[Tridacnid]

My next project is going to be the Infinity Universal Guitar Preamp for use with my bass. The designs call for a dual rail PSU that supplies +9 and -9 volts.

See the 5th page of this: http://www.techniguitare.com/projets...ms-V1.1-En.pdf

How come I can't just use a PSU that supplies +18 volts and a 0V "ground" line? The description is a bit vague, which leads me to believe that it doesn't really matter how, so long as the opamps get the correct voltage.

Thanks,
Tri

 

[Nemo de Monet]

Quote:

Originally Posted by Tridacnid /img/forum/go_quote.gif My next project is going to be the Infinity Universal Guitar Preamp for use with my bass. The designs call for a dual rail PSU that supplies +9 and -9 volts. See the 5th page of this: http://www.techniguitare.com/projets...ms-V1.1-En.pdf

Eh, no, read the schematic again. It uses a +18V power supply and creates a fairly crude virtual ground of +/-9ish VDC. See the bit under the schematic where it says "+/-9V from a 18V DC Regulated PSU"? It already does exactly what you want it to...

 

[Tridacnid]

Oh. Oops. When you say crude, does that imply a bad design, or a non-optimal, yet still on par, way of doing it?

But what's the opamp doing, then? And why can't I just bypass this all with a +18V regulated power supply? Isn't that what this is doing?

 

[guzzler]

It's actually a very sensible way of doing it. The output from the voltage divider R1 and R2 creates a virtual 0V. The impedance of a voltage divider is quite high (R1||R2) and so the voltage will change under load. The opamp has extremely high input impedance and is acting as a buffer.

You can't just use an 18V supply without this section as the circuit requires a bipolar supply (ie, +9V--0V-- -9V) as it's DC coupled. You would have to completely redesign it if you wanted to use a single supply without the rail splitter.

 

[Nemo de Monet]

By calling it crude, I wasn't trying to imply anything particularly negative; it's simply, oh, maybe $0.45 worth of parts (the op-amp acting as a buffer) more complex than the absolute minimum required, that's all. The whole design kind of looks that way - like it was built down to a price for manufacturing, rather than as a DIY project. Again, not trying to sound negative - Behringer, for example, thrive by building things to a similar standard, and who am I to argue with their commercial success?

You can learn more than you ever wanted to know about this kind of virtual ground design right here, if that's the kind of thing that floats your boat...

 

[Tridacnid]

Thanks for the link. It looks like that's exactly what I need.