Neumann TLM circuits

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Monte McGuire

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So, according to that, any unbalanced stage is asking for trouble?
Unbalanced signal into a balanced input... sure. I would not disagree. The typical 6dB per octave rising CMRR curve would seem to add that coloration directly to such a signal. Sure, the mode converted signal is at a lower level, but it's far from flat, so it has the potential to make a difference. It's all perfect in theory yet none of our devices work that way :)
 

Khron

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abbey road d enfer

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Unbalanced signal into a balanced input... sure. I would not disagree. The typical 6dB per octave rising CMRR curve would seem to add that coloration directly to such a signal.
Not sure I understand.
Let's assume gain G, input voltage Vin, output voltage Vout.
For balanced connection, Vout=G.Vin.
When presented an unbalanced signal, the CM voltage is half the input.
Thus the new output signal is G.Vin +/- 1/2.Vin.-CMRR
After reduction, the gain error is +/-1/2.- CMRR.

Sure, the mode converted signal is at a lower level, but it's far from flat, so it has the potential to make a difference.
E.G. When CMRR falls at 40dB -CMRR=1/100), the gain error is 0.04dB. Is it really a concern?

I would think the issue should be analyzed in regard to distortion. It is clear that driving a differential amp with an unbalanced voltage tends to modulate the input stage's operating point, but it's certainly not worse than driving an unbalanced stage.
 

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