[sdiy] Introduction and a couple of noob questions

[Marc Nostromo [M-.-n]]

Hi everyone,

My name is Marc and I'm mostly a software developper. I've been doing
music software for quite a while now but have always been interested
into 'odd' platforms that would provide a single 'musical' environment
rather that do plug ins or other software that run in multiple purpose
computers. Even if it is purely running software, I find the
relationship to an object that exclusively generates sound
fascinating. Across the years, I've done trackers for gameconsoles,
synth code for the arduino and recently ported a software synthesis
framework I'm working on to both the beagleboard and RPi.

However, while working with the arduino, I've started to try to
understand the simplistic shields that gave me access to audio/midi,
then got interested in digital electronics a bit then in Olivier's
Shruti line, built one and got bitten by the synth-diy bug.

At this point, I'm very much of a noob. Thinking in terms of
schematics is way different than doing programming and I'm learning
about simple stuff, op-amps, otas, grabbing schematics from the web,
making them and (trying to) figure out how they work.

There's a couple of basic questions related to voltages that I can't
really find out and since it still puzzled me, I thought I'd jump in
and ask:

1) Audio voltage: What is the range of an audio voltage and it's 'polarity' ?

I'm expecting it to be +/- a few volts around ground value. But what
happens when you use an op-amp where the V- is connected to the ground
(like in the single supply amplifier from forrest mims) ? Is it the
same ground reference as the audio input ? In that case, what happens
when the audio voltages is negative ? Can it be transformed by the
op-amp to a voltage lower than v- ? Or should the ground lead from the
audio be connected to a virtual ground point ? Somehow I feel I'm
missing a point there.

2) Circuit supply voltage.

A lot of circuit schematics I see expect a voltage reference of 15v.
But I also know some of them will work with 9v. How does one know how
low the voltage can be ? Is there a rule of thumb ? Assuming I can use
+9/-9 as source and all I have is a 9v dc source, is there a way for
me to generate the +9/-9v from +9/0 ?

Thanks for any hint and looking forward to progress.
/M