Actually most of your analysis is wrong, in particular the formulae that ignore the interaction between R7, R8 and gm. Indeed the cut-off frequency changes, but not in the way you describe. The original circuit has a rather flat LF response because the gain outside LF is not great, about 18dB. ther is not much difference between LF and HF because the capacitor acts on half of the source resistor, so the gain variation cannot be more than 6dB. In addition there is some selective feedback that boosts LF.VictorQ said:Unfortunately making the suggested modifications would indeed change the frequency response.
The RC combination in question before modification is ((1/gm) +R7 ) C3,
where gm is the transconductance of the FET.
After modification we have (1/gm) C3
Isn't it a bit overoptimisic? Any level loss at the input results in loss of S/N ratio, since the FET's noise is constant.VictorQ said:The problem with excessive gate capacitance is not actually loss of signal (since a capacitive voltage divider is noise-less and gain is cheap
Agreed, but distortion resulting from modulated Cgs is even more significant.but distortion generated from loading the capsule with "deadweight" capacitance.
Yes.VictorQ said:... right?
Just observe the frequency response. The VLF response is almost equal to the midrange response, and extends so down to DC. There is no -3dB point, so the response is actually DC. The capsule and coupling capacitors determine the actual overall -3dB point, not the source decoupling cap.I don't see where your "impossibly low" figure comes from..
I used the 2N3819 model in LTspice, that has Gm more or less equal to 2mS.What are you using for gm?
You raised the subject, rightfully. It's a can o' worms. Who am I to criticize Neumann on the basis of a mere physics formula? Anyway, by all accounts, the other distortion mechanisms (mechanical as well as electrical) seem to be dominant.But who cares about microphone distortion indeed? Some people seem to like it even. You can't claim that the Neumann U89 for instance (which uses a charge amp) sounds severely distorted. It definitely does impart a 'color', though, and that's not ideal for everything. Sometimes you need a sharper knife. Sometimes you want to record a tambourine without having to remove sub-sonic crud after. But I digress.
That's the case with most inexpensive condenser mics; when one single FET can do the job, why complicate things for performance most buyers won't appreciate...Anyway this circuit is not really worth spending too much time on. Oktava has other designs that are better. If the goal is a quiet mic without much color I think the first step is to start over.
No, it's not NFB. At least, not AC NFB, but actually DC NFB.Greetings to all.
I removed the R3 - 200k resistor from the circuit.
I liked the sound better than with the resistor.
What does the R3 resistor affect in this circuit?
Not much, except if it puts the stage in a non-linear operation, which would result in distortion.I see, but that might change to sound?
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