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In Reply to: RE: hot amp switch box posted by vinnie2 on June 06, 2017 at 18:15:42
If you put a 10 ohm resistor across the speaker terminals then, as you say, the amplifier will be seeing a 4.4 ohm load, instead of 8 ohms, if your speaker is 8 ohms. This is a very significant change to the loading.
I would have thought you would want a shunt resistor that was considerably higher than 10 ohms. Essentially, you would want "as high as you can get away with" while still achieving the goal of keeping the amplifier OK in the event that it is running without the speaker load. 10 ohms would certainly be OK from that point of view, since it is barely more than the 8 ohms of the normal speaker load. I have no experience of how high you can get away with, but it must be a good deal higher than 10, I would think.
I've not thought through the details, but I would think an "ideal" arrangement would be one where there is no resistor shunting the amplifier when it is the one selected for listening. A make-before-break switch is needed for switching the amplifier either to the speaker, or to the substitute resistive load. And arrange, maybe by using a multi-gang 3-way switch, so that you can do this for each amplifier.
Assuming that you don't use permanent shunting resistive loads, you need to use make-before-break switching so that an amplifier never operates with no load. But you also need to make sure that you don't end up with an intermediate switching state where the two amplifier's outputs are connected together, since in principle there exists the possibility that the amplifiers are in antiphase (one inverting, the other non-inverting) and so connecting their outputs would be like a dead short. I think some arrangement with (maybe ganged) 3-way make-before-break switches would achieve all this.
It must be a standard problem, with a standard solution, I imagine. There should be schematics floating around on the internet somewhere. Otherwise, one can work it out with bit of thought, keeing in mind the two essentials: no amplifier ever open load, and the two amplifiers never connected together.
Just enough to keep the transformer primary impedance to a safe level.
Your last sentence is exactly why I think I will try the resistors on each amp. The old KISS principle once again. With the resistors there is no chance of damaging equipment with a dead short or no load. At worst the sound will be funky, but I ought to be able to notice that. If you happen to run across any plans please let me know as my searches have come up empty for a unit that will allow switching with both amps running.
Sorry for the poor image quality, but attached is a sketch of the sort of thing I had in mind. Two ganged 3-way Make-Before-Break switches, wired as shown. I've had a couple of beers (in China right now, where it is evening), but I think it does the job of ensuring neither amplifier ever operates with no load, and that the two outputs from the A and B amps can never be connected together as the switching occurs. The indicated positions of the switches are for Amp. A playing through the speaker. The middle position is neither A nor B playing through the speaker. R would be 8 ohms or so. Of course another 2 gangs are needed for the other channel of a stereo amplifier.
The schematic assumes it is allowable to have a common ground for the outputs from amplifiers A and B. If this is not allowable (e.g. circlotron OTL amplifiers) then I suppose it would need a similar switching arrangement for the other end of the speaker too.
I could imagine there are more efficient ways to achieve the desired goals, but I think this would achieve it (unless the beer was too much!).
I appreciate the effort, but I tried just adding a 10 ohm/30 watt resistor to each set of speaker terminals on the 6bq5 amp and played it that way for an hour or so. Did not notice any major difference in the sound from without the resistor.
Going to have to put the amps side by side with the resistors mounted and then use a regular switch unit between them to go back and forth. Just too much trouble to route all the wires that would be needed to go to a switch from the equipment rack.
I'll do a follow up post when I get everything hooked up and working.
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