
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

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 

Thread Tools  Search this Thread 
8th April 2014, 04:01 PM  #1 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Measuring Power Amp Output Impedance
I usually measure output impedance of a completed amp fed from the mains supply via the installed PSU.
I usually get around 40milliohms to 80milliohms. Today I am measuring a 3886 chipamp while it is being fed from a dual polarity Lab Supply. Results, all at 1kHz sinewave, for 3 different supply voltages: Supply Vccee . output into 8r0 . into open circuit . . 20V . . . . . . 2804.1mVac . . . 2806.2mVac . . 40V . . . . . . 2822.9mVac . . . 2824.4mVac . . 63V . . . . . . 2822.9mVac . . . 2824.4mVac Output impedance = [Vopen / Vinto8r * 8r]  8r for 20V, 6.0milliohms for 40V & 63V, 4.3milliohms Q1.) is the output impedance really that low? Q2.) is the formula correct? Q3.) why does the low supply voltage impedance increase by 1.7milliohms? The test (~1W into 8r0) output voltage and the output current are well below the limits for the chipamp output stage. Yet the 20V result shows the amp gain has dropped from 10.25times to 10.18times. The supply voltage is 10Vdc and the output voltage is 3.96Vpk This result contradicts what I was being told in other Threads a few weeks ago. I asserted that amplifier gain falls as the supply rail voltage moves down towards the output voltage. Comments welcome
__________________
regards Andrew T. Last edited by AndrewT; 8th April 2014 at 04:23 PM. 
8th April 2014, 04:35 PM  #2 
diyAudio Member
Join Date: Mar 2001
Location: Lyon, France

Q1.) is the output impedance really that low?
Yes, and 6 mOhm is a bit high for an amp with lots of feedback (not very important, since the cable impedance will swamp it anyway). Your way of doing it has the inconvenience of needing an extremely precise AC voltage measurement, which can be a problem. It is much simpler to connect the amp output to GND and inject an AC current in the amplifier output, for example using another amp and a 8 ohm resistor. The voltage at the amp's output gives the impedance, an you can also visualize it on a scope, FFT it, whatever, to reveal all sorts of gremlins (crossover distortion has nowhere to hide). You can also inject, for example, a 50 Hz 1 amp current and a 1kHz 10mA current, with 2 extra amps. Then you can measure the intermodulation at the amplifier output. Etc. 
9th April 2014, 04:58 AM  #3  
diyAudio Member
Join Date: Apr 2010
Location: Coffs Harbour, on the east coast

Quote:
The method may vary among test authorities but this detail from a review tester at least clarifies how it may be done commercially. The relevant section is the editorial in the conclusion. Basic Amplifier Measurement Techniques  Audioholics
__________________
regards 

9th April 2014, 09:17 AM  #4 
diyAudio Member

Andrew, the small perturbations on the supply voltage should not change voltager gain, except when you get to clipping.
Remember that Zout is really a differential value. The correct way to measure it is as follows:  measure Vout with a specific load, say 8 ohms, and calculate Iout. Example: 8V out, at 8 ohms is 1 amp;  change load to say 4 ohms, measure Vout again, and calculate Iout. Example: now Vout drops to 7V, which gives Iout = 7 / 4 = 1.75 amps. Now calculate Zout = change in Vout (87) / (change in Iout (1  1.75) = 1.3333 ohms. Jan
__________________
Never explain  your friends don't need it and your enemies won't believe you anyway  E. Hubbart Check out Linear Audio Vol 7! 
9th April 2014, 09:32 AM  #5 
diyAudio Member
Join Date: Dec 2004
Location: Next door

When I need to measure an amp output impedance, I first set the output voltage with a load, both having simple numbers.
For example, 8 Vrms on a 8 Ohm power resistor makes the amplifier delivering a current of 1 A. Then I remove the load. The output voltage changes, usually only slightly. The voltage difference is equal to the value of the output impedance. No calculation. 
9th April 2014, 11:05 AM  #6 
diyAudio Member
Join Date: Sep 2006

Peufeu's method is the only valid way to measure the impedance, for a number of reasons.
One the most important is that for amplifiers with feedback, the output impedance is not real: it is complex, almost purely inductive. When the small inductive drop is subtracted from the huge quadrature output voltage, it is completely overwhelmed, leading to enormous errors. The differential load method is a remnant of the tube era, when the output impedance of an amplifier was of the same order as the load, or even much larger in some cases, but it is totally unapplicable to modern semiconductor amplifiers.
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 
9th April 2014, 11:40 AM  #7 
diyAudio Member
Join Date: Jan 2009


9th April 2014, 11:49 AM  #8 
diyAudio Member
Join Date: Sep 2006

He means the input (= no signal)
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 
9th April 2014, 03:09 PM  #9 
diyAudio Member
Join Date: Jan 2009

Thanks for that  it makes sense now.
Brian 
9th April 2014, 03:22 PM  #10 
diyAudio Member

Mr. Pass has an article entitled "Mosfet Citation 12" at www.firstwatt.com. He described at the end of it the method as posted earlier by peufeu.

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Measuring preamp output impedance  kacernator  Tubes / Valves  31  10th October 2012 07:11 PM 
help with mosfet power amp output impedance calculation  kasey197  Solid State  5  10th September 2012 10:51 AM 
Measuring Output Impedance of Power Supply  korneluk  Tubes / Valves  21  27th December 2010 09:58 PM 
Measuring amp output power  fragman56  Tubes / Valves  15  26th May 2003 08:26 AM 
Easy ways of measuring output impedance of tube stages  stigla  Tubes / Valves  4  16th May 2003 11:37 AM 
New To Site?  Need Help? 