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Measuring amp output power
Measuring amp output power
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Old 23rd May 2003, 06:54 PM   #11
Gabevee is offline Gabevee
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My

If memory serves, the speaker efficiency with regards power transfer is a major factor. If you put out, or measure, 1 watt with a watt meter, or the non-inductive load, it may not be the same at the speaker.

I mean, you can have a speaker that is designed to handle 10 watts (1 inch voice coil, 5 ounce magnet) be 90dB/watt, and a speaker designed for 500 watts (2.5 inch coil, 10 pound magnet) be 90dB per watt. Will the 1 watt amp drive them to the same actual dB? Nope. That is because there is power loss in the bigger power speaker.

So, the rating really means that if you have enough power that the speaker processes 1 watt, you will get 90 dB. The 500 watt speaker may need 50-100 watts of amplifier power to give you close to 90dB.

In my opinion, if memory serves. Haven't read about this in 15 years. Could be starting another war!

Gabe
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Old 23rd May 2003, 07:08 PM   #12
fdegrove is offline fdegrove  Europe
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Default RE:Could be starting another war!

Hi,

Gabe, I don't think that's correct.

Speaker effeciency as stated by the manufacturer is, or shall we say should, be the same regardless of its size or powerhandling.

Actually the poweroutput capabilty of an amp should be calculated by using a resistance of say 8,4 or 16 Ohm to get the rating.
You can measure it with a speaker connected but that will only tell you how much the amp is delivering with that particular load.

I don't know the formula by heart but I'm sure it's derived from Ohm's law.

Cheers,
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Old 23rd May 2003, 10:05 PM   #13
fdegrove is offline fdegrove  Europe
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Default OHM.

Hi,

Tada....

P = I ^2 *R.

Cheers,
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Old 24th May 2003, 02:59 AM   #14
PassFan is offline PassFan  United States
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Quote:
Originally posted by Gabevee
My

The 500 watt speaker may need 50-100 watts of amplifier power to give you close to 90dB.


I don't think anyone will be lining up to buy that driver. Don't know if this applies or not but the formula for wattage to db is: W=logx10. This is added to your 1 watt spl rating for db output at any given wattage. The formula for power is: power=logx20.
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Old 24th May 2003, 03:29 AM   #15
Brett is offline Brett
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Quote:
Originally posted by Gabevee
So, the rating really means that if you have enough power that the speaker processes 1 watt, you will get 90 dB. The 500 watt speaker may need 50-100 watts of amplifier power to give you close to 90dB.
That's incorrect. Read Theile and Small.
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Old 26th May 2003, 08:26 AM   #16
Gabevee is offline Gabevee
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OK, OK already, I did say if memory serves, doggoneit!

I also did a few searches on the web, and one place did say something similar to what I said. However, what the person said was that one has to look at the overall efficiency (I forgot the exact term). Remember that generally the speaker is tested for that SPL at 1kHz. It may not be 90 dB at other frequenies.

Now, I am sure we all know this.

Thanks for the correction. As I said, I read about this stuff 15 years or so ago, when I built my first pair of speakers,which BTW I still listen to. I haven't thought much about speakers since, except for the crossover networks, which I readily build and put into speakers I buy, improving their quality.

So... ignore what I said!

OTOH, one must not think that either one watt is enough or that 2 watts will give one 180 dB. Doubling the power only gives a 3dB boost, hardly loud enough to detect. Perceptible doubling required 10dB, if memory serves (Uh, oh, I am in trouble again! ).

Later!
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Old 19th November 2017, 09:31 PM   #17
pbpix is offline pbpix  United States
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Originally Posted by dhaen View Post
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,
This is a rather old post so I'm hoping to revive it to get an answer to my recent experience and question.
So ...I have a question about this procedure.
I was using this method just yesterday to measure power on my cathode-biased 6V6 PP amp. (class AB1)
I injected 1khz and cranked it to just the edge of distortion/clipping point on my scope. Then I measured the RMS voltage across my 4 ohm dummy speaker-load.
However, my friend advised me that I cannot accurately measure power that way because the by-pass caps will keep charging up and increase the bias voltage reducing the effective output power.
And sure enough, when I put my voltmeter on the cathode voltage and I saw the bias voltage creep up over 2 volts!
So.. what is the proper way to dynamically measure power?

Last edited by pbpix; 19th November 2017 at 09:39 PM.
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Old 19th November 2017, 09:48 PM   #18
audiowize is offline audiowize  United States
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This is a big reason to go with fixed bias!

You can temporarily solder in some zener diodes to clamp the cathode voltage.

You can also put a normally closed momentary switch across your signal source so that you can blip the input and do a peak hold on the output.
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Old 19th November 2017, 10:05 PM   #19
petertub is offline petertub  Sweden
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Quote:
Originally Posted by pbpix View Post
This is a rather old post so I'm hoping to revive it to get an answer to my recent experience and question. <snip>
Just like you did.
The fact that cathode voltages rises is just a function of - cathode bias. And
this is the amp you have, this is the power you will get.
Building an amp with fixed bias will get more power, since the voltage drop
across the cathode resistor is gone. It's not all benefits, fixed bias also
introduces the risk of "redplating" as the feedback mechanism is gone.
In cathode based amps, if the current through the tube increases, the bias will
increase negating the current increase. When using fixed bias no such
mechanism exists.
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Old 19th November 2017, 10:19 PM   #20
pbpix is offline pbpix  United States
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Thank you I get it. So I guess... "just live w/it"
And also yes.. thanks I'm aware of the fixed bias possible-dangers too.

Thanks for speedy reply
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