[pummer]

I'm a noob to DIY audio, and I've just "finished" building my first CMOY. I don't have a multimeter, but I should be able to get one within a few days.

The problem I'm having is that I'm not getting any sound from the CMOY, except when I turn the volume all the way up on the source. When I do that, the only sound I get is an extremely quiet buzz in the left channel corresponding to the bass in the music. This sound is present no matter if the switch on my amp is on or off. The LED only lights up if the power switch is set to ON.

Please help a noob out. I honestly can't figure out what's wrong. My soldering might be kind of shoddy, but I've looked and looked for soldering bridges, and none are present, at least as far as I can see, and i've looked at least 5 times with a magnifying glass.

My source is a Creative Zen Xtra, and my test cans are MX400s.

PICS:

 

[Pars]

From what I can see looking at your pics, it appears to be built correctly. A more straight on shot of the bottom would help, but nothing I saw looked out of place. Without a meter, this one might be a bit tough to troubleshoot. Reheat the solder joints a bit (if you haven't already) to ensure good contact. Look for any joints where the lead/wire is not protruding far enough out of the bottom of the board to ensure contact when soldered. Otherwise, check your jack wiring (although I didn't see anything there either).

 

[pummer]

The jack wiring is correct now, but I had it wrong before I took the pic. I haven't gone over the board joint by joint, but I think that's the next step. A couple more op-amps are on the way, just in case I fried mine.

Assuming I scrounge up a meter (It'll be an analog one), what do you suggest I do with that? Check the power rails and make sure of correct resistance?

 

[Heady]

I don't think you can measure the resistors after soldering them, as they are now interconnected in some way. At least that's what I found. You can however look at the bands on them to see what the values are. Measuring the supply voltage and DC offset will probably be more helpful.

 

[pummer]

OK, I got ahold of a multimeter. But I'm not sure where to measure exactly.

 

[Dreamslacker]

You're using a dual opamp like the OPA2134/ 2132 rather than a single opamp like the OPA134/ 132, I hope.

Try measuring the voltage from Pin 4 of the opamp to ground (vertical strip) and from ground to Pin 8. They should be fairly close in value.

 

[pummer]

Yes, it's the OPA2132PA.

I'm not getting any voltage from either pins 4 or 8 to the virtual ground. Yes, I'm sure I'm measuring the right pins. And I'm pretty sure I have the multimeter set up right. I measured a 9v battery from positive to negative and got 9v, and I'm using the same settings I used then, so it should work.

What does this mean? Where's the faulty wiring? Blown power caps maybe?

 

[Dreamslacker]

Quote:

Originally Posted by pummer Yes, it's the OPA2132PA. I'm not getting any voltage from either pins 4 or 8 to the virtual ground. Yes, I'm sure I'm measuring the right pins. And I'm pretty sure I have the multimeter set up right. I measured a 9v battery from positive to negative and got 9v, and I'm using the same settings I used then, so it should work. What does this mean? Where's the faulty wiring? Blown power caps maybe?

Wait, the multimeter is an analog model, yes?

In that case, the red probe goes to Pin 8 and the black probe to virtual ground. You should get a reading of 4.0v - 4.5v. Do the same check with the red probe at virtual ground and the black probe at Pin 4.


Also, check the voltage across the leads from the 9v battery pad. It may be faulty.

 

[pummer]

Quote:

Originally Posted by Dreamslacker Wait, the multimeter is an analog model, yes? In that case, the red probe goes to Pin 8 and the black probe to virtual ground. You should get a reading of 4.0v - 4.5v. Do the same check with the red probe at virtual ground and the black probe at Pin 4. Also, check the voltage across the leads from the 9v battery pad. It may be faulty.

Yes, it's an analog multimeter, and that's what I was doing.

The 9v battery pad works as expected (9v), and everything seems to work fine at least up until the power caps / resistors. remember, the LED works fine. I'm gonna head over to ratshack tomorrow and grab a couple replacement capacitors. Also I'm going to replace the power resistors.

 

[Pars]

Quote:

Originally Posted by Heady I don't think you can measure the resistors after soldering them, as they are now interconnected in some way. At least that's what I found. You can however look at the bands on them to see what the values are. Measuring the supply voltage and DC offset will probably be more helpful.

That's not really true, particularly on a CMoy. Pull the opamp, and measure from each pin in the socket to virtual ground (it helps to have the schematic handy). Most of the resistors in a CMoy go to ground, and you will (should) see basically that resistance to ground (the input 100K resistors, for example). Likewise, in the feedback loop, R3 (I think, the 1K in the feedback loop) goes to ground. The V+ and V- pins will be through the 4.7K resistors in parallel with the electrolytic caps, so you won't see anything meaningful there (other than you may see the meter charging the cap up).

Also, measure pin to pin on the opamp socket and look for anything that appears unusual.

Not having any voltage on pins 4 and 8 is a problem. I would doubt that you have done anything to hurt the opamp... short of reversing the power they are pretty resilient. I would doubt your PS caps or resistors are the problem. Do you have the power input pads jumpered to the caps as in Tangent's guide? An analog meter will make these measurements more difficult, but will work.

 

[Earwax]

If you pull the opamp out of the socket and still measure 0 volts on pins 4 and 8 relative to ground there's no reason to think about opamp problems, something more basic is wrong. From the photos, everything seems to be in the right place, so I wonder about a cold solder joint somewhere. Check continuity at various points on the bottom of the board, especially the red and black power wires.

 

[pummer]

YAY! it works!


It turns out the problem was a poorly flowed solder joint between the power input and the cap. I suck

Thanks guys for your help! It sounds amazing

 

[pummer]

I figure I may as well ask questions in here rather than starting a new thread.

I've got my cmoy in an altoids tin, but by no means is it in its final form. I'm going to cut the wires to size and set everything up exactly how i want it before moving to a new tin.

What I'm wondering is this: do I need to ground the case? For example, take a wire and run it from the metal case to the virtual ground? it seems to me like this would be worthwhile. And how well do I need to insulate? Right now all I have is a single layer of duct tape on the bottom of the case.

 

[amb]

If your input and output jacks are not isolated then it will "automatically" ground to the case. I suggest isolating the output jack and only let the input jack ground to the case. This removes any possibility of ground loop.

 

[pummer]

How exactly do I isolate a jack?