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RE: hot amp switch box

Posted by cpotl (A) on June 6, 2017 at 19:27:19

In Reply to: RE: hot amp switch box posted by vinnie2 on June 6, 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.

Chris

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