My First Two Stompboxes...I'm zeroing in on the hum.

[kpfeif]

Hi.

I built a BMP clone as well as a Korg SDD Delay preamp section via PCBs from Aion Electronics.  Both have a good amount of hum...bypassed they're silent, engaged they hum.  They're being powered by a single AC wallwart adapter.

I just wired up a negative tip plug and hooked it up to a 9v battery.  They're both dead silent.  Whatever the hum is, it's coming from the AC side of the equation....it's not the pedals.  Good, because I've triple checked all the ground points in the pedals and all should be good.

Cheap adapter?  Should I build a filter?  Maybe the ground loop thing?

Thanks,
kris

[bloxstompboxes]

You most likely want a DC wallwart and not an AC one unless you have an on board rectifier and filtering on your pedal PCBs. If not, I am surprised the boards aren't fried.

[kpfeif]

Sorry, I wasn't clear.  It's a wallwart transformer, plugged into AC power, which converts it to DC 9v, which is then plugged into the Stompbox. 

[antonis]

There are some tricks to eliminate hum but the most simple and effective is to reduce the GND loops...

i.e. GND from PS to Out sleeve -> In sleeve through case -> Only one cable to PCB -> extra GNDs with the minimun length cables - in a non circular way..

Maybe you'll need to add a small resistor (33-100Ω) and a 47-100μF cap just before the +9V connection to the first active element on your PCB..

[induction]

Use a regulated adapter.

[bool]

Lots of hum (in case of an unregulated) wallwart can be shed away by using a simple capacitance multiplier. When done right, it can be unusually effective.

[smallbearelec]

I did an FAQ about this, hope it's helpful:

Why do I get hum when I power my pedal with a wall wart?

Consumer wall transformers, like the unregulated types in my Stock List, include only a bare minimum of filtering to keep them as inexpensive as possible. This is OK for powering computer speakers, for example, but the ripple current shows up as hum in high-gain circuits.

Some pedals include extra filtering and regulation and so are OK with an unregulated wall wart; a number of the old E-H pedals were designed this way, for example. If your pedal needs cleaned-up power, you have choices:

◦Buy an unregulated wall wart, and build an add-on that includes a three-terminal regulator and filter capacitors. There are many on-line references that describe how to do this.

◦Buy a wall wart that includes regulation and filtering, like the Morley, SKU 2401K.

◦Build a Small Wart.
http://diy.smallbearelec.com/Projects/SmWart/SmWart.htm

Regards
SD

[amptramp]

I took one wall wart apart and found the expected transformer and diode bridge and filter capacitor, but there was a resistor in series with the capacitor to limit inrush current.  It was only in series with the capacitor, not the positive or negative line, so ripple could be guaranteed to be above a certain amount.  Buy or build a regulated supply and eliminate the problem.

[GibsonGM]

Another maybe-issue with the generic wall wart is that they tend to be 'sloppy', since they're unregulated. This may or may not matter to whatever it is you're building, but hey - we try to control what we can, right?   In there with regulation is all the filtering you want....as it keeps a steady output voltage, by definition it won't have that nasty ripple going "Huuuuummmmmmmm"...

Generic wallwart:
Ex: it says "9V 350mA" on the box.  So it will deliver about 9V when asked to provide 350mA.   If you draw say 100mA, you might get 13.2V.  If you exceed 350mA, maybe you drop to 8V or even less.    This is the problem with UNREGULATED supplies.   They can only give you a rated current.

A nice wall wart designed for audio use will be regulated, and will provide you with UP TO some amount of current, holding the voltage just right about where we want it.   Many of them are rated to 1A, or even more.    Or build your own with a LM7819 regulator IC or other fun silicon product!

[nate77]

Not to be totally redundant here, but all the answers here are spot on. A regulated power supply will fix this for you. The aion boards are designed really well, but the power supply is the "other" part of the equation. I still rely on archaic batteries because of this. It makes testing and isolating power supplies much easier, (although much more costly for bothe and Mother Earth) but since I love in a very old house that has only 1 outlet in the entirety of the upstairs where my music room is, it solves many problems for me. This hum is eggsaserbated when running several distortion pedals, compressors or preamps in series. Kevin designs all of his boards for only power supplies, without using a battery. I believe this is to save precious real estate on the users pedal board, but I build all of mine with a 9v adapter, because of the way my house is set up, as well as to avoid Murphy's law. When I gig, I can't afford to worry about crappy power fluctuations or lack of power supply.