Need schematic for a linear amplifier for REW measurements

[bobinuta]

Hi all,
First of all, please allow me to congratulate the author of the REW software, and also all the contributors.
It's a great software and I am really impressed !

For the moment, I am trying to use REW for RLC measurements and I would like to try REW to check the behaviour of some magnetic toroidal cores made from different materials, in the 10Hz-100KHz domain.
My soundboard is an ASUS Xonar D1 PCI.
The output level of Xonar D1 is not enough for measurement of electrolytic capacitors bigger than 2200uF and I would like to ask here for help in finding a schematic/project for a small power amplifier (max. 3W) but with a very good linearity in the 10Hz - 100KHz domain .

Xonar D1 has 100 ohms output impedance and 3.6Kohms line input impedance.
The output level is 2Vrms on 100 ohm load.
A 4700uF capacitor has about 0.7 ohms capacitive reactance at 50Hz...

The original REW measurement circuit setup is here :
https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html

... and I think that I should use something like this:

Well, in order to measure electrolytic capacitors (especially for ESR) I think that I could use any ordinary amp that could cover the 10Hz-300Hz domain, but this will became uncomfortable : for small values of capacitance (< 2200uF) I should switch out the amp from the measurement circuit, and after that I must recalibrate the entire system

So, I think that a linear amplifier in the 10Hz-100KHz domain should do the job - but I didn't find a proper concept.
That's the reason for I am writing here, asking for your help.
If anybody could help me with some ideas and schematics, I will be grateful !

Thanks and all the best!

Bob

 

[sm52]

Read here.

Sound Card Calibration and Impedance Circuit

On the old version of REW, I was able to calibrate the sound card and measure the impedance of the speakers. On the latest version, I can’t calibrate. I use an amplifier and a voltage divider to protect the sound card inputs. Questions. 1. If an amplifier and voltage divider are used, is it...

www.avnirvana.com

 

[bobinuta]

Read here.

Sound Card Calibration and Impedance Circuit

On the old version of REW, I was able to calibrate the sound card and measure the impedance of the speakers. On the latest version, I can’t calibrate. I use an amplifier and a voltage divider to protect the sound card inputs. Questions. 1. If an amplifier and voltage divider are used, is it...

www.avnirvana.com

Thanks for the hint!
I wrote you a PM.

Regards,
Bob

 

[dcibel]

It should be clear from the measurement schematic that impedance measurement is a differential measurement between left and right input, as a result the linearity of your amp is not as important as you might think, just use whatever you have around the house, DIY build of a custom amp for measuring a capacitor seems very counter-intuitive. Further, amplitude linearity beyond 20kHz is usually limited by DAC output filter, expect roll off beyond 40kHz regardless of sample rate for most DAC, a simple loopback measurement will show you how your own equipment performs in this regard.

For the large capacitors you are trying to measure, high frequency is unimportant anyway except for very small values. Large caps will be captured by low frequency measurement. Consider the value of Rsense as well, since the most often use for measuring capacitor in REW would be for crossover parts 1-100uF, not 2000uF+.

 

[bobinuta]

It should be clear from the measurement schematic that impedance measurement is a differential measurement between left and right input, as a result the linearity of your amp is not as important as you might think, just use whatever you have around the house, DIY build of a custom amp for measuring a capacitor seems very counter-intuitive. Further, amplitude linearity beyond 20kHz is usually limited by DAC output filter, expect roll off beyond 40kHz regardless of sample rate for most DAC, a simple loopback measurement will show you how your own equipment performs in this regard.

For the large capacitors you are trying to measure, high frequency is unimportant anyway except for very small values. Large caps will be captured by low frequency measurement. Consider the value of Rsense as well, since the most often use for measuring capacitor in REW would be for crossover parts 1-100uF, not 2000uF+.

Hello, Dcibel !
And thanks for the reply and your comments.
I used Rsense = 10 ohm for measuring 4700uF or 10000uF, and I think that I should use a smaller value (4.7 ohm ).
At 120Hz, the capacitive reactance of a 4700uF cap is aboud 0.28 ohm and on the line input of the soundcard, the level is too small (few millivolts).
Probably, I will apply your idea, to use any decent low power amp ( by my calculations, a 5W one will do the job) in order to test such capacitors.

Yes, you're right, about the most often use of the REW at measuring capacitors values, but I would like to use REW for some other things, like study of the behaviour of such capacitors, or damaged one ... and even for the study of the behaviour of some magnetic toroidal cores, in the low frequency spectrum (10Hz - 50KHz).

If you don't mind, I will ask here for another problem: did you ever used REW with ASIO drivers at 96KHz sampling rate?

Thanks a lot and best regards,
Bob

 

[dcibel]

did you ever used REW with ASIO drivers at 96KHz sampling rate?

Yes