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Old 18th March 2013, 06:40 PM   #2311
Eric is offline Eric  United States
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Thanks for the encouragement, William. It's times like this that I start feeling foolish for fiddling with something that is already working...

I'll just have to be a bit more methodical in my approach.
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Old 19th March 2013, 12:32 PM   #2312
Eric is offline Eric  United States
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OK - here is where things are. I checked each of my 550's and the SJ74's in the input differential - all measure good. I put the differential back on the PCB - I think I mixed up the wires on the Q7 side of the differential last time around.

I powered it up and here are the voltages I was able to read:

Voltage across source resistors for output mosfets Q1, Q10, Q2, and Q11 each start at about 0.6v and decreases quickly and begins to stabilize around 0.20v

Voltage across drain resistors for the input differential R23 and R25 measures 4.6v - same as it always has.

Offset across speaker terminal starts at about 2.0v and increases rapidly.
Absolute offset starts at about 3v on +speaker terminal and about +6v on -speaker terminal and increases rapidly.

Voltage across R44/R45 starts at about 7v and quickly rises to rail voltage. Same for R1/R4 on the other side.

Voltage across R2/R3 = 0v, same for R42/R43.

Any ideas on where to go next?
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Old 19th March 2013, 03:19 PM   #2313
Eric is offline Eric  United States
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Thinking about this more leads me to believe that Q6 has been damaged and this is what is accounting for the quick change in offset after power up. But since this is connect to the differential, why is my increasing offset not symmetric across the speaker terminals. Wouldn't a damaged Q6 affect both sides of the amp in the same manner?

I think I'll start tonight with replacing Q6, Q4 and Q9 to see happens...
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Old 19th March 2013, 07:25 PM   #2314
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Hi Eric,

if the current source doesn´t work anymore absolute dc offset will vary. The relative offset is not affected.
Since you measure around 4.5V over the drain resistors the current source probably works.
Having a wandering rel. offset points to the input diff pair (pairs)

William
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Old 19th March 2013, 09:31 PM   #2315
Eric is offline Eric  United States
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Hmmm.. When I tested the SJ74s, I just connected them to my matching circuit for a moment to see that they came up close to their original value. Perhaps I should retest them for a longer period of time and see if they all still match as closely as they did originally.
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Old 19th March 2013, 10:34 PM   #2316
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Hi Eric,

it is also possible that somehow they are not connected to each other in the right way or there's a loose solder joint or something like that. I don't think the jfets are broken.

William
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Old 20th March 2013, 01:43 AM   #2317
Eric is offline Eric  United States
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Hi William,

You are correct about the JFETs not being broken. I pulled the differential again, fully disassembled it and measured the 550's and the SJ74's. Each measures correctly as a diode across the pins and the matched JFETs still match well. I also measured the empty holes on the PCB to make sure there are no hidden solder blobs on the back side of the board that are shorting across the pads.

Everything checks out. I also didn't find any loose solder joints among the input differential when I removed it from the board. I am happy to learn that I haven't blown up any of my parts, but this leaves me confused about what is causing this behavior.

With my first effort to replace the input differential, I did make a minor adjustment to the VR2 pot when I moved from the 9v to the 12v zener when things were working for a short period of time. Relative DC offset was in the 40-50mV range and Absolute DC offset started around 17v and declined quickly toward zero.

With my second effort to replace the input differential, I didn't make any adjustments to VR2 at all - I expected offset would just behave like the last time around and not need major adjustment.

So, here's a dumb question now that I have everything disassembled for a second time: Could this behavior just a result of needing to re-adjust VR2? It doesn't make sense to me that it would be this far out of whack from the last time around.

Eric
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Old 20th March 2013, 01:47 AM   #2318
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put different Jfets in place and you'll need to readjust current through LTP

in fact - just move them few mm one from the other , and you'll need readjusting

nature of the beast ; you must count on mucho patience and temp equilibrium as a must ..... or go servo route
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Old 20th March 2013, 02:40 AM   #2319
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Hmmm... Interesting insight, Zen. I didn't realize the input differential would be quite so sensitive. I was expecting it to behave as before, thus I didn't even think to adjust the current. I did use a second set of JFETs the second time around. It seemed so far out of whack, though, I was assuming something much more drastic was wrong. This is making me think I took it apart when all I needed to do was adjust the pot...

I guess I'll put it all back together again tomorrow night. I'm getting lots of practice designing a nice compact point to point circuit... More results in 24 hours...
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Old 20th March 2013, 08:57 AM   #2320
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Hi Eric,

VR2 is for the bias current and sets absolute offset not relative offset.

If the relative offset is off the current through the diff pair is not distributed evenly.
This happens when they are not matched right or the source resistors are off or something else is not right. I saw your fets are thermally coupled. This means that normally temperature has no influence.
I can cook or cool my input fets without any influence on absolute or relative offset.

William
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