Measuring audio signal voltage with DMM

[1145climber]

Hey all. i have a basic DMM that i got from radioshack. i just finished building a cmoy, and i have a few questions about how i can measure the voltage of each channel... is this possible? also, how do i measure dc offset?

ive tried plugging in my mini-to-mini interconnect and measuring voltage from ground to each channel, but that doesnt work. i dont think its my meter, because i can measure the voltage for each power rail. i dont know

any help? thanks

 

[rickcr42]

audio is an AC voltage .In order to measure it you need a meter capable of measuring Ac voltages from the low millivolt range up to the ten volt range or so in the audio frequency range across a known load (IHF 10K is good for most line level measurements)

 

[JahJahBinks]

Nope, DC offset is a DC voltage, it has nothing to do with audio being an AC signal. To measure DC offset, turn on your amp, do not connect it to any source or load, measure the voltage difference between the left channel and ground, then the right channel and ground. Basically what DC offset means is that when the source is not playing any sound (not sending any AC signal to the amp), the amp should output ideally 0 volt because a certain amount of DC voltage can destroy headphones. In reality, the DC offset that you measure should be in the mV range.

 

[1145climber]

aright, thanks both.... yeah, my meter only goes down to 200V AC, so i guess ill pass on that one.... on the other hand, my offset is.... (lemme look)...

.002 V on the right, .003 V on the left.... not too bad, i hear its only a problem above 10 or 15 millivolts.... or maybe .002 is really 20 millivolts... ummm.... milli.... multiply by 1000..... yup, 2.5 millivolt offset average.... not too bad, right?

i mean, it cant be too bad, seeing as my right-to-left hand offset is around .05 V.... guess i make lotsa electricity...

yeah, my external sound blaster extigy gives off .003 average offset... cool to know

 

[rickcr42]

Quote:

Nope, DC offset is a DC voltage, it has nothing to do with audio being an AC signal

what the heck are you talking about man ?

the question was how to measure an audio signal which is AC and NOT DC .

the DC offset was a secondary question and not the primary.

BTW-you have no shot at measuring music signals without a scope (non steady state signal) but with a calibrated signal source (audio generator ,AC ) into the device then the output can be measured for gain or Dc offset into a standard load.

 

[1145climber]

ahhhh, sounds like expenseive equipment... ill pass, i can live without it... i dont even remember why i needed it in the first place, but whatever.... thanks for the tip on dc offset

 

[Garbz]

If your trying to measure the output Point to Point voltage of the amp don't you just run a 1khz sinewave through the input and measure the output using the AC (200) setting on the DMM? IIRC anyway.

Now that I think of it wouldn't a 50hz signal make more sense?

 

[rickcr42]

actually not expensive at all if you are up to DIY.I can provide links if interested.

option #2 would be an old Eico Audio Signal generator from Ebay for about ten bucks and maybe even an old eico scope for another ten-twenty.not state of the art but servicable in a pinch and really good for training on the cheap.

then when you replace the scope later with a better one you can move the Eico into your system to monitor the line level signal at all times for level and/or clipping or just because it looks cool like i do ! (right next to the lava lamp and color organ

)

 

[1145climber]

actually yeah, can i get a link or two? can i really measure clipping with them? i could use that.... also, i thought i saw it somewhere, or maybe i heard it from someone, but someone build an amp that had a led that lit up when that channel entered clipping... you know anything about that? or maybe another link? hehehe.... thanks


edit: garbz - i have no idea what all that was... im kinda a beginner... i think ill stick with this meter thing though (maybe)...

 

[rickcr42]

Quote:

Now that I think of it wouldn't a 50hz signal make more sense?

Most DMMs are real crap at that frequency and accurate they are not.

Sometimes it is best to "front" the /DMM with other devices such as preamp stages then either divide or multiply ,depending on measurement scale-which one is most accurate,to get the actual value.

there is a reason you see audio specific volt meters.Some with weighing circuits for noise measurements

again,really easy to build

 

[Garbz]

U'd think that'd be the frequency they are most accurate at. Consider what frequency the 240v lines run at.

Oh well.

 

[rickcr42]

Quote:

actually yeah, can i get a link or two? can i really measure clipping with them? i could use that.... also, i thought i saw it somewhere, or maybe i heard it from someone, but someone build an amp that had a led that lit up when that channel entered clipping... you know anything about that? or maybe another link? hehehe.... thanks

links coming right up man .

But first the LED "clip" light.Again simple to build and is no more than a full wave rectifier made up from an opamp with a diode in the feedback loop which converts the Ac of the audio signal to DC for voltage measurement.You clibrate it by 'scoping the output and increasing the input signal to the amp until you see the beginning of clipping on the scope screen then backing off just a bit,then setting the LED sensitivity control for the LED to light right at that point.
Even better is a bicolor LED that flashes red at actual clipping and yellow -3db below clipping so you know it is coming.Way better than a Clip/No Clip situation .

anyway-

some meter background to get your feet wet-

http://www.allaboutcircuits.com/vol_1/chpt_8/3.html

http://www.allaboutcircuits.com/vol_6/chpt_6/5.html

as simple DIY project :

http://allenk.home.infionline.net/techref/acvm.jpg

A nice group of software for non critical measurements :

http://www.moonaudio.com/softwar5.htm

there is better out there but this is a nice all-in-one solution and a good learning vehicle.Functional too !

And finally a real hardware COMPLETE DIY test set solution -well documented ansd fairly easy to build :

http://sound.westhost.com/project16.htm

http://sound.westhost.com/project22.htm

 

[Syzygies]

Great links, thanks.

Even simple books pooh-poohed here like "Practical Electronics for Inventors" have lucid explanations of how to understand capacitors, inductors, resistors, amplitude and phase of AC sources using complex numbers. It astonishes me that there aren't DMM's with signal generators and complex readouts, taking over many of the simpler applications of scopes, such as checking a feedback loop.

You'd think someone would have built something like this, along the lines of the above links... Or is everyone terrified of a+bi, but an electrical genius when they can see the curve on a clunky, expensive scope?

Sure, I'd rather see how an amp mangles a square wave graphically, but for comparing two sine waves to each other, using a scope is ridiculous overkill, a $50 DMM should be able to do the job if they would only incorporate complex numbers into their design.

 

[gsferrari]

Quote:

Originally Posted by Syzygies It astonishes me that there aren't DMM's with signal generators and complex readouts, taking over many of the simpler applications of scopes

There are a few...they are expensive and uh...not really portable

LOL

 

[rickcr42]

Quote:

Even simple books pooh-poohed here like "Practical Electronics for Inventors" have lucid explanations of how to understand capacitors, inductors, resistors, amplitude and phase of AC sources using complex numbers. It astonishes me that there aren't DMM's with signal generators and complex readouts, taking over many of the simpler applications of scopes, such as checking a feedback loop.

They are available but usually application specific equals quite expensive.

one thing-when purchasing a new DMm if possible obtain one with "true RMS" reading.This is a great aid for audio measurements due to audio being a signal in motion and not just a steady state absolute value measurement.
a good meter can perform many audio measurements with the proper "front end" device,including frequency with a Frequency-to-voltage converter plus can test caps (must be IN circuit simultated),resistors (must be OUT of circuit),transistors,Fets,diodes,heat,just about anything with the proper add on device.Many that can be made at home.