[sdiy] Preamp circuits

[Magnus Danielson]

David G. Dixon wrote:
> I'm looking to build a simple preamp circuit so I can run a microphone
> through my ring modulator and scare the neighborhood kids on Halloween with
> my rendition of "The Humans are Dead".
> 
> I was simply going to chain both sides of a TL072 with, maybe, 10x gain on
> both (exact number to be determined to achieve roughly 10Vpp output), with
> perhaps a 474 monolithic AC coupling cap on the input and an attenuator on
> the output.  The whole circuit would be about 1 square inch.
> 
> Am I missing something here, or is it just about that easy?

About that easy. I recommend using a balanced input design rather than 
un-balanced. That way you can keep the mike input slightly floating and 
use ground for shielding. It costs you two extra resistors of the same 
value as you are using, so it's not a big deal. If you want a bit more 
gain, put that on the second stage.

> While we're at it, does anybody here have a preferred circuit for an
> envelope follower?  Offline responses are OK.

Precision rectifier followed by some suitable low-pass filter gets you 
crawling. Not too hard. Buffer the lowpass filter output. Maybe not the 
most advanced one, but essentially what you need to get started. To get 
a trigger, then just use a comparator and a pot to set trigger level 
(usually not needed, input sensitivity gives you more of what you need).

Cheers,
Magnus