One system, two amps...one for tv and the other for music


My preamp has dual outputs going to a decent low power (consumption) amp and the other to my Threshold S200 space heater. I only run the Threshold while doing serious music sessions and my small amp for tv and casual listening. My speakers have dual binding posts with a factory jumper. I'm currently swapping my speaker cables from one amp to the other. Is it ok to have both amps connected with separate cables but only having one amp powered up at a time? My concern is added impedance from the other powered down amp. I'm guessing this isn't a very good idea no matter how convenient it would be.
treynolds155
I do something similar, although I use two separate but identical speaker cables. All I need do is disconnect a particular cable at the speaker input binding posts, and connect the one I wish to use.

This is done to allow the use of an SET 300B, or a Class D amp. I also use a preamp/processor, and a tube preamp with the SET 300B. A bit of an initial expense (separate tube DIY preamp, power and interconnect cables, but the convenience has made it well worth doing, for me, at least). 

Regards,
Dan

Yeah I was thinking of going that route, was looking to upgrade the speaker cable anyway, getting better ones for the Threshold obviously. But the sheer convenience of just powering up the amp I want to use is tempting. 

Thanks Dan
I don't see a problem with powering your amp seperately
Is it ok to have both amps connected with separate cables but only having one amp powered up at a time? My concern is added impedance from the other powered down amp. I’m guessing this isn’t a very good idea no matter how convenient it would be.

Your guess is correct, but for a more serious reason.  The outputs of two power amps should absolutely never be connected together, whether the connection is directly between them or is via the speaker terminals, or serious damage can result. That applies even if both amps are never powered up at the same time. For example, if a connection exists between the amps when you are using the S200 it may at times apply 40 volts or more to the unpowered output transistors of the smaller amp, in a manner that is likely to "reverse bias" their base-to-emitter junctions much of the time, and in doing so greatly exceed the maximum reverse bias the transistors are rated to be able to withstand.

A Google search will turn up various devices that would enable you to select which amp is connected to the speakers, while leaving the other amp unconnected. If you want to consider one of those, be sure it is rated to handle the power capability of the S200.

Also, if the smaller amp happens to be a tube amp, keep in mind that a tube amp that has output transformers (which means most tube amps) should not be powered up without a speaker or equivalent resistive load connected to its outputs, especially if it is being provided with an input signal. And in most and perhaps nearly all cases a switching device would leave the unselected amp unloaded.

Regards,
-- Al

Thanks Al, I've got a set of Acoustic Zen Hologram II speaker cables coming on Tuesday that I was going to use for this. Disaster averted so I really appreciate your taking the time to confirm my fears. I'm sure I'll still enjoy the new cables on the S200 though.