Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)

[Fried Chicken]

I’ve gone deeper into audio than is healthy; I know I’m not the only person who’s been afflicted by this.

I recapped an old amplifier (Yamaha M-65) and removed some corrosive glues.  I want to go in and actually test this work: channel separation, signal clarity, stuff like that.

My equipment to achieve this is limited: analog 2-channel scope, analog function generator, Fluke DMM, a smattering of coax adapters, and a couple power resistors.

The power resistors, I’m not sure can handle the full power of the amp.., I think I have two 4 ohm resistors, but the speakers I run are 8 ohm.  They are expensive!

Anyway, I’m just fishing for advice here, open to suggestions on what to do, what not to do.

I’ve heard claims that capacitors need to “break-in”; I’m not sure I believe that.  I can set the bias voltages (to center the A vs AB amplification I think this is?  What the signal rectified around?).  That and calibrating the VU meters is about all that’s offered in the service manual.

I think it’s here:
https://elektrotanya.com/yamaha_m-65_sm.pdf/download.html

Anyway, this is a fun project for me.  If anyone has any especially wild tricks, like figuring out the THD with all analog stuff, that’ll perk my ears!

Thanks in advance

[CaptDon]

Be aware of the subjective bullshit observations of 'Audiophools'!!! Remember that in a very real double blind test 12-2 NM house wire beat every other test sample wire including idiotic $30.00 per foot stuff you could use as a mooring line. And then the audiophools still insisted there must be a flaw in the testing because their expensive stuff MUST be better!!! Stop and think, how would you measure channel separation? Drive one channel to maybe 80% of full output and look for residual output on the other channel. Then calculate how many dB from the voltage ratio. THD is nearly impossible to measure with the gear you have. Audio THD meters are a specialized instrument. Look up the Potomac Instruments 'AA' audio analyzer or the older HP audio analyzers. Measure the THD of your generator and then measure the output of the amplifier. Cheap function generators and audio oscillators have terrible THD to begin with. As for 'A' vs. 'AB' vs. 'AB1' vs. 'AB2' setting of the bias, a lot of it comes down to "What sounds best to you at your listening level". So much bad information among tube amp guitar players!!!! How many amps I have seen burned up because some jackass said "set the bias to where the plates just begin to glow red". W.T.F., the whole idea of class B push pull is that the outputs really don't need to idle more than about 10% up the conduction curve. These red plate chuckle heads are around the class A push pull point of operation!! Audiophools think little wooden wire supports help the sound and expensive wall receptacles "Give a more open sound stage". Idiots!!

[Fried Chicken]

I appreciate it…

I was really really hoping not to encounter pro- or anti- audiophile bullshit here, and is a reason I asked here and not on audio forums.

Yamaha gives specific bias voltages in the manual, that’s what I intend to set them to.

How do I even know what sort of signals to apply to the amp?  Should I use the front meters?



I can find the difference between two signals on a scope by inverting one and adding them, right?

[CaptDon]

Many of the '50dB down' artifacts may be approaching the noise floor of your scope and test equipment. Many 'purpose built' audio analyzers can do THD, intermod IMD and other tests but often require the companion audio generator. Tektronix in their TM5000 (not the 500) series of modules had some audio distortion modules with matching generator. They still fetch big bucks on Ebay and NEVER buy from a seller who sells test equipment and lists something as 'untested'!!! That means they already tested it (after all, they are a test equipment dealer) and it is blown to bloody f--king hell!! I hate those crooks!!! May as well buy one laying in a bucket of rain water at a hamfest where it rained all day!! Anyway, best wishes in reaching your goals! Invest in good test gear as it will hold its resale value. Cheap junk won't!!

[Fried Chicken]

Surely something can be done with a normal scope?  I mean the scope samples at several orders of magnitude above audio signals?

Are there no analog tricks or similar?

[CaptDon]

THD and IMD are specialized measurements requiring filters and accurate clean frequencies incorporated inside the audio analyzers. I forgot about the Tektronix AA-501 audio analyzer for the TM500 series racks. Just looked at two on Ebay $500 and $840. If I were still making a living doing audio repairs I would consider that but I am retired these days from electronics. Still Master of a 140 passenger tour boat, guitarist and ham radio operator keeping me busy and somewhat sane in my 'golden years'.

[vk6zgo]

Surely something can be done with a normal scope?  I mean the scope samples at several orders of magnitude above audio signals?

Are there no analog tricks or similar?

Well, you can measure channel separation (aka as "crosstalk" in comms), Power output, pretty roughly with the 'scope, although DSOs do have a voltage readout), maybe guess on Noise, but it will be down near the bottom of the scope's range, or depending on the DMM, that may work.

If your 'scope has FFT, perhaps you could maybe.use that for THD, but what you really need is either a "Noise & Distortion Test set", which incorporates a sharp tunable filter, which you can use to notch out the fundamental frequency leaving the harmonics (plus noise), or as an alternative, a selective voltmeter, which can show you the levels of each of the harmonics.

There should be quite a number of N& D sets around, though they still maintain their price.
Selective voltmeters are somewhat rarer.

[Fried Chicken]

Surely something can be done with a normal scope?  I mean the scope samples at several orders of magnitude above audio signals?

Are there no analog tricks or similar?

Well, you can measure channel separation (aka as "crosstalk" in comms), Power output, pretty roughly with the 'scope, although DSOs do have a voltage readout), maybe guess on Noise, but it will be down near the bottom of the scope's range, or depending on the DMM, that may work.

If your 'scope has FFT, perhaps you could maybe.use that for THD, but what you really need is either a "Noise & Distortion Test set", which incorporates a sharp tunable filter, which you can use to notch out the fundamental frequency leaving the harmonics (plus noise), or as an alternative, a selective voltmeter, which can show you the levels of each of the harmonics.

There should be quite a number of N& D sets around, though they still maintain their price.
Selective voltmeters are somewhat rarer.

Yeah; I'm not getting into all of this.

I guess I should have rephrased the question: what *can* I do with my basic equipment?  I'm not against rigging up some clever circuit, but I'm also not wanting to go to unreasonable levels

[edit]

I found this nice playlist on youtube by a guy named "Blueglow Electronics":

[vk6zgo]

Surely something can be done with a normal scope?  I mean the scope samples at several orders of magnitude above audio signals?

Are there no analog tricks or similar?

Well, you can measure channel separation (aka as "crosstalk" in comms), Power output, pretty roughly with the 'scope, although DSOs do have a voltage readout), maybe guess on Noise, but it will be down near the bottom of the scope's range, or depending on the DMM, that may work.

If your 'scope has FFT, perhaps you could maybe.use that for THD, but what you really need is either a "Noise & Distortion Test set", which incorporates a sharp tunable filter, which you can use to notch out the fundamental frequency leaving the harmonics (plus noise), or as an alternative, a selective voltmeter, which can show you the levels of each of the harmonics.

There should be quite a number of N& D sets around, though they still maintain their price.
Selective voltmeters are somewhat rarer.

Yeah; I'm not getting into all of this.

I guess I should have rephrased the question: what *can* I do with my basic equipment?  I'm not against rigging up some clever circuit, but I'm also not wanting to go to unreasonable levels

[edit]

I found this nice playlist on youtube by a guy named "Blueglow Electronics":

Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.

An interesting alternative people have often suggested was that of using the Sound Card on a computer.
I have no experience with Sound Cards used in this manner, but there may be "how to" descriptions on the Internet.

[Kleinstein]

A twin T notch fitler for THD test can be build relatively easy. One may want some trim (2 pots or 1 pot and adjusting the frequency).
Builing a reasonable low THD generator is possible, but needs some knowedge and not all plans found hold the promisses.
For just hearing there is little need for THD much better then 0.1% - one can hardly hear 1%. For checking the amplifier specs it can get a bit tricky though.

Using a soundcard is a real option, especially as a low THD generator.
Quite some of the modern audio measurement systems are essentially a good sound card with a few input dividers and software.

[CaptDon]

There is a program called spectrum lab and several others, perhaps RoomEq that use your sound card and software FFT to show spectrum analysis of your signal. When I compare 'those' results to a real purely analog 'swept I.F.' style analyzer the results differ and there must be at least four types of FFT which all show different results. I think my Tektronix TDS644B scope has at least three FFT calculation algorithms. Anyway, the simple point being if you analyze the input signal to the amplifier and note its cleanliness and harmonic content and then analyze the output most forms of distortion show up as addition harmonics or elevated harmonic levels. If the sinewaves look good on the scope and sound good to your ear then you are in the ballpark. As you raise and lower the output level of the amplifier look for three important signs of problems. 1. Zero Crossing Distortion, Shows up as little flat segments where the trace crosses zero. Usually at low volume levels and they sound really bad!! 2. Flat Topping Clipping, especially if it occurs well below the max power output level. 3. Weird Kinks in Sinewave. This is sometimes found in so called 'Class G' type amplifiers where there are multiple voltage rails in the output stage and the BJT or MOSFET devices are 'stacked'. Many of the Bob Carver amplifiers are built this way and the transfer of voltage output level can see a small anomaly as the next higher device in the ladder begins to source current and voltage (P=I X E). Remember, even harmonics are musically pleasing, odd harmonics sound horrible and are very evident to the ear!!

[Fried Chicken]

This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

I can safely probe around bing bing bing bing bing?

[Fried Chicken]

Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.

Well... what other "tricks"?

Does the A minus B work off the displayed signal or the fed signal?  Can I set the voltages so that the two signals match on screen and then do A minus B?  I can alter the phasing by altering the trigger, no?  Or do I need a tunable RC circuit to change the phase?

[vk6zgo]

Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.

Well... what other "tricks"?

Does the A minus B work off the displayed signal or the fed signal?  Can I set the voltages so that the two signals match on screen and then do A minus B?  I can alter the phasing by altering the trigger, no?  Or do I need a tunable RC circuit to change the phase?

There may be tricks you can do with a DSO, but with an analog 'scope the RC phase change circuit would be the way to go.
The difference in shape of the two signals would show up as the result of A minus B, which may require some analysis to work out the relationship between that display & distortion in % (or dB).

It will, in fact be a result of THD + Noise, but usually the latter is low enough to be ignored.

In TV testing, we expressed both THD + Noise & Noise by itself in dB, making it easier to mathematically remove the latter to give pure THD.

[TimFox]

This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

I can safely probe around bing bing bing bing bing?

Usually, that is true.  It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output.

[TimFox]

Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.

Well... what other "tricks"?

Does the A minus B work off the displayed signal or the fed signal?  Can I set the voltages so that the two signals match on screen and then do A minus B?  I can alter the phasing by altering the trigger, no?  Or do I need a tunable RC circuit to change the phase?

Note that "A minus B" mode of an analog two-channel scope is not a perfect subtraction:  the tolerance on the analog combination can be comparable to the THD that you are trying to measure.
I do THD measurements (technically, THD plus noise) using an -hp- 339A analyzer that combines a low-distortion generator with a nulling filter and auto-scaling of the remainder to the input level.
The nature of the distortion products can be viewed by an analog output of the remainder to an oscilloscope.

[macboy]

With your equipment you listed, you can do some useful things.

You can determine the clipping point of the amplifiers by feeding the amp a sine wave (around 1 kHz, 1 Vp-p), and adjusting the volume up until you can clearly see the flat tops on the output sine wave, observed with the scope. Back off the volume control until the peak is just a little below that, and there you have the maximum unclipped power output. The distribution may already be a little high at this point even if the output isn't technically clipping. The load (e.g. 4 vs 8 Ohms) will alter the clipping point somewhat. Calculate power by P=V²/R . Obviously use a dummy load resistor for this, not speakers.

Now without changing the volume control, remove the function generator and short circuit the input jack (I have built shorted phono plugs for this purpose). The input is now zero. Measure the RMS AC voltage of the amplifier output. It will be low but won't be zero. Take the ratio of this compared to the full output power from earlier and you have the signal to noise ratio. Convert to decibels: SNR(dB)=20*log(Vmax / Vnoise).

You can repeat the SNR measurement at lower volume levels as well, but the noise will become increasingly difficult to measure accurately as it drops to mV levels.

As a rule of thumb, your multimeter's AC voltage reading is only sensible when it is at least 1% of full scale. So at 200 mV scale : 2 mV minimum input, regardless of how many digits it shows. Your fluke DMM should also have decent bandwidth covering the audio range, but you can verify that by feeding it a constant level from your function generator and sweeping through the audio frequency range. The AC RMS reading should remain relatively constant (remember +-10% difference is actually less than 1 dB).

If you have a computer with a decent sound card, you can measure distortion and other performance parameters with software such as RightMark Audio Analyser RMAA or others (some free, some are even free open source). With a scope and signal gen, you just can't do it, but you can see gross distortion such as clipping or zero cross distortion. The latter might result from a bias setting much too low.

[Fried Chicken]

There may be tricks you can do with a DSO, but with an analog 'scope the RC phase change circuit would be the way to go.
The difference in shape of the two signals would show up as the result of A minus B, which may require some analysis to work out the relationship between that display & distortion in % (or dB).

It will, in fact be a result of THD + Noise, but usually the latter is low enough to be ignored.

In TV testing, we expressed both THD + Noise & Noise by itself in dB, making it easier to mathematically remove the latter to give pure THD.

I feel like just looking at and dicking with the signal will start making things obvious to me.
Gah; I wish I had a more intuitive understanding of math, the relationship between Distortion and dB.  I used to be really good at this, but then I chose to study science instead of EE.

This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

I can safely probe around bing bing bing bing bing?

Usually, that is true.  It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output.

Why would someone put an H-Bridge on an amplifier output?

With your equipment you listed, you can do some useful things.

You can determine the clipping point of the amplifiers by feeding the amp a sine wave (around 1 kHz, 1 Vp-p), and adjusting the volume up until you can clearly see the flat tops on the output sine wave, observed with the scope. Back off the volume control until the peak is just a little below that, and there you have the maximum unclipped power output. The distribution may already be a little high at this point even if the output isn't technically clipping. The load (e.g. 4 vs 8 Ohms) will alter the clipping point somewhat. Calculate power by P=V²/R . Obviously use a dummy load resistor for this, not speakers.

Now without changing the volume control, remove the function generator and short circuit the input jack (I have built shorted phono plugs for this purpose). The input is now zero. Measure the RMS AC voltage of the amplifier output. It will be low but won't be zero. Take the ratio of this compared to the full output power from earlier and you have the signal to noise ratio. Convert to decibels: SNR(dB)=20*log(Vmax / Vnoise).

You can repeat the SNR measurement at lower volume levels as well, but the noise will become increasingly difficult to measure accurately as it drops to mV levels.

As a rule of thumb, your multimeter's AC voltage reading is only sensible when it is at least 1% of full scale. So at 200 mV scale : 2 mV minimum input, regardless of how many digits it shows. Your fluke DMM should also have decent bandwidth covering the audio range, but you can verify that by feeding it a constant level from your function generator and sweeping through the audio frequency range. The AC RMS reading should remain relatively constant (remember +-10% difference is actually less than 1 dB).

If you have a computer with a decent sound card, you can measure distortion and other performance parameters with software such as RightMark Audio Analyser RMAA or others (some free, some are even free open source). With a scope and signal gen, you just can't do it, but you can see gross distortion such as clipping or zero cross distortion. The latter might result from a bias setting much too low.

These are some clever tricks.  I know the old analog guys had some wizardry that probably surpasses what the digital people can do.

I suspect my function generator doesn't have a linear output on a sweep... at least it didn't when I hooked it up to my scope, I think... low frequencies it was very high p-p, and then it went down as I went up the frequencies... I think.
Why measure with the multimeter and not the scope for the sweep?

I don't have a soundcard in my computer, but I think I might get one as my USB headphone amplifier thing from Behringer caused bugs in software (I suspect).


I look forward to getting some free time to hook up the amplifier.  I suspect once I start dicking around something obvious might appear.  I guess I should mention the model, it's a Yamaha M-65.  I recently did a recap on it, and while I heard a difference, I'm not convinced it was an improvement.  I feel like the right speaker is fuller and better, but only across certain frequencies.  I should have measured the caps before installing them... possibly I screwed something up, but I don't think so, I worked carefully.


If I want to measure a crossover, do I also need a dummy load?

[TimFox]

"Quote from: TimFox on Yesterday at 01:03:44 pm

    Quote from: Fried Chicken on Yesterday at 10:19:59 am

        This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

        I can safely probe around bing bing bing bing bing?


    Usually, that is true.  It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output.


Why would someone put an H-Bridge on an amplifier output?"

To work with a low DC voltage.

[Fried Chicken]

"Quote from: TimFox on Yesterday at 01:03:44 pm

    Quote from: Fried Chicken on Yesterday at 10:19:59 am

        This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

        I can safely probe around bing bing bing bing bing?


    Usually, that is true.  It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output.


Why would someone put an H-Bridge on an amplifier output?"

To work with a low DC voltage.

How is that different from simply flipping the polarity?  (Red --> Black, Black --> Red)?
How does an H-Bridge give a low DC voltage?




Dumb question: if I'm measuring/testing a crossover, do I need a dummy load?  Dummy load on all outputs or just on the output I'm testing?

[TimFox]

With an “H”, the load is located on the horizontal line between the two uprights, where each upright has a “pull-up” and a “pull-down” device.  Neither end of the load is grounded, so you can’t put the ground lead of the ‘scope on one side of the load.  The two sides move up and down with respect to the power supply, with 180 deg phase difference.
My “low DC voltage” comment refers to the power supply.  The peak-to-peak output voltage is twice the DC supply voltage, while the usual circuit gives a pk-pk voltage equal to the DC supply.
This is handy for annoying other drivers when blasting audio from a 12 V automobile supply.

[Fried Chicken]

I got my amp out of the system and hooked up.  I carefully set up my signal generator and at its lowest output it's within a safe regime, but still slightly high lol.

I turns out I have two 8 Ohm 200W dummy loads, so perfect!

I carefully got everything wired up, here's my setup (apologies for the out of focus picture):







Immediately I notice the right channel has a lower p-p than the left channel, which is unusual because I swear the soundstage is shifted to the right.  I can now start dicking around.  I'm just getting my feet wet here, so I'm being stupid cautious.

[Fried Chicken]

It's pouring rain outside (actually thunderstorming so I've had to pause my dicking around).

The left channel is definitely slightly stronger than the right channel, as reported by both the scope and the lights on the front of the amp (left rises first).  I swapped the scope probes as a test, and indeed, the left channel on the amp reports stronger.  I didn't measure the resistance of the loads I'm using, maybe I should do that.  This is weird because the soundstage is off to the right when listening.

I drove the amp into clipping, and.. oh boy those power resistors got hot fast. (I really am doing this for the first time)
The front of the amp reported somewhere between 30 and 90 watts into clipping.

The p-p voltages measured on the scope are ~110-120V since I have the probes attenuating 10x...  This doesn't sound right to me, that's almost line voltage??!







I guess this would make sense for power at a single frequency?

[Fried Chicken]

So now I'm dicking around and things are starting to get interesting.
I inverted the B channel signal and set the scope to add... and the difference was quite stark.  This difference changed significantly as output on the function generator was increased, but remained almost identical as the frequency was changed: it was like a sine-wave squishing and expanding apart.



I disconnected the power resistors and measured their resistance with the Fluke.
I came up with 8.22Ω and 8.20Ω so they're decently close to one another, then I swapped them around Left/Right, cut off a bit of wire and reattached the cleaned wire to the binding post yielding these new different results:





Weirdly, after cleaning and swapping around the resistors, the outputs look almost identical, Vp-p wise.  The behavior of the difference of signals remains the same (changes on output from signal generator, squishes with changes in frequency).


The probes are on 10x for this.  The I'm thinking it might not be stupid to open the amplifier and start probing to find where that difference of signal might be coming from, but first I think it would be smart to set the biases appropriately.



One final note worth adding (I am writing notes to myself as well as everyone else, I'm not expecting everyone absorb and interpret every detail on my behalf!), depressing the "Auto Class-A" button makes no difference in the signal.  None.  I tried at different power levels, low, high, different frequencies, measuring the subtracted signals, I saw zero change to the signal.

I think I'm going to open the amp up and set the adjustments according to the manual... One final thing I've noted, I need longer probe cables for my fluke multimeter!

[Kleinstein]

The observed 120 V peak to peak are some 43 V RMS. So still quite a bit lower than mains voltage. Higher power amplifiers, especially when made for 8 ohms can indeed provide quite high voltages. They may have a warning about potentially hazardous voltages at the speaker output.

The amplifier output is normally low impedance. So the difference in the loading should not be that relevant.

An amplitude dependent effect would point to a linearity problem - maybe also loudness enabled.

[Fried Chicken]

The observed 120 V peak to peak are some 43 V RMS. So still quite a bit lower than mains voltage. Higher power amplifiers, especially when made for 8 ohms can indeed provide quite high voltages. They may have a warning about potentially hazardous voltages at the speaker output.

The amplifier output is normally low impedance. So the difference in the loading should not be that relevant.

An amplitude dependent effect would point to a linearity problem - maybe also loudness enabled.

DOH, of course mains voltage is 120VRMS.  I didn't think about that.  My head is spinning slightly.  To convert from Vp-p to VRMS you divide by two and then divide by the square root of 2, correct?

There's no loudness enabled, the amplifier is set for 8Ω speakers, the pots for the volume control flatten out at the top 1/32nd of a turn so they should be working fine (bypass, 0Ω or whatever).

[Kleinstein]

The difference measurement on the scope also includes the linearity of the scope input amplifier. Chances are the audio amplifier (at least if not broken) is more linear than the scope.

With the audio output, one may have to generate the difference in a different way, e.g. with a transformer or connecting 1 scope probe between the 2 outputs.

[Fried Chicken]

I just shorted the input.  The service manual says I should have 127 dB SNR with the input shorted, and 120dB with the input not shorted.  I could not see a discernible difference when shorting the inputs to ground, however the Left channel (inexplicably louder channel) is sitting at 1 mV with some signal at 100mHz?  The right channel at 0.75mV.  I swapped the probes to verify, and the reading stands, but later dicking with the probes on the scope changed the amplitude.  I think I'm beyond what the scope/probes can do.  Maybe I need better probes.

Scrolling now down the service manual, there is an "Idling adjustment" that should be at 10mV +/- 1 mV DC.
Doing so for the left channel I get a hovering from 9.33mV to 9.49mV so in spec, but I brought it to hover around 10mV DC.

Checking the right channel, however, it was hovering at 11.7 mV DC, so out of spec.  Result! I now brought it back into spec.  I really don't know what exactly I'm adjusting here.  What does the idling adjustment do?

The next adjustment is to push "Auto Class A", so the unit will go into Class-A exclusively under a certain output level.  This brought the idling to ~89mV on the right channel and ~84 mV on the left channel.  Spec says "more than 20mV DC".  Ok.

Issue has now emerged, disabling "Auto Class A" brings the idling voltage down quite a bit and it took a while for it to rise again... then again yamaha said "leave on, no input, for 2 minutes prior to testing", ok.  It went back within spec.

Last adjustment the VU meters: select 2 Ohm speakers, apply no load or an 8 Ohm load, put a 1 kHz signal... Read a 28.3V output on the speakers and adjust so the 0dB LED "Adjust to the lights up point".  reduce signal by 1dB to 25.2 V and "Confirm the 0dB - LED fades out".

I assume a vRMS, and for this I used my Fluke 45.  My power resistors are very hot right now.
Weirdly, the left channel indicates higher output than the right channel, even though it sounds quieter.  This is even after setting the idle adjustment.

[Kleinstein]

Aiming for some 10 mV sounds like the adjustment of the standing current (drop at an emitter resistor) for a class AB amplifier. 10 mV+-1mV looks quite small tolerance. Normally I would not complain about 12 mV. AFAIR the rule of thumb for a class AC is something like 0.5 kT/e ~ 13 mV per resistor.  This setting could effect the cross over distortion a little, but amplifier should still work OK if not totally off.

The settings can be a bt temperature sensititve. This can cause the slow settling.

[Fried Chicken]

I am about to blow a fuse here trying to interpret the difference of signal between Ch1 and Ch2...  The right channel looks lower output than the left, but....


When I see the signal on the screen, it looks like Ch2 is smaller... but if I invert Ch2 and then add the two signals together, then boost the sensitivity of the reading and/or increase the signal from the function generator.

I figured it out, but now looking at the difference of signals, the left channel being normal, starting on a rise and crossing the 0 at the zero:




As I turn up the output, I get what I initially expected, the left channel slightly pulls the signal up from zero (at the left side of the axis)...



as I keep increasing volume, however, suddenly the inverted right channel signal becomes dominant and the inverted signal becomes dominant...



What's *really* weird, is that this happens not only when I do the output adjustment, but even when I increase the volts/div sensitivity of the scope.


If someone has the patience to interpret this, and either explain something in my knowledge that's missing, or has an explanation for this phenomena, I would be very grateful.  I'll post pictures/a video to go with this post shortly.


[edit update]

I flipped the board over and noticed a number of bothersome things: when I did the recap I didn't do a particularly good job.  I only had a flux pen and not actual flux I could apply onto the board.  As a result there were some extra solder whiskers floating around and a number of my solder joints could probably have been better.

I went in and cleaned all of them up, reflowed almost all of them, re-checked the biases and repeated my measurements from before.  I'm not sure it electrically made a difference, because I didn't take notes of my measurements, and the phenomena still arises where the left channel pulls stronger at lower voltages (~2v) and as I crank it up to ~20v output the right channel pulls stronger (when taking the difference between the channels).

However.. measuring with the Fluke 45, I get 20.12v on the left and 19.9v on the right (this is maxing my function generator when attenuated -20dB).

This doesn't make sense.  When looking at the difference of channels, and inverting the right channel, as I crank the voltage up, the waveform begins to follow the inverted right channel suggesting it is "stronger".  My ears agree: the soundstage sounded shifted to the right.  The Fluke 45 DMM disagrees.  This phenomena stays even at different frequencies, although my ears hear a frequency dependence to this phenomena.

[Fried Chicken]

I'm calling it quits for tonight;

I'm going to put the amp back together and actually use it, b/c that's why I have it.  Maybe something will change.  Maybe I'm chasing a nothingburger, but if the scope is lying, the Fluke 45 is not.  What a thing.  It's pinpointing that ever so slight less voltage.

[bdunham7]

Surely something can be done with a normal scope?  I mean the scope samples at several orders of magnitude above audio signals?

Are there no analog tricks or similar?

I missed this thread or I'd have given you the short answer first:  A scope and ordinary tools are useful when testing and repairing a malfunctioning audio amplifer but are nowhere near good enough to quality-test the final product.  You might aim for 0.1% THD or better, but you won't be able to even see 1% on a scope even using offset or null methods.  10% THD maybe by comparison.  Have you verified that if you feed the same signals to both channels of your scope that you can adjust it so that they overlap perfectly?  And if you invert one channel, you get a flat trace? 

[Fried Chicken]

WOW.

I THINK I FOUND THE PROBLEM.

The amp is completely anemic and shitty, not like it was before.  It completely lacks power, and the reason for that, I suspect, is the amazing Elna and Nichicon filter caps I replaced with mouser p/n: 598/383LX333M050B052

I hooked up an old sony amplifier thinking maybe there's something else that could be wrong (speakers, preamp, etc.).  OH BOY HOWDY WAS THE POWER BACK.

I'm pretty sure the filter caps I installed are garbage; luckily I still have the originals. Way to big to even test with my LCR meters.  I could charge it and hook up a resister and find the time constant and back calculate RC whatever, but I am disinterested.

I'm just going to put the old caps back in and cry that they're NLA new... manufactured with real pride, gold cursive lettering on black with FOR AUDIO prominently displayed across the front.

[Coordonnée_chromatique]

mouser p/n: 598/383LX333M050B052

"readily handles tough switching power supply input and output circuits and motor-drive"
There are not designed for a linear PSU, do you hear an energization noise when you power up the PSU (like a soda can noise opening) ?

[Haenk]

Surely something can be done with a normal scope?

I guess, the short answer is "no", at least not as a precise measuring device.
On a budget, you could use a good external PC/USB sound card to measure the performance (200+ USD). That is probably the cheapest option.
Next step up would be COSMOS ADC+precision generator (that would be like 400+ USD) oder QuantAsylum (even more expensive, at 600USD).
Next up would be a used Audio Analyzer, depending on brand and quality, that's 1000+ USD, up to a couple of k.
If money does not matter, an "Audio Precision" would be the top of the line, at tens of k.
Those are the instruments "everyone" uses for measuring audio devices, no shortcut or cheap option there.

[Tation]

I think that, in order to measure THD using a reasonable amount of money, the best solution goes thru an external (USB) sound card. Focusrite seems to be one of the preferred brands, but there are many others, and people have reported good results with more affordable brands, like Behringer.

DIYAudio site maybe of interest for the OP.

[bdunham7]

I'm pretty sure the filter caps I installed are garbage; luckily I still have the originals. Way to big to even test with my LCR meters.  I could charge it and hook up a resister and find the time constant and back calculate RC whatever, but I am disinterested.

I'm just going to put the old caps back in and cry that they're NLA new... manufactured with real pride, gold cursive lettering on black with FOR AUDIO prominently displayed across the front.

The filter caps are 33000µF?  And only 50V?  What is the power supply voltage and what is the rated power of this amplifier?

If there's a clearly audible problem then you should be able to track it down--with some help and guidance perhaps--using the tools you have plus a pair of 8R power resistors as test loads.  If you recapped it and the problem started then, you've probably made an error somewhere.  I doubt it is the filter caps, but if you suspect them or some power supply issue then put your scope on the power rails and look at them when you load the amplifier.  With a signal into a test resistor at higher volume you'll see a sawtooth pattern with an inverse of your signal superimposed on it.  As long as the rail doesn't drop too much everything is OK.

[Fried Chicken]

I'm pretty sure the filter caps I installed are garbage; luckily I still have the originals. Way to big to even test with my LCR meters.  I could charge it and hook up a resister and find the time constant and back calculate RC whatever, but I am disinterested.

I'm just going to put the old caps back in and cry that they're NLA new... manufactured with real pride, gold cursive lettering on black with FOR AUDIO prominently displayed across the front.

The filter caps are 33000µF?  And only 50V?  What is the power supply voltage and what is the rated power of this amplifier?

If there's a clearly audible problem then you should be able to track it down--with some help and guidance perhaps--using the tools you have plus a pair of 8R power resistors as test loads.  If you recapped it and the problem started then, you've probably made an error somewhere.  I doubt it is the filter caps, but if you suspect them or some power supply issue then put your scope on the power rails and look at them when you load the amplifier.  With a signal into a test resistor at higher volume you'll see a sawtooth pattern with an inverse of your signal superimposed on it.  As long as the rail doesn't drop too much everything is OK.

I have two 8Ω power resistors that I've been running all these tests on.
Yes the originals were 50v and 35v Nichicon and Elna.

The Amp was working, but something was off and it got progressively worse after the recap (that included the filter caps)... that I didn't notice until I hooked up the Sony and realized what real sound is.  It's textbook not enough power... I also have a hard time believing the filter caps would go bad and cause that, but that's my immediate instinct

[CaptDon]

120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

[Fried Chicken]

120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

ok?

[Coordonnée_chromatique]

120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

Excuse me but could ou describe your numbers for idiots like me !
10dB is the headroom of the : amplifier gain ?

[bdunham7]

The Amp was working, but something was off and it got progressively worse after the recap (that included the filter caps)... that I didn't notice until I hooked up the Sony and realized what real sound is.  It's textbook not enough power... I also have a hard time believing the filter caps would go bad and cause that, but that's my immediate instinct

Since you only have two channels of scope to work with, try this:

Set up your amp with your function generator putting out a 1kHz 500mV_RMS signal feeding both LINE, CD or AUX inputs of your amp.  Use a Y-cord or similar, an actual parallel connection.  Connect your 8R resistors to the output and then your scope to the power rails.  Adjust your scope with the same V/div setting on each channel and the vertical positions so that you have both traces on screen.  Then slowly turn up the volume and see what you get on the scope.  Post a photo if you can.  You can use your DMM on one of the 8R resistors to make sure you don't go over the maximum output (what is the amp rated for??).  Then do the same thing with a 40Hz signal. 

I looked up your M-65 amplifier and it has a slightly unusual setup with all 4 of those 33mF caps are in series.  When I get a chance I'll have a look at the schematics and see if I can come up with some other things to test.  This is a pretty powerful amplifier so there's definitely something amiss.  Unfortunately this happens sometimes with recaps where an error is made.  The fact that it affects both channels seems interesting though.

One additional question--is the Auto Class A turned off?  It should be for now.

[Fried Chicken]

The Amp was working, but something was off and it got progressively worse after the recap (that included the filter caps)... that I didn't notice until I hooked up the Sony and realized what real sound is.  It's textbook not enough power... I also have a hard time believing the filter caps would go bad and cause that, but that's my immediate instinct

Since you only have two channels of scope to work with, try this:

Set up your amp with your function generator putting out a 1kHz 500mV_RMS signal feeding both LINE, CD or AUX inputs of your amp.  Use a Y-cord or similar, an actual parallel connection.  Connect your 8R resistors to the output and then your scope to the power rails.  Adjust your scope with the same V/div setting on each channel and the vertical positions so that you have both traces on screen.  Then slowly turn up the volume and see what you get on the scope.  Post a photo if you can.  You can use your DMM on one of the 8R resistors to make sure you don't go over the maximum output (what is the amp rated for??).  Then do the same thing with a 40Hz signal. 

I looked up your M-65 amplifier and it has a slightly unusual setup with all 4 of those 33mF caps are in series.  When I get a chance I'll have a look at the schematics and see if I can come up with some other things to test.  This is a pretty powerful amplifier so there's definitely something amiss.  Unfortunately this happens sometimes with recaps where an error is made.  The fact that it affects both channels seems interesting though.

One additional question--is the Auto Class A turned off?  It should be for now.

Yeah Auto Class A is off except for testing with the service manual.

I did a recap from a kit from a guy in Canada off a recommendation of the audio forum members.
Here is a link to the thread, but don't feel obliged to read through all that.
There are a few people that are quite enamored with these amps, here's a guy who has single-handedly created a cult following through publications on his website. (it worked for me).

The 120dB of the amp is real, the thing is silent.  If you hear any noise coming from the amp, you better be very very careful what source material you're about to start on it, b/c it doesn't mess around.

I'll get around to the setup, but I have other things in the way before I can re-do this.  I already had everything set up as described:



It wasn't until I hooked it up again but compared it to the sony I had laying around, that I realized I had missed the forest for the trees and something else is going on.

Unfortunately I'm not an electronics guy.  My guesses for what causes low/shitty power are: 1. filter caps, 2. volume level pots, 3. some circuitry controlling rail voltage?

I don't think it's the volume pots, b/c I've noticed additional inconsistencies and weirdness (like transistors not reaching threshhold voltage?) until a certain volume is reached or something.  My guess remains with the filter caps, but removing them is a bit of a hassle.

[bdunham7]

Unfortunately I'm not an electronics guy.  My guesses for what causes low/shitty power are: 1. filter caps, 2. volume level pots, 3. some circuitry controlling rail voltage?

Hopefully we can fix that! 

1. Filter caps seem unlikely as their function is pretty straightforward  2. Volume pots are unlikely to affect both channels and 3.  The rails are unregulated on these amplifiers. 

Check the power rails with a scope as I mentioned and that should remove all doubt.

[vk6zgo]

120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

There is no such thing as watts rms.
You can have rms volts, & rms amps, but when you multiply one by the other, you get "average power".

[TimFox]

120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

There is no such thing as watts rms.
You can have rms volts, & rms amps, but when you multiply one by the other, you get "average power".

This is one of my pet peeves:  when you start with rms voltage to calculate the power into an 8-ohm load, you get "mean" or "average" power, as you said.
However, there is such a thing as watts rms, it's just useless.  One could lie about the power of ones amplifier using rms power:  for a sine-wave it's (3/2)^1/2 = (1.225) x P_avg.

[CaptDon]

Yeah, yeah, I get it, RMS volts times RMS current isn't RMS power. The audio industry has used this term for so long it is ingrained in me. And the industry tends to use the term 'average' as an indication of the average amount of musical power being delivered to a speaker and we see speakers with ratings like 300 watts RMS 400 watts peak or 300 watts musical power 200 watts RMS and tons of other weird specs. My calculation maybe should simply be called 225 watts 'balls out' and 22.5 watts with enough headroom to not have the 'hammered shit sound'. I think I will propose that!! 225 watts B.O.  22.5 watts before H.S.S., yup, gonna start writing that spec on all the amps I repair from now on! Sadly, when we allow for 10dB of headroom for today's musical peaks a 1KW 'average' amplifier suddenly looks like a 100W amplifier with adequate headroom for quality listening pleasure. And what was that weird comment by a previous poster "10db headroom set by amplifier gain"? Oh well, it's all subjective to some degree. I don't think I have ever seen an audio amplifier with a rating like "100 watts average", they always state "!00 watts RMS per channel both channels driven" or something like that. I f--king hate these '7 pound 4 kilowatt' stated D.J. amplifiers that would shit the bed trying to do a mere 100 watts 'average' continuous sinewave!!! Cheers mates!!! I think I was playing guitar at '3 watts average' in church today, oh course that was an average guitar player playing an average guitar under average conditions.

[TimFox]

When I first became interested in "high-fidelity" equipment (late 1960s), the industry went through a period of ever-more fanciful ratings of power.
Unfortunately, someone in regulation did not understand the meaning of "rms", which is well-defined mathematically, and imposed "rms power" as a legal description.
Before that, there was an imaginative definition of "IHF power":  since vacuum-tube amplifiers had ill-regulated plate power supplies (good regulation is not required for musical application), they allowed the manufacturer to substitute a stiff supply for the static test.  That made some sense.
Past that, some manufacturers went into full-scale lying:  specifying power "+/- 1 dB", which automatically inflated any power rating by 26% (unless you naively thought that was a normal distribution).
However, no manufacturer of whom I am aware tried to get away with using the actual definition of "rms power' to pick up another free 22.5%.
Flogging the dead horse:  "rms" means "the square root of the average value of the variable squared", and is very useful in electronics and statistics.

[Alex Nikitin]

The amp is completely anemic and shitty, not like it was before.  It completely lacks power, and the reason for that, I suspect, is the amazing Elna and Nichicon filter caps I replaced with mouser p/n: 598/383LX333M050B052

I hooked up an old sony amplifier thinking maybe there's something else that could be wrong (speakers, preamp, etc.).  OH BOY HOWDY WAS THE POWER BACK.

I'm pretty sure the filter caps I installed are garbage; luckily I still have the originals. Way to big to even test with my LCR meters.  I could charge it and hook up a resister and find the time constant and back calculate RC whatever, but I am disinterested.

I'm just going to put the old caps back in and cry that they're NLA new... manufactured with real pride, gold cursive lettering on black with FOR AUDIO prominently displayed across the front.

1) Unless the old caps leaked or died in some other fashion (open circuit for example), or lost capacitance, or substantially increased ESR, I would be very reluctant to replace these, especially the power supply ones. There are some cases when a complete re-cap is advisable, but in general it is not a good idea, IMHO.

2) New capacitors need at the very least 24h under power before you could listen to the amplifier, some caps need a week or more. Nothing esoteric, just forming the isolation layer, with measurable changes.  Leave the amp powered up for a couple of days and then listen to it again, before reinstalling the old set of capacitors (which might be a good idea anyway, as long as these measure OK on the value and ESR). Ideally you should run the amp at 1/4 power output (at, say, 1kHz) into a dummy load for these 24-72 hours.

Cheers

Alex

[bdunham7]

2) New capacitors need at the very least 24h under power before you could listen to the amplifier, some caps need a week or more. Nothing esoteric, just forming the isolation layer, with measurable changes. 

Do you mean measurable changes that you can observe (measure, with instruments) during amplifier operation or measurable changes if you had the capacitor isolated and were testing with an LCR meter or similar?  I'd be very suprised if you could tell the difference between a capacitor that has been "formed" for 1 minute vs 72 hours in either case, but causing gross changes in amplifier operation seems very unlikely.

[Coordonnée_chromatique]

10dB of headroom for today's musical peaks weird comment by a previous poster "10db headroom set by amplifier gain"

Thanks i've got my answer, i'm really happy with the clarification.
Sorry but you are all hard to follow when you speak to others EE

[Alex Nikitin]

2) New capacitors need at the very least 24h under power before you could listen to the amplifier, some caps need a week or more. Nothing esoteric, just forming the isolation layer, with measurable changes. 

Do you mean measurable changes that you can observe (measure, with instruments) during amplifier operation or measurable changes if you had the capacitor isolated and were testing with an LCR meter or similar?  I'd be very suprised if you could tell the difference between a capacitor that has been "formed" for 1 minute vs 72 hours in either case, but causing gross changes in amplifier operation seems very unlikely.

Either is possible, the second is obviously much simpler. A good indication of the capacitor changes with time after a DC voltage applied to a capacitor freshly from storage is the "leakage current" which would decrease steadily in time for most capacitors (at a constant temperature).

On the need to "run-in" an amplifier after replacing capacitors (especially power supply ones) I speak from personal experience * . That requirement also makes it difficult if not impossible to do a quick comparison when swapping various capacitors in the same circuit.

Cheers

Alex

* - I did design Hi-Fi amplifiers professionally for many years and was heavily involved in production and quality control, including listening tests. So I had many opportunities to compare a fresh from the assembly line amplifier with a well run-in unit of the same model. You have to be completely deaf not to hear the difference a 24h run-in makes. At that company we ended up running every amp for 48 hours at 1/4 output power before shipping (which incidentally helped also to catch occasional quality problems).

[bdunham7]

Either is possible, the second is obviously much simpler. A good indication of the capacitor changes with time after a DC voltage applied to a capacitor freshly from storage is the "leakage current" which would decrease steadily in time for most capacitors (at a constant temperature).

In order to cause a measurable difference in the output of the amplifer there would have to be some observable difference in the power supply rail voltage...or something.  How is the leakage current going from 5mA to 5µA going to affect the power rail voltages?  I'm asking how I could observe this with test instruments, not whether you or I can hear the difference.

[Alex Nikitin]

The “leakage current” is only an indicator of changes, not the reason for change in the sound*. In a common “traditional” power supply (iron core transformer, split secondary winding, rectifier and a pair of reservoir capacitors) the dynamic of what is happening when it is used with an audio amplifier, especially class AB/B, is quite complicated and every component can affect the sound. A possible approximation would be a diode mixer producing a complex interference pattern by mixing the pulsed mains frequency current with a distorted amplified signal current and all that goes through reservoir capacitors (which are not completely linear and nonlinearity changes in time during run-in. The simplest way to observe the difference is to look at the intermodulation products for an amplified multi tonal signal.

Cheers

Alex

* - however it is a useful parameter as the speed of the “leakage current “ changes is a good indicator of how fast a particular capacitor could settle .