Re: OTish Distortion in Conderser microphones
- From: kensmith@xxxxxxxxxxxxxxx (Ken Smith)
- Date: Wed, 15 Mar 2006 03:38:52 +0000 (UTC)
In article <b3ge1210f2uhe7dg5omctkf7m2b94k6o8e@xxxxxxx>,
Pieter <pieterNOSPAM@xxxxxxxxxxx> wrote:
On Mon, 6 Mar 2006 15:06:10 +0000 (UTC), kensmith@xxxxxxxxxxxxxxx (Ken[...]
Smith) wrote:
Actually, it is more likely to act as a slightly charge sensitive RF
oscillator with a lot of noise.
Not when you do a proper design. The opamp is feedback capacitive,
making it very stable. A small series resistor at the input may be
wise.
Putting a feedback capacitor on a high performance op-amp does not make it
"very stable". You have to use a "unity gain stable" one in this sort of
application. Without the small resistor you suggest, the op-amp is fairly
likely to oscillate even if it is a "unity gain stable" one if the input
cable is very long. The cable and capacitor look like a tuned circuit.
Another advantage is that the signal is very low-impedant, this means
low disturbances, low noise.
Low sensitivity to outside disturbance.
No, this is not the case. The circuit you suggest is this:
Noise
!
---Cnoise
---
! A -------------
Signal -[Zgen]---+-------! Z Amplifier !---- GND
-------------
You can't improve the signal to noise at "A" by making Zamplifier low.
How ever much you lower the amplifiers input impedance, you must also
raise its voltage gain by too to maintain the same signal output.
Just for fun, lets assume that Zgen=Cnoise and the Signal(RMS) is equal
to the noise(RMS).
High Z amplifier case: SNR = 1:1
Low Z amplifier case: SNR = 1:1
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.
The other circuits are sensitive. The one is decribe here isnt. When
then is a short-circuit, there is no voltage. Where there is no
voltage, that voltage can't change either.
When there's no impedance, the current is infnite and thus the noise is
infinite. It was exactly the sort of circuit you are suggesting that
required the shock mounting of cables.
But it is best not to use
phantom feeding, but a separate power supply cable. The phantom's
voltage does interfere.
Try tapping of a length of COAX even with "no voltage" on it. You will
still see spikes when you strike it. Just hook a BNC-BNC cable onto your
scope's input and wack it against the work bench.
--
--
kensmith@xxxxxxxxx forging knowledge
.
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