how powerful a preamp do i need for this

[batman]

i want to have a 555 time activated by speech, but it has to be able to pick up a normal, soft voice from about 15 feet away. what kind of amp do i need to build? preferably one with a variable gain, so the trigger threshold can go from a pin drop to a bull horn!

also, how can i modify the same circuit to have a variable frequency trigger threshold, so i can go from really deep to a really high pitched sound?

THANKS!

 

[audioguru]

Hi Batman,
For your microphone to be able to pick up a normal, soft voice from about 15 feet away, it must compete with phones ringing, dogs barking, TV, etc. from the next room. If you really want such a high sensitivity then the mic preamp (electret mic) must have a voltage gain (not power) of a few hundred. A low-noise opamp or two will do it and the feedback resistor can be variable to reduce the sensitivity (bull-horn?).

For a variable frequency pickup, it needs a complicated variable bandpass-filter circuit.

Google "VOX circuit" for schematics.