Headphone Line-Out Voltages - (Newbie DMM AC Voltage Reading) - diyAudio
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Old 6th May 2008, 06:57 AM   #1
trelin is offline trelin  United States
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Question Headphone Line-Out Voltages - (Newbie DMM AC Voltage Reading)

Greetings all,

(First, I apologize because this I'm confident this is a really basic question, but I've spent all afternoon trying to figure out the right search terms; if you know the right search terms, I'd be happy to do the research myself.)

Background: I'm a newbie to DIY. I just got some Audio Technica ATH-AD700 headphones and am building a Cmoy to drive them. I'm following the tangentsoft.com guide.

I'm trying to measure the maximum likely voltage that my amp will receive from my computer. (See http://tangentsoft.net/audio/opamp-wv.html ) I set my computer volume levels to 90% and generated a 1kHz sine wave. (I cautiously hooked up my headphones to ensure it sounded correct.) I plugged in a 1/8" extension cable and tried to measure the output voltage, but my DMM read 00.0v. After much experimentation I found that pushing the volume levels up to 95 then 100% resulted in 0.1V then 0.2V; I expected at least 1-2volts, so I'm confused.

I'm using a Mastech MAS830L DMM and a SoundBlaster X-Fi soundcard. The manual says it can handle 40-400Hz AC and has AC voltage settings for 200V and 600V with accuracy of 100mV. (I'm using the 200V setting.) I've heard some cheaper multimeter's have trouble with AC, but can't find anything about this particular meter. When I discovered the meter's 40-400Hz range I tried other frequencies like 60Hz and 120Hz (and variations of those). I tested the wall socket power for a reading of 122.46V so it appears the multimeter is working, so I assume operator error.

Looking to learn whatever I can, any help is appreciated. I'm sure I'm missing something basic, feel free to "let 'er rip".
-- trelin
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Old 6th May 2008, 04:11 PM   #2
Mooly is offline Mooly  United Kingdom
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Were you trying to measure the voltage while the 'phones were plugged in or were you measuring "open circuit". Meter on AC volts ? . A meter such as this is not ideal for measuring small AC signals, it just has not got the resolution.
Looked at your link for the headphone amp, it's very straightforward and you should have no problems geting it to work.
You can easily alter the gain of the amp to match whatever source you are using it with.
Regards Karl
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Old 6th May 2008, 04:33 PM   #3
Minion is offline Minion  Canada
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I might be totally off base here but maybe you aren"t getting a good reading because your DMM has a resolution of 40hz-400hz but you are feeding a 1khz signal through it which is way above the 400hz resolution of your DMM.....

Just a thought...

I have built a couple CMOY head amps but useing a dual regulated PSU and LM4562 opamps...They sound pretty good and are really easy to build....

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Old 6th May 2008, 09:39 PM   #4
trelin is offline trelin  United States
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Thanks for the replies!

I used headphones to make sure that I was only getting a pure tone (as far as I could tell, volume full on was silent before I started the tone generator). Then I unplugged the headphones and plugged in an 1/8" cable extension, so I guess that's "Open Circuit". Then I set my meter to the 200V AC setting and tried to measure from the ground ring to both the right and left channel rings (and even voltage between the right and left channels themselves).

The reason I'm doing this is I'm trying to figure out why my amp is clipping (at least I _think_ it's clipping, all I know is it crackles when I turn it up). The amp is currently build on a solderless breadboard, and I didn't have a potentiometer yet; it's just amping at max volume. Because of this, I have to keep my soundcard volume at about 4% to balance the overall volume and the SNR is unbelievably bad (hisses and pops galore). So, figuring that my potentiometer is from 0-10kOhm I just stuck a 4.7k resistor in-line with the left and right output channels, which worked rather well. The problem is when I raise my soundcard back to 65% it crackles really bad (and the headphones aren't even particularly loud, far quieter than my soundcard alone can drive them), so I decided to study what voltages I should see all the way through the amp.

Thanks, but I actually had the same idea. I tried a bunch frequencies all through 20Hz through 250Hz (especially 60Hz since I thought maybe the meter would like that standard more than others). I also tried generating two tones in stereo in case my soundcard was doing something weird to the monotone. I thought maybe my meter just wouldn't do AC voltage, but when I measured an open wall socket I got fluctuating between 122.4-122.6V, so it seems to work.

Could it be that although it _sounds_ like a pure wave it's messy enough to confuse my meter? Or maybe my meter doesn't like low voltage (0.5-5V) measurements?

I appreciate your time!
-- Trelin
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Old 6th May 2008, 09:40 PM   #5
trelin is offline trelin  United States
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Default More Info, Tests

(Sorry to self-reply, but I figured while I wait I will keep trying fiddling.)

I wasn't sure if it would make a difference to a voltage measurement if the line-out circuit was open or closed, so I did some measurements from the L/R channels in and out of my amp at maximum amplitude (no volume control potentiometer, and I removed the 4.7k resistors, and the soundcard is at 100% volume).

With the soundcard and the cmoy at maximum volume, the input channels are 00.2V and the output is 00.4V. If I play with the soundcard volume up and down the cmoy output does move between 00.0-00.1-00.2-00.3-00.4V. I'm using a cheap pair of earbuds on the cmoy output.

Out of curiosity I measured the resistance of the L/R channels to ground on my headphone connectors. The AudioTechnica's are rated at 32ohm, and they came up at 33.1/32.5, the earbuds came up at 16.4/16.8. I tried just putting a simple 100ohm resistor between the soundcard line-out's right channel and ground to see if the increased resistance changed the voltage but it didn't (just sat at max 00.1V).

-- Trelin
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Old 7th May 2008, 07:08 AM   #6
Mooly is offline Mooly  United Kingdom
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Can we rewind back to the beginning.
You say you have built this CMOY amp, from the circuit from your web link- yes. And it is built on a breadboard, thats good for now.
You need to make sure the amp works first, forget computers and soundcards.
Have you built it exactly to the circuit.
Can I ask what OpAmp you are using.
Have you fitted anything for R5 and if so what.
The schematic does not have I/C pin numbers on, do you know what pin is what, or are you unsure.
You can not wire a volume control in series with a source.
Do you know how to wire a pot correctly.
I would get the amp tested by using it with a C.D. player line output or similar to begin with. It should drive headphones to a high level with no distortion.
The circuit is "unusual" in that it creates a "false ground" at the junction of the two 220mfd caps. Not the way I would do it, but there you go, it will work O.K. You need to check with meter on DC Volts the output of the Op Amp, it has to be ZERO as measured from the junction of the two 220 mfd caps. Plus 9 volts on pin 8 and minus 9 volts on pin 4.
Back to volume control, if you have no pot and R2 is 100 K as shown, then if you do add a resistor "in series" with C2 the input then adding 100k will cut the signal by half.
Sound like you need to read up a bit on basic theory, and get a pot ! 10K 47K 100K all will work.
Regards Karl
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Old 8th May 2008, 08:30 PM   #7
trelin is offline trelin  United States
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Sorry, I think I braindumped any information I thought might be useful. In short, the amp is connected and working "well"; it can take a clean source and deliver a clean output. I'm using an OPA2132P and I'm familiar with the orientation/pinout.

At the moment I'm struggling with three main problems. (And sorry, I'd run some tests with a simple CD player if I had one, but for the moment all I've got is my computer.)

1) My original question: why do I get a maximum AC voltage reading of 0.2V (~200mv, my meter only has 100mv AC precision) from my computer lineout? This is with the volume completely maxed, whereas the tangentsoft article implied I should see between 500mV and 6V (I think). I'd like to know if I'm measuring wrong or if my meter is simply wrong.

2) With the default gain of 11 the cmoy output gets "scratchy" if my computer volume is above 10% or so (but it's clean if I do keep the input volume low, it amps it just fine). If I lower the gain on the cmoy I can raise the input volume from my computer without it scratching during loud parts of music. Is this a "clipping" issue?

3) Really bugging me. I bought a pot (Panasonic, 10k, EVJ-C20) and wired it in series with my cmoy output channels. It "works", but it's quality seems terrible. There's actually three problems here. (And I swear, I'm trying hard not to be picky, but these are "in your face" enough they're driving me nuts.)

3a) Around near-most-quiet and near-max the left channel gets very noticeably louder than the right channel. (If it's setup with 50% being directly vertical, I'm referring to 215deg and 325deg positions, roughly.)

3b) If I had my computer volume at a level that was pleasant when my amp was at 75%, during quiet songs I would find myself pushing it up to 90%, but then my left ear would be much louder than the right. Pushing my pot up to 100% would even them back out again, but the volume jumps dramatically during that last 10%, so to even out the left and right channels makes it too loud. I'd increase the gain so I could keep the nominal volume at 45-50% and just not turn the pot so high but...

3c) Everything from ~25-75% sounds pretty much the same. It's only during the initial and last 20% of the pot's movement that affects does much; you can spin the pot up and down the middle 70% with only a small change in volume. (Of course it's really touchy in the 85-100% range.)

Hope those were more clear.

-- Trelin
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Old 9th May 2008, 07:15 AM   #8
Mooly is offline Mooly  United Kingdom
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Hi, Erm right then, point 1, I know NOTHING about soundcards and using a PC to generate test tones etc. Having said that, your meter won't be wrong within it's frequency limits. There a few things you need to read up on. When we talk of AC voltages we need to know whether we are refering to peak values, peak to peak, or RMS. 6volts RMS is nearly 17 volts peak to peak. Is it sine or square, another thing called form factor applies as well then.
Point 3, you cannot wire a pot to the output, it has to be to the input with the centre pin to the OpAmp side, one of the others to "ground" and the input between ground and the remaining pin.
How have you tested this when you say it is "clean" what did you use for an input.
So the phones wire to the output directly, but BEFORE you do this CHECK the DC voltage from "ground- this false ground" to the output pins of IC and make sure it reads ZERO. If it does not there is a construction fault.
Just looked at the circuit again and the pot is shown correctly wired, as I describe above.
Point 2, with the pot incorrectly wired to the output you can not alter the gain of the circuit, you could well be overdriving it. The pot on the input side allows any input voltage, within reason to be applied to the amp.
Point 3 abc, All to do with pot being wired up wrong.
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