[sdiy] Preamp circuits

[David G. Dixon]

Thanks for that, Magnus.  I think I understand the concept of balanced input
(differential amp to reject common mode noise, etc.), but given that I just
have a cheap-ass microphone with a standard "mono" phone jack on it (from my
kids' karaoke machine, as it happens), how would I bring this in as a
balanced input?  Would the tip be on input, and the sleeve be the other?
Then I could choose whether or not to hook the ground to a shield?  

Also, I take it you are advocating bringing the signal directly into the
differential amp.  However, I notice that others are sometimes filtering and
buffering the differential inputs first.  What are your views on this?


> -----Original Message-----
> From: Magnus Danielson [mailto:magnus at rubidium.dyndns.org]
> Sent: Friday, July 10, 2009 3:15 PM
> To: David G. Dixon
> Cc: 'sdiy diy'
> Subject: Re: [sdiy] Preamp circuits
> 
> 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