PPA v2 construction discussion

[tangent]

Quote:

Originally Posted by mik000000 /img/forum/go_quote.gif Would shorting the input and output grounds have damaged the opamp?

Much more likely, you damaged the ground buffer. If not that, then one or more TLEs. That also explains the audio problems.

 

[tangent]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Is 111mA quiescent input current in a V2 too much?

Probably. Since you've accounted for 60 mA of it, that leaves ~50 mA for the op-amps plus assorted other things. The piggiest op-amps I'm aware of (AD843) aren't quite enough to soak all that up.

Quote:

I've rechecked solder joints, etc. and can find nothing wrong so far. The amp board is not cased yet. Nothing gets hot at all. I have an Alps pot connected for the bass boost, but no C7 yet

I wish you'd waited to install the second ALPS pot, like the documentation tells you. The bass boost reduces the amp's stability, and you've got an instability problem.

The best you can do to try and patch over this potential problem is short both S2s, to take the pot out of the circuit.

If that doesn't help, trace a signal through the circuit as discussed in my troubleshooting guide.

Also, is C6 installed? What value did you use?

 

[mik000000]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Much more likely, you damaged the ground buffer. If not that, then one or more TLEs. That also explains the audio problems.

thanks tangent. I will try the tle's first as i socketed those. if that does not do it, any way to tell which of the transistors is damaged without removing them?
thanks for the help

 

[tangent]

Quote:

Originally Posted by mik000000 /img/forum/go_quote.gif any way to tell which of the transistors is damaged without removing them?

Not in any sane fashion. But in my limited experience repairing PPAv2s, a bad transistor in a buffer is likely to take down others, too, so it ends up being simplest to just replace everything in the channel. The exception is the output transistors, probably because they're much hardier.

This is a difficult enough thing to do, with a serious risk of damaging the board, that it makes sense to test the buffer before you rebuild it. Just as in my previous post, I refer you to my troubleshooting page. If you have a scope, that would be ideal: if you see the test waveform visibly distorting, you can bracket the problem location.

 

[funch]

Please excuse my sorry computer skills; I don't know how to post the quotes. This reply is for tangent.
I do have C6G installed; it's a 100pF silver mica cap. I also forgot to list that I have an OPA627UA in the ground channel, and 2 OPA637AP's in L/R.
I realized that I only had the gain at 4.9, so I changed R4 to give me a gain of 11. I also jumpered both S2's per your suggestions.
Now, the hiss isn't quite as bad (still very audible), and it will play music, but with a lot of crackling, which get worse as I add volume, so I guess I still have an instability issue. Time to up the learning curve. I'm one of those guys that's smart enough to get himself into trouble, but not smart enough to get himself out.
(By the way, how did you know I have an instability issue? You've never met me!

)

 

[tangent]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Please excuse my sorry computer skills; I don't know how to post the quotes.

What, you mean other than by clicking that big blue Quote button at the bottom of the post you want to quote?

To learn more, click on the links in the box at the bottom left of the page. ("vBCode", "Smileys", and "IMG")

Quote:

I realized that I only had the gain at 4.9, so I changed R4 to give me a gain of 11. I also jumpered both S2's per your suggestions. Now, the hiss isn't quite as bad (still very audible), and it will play music, but with a lot of crackling, which get worse as I add volume

That definitely sounds like it's marginally unstable. Have you cleaned the flux off the board?

 

[funch]

Hey, no fair picking on the mentally infirmed

Seriously, thanks. I'll have to spend some time figgerin' all this out.

No, I haven't cleaned the board yet. Can I use rubbing alcohol, or do I need straight isopropyl? I used rubbing on my M^3 and it dulled the board, but the amp workes fine.

I've noticed something else a little weird. At about one-third volume, there is a very small spot where all the noise goes away. Of course, it returns as I continue to increase the volume.

I just tested the opamps in my M^3, and they all work fine.

Actually, at this point, I'm considering just replacing all the transistors and hope for the best. Other than a DMM, I don't have any other test equipment, so that may be my only fix. We'll see. No way I'm gonna dump this amp after all this!

 

[n_maher]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Actually, at this point, I'm considering just replacing all the transistors and hope for the best. Other than a DMM, I don't have any other test equipment, so that may be my only fix. We'll see. No way I'm gonna dump this amp after all this!

Take your time with the trannie replacement. I had to do it multiple times to the same PPA board and that amp survived just fine and is still producing wonderful sounding music nearly a year later. Also bear in mind that most, if not all of the truly expensive bits can be moved to a new pcb if something does go wrong.

Best of luck,

Nate

 

[funch]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Take your time with the trannie replacement. I had to do it multiple times to the same PPA board and that amp survived just fine and is still producing wonderful sounding music nearly a year later. Also bear in mind that most, if not all of the truly expensive bits can be moved to a new pcb if something does go wrong. Best of luck, Nate

Thanks for the reply. It's both encouraging and a little disconcerting. May I ask why you had to do it multiple times?

Allen

 

[n_maher]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Thanks for the reply. It's both encouraging and a little disconcerting. May I ask why you had to do it multiple times? Allen

Allen,

For the full explanation see this thread. The short version of that story is that it was a PPA that a friend had built and while he was doing final adjustments on the buffers he shorted the ground channel output and it cooked that channel. So I rebuilt it once, got it all working and then did something really dumb. Thinking that I had once again managed to cook the ground channel I replaced all of the transistors a second time only to find that I'd tied V+ to IG and that the ground channel was fine all along. The good news for you in all of this is that it proves that the PPA pcb is rugged enough to handle a lot of soldering an de-soldering and that just about any problem with it can be fixed if you're up for the challenge.

Be careful though, you just might wind up with a custom title as a result

.

Nate

 

[villekille]

Is it normal that there is a lot of background hiss with sensitive low impedance phones as Ety ER-6i (16 ohm)?

I can't even plug them into my ears as the hiss is so loud..only way to get it down to a more reasonable level is to turn the ground channel buffer bias to 170mV and that's with 2.2ohm output resistors! And it's still uncomfortable to say at least.

With Senn PX-100 the hiss is nearly inaudible and the amp sounds great.

Another question about gain, my CDP has a loud output and I can just barely move the volume control on PPA as it gets too loud. The gain calc doesn't work on my computer but I tried to change R4 from 10K to 1K and it helped a little but not nearly enough..what should I do?

 

[kklee]

Quote:

Originally Posted by villekille /img/forum/go_quote.gif Is it normal that there is a lot of background hiss with sensitive low impedance phones as Ety ER-6i (16 ohm)?

I had exactly the same problem when I built my PPA. It was oscillating when I plugged in any low impedance phones. Upping the bias would lessen it but not get rid of it. I played with C6G until I found a value that got rid of the oscillations. However, I don't remember what I ended up sticking in there.

 

[n_maher]

Quote:

Originally Posted by villekille /img/forum/go_quote.gif Is it normal that there is a lot of background hiss with sensitive low impedance phones as Ety ER-6i (16 ohm)?

Quote:

Another question about gain, my CDP has a loud output and I can just barely move the volume control on PPA as it gets too loud. The gain calc doesn't work on my computer but I tried to change R4 from 10K to 1K and it helped a little but not nearly enough..what should I do?

Is the hiss the same with the source connected and disconnected? What opamps are you using? And as kklee suggested the solution may lie in the value of C6G so what do you currently have in there?

 

[villekille]

The hiss is there with or without the source connected.. I have the recommended 10pF silver mica at C6G. I don't have any other silver micas so I tried to put a standard ceramic cap alongside that silver mica, 27pF and 100pF but with no effect. I also tried to desolder that silver mica completely and only with those ceramics but same hiss was there. Finally I took a 2200pF film cap but now there was LOTS of noise. Without a cap I was unable to plug in my headphones as the offset was many volts.

I have used it with OPA134 x 3, OPA134 +OPA227 in G channel, and AD843 in L and R + OPA227 in G.

AD843 does not seem to be stable. If I'm running with a 12V supply it works but sometimes I get a many volts offset when I turn the amp on if the bass boost is on. With a 20V supply I haven't been able to use it because the offset is always there. Maybe it's because I have OPA134 or OPA227 in G channel as I have only two AD843s?

 

[villekille]

as a side note I have to say that the AD843 does sound good when it works, compared to OPA134 the bass is much better, deeper and more defined..that's so obvious that I'd soon like to get that setup working.