Author Topic: Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)  (Read 2767 times)

0 Members and 1 Guest are viewing this topic.

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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).

« Last Edit: December 28, 2024, 08:21:22 pm by Fried Chicken »
Google is spyware
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15149
  • Country: de
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.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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.


« Last Edit: December 28, 2024, 10:40:07 pm by Fried Chicken »
Google is spyware
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15149
  • Country: de
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.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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:

2471563-0


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)...

2471555-1

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

2471559-2

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.
« Last Edit: December 29, 2024, 03:54:31 am by Fried Chicken »
Google is spyware
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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.
Google is spyware
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
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? 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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.
Google is spyware
 

Offline Coordonnée_chromatique

  • Regular Contributor
  • *
  • Posts: 208
  • Country: fr
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) ?
 

Offline Haenk

  • Super Contributor
  • ***
  • Posts: 1301
  • Country: de
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.
 

Offline Tation

  • Regular Contributor
  • *
  • Posts: 106
  • Country: pt
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.
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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
Google is spyware
 

Offline CaptDon

  • Super Contributor
  • ***
  • Posts: 2181
  • Country: is
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.
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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?
Google is spyware
 

Offline Coordonnée_chromatique

  • Regular Contributor
  • *
  • Posts: 208
  • Country: fr
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 ?
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
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 500mVRMS 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.
« Last Edit: December 29, 2024, 07:31:49 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Fried ChickenTopic starter

  • Regular Contributor
  • *
  • Posts: 160
  • Country: us
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 500mVRMS 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:

2472063-0

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.
« Last Edit: December 30, 2024, 07:03:46 am by Fried Chicken »
Google is spyware
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline vk6zgo

  • Super Contributor
  • ***
  • Posts: 7854
  • Country: au
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".
 

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
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 Pavg.
 

Offline CaptDon

  • Super Contributor
  • ***
  • Posts: 2181
  • Country: is
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.
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 
The following users thanked this post: Smoky

Offline TimFox

  • Super Contributor
  • ***
  • Posts: 8995
  • Country: us
  • Retired, now restoring antique test equipment
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.
« Last Edit: December 29, 2024, 11:54:46 pm by TimFox »
 

Offline Alex Nikitin

  • Super Contributor
  • ***
  • Posts: 1330
  • Country: gb
  • Femtoampnut and Tapehead.
    • A.N.T. Audio
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
« Last Edit: December 30, 2024, 08:00:51 pm by Alex Nikitin »
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf