Re: OTish Distortion in Conderser microphones
- From: kensmith@xxxxxxxxxxxxxxx (Ken Smith)
- Date: Mon, 6 Mar 2006 15:06:10 +0000 (UTC)
In article <925m02588l3ntpk2l4q25khrk8r339iv7j@xxxxxxx>,
Pieter <pieterNOSPAM@xxxxxxxxxxx> wrote:
On Sat, 04 Mar 2006 12:37:02 +0100, martin griffith
<mart_in_medina@xxxxxxxxxxx> wrote:
Found this, maybe of interest to audio dudes here
http://lavryengineering.com/forum_images/Condenser.pdf
martin
The linearity changes with capacitance change. The capacitance does
not change when the voltage does not change. So to prevent voltage
changes!
This statement is simply incorrect. It is also incorrect in a complex way
too but we'll ignore this for now.
To do this, the microphone must be short-circuited (can also be into a
DC offset). The way to do this is feed the signal into the - input of
an opamp that has capacitive feedback. This way this circuit does not
function as voltage amplifier, but as CHARGE amplifier. The charge the
sound delivers is amplified.
Actually, it is more likely to act as a slightly charge sensitive RF
oscillator with a lot of noise.
Another advantage is that the signal is very low-impedant, this means
low disturbances, low noise.
No, it means very high noise. The input noise voltage of the op-amp
divided by the impedance gives a noise current. Also the lower a resistor
is, the more its noise current. When it comes to noise, you can't win,
you can't break even and you can't get out of the game.
And it is not sensitive to the length of
the cable (the cable only sees a short-circuit), and it isn't
sensitive to movement of the cable.
Try tapping on a coax with a little bias voltage on it connected to such a
circuit. You will find that the circuit is sensitive to moving the cable.
I've had to shock mount cables.
Feel free to ask any questions.
If a train is leaving the station at 11 miles per hour ..........
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