Electret microphone without FET transistor?
Anders Heerfordt
I need some cheap low frequency, low noise microphones.
Electret microphone capsules are cheap, but they are generally not
low noise and not low frequency. They typically go down 50 Hz. I need
to go lower.
If I could find a source for electret capsules without the internal
FET, I could connect a low noise FET amplifier externally, and
conceivably make a lower noise, lower frequency microphone.
Does anyone here know a source on electret microphone capsules with
no internal FET. Alternatively a low noise low frequency electret
capsule?
Regards, Anders
jup...@europa.com
In article <5k9f1l$k7t$1...@news.uni-c.dk>, i3...@csc.dk (Anders Heerfordt) wrote:
:I need some cheap low frequency, low noise microphones.
I believe that the low freq response of those mics is a function of their
physical size and has nothing to do with the FET. I've taken some apart,
and no coupling caps.
Because the Electret has such a high impedence, you would have to feed it
with antenna ladder line :) to get enough impendence. With coax, the
cap/unit lenght would severely limit the upper frequency. Maybe you would
have a mic that would work from 50-1khz. :)
You may be stuck with dynamic elements, Rob.l
Fred McKenzie
In article <jupiter-ya0235800...@news.europa.com>,
jup...@europa.com wrote:
> I believe that the low freq response of those mics is a function of their
> physical size and has nothing to do with the FET. I've taken some apart,
> and no coupling caps.
Rob.l-
I agree. I recall a variation of the Electret microphone, called the
"PZM" (Pressure Zone Microphone), that was popular several years ago. It
was mounted in/on a square plate, about 5" X 5". For best low frequency
response, it was recommended that it be mounted on a larger surface.
Perhaps the PZM technology would have a frequency response low enough to
meet Anders' needs.
73, Fred, K4DII
jup...@europa.com
In article <frederick.mckenzie...@k4dii.ksc.nasa.gov>,
frederick....@kmail.ksc.nasa.gov (Fred McKenzie) wrote:
:In article <jupiter-ya0235800...@news.europa.com>,
I think Radio Shack still sells one of those
Tom Bruhns
Fred McKenzie (frederick....@kmail.ksc.nasa.gov) wrote:
: In article <jupiter-ya0235800...@news.europa.com>,
: jup...@europa.com wrote:
: > I believe that the low freq response of those mics is a function of their
: > physical size and has nothing to do with the FET. I've taken some apart,
: > and no coupling caps.
: Rob.l-
: I agree. I recall a variation of the Electret microphone, called the
: "PZM" (Pressure Zone Microphone), that was popular several years ago. It
: was mounted in/on a square plate, about 5" X 5". For best low frequency
: response, it was recommended that it be mounted on a larger surface.
I believe you will find that the low frequency response of a capacitor
mic is determined by some other factors: the main one usually is the
capacitance and the preamp input impedance. Obviously, smaller mics
would generally have less capacitance, so that's a factor, but if you
can get a higher preamp input impedance (up to where the insulation
in the mic shorts things out), you can get to lower frequencies. I'm
not sure how realistic the expectations of the original poster are
with respect to improvement, though. I think the fets used in the
cheap capsules aren't too shabby. You can get to better response with
instrumentation capacitor mics, but then you are up in the $1000 range
right away. (A lot of our customers use capacitor mics for acoustical
measurements.)
--
Cheers,
Tom
to...@lsid.hp.com
Ed Tanton
Hi Anders... as has already been said-more or less-the other name for
an Electret mike w/o the FET is a: condenser microphone. It seems to
me that one of the surplus dealers was selling high quality condenser
microphone elements the last time I looked through their catalog.
Problem is: I don't remember which. It could have been Hosfelt
Electronics I think. Try them and see. If not, maybe M.P. Jones, or
All Electronics, or Altronics. It almost certainly WAS one of those.
GL
72/73
__________________________________________________________
Ed Tanton N4XY EMAIL: n4...@bellsouth.net
189 Pioneer Trail
Marietta, GA 30068-3466 TEL: (770)579-3933 V/MBX/FAX
__________________________________________________________
QRP-ARCI #7663 G-QRP #6779 OK-QRP #172 QRP-L #758
AdvRC #140 NORCAL #1779 NCDXF SEDXC
Life Member: ARRL AMSAT INDEXA QCWA
__________________________________________________________
INTERESTS: DX QRP BoatAnchors Test Equipment Photography
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Think you can, think you can't: either way you're right!" Henry Ford
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Anders Heerfordt
In article <5kd7si$7...@hpcvsnz.cv.hp.com>, to...@lsid.hp.com says...
>I believe you will find that the low frequency response of a capacitor
>mic is determined by some other factors: the main one usually is the
>capacitance and the preamp input impedance. Obviously, smaller mics
>would generally have less capacitance, so that's a factor, but if you
>can get a higher preamp input impedance (up to where the insulation
>in the mic shorts things out), you can get to lower frequencies.
I agree to that.
> I'm
>not sure how realistic the expectations of the original poster are
>with respect to improvement, though. I think the fets used in the
>cheap capsules aren't too shabby.
The lettering on the transistor inside the electret microphone, that I
took apart, was Q169. I am not familiar with a transistor of that name.
Does anyone know this one?
>You can get to better response with
>instrumentation capacitor mics, but then you are up in the $1000 range
>right away.
True. Having destroyed a few of the $1000 microphones, I feel a need for
a cheaper type of microphone. Perhaps a homebuilt capacitor microphone
10 inches in diameter would be more realistic than modifying the small
electret microphones. I could achieve a higher capacitance, and I would
not need an amplifier with > 100 Mohm input resitance.
Regards, Anders
jup...@europa.com
Has anyone ever tried using a speaker with whizzer cone as a mike? You
should get great low freq response.
I've used them before just for experimenting sake. You need an preamp or
input designed for a dynamic type mike.
Rob.
Roland Burgan
>
> :In article <jupiter-ya0235800...@news.europa.com>,
> :jup...@europa.com wrote:
> :> I believe that the low freq response of those mics is a function of their
> :> physical size and has nothing to do with the FET. I've taken some apart,
> :> and no coupling caps.
> :
> :Rob.l-
> :
> :I agree. I recall a variation of the Electret microphone, called the
> :"PZM" (Pressure Zone Microphone), that was popular several years ago. It
> :was mounted in/on a square plate, about 5" X 5". For best low frequency
> :response, it was recommended that it be mounted on a larger surface.
> :Perhaps the PZM technology would have a frequency response low enough to
> :meet Anders' needs.
> :
> :73, Fred, K4DII
>
> I think Radio Shack still sells one of those
As I recall, they used about 3v on these mikes...try increasing
voltage to 5 volts for better output and response.
Anders Heerfordt
...
>I believe you will find that the low frequency response of a capacitor
>mic is determined by some other factors: the main one usually is the
>capacitance and the preamp input impedance.
> I'm
>not sure how realistic the expectations of the original poster are
>with respect to improvement, though.
Having looked at the specs of some microphone preamplifiers for
condendsor and electret microphones, I see that to reach down to
2 Hz, a (half-inch) microphone-preamplifer typically has an input
impedance of
32 GigaOhm, and a stray capacitance of 0.4 pF. And a noise voltage
of 2 microVolt.
These specifications are extreme compared to most quickly made
electronic equipment. I think it is unlikely that I could easily
build a preamplifier with such specifications.
With a 10 inch diameter microphone, maybe one could use a preamplifier
with 32/20/20 = 80 MegaOhm input resistance. That may be more realistic
for quick construction.
Regards, Anders
Anders Heerfordt
>:I agree. I recall a variation of the Electret microphone, called the
>:"PZM" (Pressure Zone Microphone), that was popular several years ago.
It
>:was mounted in/on a square plate, about 5" X 5". For best low
frequency
>:response, it was recommended that it be mounted on a larger surface.
>:
>:Perhaps the PZM technology would have a frequency response low enough
to
>:meet Anders' needs.
>:
>
I imagine the PZM microphones utilize a sheet of
poly-vinylidene-difluoride?
And have a relatively low sensitivity?
Presumably the problems with preamplifier input impedance would be
similar to those of more normal electret microphones, and the noise
problems would be greater?
I couldnt find a radio shack in the telephone book. How would one contact
a radio shack office? Do they perhaps have an Email address or FAX
number?
Regards, Anders
jup...@europa.com
In article <5kn0e0$psm$1...@news.uni-c.dk>, i3...@csc.dk (Anders Heerfordt) wrote:
:
:
:I couldnt find a radio shack in the telephone book. How would one contact
:a radio shack office? Do they perhaps have an Email address or FAX
:number?
:
:Regards, Anders
Their parent corporation is the Tandy Corp. I've never seen a web site,
but I bet they have one.
Rob.
Paul Keinänen
frederick....@kmail.ksc.nasa.gov (Fred McKenzie) wrote:
>In article <jupiter-ya0235800...@news.europa.com>,
>jup...@europa.com wrote:
>> I believe that the low freq response of those mics is a function of their
>> physical size and has nothing to do with the FET. I've taken some apart,
>> and no coupling caps.
>
>Rob.l-
>
>I agree. I recall a variation of the Electret microphone, called the
>"PZM" (Pressure Zone Microphone), that was popular several years ago. It
>was mounted in/on a square plate, about 5" X 5". For best low frequency
>response, it was recommended that it be mounted on a larger surface.
>
So this is the equivalence of mounting an loudspeaker element on a
small baffle. When the cone moves outwards, some air slips around the
edge of the baffle to the opposite side of the baffle to the opposite
side of the cone. This is known as an acoustics short circuit. At
frequencies below and wavelengths above, where the total round trip
distance is less than half a wavelength the frequency response drops
rapidly. At such a small baffle, the frequency response starts to drop
at several hundred Hertz, remembering that the wavelength at 100 Hz is
3.4 m. This applies to a loudspeaker element installed on a baffle and
I do not see any reason why this would not apply to microphones as
well.
To prevent the acoustic short circuit, loudspeakers are usually
mounted in closed boxes. When a microphone is installed on a closed
box, it actually measures the instantaneous pressure difference
between the static pressure within the closed box and the
instantaneous pressure in the sound field. To prevent blowing the
membrane when the external static pressure varies (due to change of
weather or change of altitude) some means to bleed off the static
pressure difference must exist. This "leakage" will have a cut off
frequency at some low frequency.
Before going too deeply into the electronic circuit of the electret
microphone, the acoustic behavior should be checked.
Paul OH3LWR
Anders Heerfordt
>Does anyone here know a source on electret microphone capsules with
>no internal FET. Alternatively a low noise low frequency electret
>capsule?
Sennheisers industrial electret capsules have connections so that
you can use them without the internal FET, or you can couple the
internal FET into your circuit in a variety of ways, which gives many
options.
You can use your own FET, if you find a lower noise one, than the
internal.
Can anyone recommend a low noise N-channel JFET?
I cannot find noise data on FETs..
Regards, Anders
Anders Heerfordt
>Has anyone ever tried using a speaker with whizzer cone as a mike? You
>should get great low freq response.
>
Loudspeakers and earphones are sometimes used as makeshift microphones.
A 10 inch speaker (with a baffle) gives a similar output at 70 Hz as
a normal microphone. But the frequency response of the loudspeaker is
very unflat, and quickly drops off at lower frequencies.
It is very easy to interface to loudspeakers because of the low
impedance.
Possibly some earphones are better suited than a 10 inch loudspekaer.
Regards, Anders
Anders Heerfordt
> When a microphone is installed on a closed
>box, it actually measures the instantaneous pressure difference
>between the static pressure within the closed box and the
>instantaneous pressure in the sound field. To prevent blowing the
>membrane when the external static pressure varies (due to change of
>weather or change of altitude) some means to bleed off the static
>pressure difference must exist. This "leakage" will have a cut off
>frequency at some low frequency.
>
True. In the case of cheap electret microphones, the closed box
is the small aluminium tube housing the internals.
The venting seems to be around the edge of the small circular pertinax at
the bottom of the tube.
This presumably can be sealed with an application of Loctite Multibond.
Alternatively, the microphone can be mounted inside a metal tube which
can be sealed behind the microphone. Still some venting is necessary
for safety reasons. Perhaps a small stopcock, in the form of a screw in
a hole?
Regards, Anders
Tom Bruhns
Anders Heerfordt (i3...@csc.dk) wrote:
: The venting seems to be around the edge of the small circular pertinax at
: the bottom of the tube.
: This presumably can be sealed with an application of Loctite Multibond.
: Alternatively, the microphone can be mounted inside a metal tube which
: can be sealed behind the microphone. Still some venting is necessary
: for safety reasons. Perhaps a small stopcock, in the form of a screw in
: a hole?
Perhaps a (relatively long) capillary tube? You can draw extremely fine
capillary tubes from glass tubing. Just how low a frequency do you
need to get to, anyway? Also, it would be good to consider that
the noise of the amplifier will be going up at low frequencies
because the source impedance of the capacitor will be getting quite
high...a big advantage to using larger diameter microphones. Perhaps
a 2.5cm diameter?? We had some B&K 'one-inch' capacitor mic capsules
around here for a while.
--
Cheers,
Tom
to...@lsid.hp.com
Anders Heerfordt
>Perhaps a (relatively long) capillary tube? You can draw extremely fine
>capillary tubes from glass tubing. Just how low a frequency do you
>need to get to, anyway?
>Also, it would be good to consider that
>the noise of the amplifier will be going up at low frequencies
>because the source impedance of the capacitor will be getting quite
>high...a big advantage to using larger diameter microphones. Perhaps
>a 2.5cm diameter??
At the moment I intend to use 1.5 cm diameter electret capsules. I havent
seen larger electret capsules.
I intend to use a Burr Brown OPA121KP DIFET opamp. With an input
resistance better than 1 TeraOhm, and a noise around 6 nanoVolt per
squareroot Hz, it is difficult to beat.
Regards, Anders
Paul Keinänen
i3...@csc.dk (Anders Heerfordt) wrote:
>>Perhaps a (relatively long) capillary tube? You can draw extremely fine
>>capillary tubes from glass tubing. Just how low a frequency do you
>>need to get to, anyway?
>I aim at 2 Hz.
I did not realize that you needed to go down that far, what is the
required upper frequency range ?
Long time ago there were condenser microphones in which the variable
capacitance of the microphone capsule formed part of an oscillator
tank circuit. The output was a frequency modulated carrier and the
audio was recovered with a frequency discriminator.
The early radiosondes (at least those made by Vaisala) used capacitive
pressure and humidity sensors that directly modulated the transmitter
frequency.
I have not heard if anybody has tried this with an electret capsule,
but if it works, the electric (but not acoustic) frequency range would
extend down to zero.
Have you looked at different kind of pressure sensors, if you must go
this low ?
Paul OH3LWR
<posted and mailed>