Mike

Thanks for the circuit and the great description. The description almost
makes me believe I understand analog electronics. The circuit is similar to
one posted by Wil Reeder so I think we have a consensus. I will give it a
go.

Steve

-----Original Message-----
From: Mike Keitz <mkeitz@JUNO.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Wednesday, February 03, 1999 3:13 PM
Subject: Re: [OT] electret microphone preamp


>On Wed, 3 Feb 1999 11:08:25 +1100 Steve Ridley <stever@NSW.UNILINK.OZ.AU>
>writes:
>>So I now need a
>>simple but
>>low noise if possible, preamplifier for an electret microphone.
>>Something
>>using an op-amp or just a transistor would be fine. The input
>>impedance of
>>the ISD33000 analog input is 3k and it takes a maximum of 32 mV p-p
>>when fed
>>in single ended input mode.
>
>If you have an extra op-amp, may as well use it.  Otherwise this
>deceptively simple transistor circuit should meet your needs:
>
>    Vcc           Vcc
>     |             |
>     |             R3
>     |             |
>     |        -----*-------C2---- Out
>     R1       |    |
>     |       R2    |  C
>     |        |  |/
>     *---C1---*--|
>     |           |\
>    MIC             >  E
>     |              |
>    GND            GND
>
>
>The transistor is any old NPN you can find.  To start with, try 4.7K for
>R1 and R3 and 470K for R2.  C1 and C2 should be at least 0.1 uF.  Higher
>capacitance will increase bass response but that may not be what you
>want.  If you have a 3-wire microphone, R1 and maybe C1 are built into
>the microphone itself.
>
>If you have too much gain, make R2 smaller.  This also changes the DC
>bias on the transistor, but since only 32 mV of output is needed it is OK
>to operate the transistor close to saturation.  You can also add a
>resistor in series with C1 to reduce the gain.  If someone will be
>speaking directly into the microphone, it may be able to drive the ISD
>chip without an amplifier at all.
>
>Unless you're trying for very high sensitivity, noise inherent in the
>microphone or amplifier won't be a problem.  But, the Vdd supply needs to
>be quite clean or noise from it will get into the output.  When this is a
>problem, filter the Vdd supply going to the microphone and amplifier by
>adding a 220 ohm or so resistor in series and a large capacitor to
>ground.
>
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