another 'basic' question: power supply filtering

[plexi12000]

i was looking at a popular distortion pedal schematic from around 1970.  then was looking at a new pcb for it, for sale online.  they added an electrolytic in the power supply the original didnt have.

in tube amplifiers, filter caps are needed to smooth the ripples after the AC is rectified.  this i understand, etc.

but with a pedal....the circuit "starts" out with DC.  does it really need to be filtered?  

[Brisance]

most wall warts are crap, with a lot of ripples going through. Provided you only use good filtered power supplies or a battery, no need, but I'd rather have my toys capable of running from the shit ones as well.

[antonis]

In many cases there is also RF and/or AC power supply frequency filtering with common caps so it's quite tricky to identificate any individual filter circuit..

[GibsonGM]

It's a good practice, even if running just from batteries.   Our toys LOVE a nice clean power supply, even tho they'll typically do fine with less than ideal.   And one day you may wish to convert over to wall wart operation - one less thing to think about.   

Things with a clock/ LFO (tremolo, phaser....) really should have a reservoir for those pesky spikes (and sometimes require even more).     I put one in regardless.  Such a small thing, and being consistent about it means I don't have to go back later and add one should the need arise.     

[plexi12000]

awesome, thanks guys.  Just wondering!....i knew it certainly doesn't "hurt" anything... but i value the experience and opinions of others!

[greaser_au]

When batteries run down, their internal resistance increases.  With certain devices this can cause 'motorboating' (a self-sustaining low frequency 'pulsing' as the supply voltage repeatedly sags with increasing current draw, and recovers as the current drain falls).  A largish electro across the battery reduces the effective power supply impedance  by providing a reservoir of energy to limit the sag.

Like power supply design, there are methods and maths for optimum values - but at these small levels of V & I, arbitrary selections are usually fine  

david

[teemuk]

The filter caps are there also for decoupling audio signals from the power supply. Without low impedance shunt to ground (via capacitor) the audio signals would "ride atop" the DC component and feed back - both in and off phase - to each and every gain stage supplied by that particular power supply node. Decoupling capacitance prevents that.

[greaser_au]

The filter caps are there also for decoupling audio signals from the power supply. Without low impedance shunt to ground (via capacitor) the audio signals would "ride atop" the DC component and feed back - both in and off phase - to each and every gain stage supplied by that particular power supply node. Decoupling capacitance prevents that.

This is why a 100n->1u poly/monolithic should be there also 

david

[Fp-www.Tonepad.com]

A cap in the pedal will form a lpf with the resistance of the cable, so it does help with power supply noise. Caps are way cheaper and smaller in size, so it makes sense to take advantage of that nowadays, It's been 40 years since the 70's.