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PPA V2 problem... Please HELP!

post #1 of 10
1/7/10 at 4:14am
Thread Starter 
Hey,

I've got some problems with a PPA v2. I've already build a perfect-working YJPS, but can't figure this out just yet.

I am measuring 0,000 V DC over the output resistors. I'm using AD8610 SOIC op-amps with an adaptor, the op-amp hangs on the down side. The PPA gives no or very distorded sound, but when I push the upside of the adaptor it sounds good. I thought it might be some loose contacts. However, when I do the same with a piece of plastic between my finger and the adaptor, it has no effect. This happens only with the right channel, but the left channel is also affected if I don't have my finger on the adaptor. It makes the difference between no or the most distorded sound I ever heard (including guitar overdrive amps) and a smooth, clean sound. I already tried adjusting the gain, which is now set at 3. This had no noticable effect.

Any ideas what the problem could be?
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post #2 of 10
1/7/10 at 5:19am
I'm not sure what you mean by pushing on the upside and downside of the adapter, nor what you mean by the opamp hanging "on the down side."

Have you checked the connectivity with a multimeter between each pin on the opamp and the appropriate connection on the board, particularly pins 2, 3, and 6? It sounds like you might have a cold solder joint somewhere.

Did you perform the initial tests from step 6 of the assembly guide? I'd also be interested in hearing if the correct voltages are being supplied to the opamp.
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post #3 of 10
1/7/10 at 5:45am
Thread Starter 
Quote:
Originally Posted by jezz View Post
I'm not sure what you mean by pushing on the upside and downside of the adapter, nor what you mean by the opamp hanging "on the down side."
I mean that the op-amp is mounted between the adaptor's pins, so it it not visible from above. If I push on the upside of the adaptor (so pushing it down into the socket) I get a good result.



Quote:
Originally Posted by jezz View Post
Have you checked the connectivity with a multimeter between each pin on the opamp and the appropriate connection on the board, particularly pins 2, 3, and 6? It sounds like you might have a cold solder joint somewhere.
I thought of a cold solder joint at first too. That's why I tried pushing it down into the socket with a piece of plastic. It is a good idea to check it with a miltimeter indeed, I'll try that first thing when I get home later today.

Quote:
Originally Posted by jezz View Post
Did you perform the initial tests from step 6 of the assembly guide? I'd also be interested in hearing if the correct voltages are being supplied to the opamp.
Yes, I did perform that. I measured everything that is stated there, they were all right. I'll measure the voltage supplies to the op-amp.
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post #4 of 10
1/7/10 at 2:36pm
Putting your finger on the op-amp contacts can fix things like oscillation and other strange behaviors. You're adding some low-current resistive and capacitive links between power and some of the most sensitive nodes in the circuit.

That doesn't help us figure out what's broken, though. Best to post pics and more measurements.
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post #5 of 10
1/7/10 at 4:35pm
Thread Starter 
I tried to post a message earlier today, but it's apparently waiting for the approval of a mod...

What we hear is a distortion on both channels, this distortion decreases enormously with a finger on the channel op-amp pins. (specially pins 2-3)
Low volumes are better than higher volumes.
Hitting pin 3 op-amp left gives a loud tick on that channel. The resistance of the pin to board is only 0,3 ohms

Measurements:
Across R24/R34 )V. (with and without load/input signal)
op-amp voltages:
across left ground right
4/7 21,4 21,2 21,8
4/2 10,7 10,6 10,9
4/3 10,7 10,6 10,9
4/6 11 11,2 10,7
mass/4 -10,7 -10,6 -10,9
mass/7 10,6 10,6 10,9

We are planning to add the optional bias channels for the op-amps. Could that be a solution? (installing the caps c5 didn't solve the problem)

I could take some pics if you really need them, but I don't think you would be able to see anything on a pic I would take in the dark...
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post #6 of 10
1/7/10 at 6:01pm
What's the resistance between IL and IG with the opamps uninstalled? If they're disconnected, that could be a source of oscillation (amongst a bunch of other things).

A picture really would be best, particularly with the lights turned on.
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post #7 of 10
1/7/10 at 6:50pm
Thread Starter 
The resistance between IL and IG with the opamps uninstalled is 48K (both are the same), which comes very near the potentiometers value of 50K.

Some pics:


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post #8 of 10
1/8/10 at 12:37am
How confident are you that you copied the layout exactly?
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post #9 of 10
1/8/10 at 2:11am
Thread Starter 
About 99,9 percent.

Unless there is some connection between the under and upper layer where I didn't mount a part I didn't notice. I made two under to upper connections (at the left connections where R7 could be mounted).
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post #10 of 10
1/8/10 at 5:29pm
Thread Starter 
IT WORKS!!!

And it sounds amazing. It is really, REALLY good. A stock MF is really no match for it.

As it turns out, there were a few cold solder joints at the power supply part. Furthermore, and there was a FET (Q3) broken. In the meantime, I biased the op-amps to class A.

Thanks guys for your help, it's really appreciated.
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