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Old 22nd May 2003, 06:28 PM   #1
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Default Measuring amp output power

Hi everyone,

What is the best/proper method for measuring amp output power?

And, once this is determined, can you really expect to see the db levels from published speaker/driver specs? That is, if the spec says 95 db/1w/1m, is this actual? Can you really rely on a sould level meter reading from say a hand held Audio Dynamics meter?

Thanks,
Rick
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Old 22nd May 2003, 07:11 PM   #2
dhaen is offline dhaen  Europe
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Default Power measurement

Hi,

Inject 1KHz sine-wave.

Increase the input level until just before clipping starts to occur. The distortion is likely to be 2 to 5% at this point. Confirm it if you can.
Distortion onset rate depends on topology and design.

Use a dummy load, and measure the voltage across it.

P= V*V/R where V is the RMS value.

Be aware that DVM's will read a little low at 1KHz.

Cheers,
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Old 22nd May 2003, 07:21 PM   #3
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Default Re: Power measurement

Quote:
Originally posted by dhaen
Hi,

Inject 1KHz sine-wave.

Increase the input level until just before clipping starts to occur. The distortion is likely to be 2 to 5% at this point. Confirm it if you can.
Distortion onset rate depends on topology and design.

Use a dummy load, and measure the voltage across it.

P= V*V/R where V is the RMS value.

Be aware that DVM's will read a little low at 1KHz.

Cheers,
Ditto, except I use a THD meter and spec performance at 1% and 5%, and will sometimes do a number of frequencies to check performance at either end of the spectrum. It's hard to pick 1% THD on a CRO.
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Old 22nd May 2003, 08:32 PM   #4
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Default Thanks guys.

So I use a signal generator input and an o-scope at input and across a 8 ohm resistor (dummy load) after the OPT? Measure V there (across resistor) and use the power equation, right? Then, that is true power to speaks?

Or, are you saying forget the OPT and measure V across dummy load (resistor) used in lieu of OPT? Then, convert to Pout considering trans ratio, etc?

Brett, I don't have a THD instrument--just have DVM, O-scope, signal gen-but thanks anyway. What's a CRO?

Assuming I get Pout, can I then expect to hear 95 db at 1 m at 1 watt using a speak of 95 db/m/w? That's pretty loud isn't it?

Thanks,
Rick
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Old 22nd May 2003, 08:44 PM   #5
dhaen is offline dhaen  Europe
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Rick,

Quote:
So I use a signal generator input and an o-scope at input and across a 8 ohm resistor (dummy load) after the OPT? Measure V there (across resistor) and use the power equation, right? Then, that is true power to speaks?
Yes and no
Your speakers won't be the exact impedance quoted, so there'll be a small error.

CRO = cathode ray oscilloscope. - He must be old

I thought you might not have a distortion meter. You should buy or build one if you're serious.

I'll leave the loudness measurements for someone more expert than me to answer.

Cheers,
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Old 22nd May 2003, 09:35 PM   #6
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Hi,

Quote:
CRO = cathode ray oscilloscope. - He must be old
Younger than you and me, John.

But let me add he knows what's worth keeping around.

Cheers,
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Frank
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Old 22nd May 2003, 09:41 PM   #7
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I use my distortion analyzer and generally check for power output and distortion at 100hz, 1K, and 20K.

Same method as others... scope and RMS meter.


Speaker output....

Is generally measured in a room set up for testing. Conditions can affect the output. Don't be surprised if the efficiency isn't quite the same as specified as you won't be able to duplicate the testing conditions.

The meter itself will have a bearing on the testing. Is it a type 1, or type 2 meter? Is the mic calibrated? Has the meter been certified this year?

I sincerely doubt that the meter in question is capable of this precise measurement. There are too many variables involved because you probably don't know what frequency/s was used. Without knowing how the testing was setup you will have a slim chance of being able to duplicate the test.

Be happy and enjoy your speakers.... I'm sure they are every bit as good as their specifications.

Joe
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Old 22nd May 2003, 10:23 PM   #8
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Default Thanks all,

Here's where I am: I have "built" a few tube amps (SET and SEP) in the 1.4 to 4.5 watt range (12eh5 to 6bq5 to 6aq5). I have plans for more such as 6bm8, 6L6, 807, and 300b. Right now I really like the sound of the single tube amp (that is, the 12eh5 is biased at -3v and puts out reportedly 1.4 watts). Oh, almost forgot, I have a 45 SET, too.

I run a cdp straight into the grid of the 12eh5 (pentode) and output plate in series with the OPTs. (BTW, I have tried the 6cl6 but did not like it at all).

I run either some Altec 605a's closed boxes (103 db) or JBL Control 28 (92 dbl) with these amps. Not really satisfied with either. I want to confirm that I am getting all the power of the tube before buying more speakers (maybe pro speakers, looking at more horns). I really like the sound and imaging of the se 12eh5 with the JBL's but just not enough SPL.

Rick
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Old 23rd May 2003, 01:08 AM   #9
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Rick, just in case you have a need to check your speakers rating you can do the pout calculation backwards to derive output voltage for 1 watt.
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Old 23rd May 2003, 03:51 AM   #10
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Default Wattage

If your scope is calibrated and checked for accuracy, you can measure the p-p voltage and then divide by 2.82 to obtain the RMS voltage.
This voltage will also include the harmonic voltages. But it should be fairly close to accurate.
Then take the RMS voltage and divide by the load resistance to obtain the RMS current. Multiply the RMS voltage by the RMS current.

This should give you a rough estimate of the power output.

Hope this helps.
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