Not really, because the top of the pot is zero ohm again, although at a different voltage (whatever the source outputs). The worst case is 6dB below max where both halves of the pot are 50k. Try it - short the inputs to ground or connect some source which plays silence and see which volume setting makes the worst noise.

Or maybe changing just the input resistors on the base of Q501 & Q502 would help?

Surely, but not without side effects.
They form a divider with the output impedance of the pot which is two 50k halves in parallel worst-case, i.e. 25k. A tiny bit less because there is also R505 in parallel with it on the preamp board. So if you make R507 25k, signal at this volume position will be reduced by half and then it will skyrocket as the pot is turned to 100% and its output impedance drops back to zero. I hope you can see how it happens.
Also, they form a high-pass filter with C507 so C507 might need to be adjusted to retain the same cutoff frequency.
But you can try it, connect something in parallel with those Rs already there and see what happens and how bad it is.

[xwarp]

You know, the problem I have with this post about this amp in question is that you started it off with "just fixed this amp, changed all caps, some resistors for metal films for lower noise, better therm stability and audiophoolery."

You never stated the fault that you fixed and then you claim to be a "beginner" without an oscilloscope.

In my opinion, you aren't doing yourself any favors by trying to fix a problem that you could have very well introduced yourself.

Not trying to be a jerk here, but when I work over vintage audio equipment, I diagnose the failure, fix it, then proceed to correct marginal issues that affect the base performance of the unit before doing any mods.

[ExtraThiccBoi]

You know, the problem I have with this post about this amp in question is that you started it off with "just fixed this amp, changed all caps, some resistors for metal films for lower noise, better therm stability and audiophoolery."

You never stated the fault that you fixed and then you claim to be a "beginner" without an oscilloscope.

In my opinion, you aren't doing yourself any favors by trying to fix a problem that you could have very well introduced yourself.

Not trying to be a jerk here, but when I work over vintage audio equipment, I diagnose the failure, fix it, then proceed to correct marginal issues that affect the base performance of the unit before doing any mods.

When first got it, the amp was messed up (bad ouput Qs in Rch) and Rch idling current couldnt be set to spec. So far i have fixed all of that, didnt bother to mention that since it doesnt have any relation to this problem. I had these noise issues before, they were even worse before replacing Cs and Rs. The resistors, well, i was recommended to changed them to metal films by a friend who does vintage restorations.
I hoped the things above would get rid of the hiss, but as it turns out, it's probably a design flaw. I did not know that because as a beginner i don't know a whole lot about these circuits. I started this project to learn about this stuff originally

[elecdonia]

When first got it, the amp was messed up (bad ouput Qs in Rch) and Rch idling current couldnt be set to spec.

I have often discovered damaged input transistors in audio power amplifiers where the output stage transistors had previously failed. By "input transistors" I am referring to the differential pair of small transistors located at the input side of the power amplifier stage on the schematic.  Sometimes this pair of differential input transistors are packaged together in an IC chip type case, or they may be in the form of two separate transistors which are usually mounted very close to each other.

Excessive noise is one symptom of a damaged differential input transistor pair.  It may be heard as a steady white noise or as "popcorn" noise which occasionally stops and starts. For the common case of a stereo 2-channel amplifier repair where only one channel failed and the other still worked, there will be a considerable difference in noise level between the "repaired" channel and the other channel that didn't need repair because it was initially still working.  The "repaired channel" will be noisier.

Another common symptom  of a damaged differential input pair is excessive DC output offset voltage at the speaker terminals. Some amplifiers contain offset adjustment trimpots. Others do not. In every case there should be less than +/- 50 mV of DC offset at the speaker terminal. If the amplifier has an offset adjustment trimpot then it should be possible to reduce the DC offset < +/- 10 mV without needing to move the trimpot very far from its middle.

When replacing the input differential pair, always replace BOTH transistors at the same time. If the PC board layout has 2 separate transistors, it can be useful to position the replacement transistors so that they touch each other, and perhaps add a tiny lump of white heatsink grease bewteen the 2 transistors. Keeping both input transistors at the same temperature reduces output offset voltage drift caused by temperature changes.

In some Japanese amplifiers, the tone control circuits are integrated with the power amplifier stage. Basically they treat the power amplifier as a "large op-amp."  The architecture in this case has all the line-level RCA inputs switched into the volume control/balance control section, and this then goes straight into the power amplifier section without having any other "active" circuitry.

Other amplifier designs contain an active gain stage in the '"tone control" section. Often the architecture resembles a "discrete op-amp" containing several transistors. There might be a "differential pair" of transistors in the input of the tone control amplifier.  Other (older) circuit designs may have only 2 transistors in the tone control stage. The first transistor after the volume control (e.g. the input transistor) is the one to check in case of excessive noise, hiss, or popcorn noise. The best way to ID a "noisy" transistor is by replacing it. And if the input state for the tone control amplifier section is a differential pair of transistors, then they should both be replaced.

[elecdonia]

The input transistors are Q501 and Q503. Both are 2SC2320. I recommend replacing both transistors on the "noisy" channel.



This circuit is a "discrete op-amp" with a differential input stage. This is a very sophisticated tone control amplifier circuit design.  The circuit design resembles high-performance discrete op-amps used in recording studio mixing desks since the early 1970's, such as the legendary API 2520 modular op-amp.

Is your tone control stage passing the audio signal OK (except for the noise) ?
If so, the cause of the noise is likely to be Q501 or Q503 (the input transistors)

The following voltage checks will confirm that all other parts of the tone control stage are working correctly:

DC voltage measured from ground (earth) to the junction between Q515 and Q517 collectors should be less than +/- 50 mV DC.
Differential voltage between emitter and base of Q515 should be in the range of 500-650 mV.
A similar differential voltage should be observed between E and B of Q517. 500-650 mV.  Note: Be very cautious to avoid any short-circuits when making these tests because E and B of Q515/Q517 are at 20V or -20V compared to ground (earth).
Finally, the DC supply voltage rails supplying power to the tone control section should be +20 and -20 V, within a tolerance of +/- 1.5 volts. Also these power supply rails should be pure DC, no noise or AC ripple.



I have one additional recommendation:
Because this tone control stage is supplied with +20V and -20V, the DC voltages at both audio input and audio output are very close to 0V.  There is no DC voltage present across the audio coupling capacitors, C507 at the input and C519 at the output. For best audio quality, it is best to use "non-polarized" aka "bi-polar" BP or bp electrolytic capacitors which are specially designed for high quality audio. I have had great results with the Nichicon MUSE. These are packaged in a bright green case. They are available from many vendors.

[schmitt trigger]

Many moons ago I finally repaired an HH Scott integrated receiver, which had a hissing sound, after a leeeeeeeengthy troubleshooting process, by replacing a transistor.

That puzzled me a lot, because on a simple transistor tester, the device tested just fine.

So I saved the faulty device, until I could travel to another city where I had access to a Tektronix curve tracer.

Comparing the transistor, vs. a known good device, immediately showed the problem. At a Vce setting of about half the maximum rating, the traces on a defective device would  start to wiggle. The transistor was going into avalanche!
The amplifier circuit operation would actually bias the transistor around half its rated voltage, so the symptoms matched.

And before someone asks, No, I don't remember the transistor part number.

[magic]

There is no DC voltage present across the audio coupling capacitors, C507 at the input and C519 at the output. For best audio quality, it is best to use "non-polarized" aka "bi-polar" BP or bp electrolytic capacitors which are specially designed for high quality audio.

Nope, you forgot about the base current of Q501 which flows through R507 and, by my quick calculation, drops about 2V across it. Therefore there is -2V on the right side of C507 and that's why it is polarized the way it is.
But you have a point. If the input pairs are broken and, for example, have lousy beta, their base current may increase unreasonably and together with it the current noise. I guess it costs nothing to check voltage across R507/R508 and R525/R526, calculate base and emitter currents and check if beta is within spec. If it isn't, they certainly are bad.

Still, I think tens of kΩ of input impedance may simply be too much for bipolar inputs, no matter how good. In my limited experience, numbers like hundreds or low thousands of ohms are thrown around as recommended input impedance for line level audio inputs.

Related to the above:

DC voltage measured from ground (earth) to the junction between Q515 and Q517 collectors should be less than +/- 50 mV DC.

Not really. Firstly, the diffpair will try to follow the -2V rather than 0V DC offset at Q501 base. Secondly, there is R535 and Q503 base current going through it, which adds another offset, hardly predictable to mV accuracy IMO.

[David Hess]

The noise is obviously Johnson noise of the Rs or current noise of the Qs producing voltage across the Rs.

Yep, the AC current noise into the differential pair multiplies across the source impedance to act as another voltage noise source at the input.  With the input volume control potentiometer at minimum, the input is shorted so this current noise has no effect.

When first got it, the amp was messed up (bad ouput Qs in Rch) and Rch idling current couldnt be set to spec.

I have often discovered damaged input transistors in audio power amplifiers where the output stage transistors had previously failed.

As is the case here, the maximum input differential voltage is about +/-12 volts.  If this is exceeded when the output stage shorts, then this may cause base-emitter breakdown of the input differential pair transistors damaging their hfe which has a direct effect on input current noise.  In applications where this is a possibility, the differential pair is protected.

Note however that the 470 ohm emitter degeneration resistors in series with the emitters means that this was not a particularly low noise design to start with which is usually the case for power amplifiers.

Excessive noise is one symptom of a damaged differential input transistor pair.  It may be heard as a steady white noise or as "popcorn" noise which occasionally stops and starts.

Popcorn noise is a different problem caused by bad semiconductor fabrication or contamination of the transistor.

[ExtraThiccBoi]

snip

Thank you very, very much for such a deep analysis!
Very interesting to know this is more of an advanced design, guess the "all discrete, no ICs could achieve this level of performance" in the manual isn't there just for marketing hotwords 
The noise is equal in both channels.
The differential pair on the main amp board you mentioned (unobtanium 2SC1775) is most likely OK. It is coupled with a heatshrink, but DC still drifts because its a small design flaw of this harmankardon design. I tested the main amplifier board separately (injected audio signal right in the power amp input), the main board is very quiet, there is a little bit of white noise but it's audible only when i put my ear against the speaker and even then, it's very quiet and from my memory equal in both channels. Actually, the channel that didnt blow up has a little bit more noise (hum-buzz) from i suppose being close to AC lines, still very quiet and inaudible from listening position even in death quiet room.
Now the tone control, it's passing audio OK and the EQ works great too, i replaced those film caps in there, i could try matching the foil ends but i won't bother since the factory config wasn't like that too. Also the resistors in the feedback circ. are metal films now.
I can't really check those low voltages since it's very finnicky to measure that, but i do know the power supply voltage should be ok. The raw voltage is 50V, filtered by 12000uF per rail, the stepped down using a zener-transistor (2SB941/2SD1266) regulator with 2 x 330uF per rail and then goes to the tone control with extra 2 x 220uF per rail. I replaced all caps for either Panasonic and Nippon Chemi-con with extra generous voltage ratings.
I will replace those two trannies with matching beta 2320s i have, see if it makes any difference.
I replaced the electrolytics in audio path with Nichicon UFG, i unfortunately wasnt aware of the UES (muse) at that time. Maybe in the future, my local supplier run by a friend has all of them available.
Thanks again for all the analysis and interesting information!

[ExtraThiccBoi]

Many moons ago I finally repaired an HH Scott integrated receiver, which had a hissing sound, after a leeeeeeeengthy troubleshooting process, by replacing a transistor.

That puzzled me a lot, because on a simple transistor tester, the device tested just fine.

So I saved the faulty device, until I could travel to another city where I had access to a Tektronix curve tracer.

Comparing the transistor, vs. a known good device, immediately showed the problem. At a Vce setting of about half the maximum rating, the traces on a defective device would  start to wiggle. The transistor was going into avalanche!
The amplifier circuit operation would actually bias the transistor around half its rated voltage, so the symptoms matched.

And before someone asks, No, I don't remember the transistor part number.

Well, ill grab some of my other 2320s and try it, see what happens, looks like they all come from one batch since they all have the same beta.
I measured all of those from the tone board and they seemed fine but as you said, transistor tester wont probably help me much.

As is the case here, the maximum input differential voltage is about +/-12 volts.  If this is exceeded when the output stage shorts, then this may cause base-emitter breakdown of the input differential pair transistors damaging their hfe which has a direct effect on input current noise.  In applications where this is a possibility, the differential pair is protected.

Note however that the 470 ohm emitter degeneration resistors in series with the emitters means that this was not a particularly low noise design to start with which is usually the case for power amplifiers.

The main output fail was in the actual power amplifier (schematic included). I didnt experience any problems (ear on the speaker - very quiet hiss, not worth concerns) there after replacing them with NOS parts (matched within production tolerance of toshiba). I don't really want to start messing with transistors in the main amp, it's quite a complicated system. It works fine and hasnt released magic smoke yet.
By the way, should i try replacing the 470 ohm resistors (R525/527) with metal film?
 

[David Hess]

Very interesting to know this is more of an advanced design, guess the "all discrete, no ICs could achieve this level of performance" in the manual isn't there just for marketing hotwords 

It is just marketing.  An integrated design would be lower noise because the emitter degeneration resistors which add significant noise would be left out in favor of a better way to implement Gm reduction of the input stage without adding additional noise.

In this design if the resistors were left out, then the tail current to the differential pair would need to be reduced to reduce Gm and maintain stablity.  But with lower tail current, the full power bandwidth (slew rate) would be reduced which is a real problem with power amplifiers.  The degeneration resistors were added so that a higher tail current could be used for a higher slew rate (full power bandwidth) while maintaining closed loop stability.

Application Note A from National Semiconductors discusses the details and math of why the emitter degeneration resistors are included and the alternatives.

[duak]

I 've also encountered small signal bipolar transistors that are noisy and have significant leakage.  Two I'm thinking of were in the input diff stage of a power amp I built in 1981 so they are from before then.  If memory serves they are Motorola MPS8099.  I didn't do any AC tests on the parts by themselves on the bench so I don't know if they are avalanching.  It took about 10 years before the noise manifested.

I understand that the leakage and noise performance of a bipolar transistor can be permanently affected by reverse biasing the B-E junction to breakdown.  I wonder if this is what is happening here.  It's easy enough to overdrive the input of a power amp when it's off if driven from a preamp.

[schmitt trigger]

Application Note A from National Semiconductors discusses the details and math of why the emitter degeneration resistors are included and the alternatives.

Extraordinary paper,......everything you wanted to know about monolithic opamp architecture, but were afraid to ask.

Unfortunately, it does not list the author's name.
Although in the references, the persons listed were the who is who of linear IC design back in the day

[ExtraThiccBoi]

Very interesting to know this is more of an advanced design, guess the "all discrete, no ICs could achieve this level of performance" in the manual isn't there just for marketing hotwords 

It is just marketing.  An integrated design would be lower noise because the emitter degeneration resistors which add significant noise would be left out in favor of a better way to implement Gm reduction of the input stage without adding additional noise.

In this design if the resistors were left out, then the tail current to the differential pair would need to be reduced to reduce Gm and maintain stablity.  But with lower tail current, the full power bandwidth (slew rate) would be reduced which is a real problem with power amplifiers.  The degeneration resistors were added so that a higher tail current could be used for a higher slew rate (full power bandwidth) while maintaining closed loop stability.

Application Note A from National Semiconductors discusses the details and math of why the emitter degeneration resistors are included and the alternatives.

Very interesting! I will go through it though i am afraid it's a little bit too complex for me. Thanks!

[ExtraThiccBoi]

I 've also encountered small signal bipolar transistors that are noisy and have significant leakage.  Two I'm thinking of were in the input diff stage of a power amp I built in 1981 so they are from before then.  If memory serves they are Motorola MPS8099.  I didn't do any AC tests on the parts by themselves on the bench so I don't know if they are avalanching.  It took about 10 years before the noise manifested.

I understand that the leakage and noise performance of a bipolar transistor can be permanently affected by reverse biasing the B-E junction to breakdown.  I wonder if this is what is happening here.  It's easy enough to overdrive the input of a power amp when it's off if driven from a preamp.

Hmm, i have not overdriven the actual power amplifier at all. It's got quite a headroom (70w 0.09% rated, 120w 1% capable).
All of the noise issues are on the preamp/tone control board.
I replaced the differential pairs with my other 2sc2320s and will test it tomorrow, im currently working on a modification that will turn one of the old phono (salvaged the components since it was bad anyway) into a direct input right into the power amplifier, so i can see how much noise is the preamp adding.

[ExtraThiccBoi]

I have an extra question, unrelated to this preamp noise issue.
When i got this amp, it was set to a bad secondary and the power supply voltage was 4.5V higher than supposed to. On the main amplifier board, there are 2 transistors that are already being run near limit (50V) when the voltage is correct. Could this incorrect voltage setting cause these transistors to go bad? I haven't done the maths but i suppose having main voltage 54V instead of 49.6V (or thereabouts) would cause the voltage at the base of this 2SA999L (50V/50V/6V) to go over the rated 50V. Schematic included

[David Hess]

Unfortunately, it does not list the author's name.

It is an old National Semiconductor application note and like most companies then and now, National did not allow the authors to put their name on work products.  Linear Technology and Analog Devices are unusual as exceptions to this and the difference indicates the company's attitude toward employees.  Think about the connotations of the term "human resources".

I suspect more than one author was involved but the application note does say "the author".  (1)

I 've also encountered small signal bipolar transistors that are noisy and have significant leakage.

I understand that the leakage and noise performance of a bipolar transistor can be permanently affected by reverse biasing the B-E junction to breakdown.  I wonder if this is what is happening here.  It's easy enough to overdrive the input of a power amp when it's off if driven from a preamp.

High gain (hfe) transistors are the most susceptible to damage from base-emitter breakdown and these are also the best transistors for low frequency low noise applications.  Lowing the gain (hfe) directly increases the input current noise.

Personally, I would change them and maybe add reverse base-emitter breakdown protection if only to see if it makes a difference.  Calculating the expected noise just from the schematic is not difficult but measuring it is.

Could this incorrect voltage setting cause these transistors to go bad? I haven't done the maths but i suppose having main voltage 54V instead of 49.6V (or thereabouts) would cause the voltage at the base of this 2SA999L (50V/50V/6V) to go over the rated 50V. Schematic included

It is the voltage across the transistor terminals which matters.  Q413 and Q415 are emitter followers which drive the following voltage gain stage and *that* stage is what sees the high voltage.  Q413 and Q415 only operate at relatively low voltages.

(1) Author found.

[schmitt trigger]

Unfortunately, it does not list the author's name.

It is an old National Semiconductor application note and like most companies then and now, National did not allow the authors to put their name on work products.  Linear Technology and Analog Devices are unusual as exceptions to this and the difference indicates the company's attitude toward employees. 

And nothing exemplifies this better than the late Jim Williams' app notes.
Not only did he sometimes employ colloquial language and subtle humor to explain a complex idea, but he would very often include deliciously absurd footnotes: "Local historians can't be certain, but this may be the only IC pin ever named after a person.", and preposterous-sounding app note names: App Note 25, "Switching regulators for poets: A gentle guide for the trepidatious" or even better: App Note 45, "Measurement and control circuit collection: Diapers and designs on the night shift"

And let's not forget his trademark tour de force, a hand-drawn cartoon on the last page, which could be related or not to the topic at hand.

[duak]

David, thank you for your comment on low noise transistors.  Do you have any further information on this particular effect?  I remember an app note for something or other from the 70s discussing this .  It used a 2N4401 with the base disconnected as a 5 or 6 V, low leakage current transient suppressor to protect the inputs of some precision device.

I should have been a bit clearer about damaging low noise transistors in audio systems.  In my case, the power amp was close to the speakers with long RCA phono ended cables from the preamp.  It's also got a safety ground that ties to signal ground through 100K in parallel with some capacitance.  The preamp was in a rack with various other equipment like a tuner, CD player and VCR; all of which have only a two pole line connection - none had a safety ground.  The big problem with RCA connectors is that the center signal pin makes contact before ground does.  When connecting the power amp, it's a damn good idea to not have it powered on or you could frap a speaker so this clearly indicates there is significant voltage there.  I suspect the voltage was high enough to damage the transistors, maybe not all at once but over a period of time.  Since I built the preamp too, I was often making changes and disconnecting and reconnecting the cables.    I mentioned I had a VCR in my system.  It was connected to the cable TV network.  I found that the VCR was injecting a 60 Hz hum into the system because it had a 5 VAC 60 Hz at 100 mA interfering signal coming from cable.  I put a couple of back to back 75 to 300 ohm transformers on the cable to break the ground loop and hum went away.

If the OP has found noisy transistors in the preamp, it would be more difficult but not impossible to have something similar happen.  The overvoltage could come from just plugging in an audio source that has an electrostatic charge on it.

Food for thought, anyway.

[David Hess]

David, thank you for your comment on low noise transistors.  Do you have any further information on this particular effect?  I remember an app note for something or other from the 70s discussing this .  It used a 2N4401 with the base disconnected as a 5 or 6 V, low leakage current transient suppressor to protect the inputs of some precision device.

I found some discussions though a search like "base emitter bipolar transistor breakdown hot carrier".  My understanding is that the breakdown produces hot carriers which have enough energy to cause dislocations in the crystal structure of the junction damaging it.  These dislocations allow minority carriers to recombine lowering minority carrier lifetime lowering current gain.  Gold doping also lowers minority carrier lifetime reducing hfe which is why RF and fast saturated switching transistors have low gain.

The damage can be repaired by annealing the junction at high temperature but of course this is hardly feasible in a typical application.  I wonder though if any space applications where radiation was an issue did this.

Since gold doped transistors and transistors operating at high current density already have low current gain, they are not affected significantly.  But high current gain transistors and transistors operating at low current density are especially susceptible.

I should have been a bit clearer about damaging low noise transistors in audio systems.  In my case, the power amp was close to the speakers with long RCA phono ended cables from the preamp.  It's also got a safety ground that ties to signal ground through 100K in parallel with some capacitance.  The preamp was in a rack with various other equipment like a tuner, CD player and VCR; all of which have only a two pole line connection - none had a safety ground.  The big problem with RCA connectors is that the center signal pin makes contact before ground does.  When connecting the power amp, it's a damn good idea to not have it powered on or you could frap a speaker so this clearly indicates there is significant voltage there.  I suspect the voltage was high enough to damage the transistors, maybe not all at once but over a period of time.  Since I built the preamp too, I was often making changes and disconnecting and reconnecting the cables.    I mentioned I had a VCR in my system.  It was connected to the cable TV network.  I found that the VCR was injecting a 60 Hz hum into the system because it had a 5 VAC 60 Hz at 100 mA interfering signal coming from cable.  I put a couple of back to back 75 to 300 ohm transformers on the cable to break the ground loop and hum went away.

Single ended consumer audio stuff always has problems with hum unless care is taken with the configuration.  Powering everything off of one outlet strip is a good start.

Protecting the input transistors can be as simple as placing a diode anti-parallel with the base-emitter junction of each transistor and some integrated differential pairs do exactly this:

https://www.analog.com/media/en/technical-documentation/data-sheets/mat12.pdf
http://www.leadelectronics.com/product_specs/LM394-DATASHEET.PDF
http://www.alfarzpp.lv/eng/sc/AS394CH.pdf

[duak]

Thank you David.  I had known about gross over current thru a zener, AKA "zener zapping" where the current is high enough to cause metal migration and shorting of the junction.  Your comments indicate that damage or degradation can occur at far lower energy levels.  I don't design anything for anyone except myself these days.  However, it's satisfying to learn something about a small mystery.

While doing some research (frobnicating on the 'net) I ran across this article on reverse avalanching in bipolar transistors: http://www.kerrywong.com/2014/03/19/bjt-in-reverse-avalanche-mode/  I had always wondered if there was anything special about the 2N4401 but it appears that there isn't; most any general purpose BJT appears to do this.  The 2N4401 seems to be similar to the 2N3904 but has a greater current rating.  I can see from the article that the effect demonstrates negative resistance which I think could be an advantage as a transient absorber, ie., it will reduce the duration of the pulse applied to the following circuitry.

[David Hess]

Thank you David.  I had known about gross over current thru a zener, AKA "zener zapping" where the current is high enough to cause metal migration and shorting of the junction.  Your comments indicate that damage or degradation can occur at far lower energy levels.  I don't design anything for anyone except myself these days.  However, it's satisfying to learn something about a small mystery.

Bob Pease wrote some articles discussing it also.

I had always wondered if there was anything special about the 2N4401 but it appears that there isn't; most any general purpose BJT appears to do this.  The 2N4401 seems to be similar to the 2N3904 but has a greater current rating.  I can see from the article that the effect demonstrates negative resistance which I think could be an advantage as a transient absorber, ie., it will reduce the duration of the pulse applied to the following circuitry.

I wish manufacturers gave more details about the fabrication of different devices.  A little can be learned from the National Semiconductor Discrete Databook.  The 2N3904/2N3906 are *not* just a smaller 2N4401/2N4403; they use a different process and a different layout.  Of course different manufacturers make different parts with the same part number and JEDEC specifications.

2N2222 1-17 20   NPN Medium Power (gold doped)
2N2906 2-10 63   PNP Medium Power

2N3904 1-20 23   NPN Small Signal (gold doped, interdigitated)
2N3906 2-13 66   PNP Small Signal (gold doped)

2N4401 1-21 13   NPN Medium Power
2N4403 2-14 63   PNP Medium Power

[elecdonia]

I have an extra question, unrelated to this preamp noise issue.
When i got this amp, it was set to a bad secondary and the power supply voltage was 4.5V higher than supposed to. On the main amplifier board, there are 2 transistors that are already being run near limit (50V) when the voltage is correct. Could this incorrect voltage setting cause these transistors to go bad? I haven't done the maths but i suppose having main voltage 54V instead of 49.6V (or thereabouts) would cause the voltage at the base of this 2SA999L (50V/50V/6V) to go over the rated 50V. Schematic included]

If the power amplifier sections are working on both channels then there is no problem with the circled transistors.  These operate at low collector current compared to other transistors in this amplifier circuit. So they shouldn't get warmer than room temperature. The fact that the power supply voltage might be slightly higher than the voltage rating of these transistors isn't likely to be an issue.

This is a rather complicated power amplifier circuit. HK was known for that during the 1980's especially.  HK designed their circuitry for very low distortion and very wide bandwidth.

This amplifier is capable of sounding good with most speakers.  But there might be a handful of speaker systems that it doesn't sound good with. Speaker systems with extremely wide ranges of impedance, from low to high at different frequencies might cause this amplifier circuit to oscillate intermittently. That could create a "harsh" or "tinny" sound quality.  This would depend on the relationship between the amplifier and the speaker.  In particular, some of the multi-driver speaker systems of UK design and origin might present issues when driven by this power amplifier.

RE: The power amplifier section:  It is very important to adjust the output offset voltage to a value as near 0.0 volts as possible. Also there should be a detailed procedure in the HK service manual for adjusting the "idle current" of the large output transistors. There are a total of 4 adjustment potentiometers:  Each channel (left, right) has 2 potentiometers:  One potentiometer is  used to adjust the DC output voltage at the speaker terminal to 0.0 volts. The other potentiometer is used to adjust the "idle current."

-EB
 

[ExtraThiccBoi]

I have an extra question, unrelated to this preamp noise issue.
When i got this amp, it was set to a bad secondary and the power supply voltage was 4.5V higher than supposed to. On the main amplifier board, there are 2 transistors that are already being run near limit (50V) when the voltage is correct. Could this incorrect voltage setting cause these transistors to go bad? I haven't done the maths but i suppose having main voltage 54V instead of 49.6V (or thereabouts) would cause the voltage at the base of this 2SA999L (50V/50V/6V) to go over the rated 50V. Schematic included]

If the power amplifier sections are working on both channels then there is no problem with the circled transistors.  These operate at low collector current compared to other transistors in this amplifier circuit. So they shouldn't get warmer than room temperature. The fact that the power supply voltage might be slightly higher than the voltage rating of these transistors isn't likely to be an issue.

This is a rather complicated power amplifier circuit. HK was known for that during the 1980's especially.  HK designed their circuitry for very low distortion and very wide bandwidth.

This amplifier is capable of sounding good with most speakers.  But there might be a handful of speaker systems that it doesn't sound good with. Speaker systems with extremely wide ranges of impedance, from low to high at different frequencies might cause this amplifier circuit to oscillate intermittently. That could create a "harsh" or "tinny" sound quality.  This would depend on the relationship between the amplifier and the speaker.  In particular, some of the multi-driver speaker systems of UK design and origin might present issues when driven by this power amplifier.

RE: The power amplifier section:  It is very important to adjust the output offset voltage to a value as near 0.0 volts as possible. Also there should be a detailed procedure in the HK service manual for adjusting the "idle current" of the large output transistors. There are a total of 4 adjustment potentiometers:  Each channel (left, right) has 2 potentiometers:  One potentiometer is  used to adjust the DC output voltage at the speaker terminal to 0.0 volts. The other potentiometer is used to adjust the "idle current."

-EB
 

Yes i adjusted it as the spec says, idling current is 41.70mV in both channels (40mV spec). I reduced DC offset as much as i could but due to that capacitor in feedback loop, it tends to drift couple tens of mV.
The power amplifier works flawlessly, sounds good and is very dynamic.
Thanks a lot for your help!

[magic]

So how is the preamp doing? Did input pair replacement fix its noise?

[ExtraThiccBoi]

So how is the preamp doing? Did input pair replacement fix its noise?

Nope it didnt fix it.
I guess my only option would be to redo that preamp stage with a op-amp, i have an old mitsubishi which is made to run at 23V for exactly these sorts of applications, but i don't have a fricking clue how to build that. I guess i would have to attach on the feedback lines and then deal with the input and output resistors and capacitors.

[ExtraThiccBoi]

I have an extra question, unrelated to this preamp noise issue.
When i got this amp, it was set to a bad secondary and the power supply voltage was 4.5V higher than supposed to. On the main amplifier board, there are 2 transistors that are already being run near limit (50V) when the voltage is correct. Could this incorrect voltage setting cause these transistors to go bad? I haven't done the maths but i suppose having main voltage 54V instead of 49.6V (or thereabouts) would cause the voltage at the base of this 2SA999L (50V/50V/6V) to go over the rated 50V. Schematic included]

If the power amplifier sections are working on both channels then there is no problem with the circled transistors.  These operate at low collector current compared to other transistors in this amplifier circuit. So they shouldn't get warmer than room temperature. The fact that the power supply voltage might be slightly higher than the voltage rating of these transistors isn't likely to be an issue.

This is a rather complicated power amplifier circuit. HK was known for that during the 1980's especially.  HK designed their circuitry for very low distortion and very wide bandwidth.

This amplifier is capable of sounding good with most speakers.  But there might be a handful of speaker systems that it doesn't sound good with. Speaker systems with extremely wide ranges of impedance, from low to high at different frequencies might cause this amplifier circuit to oscillate intermittently. That could create a "harsh" or "tinny" sound quality.  This would depend on the relationship between the amplifier and the speaker.  In particular, some of the multi-driver speaker systems of UK design and origin might present issues when driven by this power amplifier.

RE: The power amplifier section:  It is very important to adjust the output offset voltage to a value as near 0.0 volts as possible. Also there should be a detailed procedure in the HK service manual for adjusting the "idle current" of the large output transistors. There are a total of 4 adjustment potentiometers:  Each channel (left, right) has 2 potentiometers:  One potentiometer is  used to adjust the DC output voltage at the speaker terminal to 0.0 volts. The other potentiometer is used to adjust the "idle current."

-EB
 

I see you know a lot about audio amps and the circuit designs, would you be able to tell which one of those is better? One is HK6500, the amp that i have. Second is PA2200, a power amp i can get a deal on. It's rated higher but the european version of the 2200 is cut down, so it will most likely provide similiar RMS power to the HK6500.

[magic]

I guess my only option would be to redo that preamp stage with a op-amp

Or insert an opamp between the volume pot and the preamp. Its output impedance will be zero and the preamp will be happy.
Doesn't need to be a high voltage one, just add a 78L15/79L15 pair and some capacitors.

You will probably need a FET opamp. I would try OPA2134 for starters or TL072 if you feel cheap. Bipolar opamps like NE5532 may still have too much current noise for this volume pot. No idea what your Mitsubishi opamp is.

Good idea to use a socket, I guess. Just mount it on a piece of perfboard, connect and and see what happens. Beware that it may change nothing if the noise comes from the volume pot itself rather than the input transistors, but it probably comes from the trannies.

[David Hess]

Adding a JFET or CMOS voltage follower to buffer the potentiometer is probably the easiest way.

Or the bipolar differential pair could be replaced with a JFET differential pair.

[magic]

CMOS certainly isn't audiophile-approved
Not even sure if there are any suitable CMOS amps out there - low noise, high voltage, good CMRR, low distortion...

Discrete JFETs are perfectly halal but the reduction in open loop gain would have unknown influence on distortion of that stage. The degeneration resistors could be removed, but OP doesn't even have a scope to test for stability.

[ExtraThiccBoi]

I guess my only option would be to redo that preamp stage with a op-amp

Or insert an opamp between the volume pot and the preamp. Its output impedance will be zero and the preamp will be happy.
Doesn't need to be a high voltage one, just add a 78L15/79L15 pair and some capacitors.

You will probably need a FET opamp. I would try OPA2134 for starters or TL072 if you feel cheap. Bipolar opamps like NE5532 may still have too much current noise for this volume pot. No idea what your Mitsubishi opamp is.

Good idea to use a socket, I guess. Just mount it on a piece of perfboard, connect and and see what happens. Beware that it may change nothing if the noise comes from the volume pot itself rather than the input transistors, but it probably comes from the trannies.

I have a mitsubishi m5220l SIP. Pulled from an old amp. It's a bipolar opamp, would that atleast help? The thing is most modern opamps arent bueno for such high voltage rails from the datasheets i've seen.
I will have to run a jumper from output to inverting input right? Is that a voltage buffer operation?
Should be easy enough to freewire it.
Sorry, i am not really good with designing circuits and even worse with building them from scratch.

[David Hess]

CMOS certainly isn't audiophile-approved
Not even sure if there are any suitable CMOS amps out there - low noise, high voltage, good CMRR, low distortion...

The performance of the existing design is already compromised enough that using a good CMOS operational amplifier will not be a disadvantage.

Discrete JFETs are perfectly halal but the reduction in open loop gain would have unknown influence on distortion of that stage. The degeneration resistors could be removed, but OP doesn't even have a scope to test for stability.

That is why this option is more complicated compared to just adding a voltage follower; performance testing and some changes would be necessary although it is possible to just compute the new source degeneration resistor values.  With the same adjusted input stage transconductance, everything should work the same except the distortion characteristics will be different.

[magic]

OPA2604 is the only JFET opamp coming to my mind that can run on ±22V. A genuine one, because a "rebranded" TL072 could blow up
It may be cheaper to add 15V regulators and get a wider choice of opamps.

I frankly have no idea how discrete transistors compare to those in bipolar opamp input stages in terms of current noise. If they are comparable, a bipolar opamp ought to have some advantage over discrete due to about 10x lower input bias current. Whether it will be sufficient for you I don't know.

Yes, you would connect IN+ of each channel to the signal and IN- straight to OUT. Some 47-100Ω resistor between OUT and the cable to the preamp board may be a good idea to avoid risk of oscillation. Add 100nF ceramic plus a 10-100uF electrolytic between V+/V- and that should suffice for a quick noise test. I'm not even sure if it makes sense to upgrade to separate capacitors from each rail to ground later, the output current is very low.

And I came up with another option: if you set volume to max and balance to center, the preamp board will probably be more or less shorted to the input jacks. So it will see whatever impedance the external source has. So you could set the volume to max and use an external box with a low impedance (like 5~20k) volume pot. Or even an active preamp or whatnot.
It would take exerimentation to see what input impedance can be tolerated. So take some resistors and insert them into the RCA jack (from hole to ground) and see how much hiss remains when you turn volume up. Don't touch the input jacks with volume at max

[ExtraThiccBoi]

OPA2604 is the only JFET opamp coming to my mind that can run on ±22V. A genuine one, because a "rebranded" TL072 could blow up
It may be cheaper to add 15V regulators and get a wider choice of opamps.

I frankly have no idea how discrete transistors compare to those in bipolar opamp input stages in terms of current noise. If they are comparable, a bipolar opamp ought to have some advantage over discrete due to about 10x lower input bias current. Whether it will be sufficient for you I don't know.

Yes, you would connect IN+ of each channel to the signal and IN- straight to OUT. Some 47-100Ω resistor between OUT and the cable to the preamp board may be a good idea to avoid risk of oscillation. Add 100nF ceramic plus a 10-100uF electrolytic between V+/V- and that should suffice for a quick noise test. I'm not even sure if it makes sense to upgrade to separate capacitors from each rail to ground later, the output current is very low.

And I came up with another option: if you set volume to max and balance to center, the preamp board will probably be more or less shorted to the input jacks. So it will see whatever impedance the external source has. So you could set the volume to max and use an external box with a low impedance (like 5~20k) volume pot. Or even an active preamp or whatnot.
It would take exerimentation to see what input impedance can be tolerated. So take some resistors and insert them into the RCA jack (from hole to ground) and see how much hiss remains when you turn volume up. Don't touch the input jacks with volume at max

Following your recommendation, i will probably do a dirty fix. I will run the wire from the preamp board to a DPDT switch. The other set of contacts will carry signal from extra RCAs at the back with 1K resistors (harman kardon reverse engineering). If i want to run it as power amp for desk use, i will run directly from those rear RCAs into the power amp, with my Magni 2 Uber as preamp (it has buffered output). If i want to run it as integrated amp, i will just switch the DPDT switch and put signal into the power amp from the preamp and whole integrated BS-ittery. Might be the easiest solution.

[floobydust]

The JFET muting circuit is common to the inputs to both channels, you might want to check its health. I have seen a failed JFET generate noise for two channels.
You can also use freeze spray to track down a noisy BJT or component. Before you blame the design. HK was pretty good.

[ExtraThiccBoi]

The JFET muting circuit is common to the inputs to both channels, you might want to check its health. I have seen a failed JFET generate noise for two channels.
You can also use freeze spray to track down a noisy BJT or component. Before you blame the design. HK was pretty good.

The power amp is good. I just finished the mod and after checking with the volume knob, the power amp generates very little noise. If i crank the Magni 2 Uber to the max (ear-shattering scenario with -18dB RMS digital output from my Modi 2 Uber DAC), there is a little tiny bit of hiss but MUCH quiter by what is generated by the preamp. I won't blame the power amplifier design, after reading some quick articles, i can tell even with noob's eye it's good. In fact, i have purchased another power amplifier incorporating basically the same design but with twice the current headroom.
The JFETs from my understanding are used to short the inputs to ground when the protection circuit detects fault, afaik shorting inputs means turning off the power transistors (??).

[David Hess]

OPA2604 is the only JFET opamp coming to my mind that can run on ±22V. A genuine one, because a "rebranded" TL072 could blow up

44 volt JFET operational amplifiers are not very common.  My list only includes the old LF3xx and LF4xx series of parts.  Analog Devices only has the LTC6090.  The OPA604 from TI looks ideal but the LME49860 could also work.  There are some other potential parts from TI.

It may be cheaper to add 15V regulators and get a wider choice of opamps.

Or even better, shunt regulators in the form of a zener diode (or transistor base-emitter junction) and a resistor for each side.  The current demands are not great and I would not trust a 44 volt part working directly off of that supply anyway.

I frankly have no idea how discrete transistors compare to those in bipolar opamp input stages in terms of current noise. If they are comparable, a bipolar opamp ought to have some advantage over discrete due to about 10x lower input bias current. Whether it will be sufficient for you I don't know.

The only advantage integrated bipolar parts have is the availability of super-beta transistors.

[floobydust]

The JFET muting circuit is common to the inputs to both channels, you might want to check its health. I have seen a failed JFET generate noise for two channels.
You can also use freeze spray to track down a noisy BJT or component. Before you blame the design. HK was pretty good.

...
The JFETs from my understanding are used to short the inputs to ground when the protection circuit detects fault, afaik shorting inputs means turning off the power transistors (??).

The "muting protection" circuit has only output over-current detect, and power up delay timer, I don't see DC detect. I think it's mostly anti-thump.
You have to check coupling capacitors, when you recap old audio gear sometimes polarity is backwards. With age, DC offsets can change, or the orginal schematic can be wrong.
At the tone controls, C513/C514 is suspicious compared to C517/C518. Just to make sure cap leakage current isn't adding noise.

[ExtraThiccBoi]

The JFET muting circuit is common to the inputs to both channels, you might want to check its health. I have seen a failed JFET generate noise for two channels.
You can also use freeze spray to track down a noisy BJT or component. Before you blame the design. HK was pretty good.

...
The JFETs from my understanding are used to short the inputs to ground when the protection circuit detects fault, afaik shorting inputs means turning off the power transistors (??).

The "muting protection" circuit has only output over-current detect, and power up delay timer, I don't see DC detect. I think it's mostly anti-thump.
You have to check coupling capacitors, when you recap old audio gear sometimes polarity is backwards. With age, DC offsets can change, or the orginal schematic can be wrong.
At the tone controls, C513/C514 is suspicious compared to C517/C518. Just to make sure cap leakage current isn't adding noise.

Yep, it brings the amp up slowly and indeed, no pops or thumps.
All caps, resistors, transistors are correctly placed, i've checked that many times. I guess the tone circuit is just high noise design or the 2sc2320 are bad transistors.
I replaced the caps with values that were in the amp to begin with, the schematic had some differences.
Btw, that 22uF/10V cap was 2.2/50V in my amp, replaced it with 2.2 as well.
I managed to get absolutely minimal DC offset with tongue at the right angle and plastic pokey thingy at 5mV when idle around 15mV when playing (that cap in feedback loop causes this drift afaik).

[magic]

Following your recommendation, i will probably do a dirty fix. I will run the wire from the preamp board to a DPDT switch. The other set of contacts will carry signal from extra RCAs at the back with 1K resistors (harman kardon reverse engineering). If i want to run it as power amp for desk use, i will run directly from those rear RCAs into the power amp, with my Magni 2 Uber as preamp (it has buffered output). If i want to run it as integrated amp, i will just switch the DPDT switch and put signal into the power amp from the preamp and whole integrated BS-ittery. Might be the easiest solution.

Not exactly, my recommendation (and the easiest solution if it works) was to turn the volume to maximum and use an external passive/active preamp for attenuation, connected to the normal input jacks.

[plurn]

If it is anything like my Harman Kardon PM640VXi (HK6500 certainly looks similar externally and is about the same age), make sure you try cleaning out all the pots on the front panel with some electrical clean and lube (or just electrical clean) - volume, balance, tone etc. Behind the front panel there is a little hole in each pot for you to insert the spray tube. You spray a bit in and work the pots back and forth to loosen the grime, and spray a bit more to flush the grime out. Put a cloth around the pot to catch the liquid to stop it getting everywhere. Let it dry out before using it.

Over time my PM640VXi got quite noisy (hiss) and the noise got louder with volume setting. Volume and balance were very touchy with dead spots - had to fiddle with the controls to get the a setting that worked well until I eventually gave up. I thought it would need repairs and ended up buying another amp instead. Went back to it a year later when I got more adventurous with fixing electronics and cleaned the pots and it was like new. No noticeable noise any more, no dead spots. It was quite a surprise as I assumed it needed new caps etc.

I have heard this is a common problem with Harman Kardon amps of that era. They just need the pots cleaned from time to time.

Might not fix your problem, but it is very easy to try and can't hurt.

[ExtraThiccBoi]

If it is anything like my Harman Kardon PM640VXi (HK6500 certainly looks similar externally and is about the same age), make sure you try cleaning out all the pots on the front panel with some electrical clean and lube (or just electrical clean) - volume, balance, tone etc. Behind the front panel there is a little hole in each pot for you to insert the spray tube. You spray a bit in and work the pots back and forth to loosen the grime, and spray a bit more to flush the grime out. Put a cloth around the pot to catch the liquid to stop it getting everywhere. Let it dry out before using it.

Over time my PM640VXi got quite noisy (hiss) and the noise got louder with volume setting. Volume and balance were very touchy with dead spots - had to fiddle with the controls to get the a setting that worked well until I eventually gave up. I thought it would need repairs and ended up buying another amp instead. Went back to it a year later when I got more adventurous with fixing electronics and cleaned the pots and it was like new. No noticeable noise any more, no dead spots. It was quite a surprise as I assumed it needed new caps etc.

I have heard this is a common problem with Harman Kardon amps of that era. They just need the pots cleaned from time to time.

Might not fix your problem, but it is very easy to try and can't hurt.

Very interesting, glad to hear yours was a simple fix! I am afraid i can't clean the pots more than i already did. I sprayed more than a couple ml of car contact cleaner (very aggresive stuff, eats away resins and enamel from coils) into those pots and spent good half an hour turning them back and forth.
I will try it once more and see if it helps. Thanks for the comment!

[ExtraThiccBoi]

Adding a JFET or CMOS voltage follower to buffer the potentiometer is probably the easiest way.

Or the bipolar differential pair could be replaced with a JFET differential pair.

Could replacing the input pair with 2N5088 or 2N5089 help? Matching them as close as possible of course.

[plurn]

ExtraThiccBoi wrote:
"... I sprayed more than a couple ml of car contact cleaner (very aggresive stuff, eats away resins and enamel from coils) into those pots and spent good half an hour turning them back and forth. ..."

I would never recommend using an aggressive cleaner like that for pots. That stuff sounds dangerous. I have typically heard you want cleaner specifically for cleaning electrical potentiometers that is safe for plastic as they often have plastic in them.

Anyway sounds like I guessed wrong since you had already cleaned them. Was hoping it would have been an easy fix for you.

Good luck and I hope you figure it out soon.

[Richard Crowley]

Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?  Granted that we regularly see failure from dried-up electrolytic caps in vintage gear, and increased ESR and leakage in vintage paper capacitors (c.f. Mr. Carlson).  But when has that ever demonstrated to have any real impact on noise floor?  IME, the list of suspects for circuit noise (Johnson noise) has active components (firebottles, transistors, ICs.) at #1 on the list (with a bullet). followed (at a distant second place) by resistors, especially in high impedance circuits.  Do capacitors even make the Top-5 Most Most wanted on the noise suspect list?  Bad/cheap circuit design and/or component-sourcing seems way ahead of capacitors as another suspect cause.

[plurn]

Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?  Granted that we regularly see failure from dried-up electrolytic caps in vintage gear, and increased ESR and leakage in vintage paper capacitors (c.f. Mr. Carlson).  But when has that ever demonstrated to have any real impact on noise floor?  IME, the list of suspects for circuit noise (Johnson noise) has active components (firebottles, transistors, ICs.) at #1 on the list (with a bullet). followed (at a distant second place) by resistors, especially in high impedance circuits.  Do capacitors even make the Top-5 Most Most wanted on the noise suspect list?  Bad/cheap circuit design and/or component-sourcing seems way ahead of capacitors as another suspect cause.

If it is a myth - you have taught me something so thank you for that. If we accept that it is a myth I would think that it has come from the following:

- electrolytic capacitors have become the number one thing to fail in relatively modern electronics (that is what I am lead to believe - could be another popular myth) so they tend to be the first thing blamed for any fault whether they are the cause or not. It is my go to reason for faults (until further troubleshooting) so I am guilty of that.

- bad capacitors can be a cause of mains hum in audio products (that is my understanding anyway - could be wrong) which is a type of noise - not a big stretch for people to also think it is a cause of other types of noise.

- many people without much electronics experience are aware of bad capacitors as a cause of issues, yet these same people may not be aware of other causes, so capacitors typically get the blame and this information spreads.

I would be in the category of having not much electronics experience so you can judge my comments accordingly.

[magic]

Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?

It is a well known fact which some "audio objectivists" still refuse to acknowledge that modded equipment always sounds better than unmodded (unless you totally screw it up)
This explains the origin of virtually all audio voodoo and also why there is so much of it.

[David Hess]

Adding a JFET or CMOS voltage follower to buffer the potentiometer is probably the easiest way.

Or the bipolar differential pair could be replaced with a JFET differential pair.

Could replacing the input pair with 2N5088 or 2N5089 help? Matching them as close as possible of course.

If the original transistors are damaged, it would help a lot assuming they meet the voltage, current, and power requirements.  The BC549C or BC550C are also high gain and what would typically be used.