Measuring Amplifier Power Output - diyAudio
Go Back   Home > Forums > General Interest > Car Audio

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th June 2011, 12:34 AM   #1
diyAudio Member
 
Join Date: Sep 2009
Location: Arizona
Default Measuring Amplifier Power Output

I was "trying" to measure the power output of my JL Audio 1000/1 amplifier, without a dummy load attached to its output terminals, and came up with the following:

Test Meter: Fluke 87-V
Test Voltage: 12.57 Volts (engine off)*
Test Voltage: 13.99 Volts (engine idling)*
Test Signal: 40 Hz, 0 dB
Speaker Load: 4 Ohm
Distortion: <3% as tested with gain overlap

Using P=V^2/R

Power=68.5*68.5/4

=1173 Watts

What is wrong with the above, seeing that I did not use a dummy load (because I do not have one) Is the above accurate, not accurate, etc.

*Voltage across the speaker terminals is independent of battery voltage. This amplifier has very tight power supply regulation, without the dummy load, of course.

Your input would be greatly appreciated.

As usual, thanks.

Last edited by SQ ABSOLUTE; 11th June 2011 at 12:46 AM. Reason: Put asteric by voltage to reference the above and addition
  Reply With Quote
Old 11th June 2011, 01:25 AM   #2
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Your meter is measuring in V RMS I would assume.

14V drop across 4 ohms is 3.5A V=IR

P = IV so power = ~ 49W

Even this is not accurate though as you would need to know the resistance of the VC at 40Hz as it may be significantly higher than 4Ohms depending on the resonant freq of the speaker.

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 11th June 2011, 01:38 AM   #3
diyAudio Member
 
Join Date: Sep 2009
Location: Arizona
Yes, those are TRUE RMS voltage readings.

I am not following the rest of what you mentioned. I was referring to a resistive load of 4 Ohm's. A resistor's value does not change at varying frequencies. It resistance remains constant. However, an inductive load which a speaker is, its resistance (impedance) will change at different frequencies which is what I understand you seem to be indicating.

That's not the issue here.

Last edited by SQ ABSOLUTE; 11th June 2011 at 01:41 AM.
  Reply With Quote
Old 11th June 2011, 01:52 AM   #4
ppia600 is offline ppia600  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Louis y ana
If you used the speaker as a load its a fair indication, of course not exact. I've done similar for checking random amps in customers' cars and the formulas always came close to rated output and were usually slightly above for quality amplifiers.
__________________
Don't worry... you can always turn the gain down!
  Reply With Quote
Old 11th June 2011, 03:16 AM   #5
diyAudio Member
 
cocolino's Avatar
 
Join Date: May 2002
Location: Bavaria (south of veal sausage equator)
The 14V (13,99) You stated is the car`s battery DC voltage (= amps supply DC voltage) and the 68,5V are the (max.) AC-Volts (RMS) which You measured at the amps unloaded speaker terminals, right?

If so the result of 1173 Watts is theoretically correct but with a connected 4 Ohms load this is only possible if the amp can provide the corresponding amount of current (Amperes).
In reality the amps supply voltage will also "sag" quite a bit with such a heavy (>1kW) load which means with an attached load You won`t quite reach this 68,5V(RMS) out You measured and therefore You also won`t quite get this 1173W output power figure anymore. How much the voltage will sag depends on the "stiffness" of the amps power supply. When it`s regulated as You say it will (should) sag only little.

This JL Audio amp is specified at 1000W / 1,5 to 4 Ohm.
That`s slighty under 64VRMS at 4 Ohms load.

So there appears nothing wrong with Your measurement and the 1173W calculation without load (except that nobody ever needs such an insane amount of power in a car, at least not for "only" hearing music ) and it`s reasonable close to the specs with load.
__________________
Christoph
STEAL the BEST - INVENT the REST
  Reply With Quote
Old 11th June 2011, 03:29 AM   #6
diyAudio Member
 
Join Date: Sep 2009
Location: Arizona
Quote:
Originally Posted by cocolino View Post
The 14V (13,99) You stated is the car`s battery DC voltage (= amps supply DC voltage) and the 68,5V are the (max.) AC-Volts (RMS) which You measured at the amps unloaded speaker terminals, right? Correct.

If so the result of 1173 Watts is theoretically correct but with a connected 4 Ohms load this is only possible if the amp can provide the corresponding amount of current (Amperes). I understand now, when you used the word "theoretically". So to see how much deviation, if any, from the above theoretical formula, we use a dummy load attached to the amplifier's output. This makes sense, now. Thanks.

In reality the amps supply voltage will also "sag" quite a bit with such a heavy (>1kW) load which means with an attached load You won`t quite reach this 68,5V(RMS) out You measured and therefore You also won`t quite get this 1173W output power figure anymore. How much the voltage will sag depends on the "stiffness" of the amps power supply. When it`s regulated as You say it will (should) sag only little.

This JL Audio amp is specified at 1000W / 1,5 to 4 Ohm.
That`s slighty under 64VRMS at 4 Ohms load.

So there appears nothing wrong with Your measurement and the 1173W calculation without load (except that nobody ever needs such an insane amount of power in a car, at least not for "only" hearing music ) and it`s reasonable close to the specs with load.Thanks for your very clear explanation. I understand now. I have searched long and hard and never got an explanation quite like that..

Last edited by SQ ABSOLUTE; 11th June 2011 at 03:36 AM.
  Reply With Quote
Old 11th June 2011, 03:48 AM   #7
diyAudio Member
 
Join Date: Sep 2009
Location: Arizona
Quote:
Originally Posted by ppia600 View Post
If you used the speaker as a load its a fair indication, of course not exact. I've done similar for checking random amps in customers' cars and the formulas always came close to rated output and were usually slightly above for quality amplifiers.


There are two (2) problems with using a speaker as a load.

Problem Number 1
You cannot turn the volume full tilt .

Problem Number 2
The speaker's impedance varies with frequency, so you will get different voltage readings and therefore power output readings at varying frequencies. The readings will be all over the place.

With a dummy resistive load connected to the amp's output you will get a stable reading on your meter as resistance does not change with frequency, provided you use a TRUE RMS meter and the test frequency is within the AC bandwith of the meter.

I believe on Perry's website bcae1.com he has a graph of what happens when you try to use an actual speaker as a load and try measuring the amplifier's power output.

Last edited by SQ ABSOLUTE; 11th June 2011 at 03:51 AM.
  Reply With Quote
Old 11th June 2011, 04:03 AM   #8
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
Sorry I must have misunderstood your post. I thought that you were saying that you did not have a dummy load and were using a 4 ohm speaker as the load (hence the comment about not being accurate as a speaker is not a resistive load). The apparent resistance at the resonant freq of the speaker is much higher.

Secondly I thought you were saying that the voltage measured across the speaker was 13.99V which I rounded up to 14V

for 65.8V which looking again I realise is your voltage across the speaker, it is a different story

I did think 14V was rather low

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 11th June 2011, 12:41 PM   #9
ppia600 is offline ppia600  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Louis y ana
Quote:
Originally Posted by SQ ABSOLUTE View Post

There are two (2) problems with using a speaker as a load.

Problem Number 1
You cannot turn the volume full tilt .

Problem Number 2
The speaker's impedance varies with frequency, so you will get different voltage readings and therefore power output readings at varying frequencies. The readings will be all over the place.

With a dummy resistive load connected to the amp's output you will get a stable reading on your meter as resistance does not change with frequency, provided you use a TRUE RMS meter and the test frequency is within the AC bandwith of the meter.

I believe on Perry's website bcae1.com he has a graph of what happens when you try to use an actual speaker as a load and try measuring the amplifier's power output.
Your first post was a lttle back and forth and I totally understand how speaker/driver impedance changes depending on amp frequency output and speaker used. You also list a 4 ohm speaker as being used for your measurement which is better than NO load. I was simply stating your measurement was fairly accurate and now you're going to dispute what I said? Wow, nevermind
__________________
Don't worry... you can always turn the gain down!
  Reply With Quote
Old 11th June 2011, 04:51 PM   #10
diyAudio Member
 
Join Date: Sep 2009
Location: Arizona
Quote:
Originally Posted by ppia600 View Post
Your first post was a lttle back and forth and I totally understand how speaker/driver impedance changes depending on amp frequency output and speaker used. You also list a 4 ohm speaker as being used for your measurement which is better than NO load. I was simply stating your measurement was fairly accurate and now you're going to dispute what I said? Wow, nevermind
My apologies for my first post being a little back and forth and misunderstanding the point of your post.

There is really nothing wrong with how you go about your testing. People do it all sort of ways and seem quite happy with the results. If you are happy with the results, then I see no reason to change. Sorry, if you took offense to what I said.

I was just trying to find out if there was anything wrong with the way "I" was going about things, and someone explained everything to me in great detail.

Again, sorry. It was not my intention.

Last edited by SQ ABSOLUTE; 11th June 2011 at 04:57 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Measuring Output Impedance of Power Supply korneluk Tubes / Valves 21 27th December 2010 09:58 PM
Just for fun - Measuring power output ben12345 Car Audio 4 26th February 2010 07:15 PM
Measuring Output Power Rodango Tubes / Valves 5 30th September 2008 12:32 AM
measuring output power MikeZ Pass Labs 14 1st December 2003 06:16 PM
Measuring amp output power fragman56 Tubes / Valves 15 26th May 2003 08:26 AM


New To Site? Need Help?

All times are GMT. The time now is 06:44 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2