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Old 9th September 2003, 12:27 PM   #21
PMA is offline PMA  Europe
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The V+ an V- pins of the OPA627 MUST be bypassed to ground near to those pins by ceramic capacitors of some 100nF, preferably in parallel with electrolyte of some 1uF - 10uF. Probably the similar for the LM.
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Old 9th September 2003, 03:26 PM   #22
JoeBob is offline JoeBob  Canada
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I used 100nF polypropelene caps in those places, although, not on the LM.
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Old 9th September 2003, 03:58 PM   #23
JoeBob is offline JoeBob  Canada
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Moving the high-pass filter from the input of the OPA to the input of the LM doesn't seem to fix the problem any...
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Old 9th September 2003, 04:02 PM   #24
Nuuk is offline Nuuk  United Kingdom
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Quote:
1000uF panasonic FC 63V
Now I am not an electronics expert but I wonder if the 'swing' of the DC offset at switch on is affected bty how big the caps are?

The main difference between the PSU that gave me the strange DC offset readings and the one that didn't, is the size of the caps which are much larger (4700uF) in the one that works OK.

Just a thought.
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Old 9th September 2003, 04:39 PM   #25
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by JoeBob
Moving the high-pass filter from the input of the OPA to the input of the LM doesn't seem to fix the problem any...
In circuit like this, nothing at 22nF capís left side can influence DC points around the output chip. Thatís why it is used.

Where do you exactly put that cap at the input of LM?

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Old 9th September 2003, 04:52 PM   #26
JoeBob is offline JoeBob  Canada
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That's eactly how it's wired up and the moment, except that the 22nF is 100nF.
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Old 9th September 2003, 05:50 PM   #27
JoeBob is offline JoeBob  Canada
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Well, I got it to work, but that still doesn't help me to understand what made it not work in the first place (mind you, this is one channel I've been working on, the 2nd is yet to be assembled).

I ran it into a speaker (12" woofer, good thing to) with a DC blocking cap and there was also very large low frequency oscillations that were causing the woofer to bottom out.

I disconnected the input, tried that. It made it worse. Then I reconnected the input and all was fine.

I must have nudged something else, or something, because I was too lazy to disconnect all the wires running to the board, then unscrew it from where it was fixed to the box, so I just squeezed the iron under the board and un soldered the wire, then resoldered it after.

I don't think it could have been a loose input wire, otherwise how could it have worked when the CD player was turned on first...

It's driving me crazy, but at least the final result works now.

Oh, and by the way, it sounds really nice too...
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Old 9th September 2003, 06:41 PM   #28
JoeBob is offline JoeBob  Canada
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I let it sit, and when I came back it wasn't working anymore...

Ugh!!!
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Old 9th September 2003, 06:59 PM   #29
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....problem with the booze...
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Old 9th September 2003, 07:38 PM   #30
JoeBob is offline JoeBob  Canada
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Well, I seem to have found the problem, and I hope I really did this time.

As it seemed Nuuk was correct the problem was in fact the power suply.

In one of my regulators (the 24V pre-regulator) I had a resistor value too high and it resulted in a voltage that slowly rose. Now that's changed and at turn on the voltage jumps right up and reaches 24V in a about two seconds, before it was taking something like 5-6 seconds and it was apparently causing the problems.

All this after I just had a thread about my PSU problems (turned out the multimeter had low batteries and was reading wrong in that case).

But what puzzles me was why it randomly started to work some times and not others. Possibly because some times I had turned it on while the caps hadn't completely discharged? Well that's my guess and it doesn't really matter because the problem appears to be fixed.

Thanks for all the help, especially you Nuuk, I wouldn't have checked there otherwise.
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