Battery sag circuit

[888guitars]

Here is a schematic of a battery sag simulator. My question is since it is a resistive load, does the regulator keep the battery drain good? Or as good as the circuit was?

[888guitars]

Oops here it is.

[888guitars]

Maybe this one would be better as it does it's own power control.

[Mark Hammer]

Battery sag in pedals is more than just reduced supply voltage, and what you have is simple (though reliable) reduced voltage. Its not irrelevant or unuseful, but it ain't battery sag when it provides a robust, albeit lower, supply voltage.

That said, not perfectly mimicking a well-used carbon zinc battery is no sin, and neither is trying out different supply voltages. Just be aware that plenty of circuits show very little audible improvement with either higher or lower supply voltages.

[888guitars]

The only circuits I am interested in are germanium fuzz faces. Which I have heard are about the only ones worth messing around with this type of thing.

[Mick Bailey]

Voltage drop alone does not simulate a fading battery. The reason is that as the voltage drops the internal resistance increases - a bit like ESR in a capacitor. I've just measured the internal resistance of a dying battery at 15 Ohms. As the Fuzz Face doesn't use a bypass capacitor across the battery supply, the battery in effect becomes it's own inferior bypass capacitor and this is where the inceased internal resistance has an effect on sound.

In the first circuit the diodes give a switchable voltage drop and the pot will simultaneously give a further voltage drop and introduce additional resistance. In the second circuit you have the adjustable voltage output of the regulator, but the 1uf cap will act as a low-value bypass cap in the FF circuit, but you don't have the simulated resistance.

The terminal voltage of a brand-new zinc-carbon battery is higher than an alkaline battery, at around 9.3v, so this would be a good starting point for your 'fresh battery' voltage. You need a series resistor with your pot to limit the maximum voltage out of the regulator when the pot is maxed.

You have couple of options;

1. Take the second circuit and add the series pot to the output, as in the first circuit - you may need to lower the value by adding a parallel resistor (say, 100 Ohm) to give an improved adjustment range. Forget the diodes. I'd experimentally make the output cap across the regulator 10uf and switchable. Add a .1uf cap from the common regulator terminal to ground to improve regulator stability. You then have two controls to simulate battery voltage drop and increased internal resistance, which is an improvement over both of the circuits. You can also hear the difference with the switchable bypass cap.

2. Take the second circuit, increase the output cap to 10uf and add a series-connected pot between the cap and ground. Pot value should be no more than 1k, but again, you may need to reduce this. This will give you two controls for voltage and resistance. This could be the best choice for a FF.

[J M Fahey]

I'd just use a fresh Alkaline battery (forget regulated PSU in this context) and add the 1K preset in series.
And under actual use would set it to drop voltage at actual circuit between 8 and 6 volts, set it by ear.
That should cover all possible real world zinc carbon battery situations which might be found onstage.

[Mark Hammer]

The sag that gets sought after, and also gets used on recordings, is a byproduct of the manner in which older carbon-zinc batteries provide current. They have a way of recovering a little bit, delivering as much instantaneous current as they can for a moment, and then petering out after a transient, that results from the boundaries between the individual 1.5V cells inside the battery. It's like a runner who can sprint 15yds, but then bends over, hands on hips, and says "Just a sec. let me catch my breath." Alkalines don't do that, and neither does a stable regulated supply whose output voltage can be altered.

Again, it may well be that the sound of a GE-based circuit can be altered in a pleasing manner by changing the supply voltage (keep in mind how many vintage fuzzes, like Maestro and others would run off one or two penlights), but the reduced voltage of an older carbon-zinc 9v is really only a side-effect, or maybe even just a marker, for the change in its current-delivery capability when it reaches that state. I suspect that if one could actually buy a 10.5V battery (with 7 slugs inside, instead of just 6), it would yield the desirable tone on a GE fuzz, even if the supply voltage was 9v, instead of 7.5 or whatever.

Don't treat supply voltage and instantaneous current delivery as absolutely equivalent.

[Mick Bailey]

To give this effect a capacitor could be charged through a seres resistor which is greater than the 'internal resistance' resistor. The RC value would represent the sag of the battery. I wonder with a Fuzz Face just how much the current increases with playing, though. I need to put one together to experiment with this whole setup - it's got me intrigued

One of the issues with dropping the voltage of an alkaline battery through a pot or preset may be that because the FF has very little current draw, by the time the voltage had reduced enough the resistance would be very much higher than the internal resistance of a zinc-carbon battery which had discharged to that voltage level.

The other factor that occurred to me relating to sag is the way the terminal voltage drops gradually with even a very moderate and constant load and then eventually recovers again when the load is removed. This is in addition to instantanous sag effects. The whole thing looks quite a bit more involved to model than I though.

Whether any of this has a positive audible outcome is another thing.

[888guitars]

Adding bias trim pots seem to give me quite a bit of tonal adjustment. Still working on the voltage drop.