Dying Battery Simulator/starve/sag.....

[Canucker]

everywhere you read about this idea they say "works well with some effects and not so desirable with others"....so if you've every tried this.....Where did you like it and where did you not like it? http://www.beavisaudio.com/projects/DBS/

[Gus]

You need a background in electronics to understand what is happening with the added series resistance as it interacts with the circuit.  This is not meant a a dismissive post.  People go to school to design power supplies and to understand how they react with circuits.

Some circuits are more sensitive to the added series resistance some are not.

There are at least two things going on with the added series resistance one a voltage drop and two a possible stability issue.  The circuit stability is also affected by the power supply bypass cap value or ABSENCE.  Hint a multistage circuit might have more stability issues.   A 1 transistor gain stage should have less stability issues but can have a sag/bias shift tone change.

Adding a series resistance is a simple build so maybe the best answer is to try it on different effects to hear the changes

[R.G.]

Gus is correct, as always.

The devil is always in the details. Different circuits will react differently to having their power supply get stingy with voltage and current. Some could hardly care less. This property is known in the EE biz as "high power supply rejection ratio". Others, those which cannot ignore their power supply conditions so well if at all, will have immediate responses to power supply changes.

In general, opamps are best at ignoring their power supply, given only that the local impedance at high frequencies is not too high. They can oscillate if that happens. Single ended circuits like simple transistor, JFET and MOSFET amplifiers are worst at ignoring their power supply, and power supply changes will ride right through in the output signal.

I think in general the idea that a dying battery is good for sound may come from the different way a circuit clips and distorts at low power supply voltages, plus some contribution of near-oscillation from power supply induced crosstalk. Again, opamp stages will do this least, single ended stages most in general.

But there are specifics exceptions to each of these, depending on the circuit. The devil again.

[Canucker]

I very much don't have a background in electronics and I've read up on this prior to the post...and of course with out the background some of it was over my head. I wasn't looking for someone to clear that up for me...just simply trying to see who has tried it on any specific build and if they liked it or not.....so less the technical version and more of the "so did the devil get ya?" 

[amptramp]

 This thread shows Sonic Youth in a panic trying to find an almost dead battery for their effects because this provided the best sound:

http://www.diystompboxes.com/smfforum/index.php?topic=84813.0

One of the worst circuits for power supply dependence would be the Fuzz Face because it has no bypass capacitor and nearly 1:1 ratio of output to supply ripple when the volume is maxed out.  But there are a lot of other circuits that are similar.

[jymaze]

I have to disagree about opamps and power supply:

It is not true at all that opamps could not care less about there power supply: The rejection ratio in the specs is for normal linear conditions only, and distortion up to the supply rails it is not normal linear conditions at all!

A rail-to-rail opamp that clips around V+ and V- will be clipping at 9V if V+ is 9V, and at 8V if V+ is 8V after starvation. Power-supply rejection ratio is exactly 0 dB in that case, just like a single common emitor transistor clipping. The starvation can have various effects on different stages of the opamp with different designs so the effect can become a little unpredictable, but be sure that if it is distorting to the rails, it will be massively affected by power-supply variation at least at the output stage. Also, stability of opamps during recovery from V+/V- clipping varies, with different harmonics/ringing/phase reversing pattern, which is why opamps may sound very different when distorting, but sound the same during normal linear operation.

For an opamp, in my opinion, a sag resistor will not cause major stability issues as long as it is under let's say 100-470 ohms (depends on the current draw and the design of the opamp, it may as well never cause any instability at all), so if you want to experiment I would put sockets and try different values.