noise from a common wallwart?

[loss1234]

I have been messing around with different wallwarts and stompboxes as I just bought on of those 5 from 1 adapters that you plug into a 9v dc adapter from visualsound.

this setup seems to work fine except with pedals that have a digital chip in them (delays,etc)

with any of those pedals, they work fine on their own wallwart, but as soon as i add a second or third effect to the 5 in 1 adapter, the digital units start to WHINE. (sounds like an oscillator)

is this due to using a switching wallwart instead of a linear wallart? i dont think its an issue of not enough current.

and are there ways of avoiding this?

i would love to hear other peoples experiences. i suppose i could just not power those digital units off of the same chain of power


thanks

[Paul Marossy]

Interesting, I haven't heard of this problem before. If it's phyiscally small and light, it probably is a switching power supply. That might possibly be the source of your problem. What frequency would you guess the whine is?

[Mark Hammer]

This "it was quiet as a church mouse until I plugged in a second digital pedal" thing has come up here before....too many times.

In general, the clock signals in digital pedals are so high as to be non-audible.  The problem is that when you have two or more pedals that are juuuuuuuuust a little different from each other, you can get audible heterodyning. E.g., the "1mhz" clock on one of them is actually 1.012mhz, and the 1mhz clock on a second is actually 1.017mhz.  As far as clock crystal tolerances go, they are reasonably close to spec, but their combination can yield a 5khz difference in addition to >2mhz sum.  So what starts out as inaudible can become audible.  Of course, when you consider how many different clock-signal divisions there may be in any digital pedal, the possibilities for audible sideband products increases.  It reminds me of the Cynbal electronic cymbal project, in which several cascaded digital ring modulators produce a large number of HF sidebands, resulting in "tuned noise" that sounds like a cymbal splash.

I'm curious about whether this is the sort of thing that those EMI/RFI power-line filters are good at addressing.

[Paul Marossy]

Huh, that's interesting, Mark. I didn't know that having two different digital pedals in a signal chain could cause that sort of problem. I would think that those EMI/RFI power-line filters wouldn't help with that as it's due to clock noise and techically not the power supply. Well, at least theoritically it shouldn't be coming from the power supply.

[puretube]

A ferrite-bead on the 9V DC cables between each effect?
(depending on whether its heterodyning between PS & Effect, or between adjacent digital effects...)