Preamps with two main-outs

kalali,
Can you use a ohm meter to check the input of the SP331 when it is power down to confirm that the input is shorted?
 

I’ll take the measurement when I get home this evening. Sorry but I can’t make out much by looking at the schematic but it’s nice to have it so I appreciate you posting it.

P.S. Had to look up what "dog’s breakfast"..no clue what it meant. Cool slang and a fairly accurate description of how my cabling looks....

Yes, thanks for posting the schematic, imhififan. And yes, it is of course hard to read. But it appears that what is most likely a relay is connected directly between the signal pin and the ground sleeve of the RCA input connector on each channel. Presumably when the warmup period concludes a control voltage is applied to the relay which causes its contacts to open up, allowing the input signal to proceed to the input stage. And presumably when the amp is turned off that control voltage goes away and the contacts close, resulting in a short across the input.

Unless perhaps there is some means by which the design causes the contacts that are opened during normal operation by the application of that control voltage are somehow also kept open when the amp is off. And then immediately closed at turn-on, for the duration of the warmup period. In which case a defect in the relay might be responsible for the issue. But I can’t envision how that scenario could occur, with the amp still able to operate reliably. So most likely the issue is caused not by a flaw in the relay but by a flaw in the thinking of the designer.

Best regards,
-- Al

+1 Al

looks like you found it! Nevertheless an active amplifier works as specified with input impedance as stated when powered up - I am not sure what happens to the input buffering on all amps when switched off - probably most remain high impedance but that is not a given hence my warning that amps are designed to be used when switched on.

Similary for line level inputs for different members of a band you can’t just branch all the inputs directly to one amplifier input - their gear is all connected to a MIXER that not only allows volume adjustments/effects but as a minimum the mixer buffers the outputs of individual instruments from affecting each other....

shadorn's comment about input impedance triggered a question I've been meaning to ask: Can the generally published spec for amplifiers' input impedance, e.g., 47K ohms for the Vincent, be simply measured at the input ports, presumably with the amplifier turned on, or there's a lot more involved in taking this measurement? Thanks again for all the education.