Massive ground hum, but working

[Jimbob Walton]

Hi everyone, new here.

I just finished building the CB55, and it works - sort of. It generates all the correct sounds, but the signal also contains a very loud hum which sounds a lot like there's a ground loop somewhere.  When I connect the ground to mains earth (proper safety precautions taken), it is completely gone.

I tried different power supplies, the problem persists. Even when using a battery, but it is considerably quieter then.

The weird thing is that the hum is also affected by the volume and tone pots. When I turn down the volume, the hum also disappears, and the tone pot affects the hum tone.

I tried to disconnect the different parts of the signal by removing the jumpers J1 to J6 to test if any individual circuit is to blame - no luck. I also tried bigger caps for the voltage regulators, didn't help either.

Did anyone else encounter that problem or have an idea where to look for the source of it?  I'd like to avoid having to use a permanent connection to ground.

Thanks!

[Circuitbenders]

You should have a permanent connection of ground anyway shouldn't you? Via the ground connection of the power supply, the sleeve connection of the trigger input jacks, and the sleeve connection of the output jacks.

[Jimbob Walton]

Well, no, at least not if I use any of the common power bricks that don't have a ground/earth connection to begin with... :-) Here in Germany they all have these two-pin europlugs. Do they all have ground pins in the UK?
I still have to put the board in a proper case anyway, so I guess I'll just order a power supply with a proper ground connection to go with it. That will surely solve the problem, I was just hoping to get by with a power brick.

Anyway, I really like the CB55, it was great fun to build and surely will be modded in some way in the future!

[Circuitbenders]

but as soon as you plug a jack in to be able to hear anything won't the CB55 be grounded via the sleeve connection of the jack?

[Jimbob Walton]

Again, yes, if there's some device in the chain that is grounded (connected to mains earth). My mixer isn't, for example - it's power supply uses a two-prong europlug. My synths  aren't  either, they all use europlug wall warts.  As soon as my PC (which is properly grounded to mains earth) is connected to the mixer as well, I have a ground connection in my setup and the CB55 hum is gone.

It's really not a big deal, I just thought I might have done something wrong while building the board.

[zorius]

I have the same problem. About -50dB ground hum idle. And it's changing with the volume pot: if the volume is at zero, there is no hum.
Replaced the supply, checked cables, and solder pads... everything seems fine. Although, I tried it yet only with breadboard, and jumper cables. It's not boxed yet. And in some cases, male + female cables are connected, so it's maybe just the poor quality, because when I changed the way/place of grounding, the loudness of hum changed, the lowest was -50dB, the highest about -40dB.
I think, I have to switch to direct, real wire soldering, not jumper cables and headers.
Any idea about this? As I see, this case is not solved, and closed, have You find a solution?

Thanks,
regards

[blakeAlbion]

I have this problem, too. It seems like the drum signals are very weak and the output mixer needs too much gain.
I'm going to cut all the jumpers and see if the individual outputs are stronger.