i have used five cheap capsules, flush mounted on a plastic in an irregular=
cross, for over 20 years. after a lot of experimentation, i decided the be=
st solution is to parallel the capsules and treat them as one; with five ca=
psules you get a noise reduction of over six db, which pushes cheap capsule=
s right up into competion with expensive ones. in my cross, the longest arm=
is about two inches from the centre capsule, and the shortest about 1/2 in=
ch. you can vary the load resistance and supply voltage to set the overall =
gain of the capsules, but atleast with the very cheap capsules i have used =
(6 mm omnis of unknown make) leaving the resistor same as in the original (=
ab2k2) results in about the same output, but with a lot less noise.
umashankar
stoatwizard <> wrote:
--- In "tk7859" wrote:
> Yes, the use of three paralled capsule vertical line array was an
> attempt to decrease self noise of the extremely cheap capsules and,
> hopefully, at the same time increase sesitivity.
...
>
> The replacement single capsules are well matched at 1.29k and they
are
> much more sensitive than the triplets.
>
> If I add a 1k resistor in series in each of the triplets circuits
the
> DC resistance would appear the same as that of the single capsule.
I
> wonder if the sensitivity would increase accordingly?
>
> Any thoughts before I experiment?
That sounds like a dubious thing to do. Adding series resistance will
degrade the noise figure, which is probably not an issue here in
comparison to the self-noise, but it will give you a signal loss. The
capsule FET drain current will be pushed through your terminating
resistance and your series resistance. You will only get output from
the term resistance and will lose the voltage developed across the
series resistor you've just added.
The drop in terminating resistance is probably what's KO's the gain
in the straight paralleling - you are using a terminating resistance
a third of the value for a single capsule.
What you might try to do instead is perhaps use a quad opamp - use
three of them to give a moderate voltage gain of say 5 and then sum
the low impedance output of those into the remaining opamp configured
as a virtual earth mixing stage. You'd have to test that in a quiet
location against the single capsule and correct for the extra gain,
but that would be easy enough to do with a stereo recording with the
left channel carrying the output from one capsule and the right the
output from the summed three.
I'd be interested to know if that works
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