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Author Topic: 70V Speaker Taps Theory Question  (Read 1428 times)

Nathan Rooke

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70V Speaker Taps Theory Question
« on: November 07, 2012, 04:45:07 pm »

I have been testing a several speakers with a standard ohm meter just to make sure they're not completely busted and I've noticed while the resistance I get in the speaker is consistent with the same model it's not consistent across different models/brands and I'm curious how this affects spl. If, for example, If I had a hypothetical speaker which was tapped at 5W and had a resistance of 50Ω would the sound level be closer to:

a speaker of a different type tapped at 5W with a resistance of  100Ω
or
a speaker of a different type tapped at 10W with a resistance of 50Ω?

Thanks for your reading.
~Nathan
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Mac Kerr

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Re: 70V Speaker Taps Theory Question
« Reply #1 on: November 07, 2012, 05:12:18 pm »

I have been testing a several speakers with a standard ohm meter just to make sure they're not completely busted and I've noticed while the resistance I get in the speaker is consistent with the same model it's not consistent across different models/brands and I'm curious how this affects spl. If, for example, If I had a hypothetical speaker which was tapped at 5W and had a resistance of 50Ω would the sound level be closer to:

a speaker of a different type tapped at 5W with a resistance of  100Ω
or
a speaker of a different type tapped at 10W with a resistance of 50Ω?

Thanks for your reading.
~Nathan

As long as there is one remaining unknown (speaker efficiency) there is no way to know. To make installation simple, the taps are labeled in "watts", This is so you can easily add up the load on the amp. The reality is they change the impedance the amplifier sees, and you are really adding high impedance speakers till you reach the lowest impedance the amp can drive.

A 5W tap should represent the same impedance on any brand of speaker, since the impedance is what is actually important.

DC resistance is not AC Impedance.

Mac
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John Roberts {JR}

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Re: 70V Speaker Taps Theory Question
« Reply #2 on: November 07, 2012, 06:23:09 pm »

A confusing factor when trying to evaluate speakers from the voice coil DCR (dc resistance) is that it doesn't accurately reflect the impedance load they present at audio frequencies.

To properly measure AC impedance requires a different specialized meter.

JR
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Ivan Beaver

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Re: 70V Speaker Taps Theory Question
« Reply #3 on: November 07, 2012, 07:13:24 pm »

A confusing factor when trying to evaluate speakers from the voice coil DCR (dc resistance) is that it doesn't accurately reflect the impedance load they present at audio frequencies.

To properly measure AC impedance requires a different specialized meter.

JR
And when trying to measure a "70V" speaker with a ohm meter-the actual impedance of the driver does not matter.  In fact the loudspeaker driver itself could be completely destroyed and the ohm meter will read just fine.

That is because the ohm meter is using DC to measure RESISTANCE-which is just the windings on the primary side of the coil.

It doesn't matter what the secondary is-the DC does not "cross over" the windings of the transformer--B ut an IMPEDANCE will-since it is using AC to measure with.

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Ivan Beaver
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Nathan Rooke

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Re: 70V Speaker Taps Theory Question
« Reply #4 on: November 09, 2012, 02:50:43 pm »

And when trying to measure a "70V" speaker with a ohm meter-the actual impedance of the driver does not matter.  In fact the loudspeaker driver itself could be completely destroyed and the ohm meter will read just fine.

That is because the ohm meter is using DC to measure RESISTANCE-which is just the windings on the primary side of the coil.

It doesn't matter what the secondary is-the DC does not "cross over" the windings of the transformer--B ut an IMPEDANCE will-since it is using AC to measure with.
Well, thats unfortunate. At least I wont be wasting my time "testing" this equipment I suppose.

Thank you all for your insight.
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Ivan Beaver

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Re: 70V Speaker Taps Theory Question
« Reply #5 on: November 09, 2012, 06:29:02 pm »

Well, thats unfortunate. At least I wont be wasting my time "testing" this equipment I suppose.

Thank you all for your insight.
You just need to use the proper tool for the job.  An ohm meter is not the tool for a system with transformers in it-at least for finding out what the loads are.

It CAN be very useful for finding shorts in the line-open in transformer windings and so forth.

it is all about what information you are looking for.
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Ivan Beaver
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Nathan Rooke

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Re: 70V Speaker Taps Theory Question
« Reply #6 on: November 12, 2012, 12:48:58 pm »

You just need to use the proper tool for the job.  An ohm meter is not the tool for a system with transformers in it-at least for finding out what the loads are.

It CAN be very useful for finding shorts in the line-open in transformer windings and so forth.

it is all about what information you are looking for.
Well thats good, not a total loss then.

Purely out of curiosity, what would be the answer to my original question be if those theoretical values I gave were the AC impedance?
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Mac Kerr

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Re: 70V Speaker Taps Theory Question
« Reply #7 on: November 12, 2012, 01:22:16 pm »

Well thats good, not a total loss then.

Purely out of curiosity, what would be the answer to my original question be if those theoretical values I gave were the AC impedance?

If those numbers were the actual AC impedance either the measurement was bad, or the speaker/transformer combinations were mislabeled.

The power in watts that the system dissipates is inversely related to impedance. An impedance of 50Ω draws 100W from a 70V source, a 50W speaker will be 100Ω, a 1W speaker will be 4900Ω. As with all audio, these power draws are only at clipping. Since the voltage is lower at lower audio levels, so is the power.

Here is some information of 70V systems.

Mac
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Ivan Beaver

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Re: 70V Speaker Taps Theory Question
« Reply #8 on: November 12, 2012, 06:45:11 pm »

Well thats good, not a total loss then.

Purely out of curiosity, what would be the answer to my original question be if those theoretical values I gave were the AC impedance?
Is your question what the impedance would be for the stated wattage-or the stated wattage for the measured impedance.

It is either one or the other.

As Mac has eluded, one of the "numbers that should be remembered"-is 1 watt at 70V is 4900 ohms.  Or with some rounding say 5000 watts.

So 10 watts would be 500 ohms and 50 ohms would be 100 watts.

You can quickly figure out impedances in between the basic numbers.

Many things in audio are not about exact numbers-but quickly getting close to what should be answer.

If you were expecting to see 200 ohms on a line-and you read 30 ohms, then you have a problem.  It doesn't matter whether it is 30 or 40 or 20 ohms at this point.

And ALSO remember-that the impedance will vary a good bit-depending on the freq.  So a simple 1000Hz tone impedance will very often give a WRONG answer.  Yes it is correct at 1000Hz, but what if that is am impedance peak in the response?  The actual load presented to the amp could be quite a bit lower.

Once again-the simple answer is WRONG.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!
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