Novice here. Looking into 80s rack preamp GK 2000 CPL. It has a single input, 2 switchable channels, stereo outputs, and built in compressor, reverb & chorus. It sounds killer. I'm trying to answer the question: where in the circuit does this amplifier turn from mono into stereo, and what exactly is the stereo effect/content - i.e. how are the left & right channels different.
I attached a PDF with the amp's circuit. As far as I can tell, the whole guitar signal chain (first page), incl. the compressor, up to and not including the FX is all mono. The signal lead labelled "D" feeds into a four circuits (2nd page): dry-lo, dry-hi, chorus, and reverb that all end in a kind of stereo mixer. Signal lead "E" feeds the reverb gate circuit.
Observations/questions - correct me if I've misunderstood anything:
Edit: Figured I attach the schematics here as images
I attached a PDF with the amp's circuit. As far as I can tell, the whole guitar signal chain (first page), incl. the compressor, up to and not including the FX is all mono. The signal lead labelled "D" feeds into a four circuits (2nd page): dry-lo, dry-hi, chorus, and reverb that all end in a kind of stereo mixer. Signal lead "E" feeds the reverb gate circuit.
Observations/questions - correct me if I've misunderstood anything:
- I can't figure out the difference between dry-lo and dry-hi. Really curious about that split. Is that a frequency split or a volume split?
- Unless I'm missing something, the chorus seems to operate entirely in mono, which is different than i.e., the ADA MP1, a popular comparable preamp from the same era, where the original signal is split into 2, one gets mixed with the chorus signal, and the other as well but with its chorus component phase shifted by 180°. I couldn't tell if there's any phase shift on the L or R channels of the chorus on the 2000CPL.
- The reverb seems to be the only circuit doing actually doing something in stereo. It seems that the 6 taps of the MN3011 BBD chip (each providing a different timing window) are divided between the left and right channels. I am not an expert but maybe that gives an expanded stereo reverb image.
- But then later, in page 3, we can see both balanced and unbalanced sets of outputs have a mono/stereo switch each. I am not familiar with their symbol for switch, and can't really figure out what happens with either switch in mono mode.
- Beyond that, it seems the output circuits of either L/R channel are identical in both balanced/unbalanced circuits. I couldn't identify if there's any kind of phase shifting or other shenanigans going on.
Edit: Figured I attach the schematics here as images
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Dry lo/hi looks like a frequency split.
Chorus looks mono to me too.
It looks like the balanced outputs on page 3 have only one switch, S8. In one position it looks like pin 3 of each XLR connector is an inverted copy of the signal on pin2 (which would amount to balanced). In the other switch position it looks like pin 3 of both J6 and J7 are both driven by an inverted copy of the right channel; it also looks like pin 2 of J6 and J7 are both driven by an inverted copy of the left channel.
Markw4, thanks for your reply! How can you tell the balanced outputs are driven by inverted copies of each channel? And does it has any effect considering both L + R channels are inverted?
The opamps that drive those outputs are configured as inverting amplifiers. However, if the mix bus is already inverted somewhere before it gets routed to that part of the circuitry, then the opamps would invert it again thus making it noninverted relative to, say, maybe the guitar input. Either way, the two output channels will be in phase with each other.