Re: Build Hum and Enclosure Shielding

[K3yPr0gg3r]

Hi,

I'm going on my 22nd build since June, and I've picked up a lot of GREAT tips from people here on the forum!! I sent an email to Ben, (RullyWow), yesterday about build hum, and how it varies from circuit to circuit. He gave me some good tips includng shielding the enclosure. Not sure what this entails. I use aluminium 1590Bs or 125Bs from either SB or Mammoth. I always cover the inside of the bottom plate with electrical tape just in case a component is sitting a bit too high.
I use my OneSpot 9V adapter to power everything. Some circuits hum like hell, some are quite! I mentioned to Ben that one circuit of his I built feedsback if I set the "Drive" pot too high. While others of his are dead quite. Might have used a bad pot...
He also mentioned something about keeping the DC power wires on the short side. What I have been doing lately is mounting the power supply on the "front" of the pedal, so I have almost 5" to 6" of wire inside the case going from the breakout in the back to the power supply in the front. I've also been twisting the positive and negative wires around each other to keep it tidy. If anyone has any other suggestions for cutting down on hum please let me know.

Thank you

[midwayfair]

First figure out the frequency of the hum.

If it's 60Hz, you've got some sort of environmental interference problem and your hum is coming from something plugged into the wall or a power transformer somewhere or lights etc. It could easily be in your amp or your guitar and not the pedal.

If it's 120Hz, the frequency is doubled, and that means that it's DC that's not being sufficiently filtered. This could indicate bad filter caps. If it happens in a lot of builds, you might have a bad batch of caps. Low ESR caps will usually be fine. In pedals, we're usually filtering very high frequencies in the power supply from switched mode power supplies, but some high gain effects like a big muff or whatever might hum from insufficient power filtering. You can also increase the power filtering with a resistor in series with the power supply input if there isn't one already in the circuit. 47-100R is usually fine, but you can't do this on circuits that require a lot of current. (Ohm's law and all that.)

In general, using shielded wire for all audio can reduce hum, but isn't usually a problem in a grounded enclosure.

Make sure your jacks are making good contact with the case. If you aren't using metal jacks, you'll need to create a way to connect your ground to the enclosure. This is usually a problem in 1590A builds for people using plastic jacks.

Also, feedback from the drive pot is not a bad pot. A pot's just a resistor. It doesn't create noise. If you had feedback it was because undesirable noise was being introduced somewhere in the circuit to create a feedback loop.

The amount of noise you get in any particular build varies because most builds don't have identical gain, phase, and impedance values throughout. A lot of us follow certain general rules when designing our own new circuits but a lot of PCB floggers are selling known projects and that means that they will vary greatly as many different design philosophies are involved.

[K3yPr0gg3r]

Thanks, Jon. I use either SwitchCraft 111As or BLMS Lumberg Monos (if build is tight) for In / Outs. Use BLMS "Outtie" DC jacks. That way I can solder the wires to the jacks ahead of time, then "cut to fit" once I'm connecting to the breakout. However, I've never added a nut to the inside of the enclosure with these. Plastic is touching on the inside, single nut to hold it in place on the outside. Wonder if that may be a problem as well? Anyway, looking forward to building Multiplex Jr. (first "fabbed" delay...Leviathan was a floop), and Britannia. I follow the BOM to the letter as far as components, adjusting the enclosure size if I feel I need more room. Any build suggestions on these? Thanks again.