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Measuring Impedance of autotransformers - speaker selectors

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Jeremy Georges:
Hello Folks,

Sorry if this question is in the wrong category, but I couldn't think of another one to ask this question.

I have a small install in a home for 4 rooms and 4 speaker pairs and was not planning on high-impedance/constant voltage system  - just a standard 100w - 8 ohm output amplifier and use a speaker selector box that I have (Monoprice MSV-4). Its not a big system, just for background music and using what I have on hand.

So here is my question, when I use an LCR meter to test the impedance that the amplifier will see 'into' the matching network of the selector box, I'm seeing some strange results.
Using a 1Khz sine wave, I'm seeing at low volume settings of ~200ohms. Only at full volume (selector volume controls) do I see around 8 ohms.
I've never used these before so I'm curious to know if this is expected results?

I've tested two of the same model and I'm seeing the same results. When testing the speakers with the LCR they are 8 ohms and 4 ohms. Inside the selector, there are jumpers where you can tweak the impedance as it selects different taps on the autotransformer. That helped a little bit...now my impedance is around 120ohms on low volume settings and around 6 ohms at full volume.

Final question, do you know if it will harm an amplifier to run it at this high impedance setting? I know its not efficient and the transformers will probably have to dissipate the power as heat. But I want to make sure it doesn't damage the amplifier (Class D).

Thanks,

-Jeremy Georges

Ivan Beaver:
Before anybody can answer, you MUST use your full real name on these forums

Jeremy Georges:

--- Quote from: Ivan Beaver on August 16, 2017, 01:17:10 PM ---Before anybody can answer, you MUST use your full real name on these forums

--- End quote ---

Sorry, first post. Got my full name now :-).

Thanks,

-Jeremy Georges

Jeremy Georges:
So it looks like no one here has ever used one of these selectors then? Nor anyone has tested one with an LCR for comparison?

-Jeremy

Jean-Pierre Coetzee:
Wait a bit longer, I suspect if JR pops by here he will be able to answer your question, he did quite a lot of work with power amps.

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