[frogx345]

Hey everyone. I recently built a simple CMoy amp and for the life of me I cannot figure out what is wrong. I have read other cmoy troubleshooting threads and I haven't read of any problems close to mine. I do get music in both channels, but very shetcky things are happening. My problems are below.
 
 
First: when I turn on the amp there is a moderately loud click (I followed tangent's troubleshooting page on this. the DC offset from output ground to out. right is 12.1 mV, from out.GND to out.left it's 13.1 mV, below the cautionary 20mV to plug in headphones). This is not a big problem but i thought it might help someone else diagnose what is happening.
 
Second: after the click there is a horrible (sometimes very loud) buzzing/whiring sound (sounds like what an alien radio broadcast would be like), sometimes for about 5-10 seconds, sometimes it keeps going. During this time if I turn the pot it affects the weird sound a bit (like changing the alien radio channels). I think it's something with grounding because if I touch the wire I soldered to the ground (other end isn't attached to anything) the buzz starts and stops a bit. I've tried to ground it, attaching it to the tin case I will put it in, etc. but the buzzing does not stop when I do this.
 
Third: when I turn the pot, during the buzzing or not, when it's turned "all the way down" there is still some noise/music in the right channel. As I "turn it up" the noise gradually gets louder and moves to the left until it is equally very loud in both channels. there are no shorts with the wires going out of the pot so I don't know how this is happening.
 
 
There have been a few times where I play music (from laptop or ipod, neither the source of the problems above, I've checked) and as I turn the pot there are "spots" where the noise drops away a bit. When I can hear the music in these spots, behing the hissing, the music sounds really nice. I have re-checked my soldering for any shorts and re-checked the layout of the components. I'm using a TL072 opamp (crappy I know) but I'm getting a better one soon. I highly doubt this cheap opamp is the problem, but you never know.
 
I followed the resistor & capacitor values from Fred's thread: http://www.head-fi.org/t/460669/strip-board-cmoy-revisited
The general layout I have is very close to this layout that Joshatdot posted: http://www.head-fi.org/t/231414/is-this-cmoy-board-layout-going-to-work. I'll post pictures as soon as I take them.
 
Any help is greatly appreciated,
 
Kevin

 

[bcg27]

Pictures might help. What are you using for power? Do you hear the sound with grounded inputs?
 
Also recheck/reflow all your soldering. 

 

[drtturnip]

Sounds like oscillation. The TL072 is trouble. Double triple check your resistor values and connections visually and with a meter. If everything ohms out fine find a decent op amp.

 

[frogx345]

Here are the photos I took of my circuit. Wasn't able to upload them in this thread so here are the links. If you have any questions about them or want me to take another close up of a certain part, just ask.
 
General circuit (the wire on the bottom left is an unterminated wire attached to the ground).
http://img685.imageshack.us/img685/4548/cimg2886b.jpg
 
close up of the board layout.
http://img214.imageshack.us/img214/7143/cimg2887.jpg
 
close up of pot wiring.
http://img856.imageshack.us/img856/9780/cimg2890e.jpg
 
close up of soldering on back of board. this picture was with flash so it shows even the tiniest scratches and bumps. after I took it I even went back to check for shorts because it shows so many tiny specks of light, but all of these were scratches on the board (from my utility knife scratching away solder bits) or glare off of solder and the plastic of the board.
http://img707.imageshack.us/img707/5045/cimg2893j.jpg
 
so there are the pics. again, if anyone needs clarification on layout, components, etc. just ask.

 

[drtturnip]

Nice neat job. 

 

[bcg27]

Everything looks ok to me, I would do as drtturnip suggested and try another op amp.

 

[drtturnip]

IF you have done the other suggestions.

 

[Pars]

The pot wiring has a problem. You have the gnd connection from the input jack going to one deck of the pot. The gnd connection of the other deck is connected to the board gnd. These need to be tied together. I'd probably just solder a resistor lead or piece of wire between the two gnd pins on the pot.
 
Also, you appear to be using R5 resistors (as in Tangent's writeup). Make sure these are a low value (< 100 ohms). One cmoy I built I used R5 resistors in and mistakenly put in 475 ohms instead of 47. It had some bad distortion which was cured when I removed the R5 resistors or replaced them with 47 ohm.

 

[drtturnip]

I took for granted that the ground wiring was fine and not shown in the picture but it the taps are not connected that would cause all kinds of chaos.

 

[frogx345]

thanks for seeing my stupid grounding mistake. I fixed that and it sounds better, however, the music is very, very quiet when it's at low volume. When I turn it passed about a third volume it suddenly jumps very loud with a lot of noise. At this point if I slowly turn the sound down the noise goes away more and more but the music stays, and just when the music is almost perfectly clear the volume jumps back very low again.
 
bcg27: I am using one 9v Ni-Mh 200mAh battery, as seen in the first picture of my post above. Is this enough power or should I try 18v? Is this just not enough power for the TL072 and would other opamps need more power?
 
Pars: I tried to compare tangent's schematic to my own board layout and I don't think I included R5 in tangents schematic. I used the same R values and (tweaked) layout that Fred used in this thread: http://www.head-fi.org/t/460669/strip-board-cmoy-revisited. If I do have R5 than it would be too high (since the lowest R value I have is 1kohm). Could someone look at Fred's layout and tell me if he has tangent's R5? If not, would I benefit from putting one in?
 
drtturnip: With buying another opamp, there's a store near me and it looks like the best ones they have are the AD8620, AD8066, OPA2604, and OPA2134. Any recommendations?
 
One more thing I forgot to mention. The cmoy schematic calls for a 10k pot but all I could find is a 100k. I read around before I got it and saw in some places that people were using 100k pots, but if this is too much can I change the values of some other components to better incorporate the 100k?
 
Thanks a lot for all the help guys (and the quick replies)

 

[Fred_fred2004]

Is it a log pot? it should have an A in the name ie A100K or 100KA
 
if it has a B in the name its linear and will have the effect you're getting
 
cheers
FRED

 

[frogx345]

No it's linear. I didn't think log vs. linear would affect the amp like that, I thought it would only change the rate at which the volume would change. And as I turn up the volume it's not like it gets loud really fast, it just jumps incredibly loud from one instant to the next. I tried to turn it as slowly as I could and it still jumps like that.
 

 

[nikongod]

Quote:

thanks for seeing my stupid grounding mistake. I fixed that and it sounds better, however, the music is very, very quiet when it's at low volume. When I turn it passed about a third volume it suddenly jumps very loud with a lot of noise. At this point if I slowly turn the sound down the noise goes away more and more but the music stays, and just when the music is almost perfectly clear the volume jumps back very low again.
 
bcg27: I am using one 9v Ni-Mh 200mAh battery, as seen in the first picture of my post above. Is this enough power or should I try 18v? Is this just not enough power for the TL072 and would other opamps need more power?
 

Inadequate voltage is a common cause of oscillation. Try adding another 9V battery. 
 
Is there any chance the pot is wired backwards? I can never keep track of which way around its supposed to be and usually spend a good long time thinking about it/measuring. 
With the volume all the way down, measure from the center pin on one of the decks to ground, should be ~0ohms. 
Turn it up a little bit and measure ohms, they should go up very slowly until you get to "half way up" or so, where they should only be about 1/10 the value of the pot. After that they should increase fairly rapidly.
 
Edited:
Ignore the above paragraph about the pot. Thats pretty much how linear pots work in audio applications. Replace it or "shunt" it (the wrong way, as described in the tangent article about why not to do it) to fix it.

 

[drtturnip]

Could be a bad pot - open at certain spots.

 

[Pars]

9V should be enough for most any cmoy. 18V will help with higher impedance phones that need to see more voltage swing.
 
I've never seen a pot go bad, and it is wired correctly. From the front (shaft pointing towards you), the pins on each deck will be gnd - wiper (output) - input from L to R.
 
With these opamps, it is unlikely that it is oscillating. You could try measuring current draw with your DMM and compare that to the datasheet for the opamp. Tangent probably shows how to do this on his site, but basically you set your DMM up to measure milliamps (mA) and connect it in series with one of the battery leads.. If it is drawing significantly more current, then it could be oscillating.
 
PRPs in a Cmoy? Heh.
 
EDIT: also check this thread http://www.head-fi.org/t/592266/my-first-cmoy-no-sound-from-left-channel-no-amplification-in-right-channel
 
Ohm out the socket (without the power on and with no opamp in it) as I discussed in one of the posts, both to ground and to adjacent pins. This should show you if you have a connection problem (other than the input caps/wiring and the output).