[FaxData]

Hi, nice to meet you all.

I wish my first post was something besides a call for help...

I've put together my first CMOY based on the plans over at tangentsoft, with the standard radioshack board, metal film resistors from radioshack and the rest of the parts from digikey. Using test cables with alligator clips, I hear soft (but pretty loud) noise, equal in both channels. The noise is there with or without a source, and it does not get louder with a source attached. I don't notice any distortion in the source when I have my ipod hooked up.

The power supply part seems to give a stable voltage, +/- 9.54v (two new batteries), measured between ground and the two power pins on the opamp. All the resistances seem to be correct from the opamp pins to the ground as well. There's no DC to the left channel output, and there's 4mV to the rigth channel. I've checked all the different solder pads for shorts and I can't find one. The amp does not heat up, and there's no buzzing or hissing.

R2 is 100k, R3 is 2.2k and R4 is 10k. The opamp is OPA2132PA. Other than the gain of 5.5, everything is pretty much by tangentsoft's default design. I tried adding 10 ohms to R5 on one side, and I couldn't tell any difference at all, so I changed it back to a jumper. I thought I might have burnt the opamp, but switching it out with a spare didn't seem to change anything at all.

The Front The Back

I've searched but haven't really found how to fix my problem... I don't even know where to start in diagnosing this.

Any advice?

 

[Daroid]

Start out with resoldering and reflowing the solder a couple of places indicated with a red circle. Don't be afraid to fry passive components... only the opamp is the most sensitive component to five seconds of soldering iron temps.
http://kom.aau.dk/~oesteraa/cmoy.jpg
Some of them might be ok, it's hard to judge sometimes..

 

[Joshatdot]

Yeah, it looks like there are many spots that need to resoldered/reflowed. You should not see the board hole after soldering. Watch Tangents videos about soldering, it helped me out =)

http://tangentsoft.net/elec/movies/

 

[FaxData]

Thanks for the replies.

Okay, I resoldered the circled joints and any others I could find where the hole was still exposed... (some I didn't even see until I took another photo, and looked at it up close!)

It's still making noise. It sounds softer, but I think it's just because the room was actually noisier.

So I'm not sure there's any real improvement. Interestingly, now the V+ is maybe 70mV less than the V- (+9.28 vs -9.34).

Back after some resoldering

Any other ideas?

 

[Pars]

You've got a solder blob from virtual ground to pin 5 on the opamp. You can hear amplified music thru the amp, but you have a bit of noise (making sure I understand the problem)?

What phones are you using? Earbuds? And this is still hooked up with test clips? Have you tried it with a single 9V? Any change?

 

[FaxData]

saw the solder blob just as I posted it... heh. got rid of it, reshot the photo (same link above). interestingly, as soon as I saw it, I broke out the volt meter and there was no actual electrical connection there. thank goodness, or I'd probably short out the battery or something.

yes, I'm hearing amplified music through the amp, it doesn't sound distorted. I'm using cheapie $10 sony earbuds. Everything is hooked up with test-clips. I tried hooking up the earbuds to the ipod patch cable directly with both sets of clipwires, and both sets are completely noise-free. The noise only comes on when I hook up the earbuds to the circuit. Switching to a single battery makes no difference in the noise level.

Is the CMOY supposed to be noisy? the noise reminds me of an old discman from 1991 I used to use; I never noticed how noisy the headphone jack was until I switched from headphones to earbuds. I hope this isn't the case, I was hoping to use this with etymotics or something like that.

 

[Joshatdot]

looks much better, I don't seen an solder bridges. Yeah what headphones are you using? are they low ohm? (16~32 ohm). If so try higher ohm cans, 60+ ohm

 

[FaxData]

these are the earphones: cheapie sony earbuds, which have an impedance of 16 ohms and sensitivity of 108 db/mW.

The only other headphones I have access to are my roommate's sony mdr-v700, which has an impedance of 24 ohms and sensitivity of 107 db/mW. It produces the same level of noise, as far as I can tell.

I was planning to use this with etymotic er-6i... which has similar specs to the sony earbuds (strange world we live in). Does cmoy not work with these? Is my gain too high?

 

[achina]

Try replacing R5 with a larger resistor. 10ohms is just a starting point.

http://www.tangentsoft.net/audio/cmo...tweaks.html#r5

 

[Zigis]

May bee Your input vires is too long. Try conect input short ( input cap conect to ground). If amp is silent, problem is in input cabel, if You still hear noise, problem is in amp. Try to play with R5, 50-100 ohms.
Zigis.

 

[Joshatdot]

Is adding R5 in the CMoy like adding impedance adapter to the plug?

 

[NelsonVandal]

You certainly won't need that kind of gain with those earbuds (not even with full size cans). Try to lower the gain.

 

[Pars]

Quote:

Originally Posted by Joshatdot /img/forum/go_quote.gif Is adding R5 in the CMoy like adding impedance adapter to the plug?

More or less, except with the Cmoy, the added resistance is inside the feedback loop (normally). You could also try adding resistance outside the feedback loop.

 

[FaxData]

brought the gain down to 2 (2.2k resistors for both R3 and R4), and now the noise is significantly less, a very quiet hiss. I don't think there's a point to lowering it further... I guess I have to accept some noise in the circuit? Or buy bigger cans...

The writeup at tangentsoft wasn't really clear, what's the detriment of adding R5? Also, I was curious, what're your thoughts (everyone) on replacing the opa2132pa with the opa2227pa? I notice it's actually a cheaper chip on digikey, and its voltage requirements seem similar, but tangent soft rates it higher.

 

[Joshatdot]

From the frequency response in the Datasheets, the 2132 has a bass boost, while the 2227 is pretty flat.