Signal noise problem

[Jasor31385]

Hey there.

I've been working on this custom pedal board project and i've almost got it finished. basically what i've got going on is a chassis-mount 1/4" output jack (one of the switchcraft models) that runs a guitar cable through the box and then up through the top to another chassis-mount 1/4" output jack to supply the pedals with the signal from the guitar. I can get the signal through but there's SO much chatter coming through my amp. I've double-checked my soldering on the mounted inputs and they look fine and i've checked the cables that i'm running to the input and out from the output and they work fine too. the only issue might be that I had to cut small chunks of rubber from the outside of the cable so I could fit snap-clamps to the cable. I wrapped the exposed areas with electrical tape and it looks solid. I didn't cut through the shield or anything in those spots. Could that be the issue with the cable making so much noise? I get a solid buzzing sound with little pops of static occassionally.

I wish I could post pictures of this thing to show you but I can't. Please see the attachment for a quick drawing of what it looks like for a better understanding.

Any help would be appreciated. Thanks!

-Jason

 

[Gary B]

First, is the noise present all of the time or only when you touch it? If the latter, verify where moving it causes the noise. I would suspect either the plug is loose in the jack (you can bend the jack springs to tighten it if necessary, just don’t overdo it) or where the wire enters the back of the plug. If that is it, and it is not a molded plug, you can fix it. Otherwise, just cut it off and put on a new one. Just make sure you wire it the same as the old one.

If the former, then you are picking up induced AC and your chassis is not properly grounded.

 

[Jasor31385]

The noise is present all the time. How would I go about grounding the chassis? Does it matter that the chassis is made of wood with a plexiglass top?

 

[Gary B]

Yep, it sure does! First, just to make sure, the noise is a hum rather than pops and crackles, correct?

The outer braid of the cable (you are using coax type of audio cable correct?) is grounded at the amplifier end only. The rest should be floating. If there is a second ground connection along the way, you have formed a ground loop that can induce noise into the circuit. Also, make sure that the Guitar and amplifier jacks are wired the same way. The tip of the plug should be hot and the rest grounded. Therefore, the spring of the jack that the plug’s tip pushes against and then snaps into the indent of the plug should be hot and the other connection (the part the sleeve slides into) should be ground. Inside the box, there must not be a break in the shield from one jack to the other. If you have not tested it do so. The best way is with an ohm meter. If you haven’t got one, pull the end off a flash light and connect one wire to the case and another one to the battery. If the flashlight lights, you have continuity.

 

[Jasor31385]

Yeah, it's a constant hum/buzz.

How can I ground it on one end only? i thought that soldering the shield to the "outer pin" of the jack was grounding it.

 

[audioguru]

The shields of the cables must be continuous from the guitar, through the box and to the amplifier.
The grounded shield of the cable is connected to earth (don't call it ground) only at the amplifier end. If the shields for the cables connect to the metal box then the box must not be connected to earth.

Measure the resistance of the shields from the guitar through the box and to the amplifier to see where it is broken or is disconnected.

 

[Jasor31385]

OK. When I soldered the twisted shield wire to the outer ring of my chassis-mount output jack I was connecting the shield to earth and not grounding it right? There isn't a metal box. It's made of wood and plexiglass.

 

[audioguru]

When I soldered the twisted shield wire ....

The shield is not twisted. It is either braided many wires or it is a metalized plastic membrane that has a "drain" wire touching along its entire length.
Do you have shielded audio cable?

When I soldered the twisted shield wire to the outer ring of my chassis-mount output jack I was connecting the shield to earth and not grounding it right?

Only the amplifier's insides should be connected to earth. But some amplifiers use a 2-prong electrical plug and they are not connected to earth.
The shielded audio cables have the shields all connected together through the plugs and jacks and we call it a common ground.

There isn't a metal box. It's made of wood and plexiglass.

Good, then the shields of the cables cannot be connected to earth accidently which would cause a "ground loop".

I sketched a mono plug and jack. Maybe you use stereo plugs and jacks?

 

[Jasor31385]

In your sketch is the jack a male or female? It looks like a male (like one to plug into an amp or a guitar) but the ones on my board are females. There are three things on the "solder-side" of my jacks: one is the spring which the male lead connects into and then there's the center one which presumably is the "hot" pin since it is attached to the hole where the jack comes in and then there's another pin that sits on the outside of the jack: **broken link removed**


I tried switching the leads from one to the other and it didn't help at all. I've switched them back.

 

[Jasor31385]

OK, i've made a SERIOUS sketch. This is exactly what my pedalboard looks like. All of the inputs are labeled and the wires are included. I included the DI BOX because even that has a strange hum to it (although not nearly as bad as the others. I didn't include the power strip and power cables because I don't think they're related but you should know that I do have onboard power and it works fine. I have a chassis-mount IEC next to the chassis-mount XLR on the right-hand side of the picture/board. Thanks for helping out guys!

 

[audioguru]

My plug is male because it sticks out like you know what.
My jack is female because it has an opening like you know what.

You show a mono jack with two terminals like mine, but you decribe a switched jack or a stereo jack with 3 terminals.
The outer metal with the threaded hole and the big nut is where the circuit ground and the cable shield should be connected. The "spring" is the hot signal terminal which connects to the end of the plug.

 

[Jasor31385]

OK. I've verified my jacks and they are definitely two terminals like in the above picture (the big one). You mention that the outer terminal is where the circuit ground AND the cable shield should be connected. Where do I find the circuit ground? I've only got the shield soldered to that terminal.

 

[Gary B]

The ground is on the shield from the amplifier. Since I see two lines from/to the amp., connect the shield terminal from only one of the amp. jacks to all of the shields in the box. That way you are not creating a ground loop out of the two cables’ shields to the amplifier.

 

[Jasor31385]

Oh I think I understand what you're getting at. In the picture there are two outputs that run to the amp however, only one is plugged in at any given moment. The top one is for my acoustic guitar and the bottom one that runs through the pedals is for the electric.

 

[audioguru]

The circuit grounds for the pedal effects electronics must also be connected to the cable shields.

 

[Jasor31385]

They are connected together using patch cables which I didn't make myself but I assume that they are connected with a ground as well, how would I go about connecting them if they're not already?

 

[audioguru]

Maybe your patch cables do not have a shield because they were made for headphones not a microphone.

 

[Jasor31385]

I'm totally confused now. They are made for neither. They are made for connecting pedals together. **broken link removed**

OK, so here's what I am doing right now to try and make this thing work. I'm going back through all of my connections to make sure they are all soldered nice and solid and then I guess i'm gonna take the ground off of the final connection that leads to the amp and see if that helps. I guess that would "break the loop", right?

 

[audioguru]

Don't disconnect the grounded shield of the cables. The shield must be continuous from the geetar to the amplifier.

 

[Jasor31385]

Ha, yeah it didn't work. I've completely re-wired the board in the past couple of hours. I did all six outputs with new leads and new solder connections and I finally got the signal through. I can play the guitar now but that hum is horrible! It's obnoxiously loud and it's messing with the guitar sound as well, it's making it very crunchy. So, now that I know all of my connections are solid...what should I do about that hum?