low power to op amps in distortion pedals

[endmile]

I remember seeing Sonic Youth play in 1992 and when they were setting up, they were frantically testing every 9V battery they could find.  From what I understand, they were looking for batteries that were just about to die.  Apparently, the almost dead batteries were giving them the perfect tone.

Has anyone ever played with varying the power to the op amp in a distortion pedal (something like the RAT)?  I was thinking about trying a mod like this.  Any insight or experiences?

[andymac1962]

Check Dano's site,   dying battery simulator
http://www.beavisaudio.com/Projects/DBS/

[endmile]

Thanks for the link.  That's the kind of thing I'm going for.  His implementation is exactly what I was thinking.  Anyone else tried it with any distortion pedals?

[amptramp]

I would be careful about using the dying battery simulator.  It places resistance in the circuit but this does not necessarily simulate what is going on:  the dying battery may still have a low internal impedance as the voltage drops.  The simulator is purely resistive and would play havoc with effects like a Fuzz Face that has no power supply bypass capacitor.  You could use an LM317 regulator with a pot to set the voltage - the volts would go down but the internal impedance would remain low.  The actual behaviour of a battery is somewhere between these extremes - a drop in no-load voltage and an increase in resistance.

[cpm]

you can try running the thing at 5v, from a 7805 regulator or the like.
also add some series resitance for extra abuse

[WGTP]

I notice with the 4049 cmos distortions that the battery voltage has a big effect on the tone.  With a new battery I use a 1K resistor.  As the voltage drops I use smaller resistors until it drops so low no resister is needed.  This is easy on the bread board...

That is why it is good to have a bias control on Fuzz Face type circuits...

[amptramp]

A Fuzz Face is OK if you are using it in a studio, but beware of temperature sensitivity if you go touring.  You may have to adjust a few things to get the sound you are used to if it has been transported through a hot desert or a cold tundra due to the temperature coefficient of Vbe.  Single-transistor buffers and fuzz pedals are usually more sensitive because they have limited DC feedback to stabilize the bias point.  This is on top of the difference in battery voltage and impedance with temperature.  If you arrive at a gig in time to get the unit up or down to indoor temperatures, fine.  But if you arrive late, the sound will change during the gig as the unit approaches room temperature.  You do outdoor gigs?  Good luck!

I would use an LM317 regulator stage as shown in the LM317 applications schematics with a variable voltage and variable resistance at the output to simulate the battery (but you may need two batteries in series to get above the dropout voltage of the regulator).