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In Reply to: RE: update on web search posted by vinnie2 on July 08, 2017 at 05:22:16
"That is a valid point. Can you think of any way to protect the amps and not alter the load?"
You had a discussion thread on this a couple of weeks ago, I think. I remember proposing some rather elaborate switching with a 3-way make-before-break multi-gang switch. For stereo I think it could be done with just two gangs if it is acceptable to connect output ground to output ground on the two amplifiers. If complete isolation of both output terminals of both left and right amplifiers is required, it would be quite a bit more complicated.
Are your "ground" outputs for the left and right channels of each amplifier separate or joined together? Are you joining the output grounds of the two amplifiers together, or are those to be kept separate?
(An extreme case where this would matter is if you were comparing two circlotrons, for which each speaker output terminal would have to be kept isolated from each other output terminal - eight in all! Quite likely, in your case, all four "output grounds" can be common, unless that introduces hum loops.)
The important criteria are that no amplifier must ever be open circuit (no load at all) in any intermediate position as the switch is operated, and the two amplifier outputs should never get connected together in intermediate positions as the switch is operated.
A simpler option might be to stick with your present setup, but to increase the values of the two permanently-connected dummy-load resistors. From the point of view of not excessively loading the amplifier being listened to, one would like the dummy load resistance as large as possible. But it must still be low enough to prevent the amplifier that is not being listened to from having problems because of its load being too high in resistance. Since you are wanting the amplifier that is not being listened to to run for extended periods into its dummy load at normal listening output levels, it could be risky increasing the resistance too much. Someone with experience might be able to advise.
Got to stay with the Kiss principle to avoid trouble. i will do a little research on what size resistors to use.
FWIW, vintage tube amps and receivers with a headphone jack had a problem similar to yours; how to load the amp when the headphones were connected thru a dropping resistor of ~ 200-500 ohms - essentially open circuit. Typically they inserted a 25-50 ohm resistor across the output when the headphones were plugged in. I would expect a load in this range of values would be a reasonable starting point, esp if your amps are inherently stable.
But wouldn't the output required for headphones be different than that required for speakers? Would the amp still drive my speakers properly?
I'm suggesting you parallel a 25-50 ohm resistor across the output of each of your amps. When the speaker is connected to the amp it sees ~ 6-7 ohms assuming 8 ohm speakers. When you disconnect the speaker the amp will see 25-50 ohms depending on the resistor value you choose to use. Most amps should tolerate both 6-7 ohms and 25-50 ohms across 8 ohm taps with signal applied w/o issue.
I was using an 8 ohm resistor in parallel, but a quick on a calculator program shows that 25 ohms would give a more even effective resistance when the amp is in use. I had meant to do this earlier as another poster had mentioned this as well, but I forgot to do it. Thanks for the reminder, I will give it a try.
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