Looper Noise

[gbt8964]

Hi guys,

I've just finished building a relay-based looper to use as an extension for my main looper pedal. It functions perfectly well, save for some apparent AC hum induced by powering the unit on. The basic run down is as follows; 8 DPDT relays (https://www.fujitsu.com/downloads/MICRO/fcai/relays/ry.pdf) routing the guitar signal in series, being controlled by 4 TRS jacks being switched by another unit. The TRS jacks have DC hooked up to the shield (which is 100% isolated from the chassis/audio ground), and the incoming DC to the unit is running through a filter (Huminator circuit from Beavis Audio). All signal jacks are grounded through the casing.

I've wired it up as neatly as I could, and kept DC and signal wires running adjacent to each other where possible (it's in a 1032L casing, and with ~30 jacks in there space is tight), but whenever the DC is connected (or a TRS jack thats connected to a powered source) there is an increase in hum in the signal. The only way I can eliminate the noise is by using a battery to power the unit.

Any ideas on how to reduce/eliminate the added noise? I don't know if I need to re-arrange the wiring, whether the relays are cheap (which they are), or whether I would be better off transferring the relays onto a circuit board... Thanks for any help!


Gareth

[PRR]

Welcome!

> kept DC and signal wires running adjacent to each other

They should be separated as much as possible. Is that what you meant to say?

> whether the relays are cheap (which they are)

Most low-price relays available today are well made. When Panasonic or whoever puts relays in microwaves or answering machines or rice cookers, they don't want a lot of warranty returns from bad relays. And that's where the market is, millions for production, not cheapos for DIY fans. At production quantity the price comes down very-very low, and stays low even when you buy one or two.

Also, a relay can't hum because it does not know the melody. (Why would it sing 50/60Hz instead of any other pitch?)

> incoming DC to the unit is running through a filter (Huminator circuit from Beavis Audio)

"DC" is coming from what? Good power supply? Crappy power supply? Rectified un-filtered AC??

> I can eliminate the noise is by using a battery

As expected, if there's no hum/buzz coming in, there's no hum/buzz on the signal.

You want to filter the "DC", probably before it gets into the box.

That Huminator (link?) is probably intended for the smaller current of older batter-power pedals. With several relays active it may have significant voltage drop. If it is 100uFd filter cap it won't filter large currents well.

If you used 9V relays, any *good* aftermarket 9V DC battery-substitute power supply (such as 1-Spot) should be inoffensive.

More details on what your power supply is, please.

[gbt8964]

Thanks for the reply.

I forgot to mention the power supply, I'm running it off a GigRig Generator, which has been very clean when used in the past with other sensitive pedals.

When I said the wires were adjacent, I meant that when they come close to each other they meet at a right angle, otherwise they are as far apart as possible. They aren't ever that close, the closest they come is when they have to meet at a relay, at which point the power lines run in vertically and the audio lines run in horizontally.

The Huminator link is herehttps://web.archive.org/web/20150403071928/http://www.beavisaudio.com/projects/Huminator/index.htm. Hopefully the cached link will work, the actual website has been down for some time. It does use a 100uF cap, though so far I haven't tested the unit with more than one 30mA relay running. I can't see myself turning them all on, I would say 3-4 maximum (all of my modulation effects are stuck in this thing) so the large currents shouldn't be too much of an issue.

The relays are actually 5V, and I've resorted to using a 150Ohm resistor on each to limit the voltage. I was using a cheap DC regulator circuit to drop the 9V to 5V, but that proved noisier so I opted for the resistors instead.

Thanks PRR,

Gareth

[PRR]

> a GigRig Generator

http://www.thegigrig.co.uk/generator-c2x15357206

specs: "Output Hum. ePix Grade 7-SM"

Hmmmmm. I never heard of ePix grade, and can't find any reference except back to that company.

However, it is quite unlikely it has much hum. At that rating (9V 5 AMPS!) it must be a switcher, thus regulated, and regulators usually wipe hum down to small level. Also they know this isn't powering LEDs or charging batteries, you will "listen" to it, and at a hundred bucks I would take it back if it hummed.

So I would question the grounding. Normally I'd expect not-connected to be best. However if the wires are in close proximity, and not powering other stuff, connected common might be best. This could be checked with a clip-lead. If that GigRig also powers your other pedals, then it may already be common-ground (unless you use their "isolators").

5VDC wall-warts are common things. USB can power several 30mA loads without intelligence (to negotiate USB power output). Problem then is connectors and getting the polarity right. (Might not matter on 9V relays, but some 5V relays have reverse-kick diodes internally.)

Interior photos may give clues.

[gbt8964]

PRR, you may have been right about the 100uF cap not handling high currents.

As it happens, I got impatient and wired the unit into my board just to test its general function and... now there's no noise. I'm thinking that perhaps now the power supply has a larger load being drawn, the tiny filter circuit is receiving less current and is actually filtering. I could be wrong, but there's no noise. Go figure 

Thanks for your suggestions, it's great to know there's people out there willing to have a go at solving strange problems!

Gareth