Maxon OD820 voltage conversion question

[ninjadave7]

Hey all,

I've always been under the impression that the Maxon OD820 used a charge pump to bump 9v up to 16-18v, but today when I looked at the schematic I noticed that the MAX1044 is actually configured as a negative voltage converter (with osc boost). The -9v goes to pin4 and the +9v to pin 8 on all opamps (2 4558's). Even on the schematic there is no mention of 18v anywhere. How does this affect the voltages in the opamp, and why send -9v to pin4 where it would normally ground? Is the online info about 18v wrong? Just trying to understand how all this works!!

Thanks!

[Jazz P Bass]

Most opamps (consult respective datasheet) will run on a dual supply rather happily.
The 4558 is one of them.
+9 Vdc compared to -9 Vdc is an 18 volt separation.(potential)
What you must adhere to is the specification for maximum voltage swing.
As far as how hard you drive them, it is usually a good thing to stay away from the opamp 'rails' as far as max swing exscursion goes.
(Although this is not always the case. It depends on the specific opamp & the circuit.)

[ninjadave7]

That makes complete sense, but to make sure I'm really understanding: the OD820 does not actually double the voltage like say, the Klon, but instead provides 18v rail to rail via putting pin4 at -9v instead of ground, giving an 18v swing instead of 9v. When they say on the webpage "the OD-820 accepts 9 volts coming in, it then uses a DC-DC voltage converter (#MAX1044) to bump this up to 18 volts," they don't really mean that...

[diagrammatiks]

yeah they do. it's exactly what that means.

0-18 is the same is -9/9.

In fact a bipolar supply is the "proper" and "old-school" way to do it since with 0-18 you'd need a negative bias supply at the input of the op-amp and with +-9 the op-amp can be biased at 0.

[ninjadave7]

Cool. So there's an advantage to doing it like that vs. just doubling the voltage? I might try it in some pedals!

[diagrammatiks]

it really depends on the op-amp.