Rat voltage divider question

the_floyd · Post by **the_floyd** » 30 Dec 2010, 22:43

Sorry if this is the wrong place for this question, but here goes:

Recently, I tried running my '86 Rat clone at 18V. The components inside are all rated higher than 18V, AFAIK, and the voltage going to pin 3 of the LM308 in this scenario would be a bit less than 9V, if my limited knowledge of electronics serves me correctly. I really liked the sound - really tight, Marshally dirt a la Alice Cooper or Aerosmith - and nothing exploded or smelt bad. The power supply is essentially this, courtesy of Beavis Audio:

So I'm wondering, could I just run the pedal at 9V, eliminate the two 100k voltage divider resistors entirely (replacing the one coming from the power supply with a wire, and breaking the connection w/ the one going to ground), and get the same effect? Should I leave the 47uf capacitor in there, as, I believe, it's not actually part of the voltage divider? Or is there more to this than this bootweaker thinks?

Help would be greatly appreciated.

cpm · Post by **cpm** » 30 Dec 2010, 23:40

that voltage divider is meant to be a half-voltage reference rail, and is required.
Powering with 18v, mid-rail is 9v,
Powering with 9v, mid-rail is 4.5v...
you get the pattern

however, i dont think that the rat topology gets a lot of variation from supply voltage, since the clipping is done by diodes tied to ground.

the_floyd · Post by **the_floyd** » 31 Dec 2010, 00:59

Ah, I see - looking more closely at the schematic, I noticed 9V goes to the opamp's pin 7, as well as the 4.5V. Makes sense, guess I'll just use an 18V supply. The difference I noted, BTW, was less sag on the lows - it reproduced the pick attack much more clearly, and didn't 'mush out' anywhere near as much; and, as far as I know, 'mushing out' is due to inadequate power - which doubling the voltage seemed to solve. I've read that some of the Rat's clipping is a result of overloading the op-amp, as well; I wouldn't really say I'm qualified to deliver a judgement on it, though, as evidenced by this thread. Thanks for your help!

RnFR · Post by **RnFR** » 31 Dec 2010, 01:54

actually, you should hear a bit of a difference. the Rat definitely has some op amp clipping going on.

stringsthings · Post by **stringsthings** » 06 Jan 2011, 06:28

since you're now also running the status LED on a higher voltage, an additional result is that the LED will be brighter ... therefore, an appropriate silly name would be the "Light Brite Rat"

DrNomis · Post by **DrNomis** » 06 Jan 2011, 09:38

As long as the electrolytic caps are rated at greater than 18V,there shouldn't be any problems,I think the op amp will gain headroom from the increased supply voltage,that means it should be able to drive the clipping diodes harder without introducing it's own distortion harmonics....