Quantum QM2201 Channel Strip

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Michael A

Well-known member
Joined
Mar 9, 2006
Messages
67
Hi All,

I'm finally getting back to my Quantum board and I'm trying to trace out a channel strip. I tried to get my picture to post here, but I couldn't make it work. Let me know if the link doesn't work.

If it does work, does this look like a proper working input section? I'm having a bit of trouble tracing the circuit.
edit: I think the pic works now. Also is a cap that is marked 100nS 100 nanofarad? And is a smallish mica marked 10 equal to 10 pico or 10 nano?


1299779342_f06939da0c_o.jpg
 
I assume that the smallish mica you're talking about is the one in parallel with the 68k feedback resistor? If so, then do the arithmetic. In parallel with 68k, it would form a lowpass filter; if it's 10nF then the -3dB frequency is 234Hz, whereas if it's 10pF the -3dB frequency is 234kHz. I'd bet on the latter.

If you're looking at the one across the secondary of the transformer, the equivalent cutoff frequency is 663Hz for a 10nF cap. That also sounds a little low, but on the other hand if it's 10pF, 663kHz sounds a little high. My bet is on 100pF, and see if there's a little resistor in series with it to form a Zobel network. Maybe that's what the 2k is really doing rather than feeding the + input?

Peace,
Paul
 
Paul,

Thanks for bringing your vast knowledge to bear on my problem. The two mica caps are identical so I'll go with your 10p value. For the one on the secondary of the transformer, I think I had that drawn wrong and it is in series with the 24k rather than parallel. The 2k does appear to be in series with pin 3 of the 5534. All of the switches around this area, which I have not drawn yet, complicate tracing this out.

There are two diodes, 1N614BF as far as I can read, that seem to be in series with the power supply pins. Any idea on the purpose of these?

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This has been a slow slog. I'm more used to tracing point to point in vintage tube gear and this double sided board and all of these switches are giving me fits. Anyway, I think this is the way the NE5532 that is connected to the fader, and other things, is wired. See any problems??? Can you tell me what it is doing?
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I updated the input section to include the switches for the mic/line selection. I tried to edit it in the old post to just update it, but it didn't seem to want to let me.
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