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sinigersky 19th October 2006 02:27 PM

InGenius balanced - has anyone experience with it?
I have built the schematics of an InGenius line receiver:

Bill Whitlock has published a paper about the subject of achieving very high CMRR in "real-world" conditions where source imbalances exist:
See also:

Here is my implementation of the schematics from the paper:

I am using quad op-amps (tested with: MC33079, LT1125) for hopefully being best matched as well as 0.1% 10k-resistors all around the instrumentation amplifier.

...and the thing is oscillating at about 150 Hz. I power it with a Jung super regulator, but I've tried with 7818/7918 - same result. The oscillation stops if I attach a 15 meters long cable at the input. I also have a phantom power supply so I use decouling caps 1uF plastic at the input and very precise 6,80 kOhm resistors. The phantom power stab. comes from
and should be quite clean (a lot of filtering used).

Has anyone built such an input? Any ideas?


mazurek 19th October 2006 04:07 PM

What is missing in that paper I think are some caps in feeback path from output to input such as are in this paper:

Just a naive suggestion, I'm no EE, but I have had oscillation issues, where I had a ceramic capacitor in the feedback, removed it on a whim for experimentation, and then had oscillation issues under some circumstances. Oscillation usually went away when I shorted the inputs, or in circuits that were safe to short across the resistor in the feeback path.

The same site has tips on stabilizing op amps, I once went to the library to find an official reference on this, and they had an official equation, but most feedback caps were in the area of 30-100pF.

AndrewT 20th October 2006 07:09 AM

150Hz or 150kHz?

sinigersky 20th October 2006 07:24 AM

about 150 Hz - a low audible frequency, I record the oscillation with my sound blaster. It looks like spikes - up and down.

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