[sdiy] DIY for Microphones
abide at dmdrafting.com
Wed Jan 3 21:54:02 CET 2018
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ingo Debus
Sent: Wednesday, January 3, 2018 3:28 PM
Subject: Re: [sdiy] DIY for Microphones
> Am 03.01.2018 um 16:39 schrieb Jean-Pierre Desrochers <jpdesroc at oricom.ca>:
> Have a look at my Electret mike project made a couple of years ago..
> I still use it while miking drum overheads and acoustic instruments..
> Very flat frequency response. Made from very easily available parts.
Interesting! I’m working on a similar thing.
I tried Monacor MCE-404U (directional) and MCE-4500 (omni) back electret capsules so far. The omnis are very good, as far as I can tell. Both don’t cost much. Good to know that the Panasonics are still available.
The circiutry is similar to yours. Some differences are:
- the capsule is not connected directly to the bases of the two trannies, there’s another tranny that allows the capsule to be connected to ground. This is probably not important if the capsule is mounted close to the PCB, but for an instrument mic there’s often a (short) cable between capsule and electronics needed.
- the collectors of the two pnp trannies are not connected directly to the zener, there’s one 15k resistor in between (one common resistor for both). AC-wise the collectors are connected to ground via a capacitor however. In earlier versions there was even a current source IC in place of that resistor. I wonder if this makes any difference at all?
- I used a matched pair (BCM856DS) for the two pnp trannies. I don’t know if this improves anything, but it doesn’t hurt either. I wanted to go SMT anyway (my first SMT hobby PCB!)
- there’s a 68 ohms resistor in series with the each emitter of the two pnp trannies, and a diode from emitter to collector each (cathode to emitter). This should give some protection. I took this idea from the Pronomic IM-20 microphone.
- there’s a LED in series with the zener. It lights up when phantom power is present. Very handy :-)
Keep us posted if you continue with this project, I'd be interested in building one to goof around with! I like your LED idea.
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