Measuring output impedance of an unknown audio circuit? - Page 1
Forum section: electronics / projects

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 13, 2012, 12:05:08 am
content: I'm trying to characterize the speaker output of a prebuilt board that I have.Modeling the system as voltage source, Ro (output impedance) and Rl (Load). I'm looking at making some simple sine wave MP3s at a few frequencies and measure output voltage into various resistances.When I find the peak power based on V and Rl, that would be the situation when Ro = Rl for that frequency, right?Does my logic work out on this one and is there an easier way?Edit: Now I'm thinking it would be easier to measure open circuit on the speaker and then setting resistance to half that voltage would also find the equal output impedance as well.

author: Lightages
author_rank: Unknown
timestamp: October 13, 2012, 01:44:22 am
content: First of all, don't use MP3 files, use WAV. You want a known signal, not some psycho-acoustic magic file.Secondly, an amplifier is not a simple voltage source with infinite current capability. If the output impedance of the amp is 0.1ohm and you have 25 volts output peak, then it would possibly output 250watts. Not very likely with a simple amplifier. The limit to the output despite the output impedance will be the capability of the power supply and the power dissipation of the output stage.If you limit the output to 0.1volt output open circuit, then yes you can measure the output impedance by finding the maximum power dissipated at what load impedance. This would be practical. This is porbably what you meant but I wanted to point out that you need to work at very low output voltages so as to not clip the because of the power supply or exceed the power dissipation of the output.

author: jahonen
author_rank: Super Contributor
timestamp: October 13, 2012, 07:13:52 am
content: It is not practical to measure the output impedance of an amplifier by loading the output until the amplifier "gives up". This is because it then turns non-linear and may even get damaged in the process.Better way is to just short the input of the amplifier to be measured to ground, and leaving it on (i.e. "playing" silence) and then use an external generator to feed a signal with known amplitude into amplifier output via known resistance. Then measure the voltage across the amplifier output. Then you can determine the amplifier output impedance simply by using the voltage divider formula. For example, if known resistance is 50 ohms, signal amplitude is 10 volts and amplifier output impedance is 0.01 ohms, then we have about 2 mV across the amplifier output.Ideal amplifier would not allow any voltage on its output, but since practical amplifiers have non-zero output impedance, there is some residual voltage which can be measured.Regards,Janne

author: JuKu
author_rank: Frequent Contributor
timestamp: October 13, 2012, 09:42:21 am
content: On power amps: Sine wave in, measure voltage over an 8 Ohm resistor, and again with 8.1 Ohms. With small enough resistor change and not-too-exotic amp circuit, the current will be the same. So you have U1/(8+x) = U2/(8.1+x), x is the amp output impedance. On a line level amp, use 100k and 2k. On power amps you are expecting a few milliOhms, so you need fairly good meter and a stable oscillator. On line level circuits, typical values will be from tens of milliOhms to few ten Ohms. The result is somewhat frequency dependent.You want two measurements from normal operation situations. Janne's method works on some amps, but can give strange results on some circuit topologies. Your method is not reliable either, open and short circuits are the extreme end of loads, and you shouldn't be surprised about strange results in extreme situations.

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 13, 2012, 03:22:49 pm
content: This is coming from a gutted processor board, similar to a modern smart phone. So I don't think I can short input. What my ultimate goal is to figure out what speakers to spec for the best volume and quality of sound with the available power, without hurting the low power audio amp.The responses have been helpful. Thanks for all the info.

author: kaindub
author_rank: Regular Contributor
timestamp: October 17, 2012, 11:22:12 pm
content: Sacherjjwhat you are really asking for is what load impedance I can attach to my amplifier?The outut impedance on amplifiers (and any power output device ) will not point you to the correct answer, and is irrelevantif its audio stuff, then I would at first guess just use an 8 ohm speaker. Thats a pretty safe bet.