How to measure output power - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 2nd June 2012, 02:21 AM   #1
diyAudio Member
 
EddieRich's Avatar
 
Join Date: Jun 2011
Location: North of Boston
Default How to measure output power

I have just finished my second amp, a custom designed stereo ipod amp. The design was for 3.5 watts / channel and I'd like to see what I'm actually getting. I have a 1KHz sine wave mp3 file which gives me 0.71Vrms at the input. I'm thinking I can put a 0.1 ohm resistor in series with the speaker and measure the Vrms across the resistor and across the speaker. Doing the math to convert the voltage across the resistor to amps, is it simply V x A ?
  Reply With Quote
Old 2nd June 2012, 02:35 AM   #2
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
While it is not a speaker, I use a resistive load for power measurements as speaker impedance can pretty much be considered off specified value at almost any frequency.

Measure the resistor with a precision ohm meter, or measure by feeding an accurately measured voltage to get your cal value.

From there, I adjust the output power while looking at the waveform on a scope and increase input till I get the onset of clipping. This lets me measure max continuous power.

I also like to look at distortion with a spectrum analyzer (Audio Tester) at 1W out.

Then I ignore my measurements and go listen to it.
  Reply With Quote
Old 2nd June 2012, 03:08 AM   #3
Pano is offline Pano  United States
diyAudio Moderator
 
Pano's Avatar
 
Join Date: Oct 2004
Location: Milliways
Blog Entries: 4
Ditto. You have to know that you are clipping or not, or have reached a certain level of distortion. Do you have a way to determine that?
  Reply With Quote
Old 2nd June 2012, 03:25 AM   #4
diyAudio Member
 
FullRangeMan's Avatar
 
Join Date: Aug 2009
Location: Brazil
Hello,
Take this oppotunity I wonder how I calculate output power on paper??

Bias x B+ x Output Transformer=output Watts

Bias and B+ is easy to find, but which value is about the output transformer??
Thanks
__________________
>Never go to a psychiatrist, adopt a cat or dog from the streets. On the streets pets live only two years average.
  Reply With Quote
Old 2nd June 2012, 03:42 AM   #5
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
I rig the amp to a resistive load and crank up the input signal until I get 2~3 % THD on the output (measured with a distortion analyzer). Then I measure the output RMS voltage and use P = E^2/R to find the output power. I use the same method to measure output power at 1 % THD and measure the THD at 1 W out. I perform this measurement at mid band (1 kHz) with spot checks at lower and higher frequencies. I usually also run a distortion vs power vs frequency sweep.

Now, why 2~3 % THD? Well... In my 300B SET with a source follower grid drive on the 300B, I have lots and lots of grid current available. But the 300B isn't designed for A2 operation as far as I know. I have noticed that once significant grid current starts to flow, the THD basically falls off a cliff. That tends to happen around 2~3 % THD, so I stop my measurements there. At that point the output waveform is also rather ugly on the o'scope.

If you don't have a dedicated distortion analyzer, you can use a computer sound card and some software. TrueRTA comes to mind.

As far as calculating output power on paper, I'd just look it up in the datasheet for the output tube. Or perhaps dust off RDH4 and look in Chapter 13 (AFAIR).

~Tom
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

Last edited by tomchr; 2nd June 2012 at 03:45 AM.
  Reply With Quote
Old 2nd June 2012, 04:21 AM   #6
diyAudio Member
 
EddieRich's Avatar
 
Join Date: Jun 2011
Location: North of Boston
Thank you for the answers. Unfortunately, I don't have distortion analyzers, or even a scope. I was hoping it could be done with a multimeter. I'm not looking for exact, precise numbers. I just to want to see if I'm in the ballpark.
  Reply With Quote
Old 2nd June 2012, 05:58 AM   #7
diyAudio Member
 
astouffer's Avatar
 
Join Date: Jun 2004
Location: Pittsburgh, crumbling wasteland
You'll have to see what the AC frequency response of your multimeter is.
  Reply With Quote
Old 2nd June 2012, 12:49 PM   #8
diyAudio Member
 
Join Date: Nov 2002
How do you figure this? Bias x B+ x Output Transformer=output Watts
  Reply With Quote
Old 2nd June 2012, 01:10 PM   #9
Account disabled at member's request
 
Join Date: Jul 2009
maybe build this?

Power Amp Clipping Indicator

  Reply With Quote
Old 2nd June 2012, 03:13 PM   #10
diyAudio Member
 
EddieRich's Avatar
 
Join Date: Jun 2011
Location: North of Boston
@astouffer
Meter is a VC99, spec for AC volts frequency response is 40-400 Hz.
I'm assuming I can't use this multimeter to measure frequencies in the KHz ?

Let's forget about clipping and % distortion for a minute. I just want to know
how I can get a rough measure (+/- 10%) of the power being delivered to the speaker.
As TheGimp mentioned, I'll actually be using an 8 ohm 5 Watt resistor.

Last edited by EddieRich; 2nd June 2012 at 03:18 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
How to measure output transformer? Original Burnedfingers Tubes / Valves 10 5th November 2011 03:11 PM
How to measure transformer output CST10 Power Supplies 3 4th September 2011 06:32 AM
How to measure amp power output hernanstafe Parts 2 16th February 2007 01:27 AM
How can I measure dB output? Duo Multi-Way 30 24th April 2003 02:53 PM
How to measure digital output of CD. protos Digital Source 0 5th March 2003 11:50 AM


New To Site? Need Help?

All times are GMT. The time now is 08:55 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