I have built the schematics of an InGenius line receiver:
http://www.thatcorp.com/1200desc.html
Bill Whitlock has published a paper about the subject of achieving very high CMRR in "real-world" conditions where source imbalances exist: http://www.jensen-transformers.com/an/ingenaes.pdf
See also:
http://www.thatcorp.com/datashts/dn133.pdf
Here is my implementation of the schematics from the paper:
http://www.sinigersky.com/images/InGenius_stereo_comp_side.JPG
http://www.sinigersky.com/images/InGenius_stereo_sold_side.JPG
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
http://sound.westhost.com/project96.htm
and should be quite clean (a lot of filtering used).
Has anyone built such an input? Any ideas?
Thanks!
http://www.thatcorp.com/1200desc.html
Bill Whitlock has published a paper about the subject of achieving very high CMRR in "real-world" conditions where source imbalances exist: http://www.jensen-transformers.com/an/ingenaes.pdf
See also:
http://www.thatcorp.com/datashts/dn133.pdf
Here is my implementation of the schematics from the paper:
http://www.sinigersky.com/images/InGenius_stereo_comp_side.JPG
http://www.sinigersky.com/images/InGenius_stereo_sold_side.JPG
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
http://sound.westhost.com/project96.htm
and should be quite clean (a lot of filtering used).
Has anyone built such an input? Any ideas?
Thanks!
What is missing in that paper I think are some caps in feeback path from output to input such as are in this paper:
http://www.jensen-transformers.com/an/an003.pdf
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.
http://www.jensen-transformers.com/an/an003.pdf
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.
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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