[Akku]

Some week ago had problems with my PPA:
http://www6.head-fi.org/forums/showthread.php?t=90341
It was the left channel cascode JFet's (as Tangent pointed out).
I am still waiting for new JFet's to substitute the broken ones so I disconnected the cascodes in all three ch.
Everything seemed to work right but today I noticed a high offset in the left channel again and swapping op-amps didn't help.
There is somenthing wrong with the low current rails. With no op-amp I have 1 volt drop and if I put the op-amp in place the voltage collapse to +1/-2,4.
Maybe the TLE is gone?

 

[tangent]

Can you trace a a signal through the amp to find out where it stops?

 

[Akku]

What signal do you mean?
The amp seems to work correctly throug a pair of cheap plastic speakers (I don't dare plugging headphones), but:
1) With 627 in all channels I get 12,3 mv offset from left one (ground and right are ok, less than 1mv).
2) The left channel low current rail tested just after the isolation jfets is around +2/-2 volt (no drop in right and ground at the same point: +13/-13).
3) With no opamp in place tested at the same point like 2), left channel rails has one volt drop (+12/-12).
The funny thing is that I tested the amp throug RMAA before I realized that somenthig was wrong and I got pretty good results apart from IMD swept freq. that in left channel was quickly rising towards high frequencies.
Help me doctor, what's the cure?

 

[tangent]

Quote:

What signal do you mean?

I mean a test signal: http://tangentsoft.net/audio/trouble.html

 

[Akku]

But I do get signal at the output in both channel.
I have connected crappy in ear phones and I cannot tell the difference from a full working PPA.
Am I mad? Am I pointing the meter in the wrong places?
This is the RMAA result fro thd (1khz -3db).

 

[Akku]

Ok, I am a newb and I have to follow wise person advice. I must remember that that's the reason I posted this thread.

I did the test to check the amp's AC beaviour with a 0db tone.
In the left channel the voltage at -IN opamp pin is ten times smaller than +IN.

At first look nothing wrong in the feedback loop (resistors' values are correct) but tommorrow I'll give a closer look. Now I need to sleep.
Thank you Tangent

 

[tangent]

Quote:

In the left channel the voltage at -IN opamp pin is ten times smaller than +IN.

Well, that may be more of a symptom than a cause. It's definitely something to check out, though. +IN should always be nearly equal to -IN, or else the op-amp isn't op-amping.

 

[Akku]

In the feedback loop everything seems ok. I am almost sure that the problem is caused by rail isolation jfets. The voltage drop in the rails with no opamp plugged shows they are to blame. With the load (the op-amp) the drop is huge: I get +/- 2.00 volts (before the jfets is +/- 13). I wonder how is possible that I still get a decent sound from the left channel with such a poor power supply.
Next week the new jfets should arrive, let's see

 

[Akku]

I had to change all the jfets in the left channel for new ones but now my PPA is delivering its magic again.
Evviva!

 

[ppl]

It sounds like one of the Jfets in the Left Low current op amp power rail is defective. easure the voltage drop from V- at the power input to Pin 4 of the opamp do the same for the Pos rail to pin 7 of the opamp with the opamp installed this will tell you the voltge drop across each fet and it maybe only a few volts across one and the remaining supply voltage across the other fet. this indicates a defective fet. the TLE will keep the opamp active so with the exception of high DC offset everything appears normal.