Preamps with two main-outs

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.

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.

Well said, Shadorne. It may or may not be an issue from a sonic standpoint depending on the specific designs that are involved.

Also, generally speaking I'd expect that the uncontrolled and unpredictable variations in input impedance that may occur in some unpowered components would be more likely to be an issue in the case of solid state designs than in the case of tube designs, particularly if bipolar transistors are used rather than FETs. Also, it's very conceivable that in many such designs the unpowered input impedance could even vary significantly as a function of signal level, which would increase the likelihood of adverse sonic effects on the signal path that is being used. But where the active device itself has a very high input impedance even when not powered, an example being the grid of a tube, the input impedance of the unpowered circuit would usually be determined essentially by resistors. And in those cases it would presumably be little different in the powered and unpowered states.

That possibility of uncontrolled and possibly varying input impedance in some unpowered components is the main reason, BTW and IMO, that having an unpowered component connected to the signal path might in some cases have adverse effects on sonics, that wouldn't occur if the component was powered up. Not the alleged phenomenon that was referred to earlier involving "out of phase backflow current."   Simple calculations will show that for cable lengths that are typically used in a home audio system energy reflected back from a destination component to the component supplying the signal will arrive at the component supplying the signal with a phase delay in the vicinity of 0.2 degrees at 20,000 Hz, and with vastly smaller delays than even that at mid-range and bass frequencies. Not to mention that reflection effects will be essentially non-existent at frequencies below RF.

Best regards,
-- Al
   

Pick up awards at the Wolf Garcia center any time. I have had 2 to 3 things hooked up to my preamp outputs (2 RCA and one XLR) with zero apparent issues for a while now, currently using 2 (tube power amp and a SS amp used for deck speakers) as I no longer have anything that utilizes the XLR outs…the third thing was a little headphone amp…may have to get a pair of XLR adapters to get the headphone amp back in the system, although it’s rarely used. I had wondered about a related topic when Morrow noted you can break in their cables by driving them to another component that is off (preamp to unpowered power amp for example). Seems weird as I would think a component that’s off would act like something that’s not there at all and not draw any signal...but hey…maybe Morrow knows things.