How to measure output power - diyAudio
 How to measure output power
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 2nd June 2012, 02:21 AM #1 diyAudio Member     Join Date: Jun 2011 Location: North of Boston 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 ?
 2nd June 2012, 02:35 AM #2 diyAudio Member     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.
 2nd June 2012, 03:08 AM #3 diyAudio Moderator     Join Date: Oct 2004 Location: SW Florida 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?
 2nd June 2012, 03:25 AM #4 diyAudio Member     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.
 2nd June 2012, 03:42 AM #5 diyAudio Member     Join Date: Feb 2009 Location: Calgary 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 __________________ Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits. Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right Last edited by tomchr; 2nd June 2012 at 03:45 AM.
 2nd June 2012, 04:21 AM #6 diyAudio Member     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.
 2nd June 2012, 05:58 AM #7 diyAudio Member     Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland You'll have to see what the AC frequency response of your multimeter is.
 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
 2nd June 2012, 01:10 PM #9 Account disabled at member's request   Join Date: Jul 2009 maybe build this? Power Amp Clipping Indicator
 2nd June 2012, 03:13 PM #10 diyAudio Member     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.

 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 OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Burnedfingers Tubes / Valves 42 15th June 2016 11:51 AM CST10 Power Supplies 3 4th September 2011 06:32 AM hernanstafe Parts 2 16th February 2007 01:27 AM Duo Multi-Way 30 24th April 2003 02:53 PM protos Digital Source 0 5th March 2003 11:50 AM

 New To Site? Need Help?

All times are GMT. The time now is 11:06 PM.