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Author Topic: Measuring Impedance of autotransformers - speaker selectors  (Read 690 times)

Jeremy Georges

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Measuring Impedance of autotransformers - speaker selectors
« on: August 16, 2017, 01:05:02 pm »

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
« Last Edit: August 16, 2017, 01:19:27 pm by jeb »
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Jeremy Georges

Ivan Beaver

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #1 on: August 16, 2017, 01:17:10 pm »

Before anybody can answer, you MUST use your full real name on these forums
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Jeremy Georges

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #2 on: August 16, 2017, 01:28:05 pm »

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

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

Thanks,

-Jeremy Georges
« Last Edit: August 16, 2017, 01:30:24 pm by Jeremy Georges »
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Jeremy Georges

Jeremy Georges

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #3 on: August 19, 2017, 08:21:23 pm »

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
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Jeremy Georges

Jean-Pierre Coetzee

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #4 on: August 19, 2017, 11:58:01 pm »

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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Jim Thorn

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #5 on: August 20, 2017, 05:13:36 am »

Your selector box is doing exactly what it should.  The power delivered to the speaker will be inversely proportional to the impedance presented to the amp by the autoformer, so lower volume settings will necessarily present a higher impedance.  Solid state amplifiers are LESS stressed driving higher  impedance loads than they are driving low impedance loads, and there is not really any excess power that has to be dissipated as heat -- the amp simply doesn't DELIVER as much power into a high impedance load -- so the autoformer volume control will run cool, too.  The autoformer type controls are the industry standard in 25-volt and 70-volt systems because they run cool, don't waste unused amplifier power, and last almost forever.

Some TUBE amps may act badly when driving a high impedance load, but analog transistor amps don't.

On the side of caution:  You mention that the amp is Class D.  I have used autoformers with Class D amps without ill effects, but it occurs to me that there could theoretically be some ultrasonic pulses from the amp which could cause the inductance of the autoformer to generate voltage spikes.  I admit I haven't looked at the output with an oscilloscope, however I strongly suspect that the low-pass filtering built into the amp's output would prevent that from ever becoming an issue.  Although I've experienced no problems in practice, I'd be interested in hearing a definitive answer from an amp designer such as John Roberts.

Jim Thorn

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
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Jeremy Georges

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #6 on: August 23, 2017, 08:42:51 am »

Your selector box is doing exactly what it should.  The power delivered to the speaker will be inversely proportional to the impedance presented to the amp by the autoformer, so lower volume settings will necessarily present a higher impedance.  Solid state amplifiers are LESS stressed driving higher  impedance loads than they are driving low impedance loads, and there is not really any excess power that has to be dissipated as heat -- the amp simply doesn't DELIVER as much power into a high impedance load -- so the autoformer volume control will run cool, too.  The autoformer type controls are the industry standard in 25-volt and 70-volt systems because they run cool, don't waste unused amplifier power, and last almost forever.

Some TUBE amps may act badly when driving a high impedance load, but analog transistor amps don't.

On the side of caution:  You mention that the amp is Class D.  I have used autoformers with Class D amps without ill effects, but it occurs to me that there could theoretically be some ultrasonic pulses from the amp which could cause the inductance of the autoformer to generate voltage spikes.  I admit I haven't looked at the output with an oscilloscope, however I strongly suspect that the low-pass filtering built into the amp's output would prevent that from ever becoming an issue.  Although I've experienced no problems in practice, I'd be interested in hearing a definitive answer from an amp designer such as John Roberts.

Jim Thorn

Thanks Jim. What's the best signal to test for any potential voltage spikes on the output? Is this as simple as sending a single 1K test tone and looking at the output with an oscilloscope?

thanks.
-Jeremy
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Jeremy Georges

Riley Casey

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Re: Measuring Impedance of autotransformers - speaker selectors
« Reply #7 on: August 23, 2017, 10:07:13 am »

That depends on what frequency you are interested in.  One kilohertz is not where the heavy lifting is for most amplifier / speaker combinations. 

Thanks Jim. What's the best signal to test for any potential voltage spikes on the output? Is this as simple as sending a single 1K test tone and looking at the output with an oscilloscope?

thanks.
-Jeremy
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