Sag switch

[Al Heeley]

Putting together a bass fuzz with a rather large left-over enclosure and some holes to fill. I thought I'd try installing a spare dpdt switch to try a voltage sag mod as per the BYOC Fuzz project mods on the Beavis Audio website. they simply intercept the 9V line from battery to pcb with a 100k variable pot. I don't have space for another pot but thought I'd try switching in and out a resistor instead. Should I just go for half way, 50k for a reasonable effect? Anyone with experience of voltage sag mods to fuzz circuits able to advise on a good compromise resistance to give a nice additional feature?

[Mark Hammer]

You can probably expect some impact of even 5-10k.

Question to everyone else out there:  If there is a smoothing cap to ground from V+ (which there isn't in the case of the circuit used for the BYOC Fuzz), should any series resistance intended to produce "sag" come after the smoothing cap?  In other words, it will impose current limitations on whatever current can come from the battery plus what is stored in the cap.

[BAARON]

It should come afterward, would be my guess.  The resistor in series with the circuit not only limits current, but emulates the growing AC impedance of a dying battery.  Thus, if you were to put the cap after the resistor, it negates the simulated "dying battery impedance" provided by the resistor.

RG's article mentions the effect that the increasing battery impedance has on the Fuzz Face, largely because of the 330e/8k2 voltage divider at the output: the increasing impedance can add significantly to the 330 ohm resistor, thus changing the ratio of the voltage divider by a fair bit.

I seem to remember somebody saying that a 9v battery that has fallen to ~7 volts has an AC impedance of about 300 ohms, but I can't back that up.  I either saw that at Beavis Audio Research or at AMZ, I think, but I'm not sure.

[maarten]

Before the smoothing cap as you want to increase the variations voltage of your powersupply in order to have the sagging effect produced. Sometimes you will see this also done in tube amps where the rectifier has been replaced by silicon rectifiers: in order to make up for the loss of compression in the end tubes, a resistor (typically 47 to 150 ohms, 10 or 20 watts) is added after the silicon rectifiers, before the first cap.

Maarten

[Al Heeley]

I'm going to open the bidding at 22k, see what sort of sound difference I get.