Quick Question Concerning Dual Rail PSU
Aug 3, 2009 at 4:18 PM Thread Starter Post #1 of 6

Tridacnid

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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
 
Aug 3, 2009 at 5:29 PM Post #2 of 6

Nemo de Monet

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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...
 
Aug 3, 2009 at 11:28 PM Post #3 of 6

Tridacnid

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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?
 
Aug 3, 2009 at 11:36 PM Post #4 of 6

guzzler

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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.
 
Aug 3, 2009 at 11:51 PM Post #5 of 6

Nemo de Monet

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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?
smily_headphones1.gif


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...
 

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