[blackreplica]

Hi guys,

I could use a little help. This is the third CMOY i have built according to this schematic:

The first time i built it, i used a pot on the inputs because i wanted volume control. It only worked on one channel well, on the other channel, it sounded terribly distorted and that channel kept cutting in and out. I did not use an output resistor

The second time i built it, the same thing happened. In this case, i also didnt use an output resistor.

Finally i built it a third time, without a pot this time, according to the schematic above. I used 100K for the R1 resistor. For the output resistor value, i used 2 x 100 ohms in parallel on the outputs for 50 ohms.

In all the cases, i did not use an input cap.

With my third effort, things are better. For some reason, the amp works fine on my IRiver mp3 player, but once i hook it to my pc, the left channel cuts in and out and distorts horribly again. Same for when i hook it to my hifi headphone jack.

Some of my observations:

1) On my hifi, the channel seems to come back on when played at ridiculously loud volumes(although it still distorts on that channel a little). Once i reduce volume, the channel cuts out again. On my pc, both channels distort, and the left cuts in and out at any volume. As i have mentioned before, it works flawlessly with my mp3 player.

2) On the CMoy website, the optional output resistor is recommended at being 50-100 ohms for driving headphones with low impedances right? Currently, its at 50 ohms. My headphones are all 16 ohm. Could that be the cause of the problem? I'm not sure exactly what loads the CMoy can drive safely. Should i make the output resistor 100 ohm by removing one of the paralleled resistors?

Please help! I'm very dejected, i want to build this thing so much! Any advice is much appreciated!

 

[llmobll]

1. Have you tried looking at Tangents Site: http://tangentsoft.net/audio/cmoy-tutorial/

lots of step by step instructions.

2. I've heard using output resistors doesn't help that much. I've never used them and don't see the need to.

3. When stuff cuts in and out, it usually means things are shorting. Check your soldering and make sure components are touching.

4. Could be a grounding issue, make sure everything is grounded.

 

[blackreplica]

Thanks for the help...i checked the website but it was pretty ambiguous about troubleshooting and didnt offer much in the way of solutions for my problem.

I did open the amp up to check the connections again, and everything seems to be fine. Funny thing is, i had discovered i had mistakenly switched the right channel for the left channel so i fixed it since i opened it up again to check. Guess what...even though the left and right channel was fixed properly now, the problem still persisted on the same side as when the channels were swapped! Its always the left side!! makes me wonder what is going on here, seriously. Theres no shorts or anything on either channel(i am using an OPA2134, the stereo chip,BTW) and the ground looks fine to me(i did resolder it, but it didnt help)

The problem is still persisting, and i dont know what to do. At this point, my only option is to use 100 ohms for the output resistor instead but i'm starting to think it wont help any, given what you've said. I did recall reading something about the power supply that could be the problem? I think the tangent website recommended using 2 9V batteries in series or something. But i have no idea if that would help either. And my case cant fit another battery so even if it works i would be in a fix

 

[MisterX]

After reading your first post I thought you had a connector issue.... reading the part about swapping the channels and still having the problem in the left channel kinda confirms it.
You got a weird problem that is for sure.

 

[Megaptera]

Using a second 9V won't fix your problem -- the opamp you're using doesn't need that much juice. It's possible, though unlikely, that if the battery you're using is running low it can cause some strange behavior, but it doesn't sound like that's the problem here.

With your meter, go through and check the resistance of every component to everything it's supposed to be touching -- if there's a resistor in the path somewhere, check the resistance across that too. Then check the resistance between each component and anything physically close to it that it should not be connected to. Obviously, you're looking for close to 0ohms between connected components and infinite resistance between non-connected components. Also, check everything's resistance to ground and make sure it's what it should be (which depends on exactly where in the circuit something is).

If you don't find any shorts or bad solder joints that way, then turn on the power, with no source connected, and test the DC voltages at the opamp outputs, opamp inputs, and the headphone jack. All of these should be relative to ground. Speaking of ground, while you're at it, check the power rail voltages -- are you getting a fairly even voltage split?

 

[llmobll]

come to think of it i had a problem similar to this. It turned out to be a bad Stereo Phono Jack. I used to diff types for input and output. Once I got the right one, i was golden.

 

[Earwax]

Quote:

Originally Posted by blackreplica Thanks for the help...i checked the website but it was pretty ambiguous about troubleshooting and didnt offer much in the way of solutions for my problem. I did open the amp up to check the connections again, and everything seems to be fine. Funny thing is, i had discovered i had mistakenly switched the right channel for the left channel so i fixed it since i opened it up again to check. Guess what...even though the left and right channel was fixed properly now, the problem still persisted on the same side as when the channels were swapped! Its always the left side!! makes me wonder what is going on here, seriously. Theres no shorts or anything on either channel(i am using an OPA2134, the stereo chip,BTW) and the ground looks fine to me(i did resolder it, but it didnt help) The problem is still persisting, and i dont know what to do. At this point, my only option is to use 100 ohms for the output resistor instead but i'm starting to think it wont help any, given what you've said. I did recall reading something about the power supply that could be the problem? I think the tangent website recommended using 2 9V batteries in series or something. But i have no idea if that would help either. And my case cant fit another battery so even if it works i would be in a fix

You may be getting oscillation from the opamp, in part because of the low ohm headphones.

The other thing I wonder about, since you say it works with the iRiver player, is if your other source has DC voltage on the outputs, since you aren't using an input cap that could be a problem that would affect one side and not the other.

 

[fureshi]

you want to make sure that your pot and in/out jacks are also grounded. i noticed that when i built my first cmoy the sound was either really low or would cut in and out until i grounded them. also, like the other guys are saying, double check and make sure that there aren't any shorts.

 

[blackreplica]

Hi guys,

thanks for the help thus far. I know this sounds silly, but i dont have a multimeter with me(call me newbie) and i just built the amp like that without getting one!

Thanks for the advice though, i'm gonna go look around and see if a buddy has one and maybe i can test the connections.

The input jacks are definitely wired up properly, as well as the ground pin. i taped the pins individually after soldering so there is no chance of shorting there.

I suppose i could give the input caps a try. I'm not expecting much out of it though, because i did use it before in my earlier version and i still had kind of the same problem then.

The only thing in common between all three versions is my power supply(i used the same one throughout, its on a seperate board). I built the PS according to the CMOY website recommendation, with that virtual ground, and i recall reading on the tangent site that this is not the best power supply for it. Perhaps i could build another PS and try it out?

PS: i tried the amp last night on my housemate's discman. Surprisingly, it worked as well. Still doesnt work on my pc or hifi though. Could this be an amp which will only choose to function on a battery powered device?

 

[Megaptera]

Quote:

Originally Posted by blackreplica Hi guys, thanks for the help thus far. I know this sounds silly, but i dont have a multimeter with me(call me newbie) and i just built the amp like that without getting one!

Get one! If there's one single essential tool here, besides the soldering iron, wire strippers, snippers, screwdriver, dremel, and first-aid kit, it's the meter -- beg, borrow, or buy one, especially if you're interested in building anything else. If this is the only project you plan to do, you might be able to fudge it, but it really is a thousand times more effective than eyeballing the board for problems.

Quote:

The input jacks are definitely wired up properly, as well as the ground pin. i taped the pins individually after soldering so there is no chance of shorting there.

Good idea. The leads on a lot of smaller pots are pretty fragile, and I've had quite a few snap before I was even done with the amp.

Quote:

The only thing in common between all three versions is my power supply(i used the same one throughout, its on a seperate board). I built the PS according to the CMOY website recommendation, with that virtual ground, and i recall reading on the tangent site that this is not the best power supply for it. Perhaps i could build another PS and try it out?

Mmm, it's possible, but since you say it worked with your housemate's discman, I'd guess the problem is somewhere else. It's entirely possible to have the same problem more than once -- all but one of the amps I've built, including a couple Cmoys, a Pimeta, and a PPA, and a protoboard crossfeed circuit, started out missing the left channel, all for entirely different reasons

The thing with the virtual ground is that it's not the most stable or balanced design in the world, but it's on par with the rest of the Cmoy design. Without a meter, I don't know that I'd recommend getting into anything more complicated -- you could spend about $15 on parts and shipping for a buffered rail splitter PSU that might not fix the problem, or you could spend $15-20 on a meter and find out exactly what's going on with what you've got, and have the tool to use later.