Power smoothing question.

[acehobojoe]

I usually power most of my projects with a one spot to test. I have noticed a slight high hum in some of them.

I was wondering if a 100uF from 9v to ground or a few other caps would help. I've seen it on other circuits. I'm guessing the 100u stores power and releases it slowly, which helps the DC rippling a bit. Is that correct?

[deadastronaut]

correct.

the one spot should be fine though...

i always have a 100uf across +/- when breadboarding etc..


other things around you can cause hum though....pc fans/certain lighting etc...

i always test with my pc off..which is a pain, but my meagre size workspace/bench dictates that i have too..

[Mark Hammer]

It is useful to first identify whether the hum is coming from the pedal itself, or from the fact that it is sharing a supply.  Helpful to try out your One-spot with just a single pedal at a time, to see if there is any hum risk coming from the pedal on its own.

[Digital Larry]

Yes that is correct.  How effective a capacitor is in reducing ripple depends on the impedance of the thing driving it.  If the impedance is "zero" then the cap has no effect at all, however this doesn't happen in the real world.  Power supply impedance can be "really low" though, on the order of one ohm or less.  I don't know what it is for typical wall warts.  You can estimate by comparing the no-load voltage to the voltage with a load.

For example, suppose your no-load voltage measures 15 volts.  When you put a 100 ohm resistor on it, it drops to 7.5 volts.  Your power supply also has an impedance of 100 ohms.  In general:

Power supply impedance (ohm) = (Rload) * (Vopen - Vload)/(Vload).  I just did that in my head so somebody check me please! [edit - I corrected it]

The challenge is trying to figure out what the source impedance of your power supply is.  A lot of times in pedal schematics you'll see a 10 to 100 ohm resistor in series with the power source and then attached to a large cap to ground.  This provides more filtering than attaching the cap directly to the power source.  However, series resistors will drop voltage depending on the current going through them.  Some pedals take small amounts of current and some take more.  You'd probably find it more effective to add supply filtering inside each of your pedals, where you could balance the series filter resistor against the current pull per pedal and voltage drop caused by it.

[Edit - and note the other suggestions as well!]

[GibsonGM]

The biggest noise maker in my vicinity is my PC monitor, just to put that out there.  The computer itself is fine, but if I do anything with the monitor on, it sounds like really bad tube heater wiring, LOL!   

If in doubt about your supply, you could always make a little dedicated LM7809 circuit snip to feed your experiments with right at the breadboard, and do what Larry just suggested - build it in so it's always there after you box it up....

[acehobojoe]

For me, even with just a 10uF on the 9v to ground, I have great results!

[GibsonGM]

Sometimes that's all you need

[R.G.]

It's important to realize that hum creeps in from many, many directions. It's broadcast by electric field, magnetic field, AC power wire grounding in the building, as well as issues with pedals.

Hum-free is a delicate, if very desirable condition.

[Mark Hammer]

I'm pretty sure that at least 10,000 of the over 33k members here are experiencing hum from the bloody flickering fluorescent fixture in my office that desperately needs either a new ballast or all-out replacement. 

[antonis]

I'm pretty sure that at least 10,000 of the over 33k members here are experiencing hum from the bloody flickering fluorescent fixture in my office that desperately needs either a new ballast or all-out replacement.  

Especialy with that @# $% #$ inrush current.....

(I've never managed to get any Voltage Comparator - even with "insensitive" 741 kind - without changing state during switching lights on on...