Battery Supply filter cap???

[Alpha579]

Hi all,
Why is it that alot of fx designs have power supply filter caps, even when its for a pp3 battery? Surely there's no point in filtering a battery supply? Is there?
Off the top of my head, what im talking about is on the brian may treble booster, the 100ohm and 47uf cap after the battery...

Thanx for all help,
Alex

[niftydog]

you can never have too much power smoothing I say...

I guess that the cap sources the peak currents, making for a more consistent demand on the battery.

As an aside to illustrate the point; one good way of powering devices from mains that require large currents for short periods is to simply use a battery charger to trickle charge a battery. The "power supply" is then not required to source the large currents - the battery does all the work.

A battery is really just a humungous, really good quality capacitor.

[Paul Perry (Frostwave)]

In first year physics prac lab (40 years ago!) we were measuring the gain of a two trasnsistor amp. And the figure came out as WAAAAY off theory.. turned out it was being powered by a flattish battery, with high internal resistance, which (owing to the particular circuit  configuration) gave positive feedback, based on the apparent resistanc of the battery. Took a while to twig that the batt was flat, because you wouldn't expect the gain to be HIGHER from a flat battery, right? :?
Now if we had a nice fat electro across the battery, the circuit would have performed as expected! at least at the 1 KHz we were testing it at.