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After the ultra-lowtech heaphones, it didn't take long until I realised that I could try to make a microphone. (Well, the heaphones actually work as dynamic microphones if you talk into them, but I wanted to build something intended as a microphone.) So, why not building a condenser microphone? Theory is simple, but the problem is building such a gadget.
To make a long story short, here are the construction details in a keyword like fashion:
The results:
Anyways the quality is good enough to understand what I'm saying in the test recording. Reminds me somewhat of old recording from 1930's radio stations.
A thought about how to modify the diaphragm tension and simply placed an
M10 nut directly in the middle of the diaphragm. Firstly, the nut is so
heavy, that it won't move under the influence of the sound waves,
only the border areas between the frame and the nut will move. Secondly
the nut increases the diaphragm tension. Those effects should lead
to an increased resonance frequency.
Thest recording with nut.
As you can hear, we have moved from a 1930's radio to about telephone quality.
I've drawn a schematic of the whole thing.
As you might guess the second attempt is not the end of the story. I decided to make a nice diaphragm tightening device and have a look what sound quality one could get from normal aluminium foil.
I give some verbal description here, pictures or drawings may follow.
The idea was the following: I make an aluminium ring with a groove which
has diameter of an O-ring. As the O-ring is pressed into the groove,
it tightens the diaphragm. The diaphragm itself is held in place by a second
O-ring which has an about 6mm bigger diameter then the first one.
The backplate (the second electrode) is another aluminium ring and
is screwed to the diaphragm tightener with an O-ring in between.
This O-ring serves the purpose of fine-adjusting the diaphragm to backplate
distance.
The whole thing is mounted on a tin can which serves as a housing for the
preamp. I choose a tin can (again), because it is made of iron which
shields not only capacitively radiated noise but inductively radiated noise,
too. The shielding cap in front of the diaphragm assembly is mandatory
to keep the nasty hum away and is made of an old kitchen sieve which got
sewed (yes, sewed) to another (shortened can) with thin wire. Looks nice.
As you can imagine, this device is not that ultra-lowtech anymore.
The housing and shielding is and could be made in any shed, but the
diaphragm assembly has been turned on a lathe.
I think the diaphragm holder could be made of hard wood or hard plywood
too, if one's good at woodworking but I don't know if it's stiff enough.
(The gap between the diaphragm and the backplate has to be adjusted very
fine to achieve sufficient sensitivity. If the holder was not stiff enough,
this would be possible.)
Things to do/come
Although the device works works (not even bad), there are still things
to do:
Preliminary results
Well, here is what you've been waiting for, pictures and sound files.
I've made a nice preamp with a JFET input buffer and a simple bipolar amp afterwards (which has a gain of up to approximately 180 which is not bad for a single transistor). Linearity is not very good but suffiecient. You can view the schematic here. If you want to build it, note that the BC547 must be a B-Type otherwise you have to alter R4.
The diaphragm is now made of the thin, cheap aluminium foil.
The supply is aa heavily filtered (but not regulated (the output voltage can vary quite some bit but this is pointless)) linear supply which can be adjusted from 0V to approximately 200V so be very careful with it. The schematic is here. The 12V are for the preamp and are stabilized with a standard 7812.
I found that I can't go up to 200V with the bias, because very tiny dirt (or whatever) particles which sit between the screws or even the diaphragm reduce the maximum field strength so far, that I can't go over about 90V. Above 90V I start to hear the microphone "tick". A click, followed by the next one sone seconds later. The time between the clicks shortens if the voltage is increased further. I think this comes from the diaphragm capacitance which discharges suddenly over the particles. (If I remove the diphragm including the holder, the ticking stops.)
I've shortened the metal can, made a bottom cover for the can and a wooden rings which holds the wohle thing together (with the help of rubber bands), The can got a 3.5mm phone jack as output and some 4mm lab sockets for the power supply.
Last but not least some pics:
Of course, I've made a recording with the new preamp.
It's been a nice experience to build it. If you got inspired and build your own, I would be happy if you could drop me a line at w15mail(at)yahoo.de if you've finished (or simply want to ask something). Maybe we could even get up a homemade-mic-gallery. :-o
Sadly, it has the same destiny as most of the "let's see if I can do this" projects - It's gonna sit around in my cupboard. :-/
Copyright (C) 2007 by Wiesner Thomas
Last change: August 5th 2007
