Hum after adding battery snap

[PeterJ]

Hi, all -- semi-beginner here. I have a Bearhug compressor pedal (awesome pedal) that someone else built using the 1776 PC board. It came with only a DC jack, and I wanted to add a battery snap, for the extra convenience. I didn't change the way the pedal was wired, just added the leads for the 9v snap to the DC jack. Now, I get a loud hum through the amp UNLESS I insert a 1/4" plug into the input jack, but only if the pedal is powered by an external supply.

In other words:
Amp <- pedal output <- pedal, powered by wall wart = hum
Amp <- pedal output <- pedal, powered by wall wart  <-  guitar cord plugged into insert = no hum
Amp <- pedal output <- pedal, powered by battery only = no hum

Obviously, something's wired incorrectly, but I'm not sure where to start. I've been over it again and again. For wiring, I've been using this diagram from Beavis audio:

(from http://beavisaudio.com/techpages/StompboxWiring/)

As long as I have a cord plugged in before I power anything up, no problem. Any ideas where to start troubleshooting? Appreciate any help.

[Rob Strand]

There's a good chance the wiring is swapped to the ring and sleeve on the input socket.  At least some muddling up there.

When the input jack is pulled, all the following grounds should still connect together:
- output jack ground
- case
- input jack sleeve
- circuit ground

The only thing that disconnects from ground is the battery -ve (and in the case of the Beavis audio, the -ve from the power jack as well).

[PeterJ]

Thanks! I'll give that a try

[rogeryu_ph]

Because when we plug the guitar cord into INPUT jack STEREO the RING will connect to the SLEEVE and Battery is ON when there is no DC wallmart jack inserted on the DC input. Battery is giving supply.

Because when we plug the guitar cord into INPUT jack STEREO the RING will connect to the SLEEVE and Battery is OFF when there is DC wallmart jack inserted on the DC input. Wallmart is giving supply.

Hope it help...

[iainpunk]

yeah, what he ^ said, if there's no guitar cable in the input, the output floats between ground and an undefined voltage, basically acting as an antenna for noise and hum. i don't know why it didn't do this humming before the snap was installed, i'd expect it to.

cheers

[PeterJ]

Appreciate the additional input all (no pun intended) -- I need to make sure everything's going where it's supposed to -- it all looks right, so I may need to unsolder and reconnect things to be sure