"Dave Plowman (News)" wrote:
An audio circuit I found online and have been playing with has an odd
design (to me).

It's op-amp based running off a (separate) +/-15v supply.

There are on board caps across the supply, which is common enough. 10 and
0.1uF in parallel. But instead of going to ground, they are wired across
the +/-15v. Does that do the job as well? Or serves a different purpose?

Is the opamp powered off those +/- rails? In dual-rail circuits often power
is drawn from the + and - rails, and the GND rail acts as a handy halfway
reference point but doesn't actually source/sink much current. That means
switching transients are taking gulps of current between + and -, and so
decoupling caps are placed between + and - to provide them.

If instead of +/-15v you thought of the circuit as 0/+30v with a PNP/NPN
stack of transistors between +30v and 0v you might see that even if there's
a +15v wire it's largely irrelevant as most of the current is going through
the transistor pair.

Theo