# Impedance measurements w/ power amplifier



## Stereodude (Feb 6, 2010)

Has anyone done impedance measurements with a power amplifier? I read through the help file and it's a little vague.


The Manual said:


> An alternative is to drive the load via a power amplifier, which can deliver the lowest noise levels and most accurate results, but great care must be taken as the levels a power amplifier can generate can easily damage soundcard inputs. If using a power amplifier the sense resistor can be much lower, 33 ohms or less, but the soundcard inputs should be connected via a resistive divider providing around 20dB of attenuation and ideally the inputs should also be protected by back-to-back zener diodes to clamp the input to less than 5V.


What's the connection diagram look like?

Presumably the sense resistor goes in series with the driver on the output of the amplifier. The connection back to the sound card I'm less clear on. I understand why a voltage divider is used to cut down the voltage going back into the sound card. Does the soundcard input still measure the voltage across the driver in this case (after being divided)? What are "back to back zener diodes"? Two zener diodes in parallel?

Aside from smoking the sense resistor if you push the amplifier's output voltage too high, are there any caveats to this method?


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## JohnM (Apr 11, 2006)

The main risk is to the inputs of the soundcard, the back to back zener diodes are a pair of zeners connected in series but in opposite orientations, so that the voltage at the soundcard input is clamped regardless of polarity. They are connected across each soundcard input, in parallel with the bottom leg of the voltage divider.

The overall connection scheme is essentially the same configuration as when using a soundcard alone, but the soundcard output is routed through the power amplifier. One soundcard input channel is fed from the power amplifier output (through a divider network) and the other input channel is fed from the connection to the driver, after the sense resistor (through another divider network). The divider network resistors would typically be around 10k and 1k, with the 1k connected across each soundcard input, giving roughly a 10 to 1 reduction. Because REW is comparing the two input channels it doesn't matter exactly what the attenuation is, but it is best to match the two channels as closely as you can and to run the calibration step, with the driver not connected and the sense resistor shorted out, so REW can compensate for differences in the divider chains and any soundcard channel sensitivity differences.


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## Stereodude (Feb 6, 2010)

Thanks for the reply John.

Any further thoughts about the sense resistor beyond what's in the manual? I know the smaller the Rsense value the greater the current through the driver and therefore the greater the voltage across the driver will be, but there's obviously a limit because if the Rsense value gets too small the voltage across the driver won't change much from the full output of the amplifier as the impedance changes. Is something like a 4Ω 30W sense resistor for "4 ohm" driver an a 8Ω 30W sense resistor for an 8 ohm driver a pretty good place to be?

Also, should I expect any problems if I want to do an impedance sweep of a driver at say a ~10W power level (aside from keeping the mass on the driver as the cone moves) when measuring Vas?


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## JohnM (Apr 11, 2006)

I'd probably aim for 22R to 33R for the sense resistor. 10W would be pretty high level for this kind of test, bearing in mind typical driver sensitivities. Average power level of music is very low, I'd stick to a power level that keeps the sweep at a comfortable audio level.


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## Stereodude (Feb 6, 2010)

JohnM said:


> I'd probably aim for 22R to 33R for the sense resistor. 10W would be pretty high level for this kind of test, bearing in mind typical driver sensitivities. Average power level of music is very low, I'd stick to a power level that keeps the sweep at a comfortable audio level.


Point taken. I guess I'm a little skeptical of parameters derived when the cone doesn't even visibly move during the sweep when it comes to characterizing something that will be moving extensively in the end application (HT subwoofer).

Of the Klippel reports I've seen (of subwoofer drivers) the small signal parameters only hold for a few mm of cone movement before the large signal parameters take over. I was hoping that by sweeping the driver at higher power levels I could derive T/S parameters more representative of the true performance of the driver when it's pushing higher SPLs in a home theater type application.

FWIW, I have a Dayton Audio DATS and it works fine at measuring drivers, but it lacks the more sophisticated inductance derivation of REW (L2/R2 & L3/R3), and it is not capable of sweeping at variable power levels. Hence my interest in using a power amplifier with REW to do the same task.


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## Erin H (Aug 26, 2009)

I understand this is a sub-forum dedicated to REW, but have you looked in to the Woofer Tester Pro? 
http://www.woofertester.com/wtproproduct.htm

It has the ability to drive a speaker to varying levels and it also does large signal analysis to some degree (large signal being relative).


The other option is to simply make your own impedance curve with some tones, a DMM, and a whole lot of time/patience. I've done this before when I was trying to tune an aperiodic enclosure. The level of accuracy would depend on how much time you have available.


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