Sound card protection for impedance measurement

[Emile Blanc]

Hello,

First of all thanks a lot for creating REW, it is very helpful for me and really easy to use.

I've been trying to measure the impedance of loudspeakers using the software and a Scarlett 2i2. It works well when I do it using a low level in the power amplifier, but I want to measure it at high power in order to be able to tune a vent very precisely.
To protect the sound card inputs, I'm trying to use a simple voltage divider in front of the inputs but I cannot understand how to set it up.
I tried several wirings but none of them seem to work like the one I joined.
I am not sure whether I should connect the negative part of the probes to the ground or not and how to choose properly the R1 and R2 values.
Does any of you have ever tried it?

Thanks a lot,
Emile

 

[trobbins]

I suggest you change your resistive dividers to a much higher resistance.

Your soundcard is measuring two voltage levels. Any loading by the divider networks (especially at the speaker node) will divert current generated by the main amp from going through the speaker.

Your soundcard input will have an 'impedance' which will load the bottom resistance of the divider, and hence change the divider ratio, but if you use matched dividers, and the soundcard input impedances are matched, then that should alleviate accuracy concerns.

If you do a calibration, you may be able to trim the dividers to get a measurement value for the sense resistor that is pretty close to its actual value, before the calibration process then nulls the error out (if I correctly recall that process).

 

[Bernard]

Hello,

Have you read "Impedance measurement" in REW Help ?
Master John says : " 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. "

Take a look from http://www.dspiy.be/forum/viewtopic.php?p=6746#p6746 and further.

An Arta measuring box has a 27 ohm sense resistor, a resistive divider and back-to-back diodes. It should work.

Salut Emile !

 

[Emile Blanc]

Hello,
thanks for your answers, I actually tried with much higher resistors for the voltage divider (10kOhm and 47kOhm) and it works perfectly well. It was indeed a perturbation of the global impedance due to the low values of the resistors used in the voltage dividers.

Cheers