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Balanced Analog Audio Measuring Interface for the Soundcard

pma

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#1
Balanced Analog Audio Measuring interface for the soundcard

I was tired to solder divider diy patches every time I needed to measure power amplifiers with a soundcard, and also by investigating how to avoid groundloops and mains related interference spectral lines. So I decided to design and build a fully balanced universal analog interface that would change a soundcard to something that resembles an audio analyser like AP, or maybe rather like QA401. I started with using such thing 2 years ago and after getting good results, I decided to make a stand-alone instrument like this. The interface, called MSYS, has 2 channels, one for low level signals up to 4Vrms, second for high level signals up to 40Vrms (at least). The gain and sensitivity is controlled by DACT stepped attenuator. Block diagram is attached, circuit diagram is attached as well, and a photo of the instrument and its rear panel. The circuit uses OPA314, AD797, OPA627 and ADA4898 chips. Dynamic range is high, distortion very low and it makes a useful tool from a conventional soundcard. Balanced input is a must if we measure a power amplifier.

Specifications

Input impedance Lo level input ….. 200k balanced

Input impedance Hi level input ….. 22k balanced

Max. input Lo level input ….. 4Vrms

Max. input Hi level input ….. 40Vrms

Level/Gain regulation ….. stepped 2dB down to -60dB

Max. Output level ….. 10Vrms single-ended, 20Vrms balanced

Frequency range ….. 0 Hz – 1.1MHz (-3dB)

Output noise level ….. -120dBV(A)

Block diagram
MSYS_s.png


Rear view
msys_rear_2.jpg


Main PCB
main_pcb.jpg


Power amp measurement circuit
msys_powamp_SE.png
 

Speedskater

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#6
Just noticed that the schematic shows the XLR's pin #1 attached to the circuit common. Any XLR pin #1 should be attached directly to the chassis at the connector.
 
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