Electronics Wiz'/Musicians--Need your help with a build

[Josh380]

I'd like to build an AC ground loop eliminator for a guitar amplifier. I'd like to be able to wire it directly inline with the power cord, inside the box. What I'm thinking is something very similar to a HumX device, without the $75 price tag. If I can source parts from other devices (I've seen an instructable for a ground loop eliminator, for a stereo setup that uses transformers from old modems), that would be best.

I'm working with a Fender SKX15R practice amp that has a horrible AC buzz that needs to go away. The shielding and grounding is fine on the guitar..it's the amp.

Any help would be appreciated!

 

[prcguy]

A ground loop eliminator usually just disconnects the third ground pin on a grounded power plug and an adapter that does that costs under a buck. You might benefit from an isolation transformer but that's not something most people can build. You usually need an old transformer to start with then take it apart and rebuild it as an isolation transformer. Lots of work and you can buy one cheap on Ebay ready to use.

Are you sure its not your environment causing the hum like playing under a bunch of fluorescent lights that are known to induce hum? In that case a ground loop eliminator or isolation transformer will probably not do anything.
prcguy

I'd like to build an AC ground loop eliminator for a guitar amplifier. I'd like to be able to wire it directly inline with the power cord, inside the box. What I'm thinking is something very similar to a HumX device, without the $75 price tag. If I can source parts from other devices (I've seen an instructable for a ground loop eliminator, for a stereo setup that uses transformers from old modems), that would be best.

I'm working with a Fender SKX15R practice amp that has a horrible AC buzz that needs to go away. The shielding and grounding is fine on the guitar..it's the amp.

Any help would be appreciated!

 

[Josh380]

There are no fluorescent lights in the room. Just your typical room electronics..TV, DVD player, computer. Guitar amplifiers in general are noisy, and I've heard that simply disconnecting the ground, or cutting it, could solve the problem, but past experience has taught me otherwise.

Now, the isolation transformer..I think that's what I need. In fact in the instructable I mentioned said the transformers pulled off modems were isolation transformers. I assume I'm going to need one a little bigger than one of those. I guess I just need to know how to wire it properly.

Would this work for my purpose?
http://m.ebay.com/itm/271894233654?nav=SEARCH

 

[prcguy]

The isolation transformer in the link is rated at 35w so as long as your amp draws less than 35w from the AC line it should work. If your amp draws more power you will need a bigger isolation transformer.

Usually you can get a ground lift gizmo at the grocery store for about 1$ and it will remove the ground pin from the power cord without cutting anything off. You can also reverse the plug and usually one way will be a little quieter than the other. Sometimes an isolation transformer can quiet things down and sometimes not.

I just noticed you mentioned isolation transformers from a modem and that would be for use between the guitar and amp. They can alter the sound because the modem transformers are not very wideband and they are much lower impedance than the guitar pickups. There are high impedance isolation transformers made that are more appropriate for guitar pickups.
prcguy

There are no fluorescent lights in the room. Just your typical room electronics..TV, DVD player, computer. Guitar amplifiers in general are noisy, and I've heard that simply disconnecting the ground, or cutting it, could solve the problem, but past experience has taught me otherwise.

Now, the isolation transformer..I think that's what I need. In fact in the instructable I mentioned said the transformers pulled off modems were isolation transformers. I assume I'm going to need one a little bigger than one of those. I guess I just need to know how to wire it properly.

Would this work for my purpose?
Stancor P 6412 Pri 115V 50 60Hz Isolation Transformer | eBay

 

[krokus]

I'm working with a Fender SKX15R practice amp that has a horrible AC buzz that needs to go away. The shielding and grounding is fine on the guitar..it's the amp.

Have you made sure that the filter caps in the power supply are still good?

Sent via Tapatalk

 

[Josh380]

Have you made sure that the filter caps in the power supply are still good?

Sent via Tapatalk

I've yet to open up the amp..so no. I assume those caps will be found closest to the transformer?

 

[Josh380]

Ok. I took pictures of the inside. From what I can tell, there's a loose connector that doesn't seem to have a home. The second picture shows another box wired to the primary control board. This seems to be the reverb effect board, since the wires coming from it are labeled "To Reverb" and "From Reverb" on the primary control board. The two largest caps on the primary control board appear to be ok. I don't see any bulging or leakage.

The green and yellow wires on the primary control board are labeled "Primary" and the white is labeled "AC"

 

[krokus]

More pics of the circuit board would be useful. I do not see any obvious main filter caps.

Is the white wire with the terminal connected to the power cord?
Sent via Tapatalk

 

[Josh380]

Here you go..4 more pics..sorry for the size

 

[krokus]

The filter caps are the bigger "cans" that you gave a close up. The hum could be from those drying out, since the hum is always present.

Sent via Tapatalk

 

[Josh380]

Hmm. I didn't know capacitors could dry out. Should be easy enough to replace. I suppose it couldn't hurt. But I still think there's gonna be some hum left. So, back to the isolation transformer. How would I wire it?

 

[krokus]

Hmm. I didn't know capacitors could dry out. Should be easy enough to replace. I suppose it couldn't hurt. But I still think there's gonna be some hum left. So, back to the isolation transformer. How would I wire it?

Electrolytic caps dry out with age. They are rated for a number of hours, at a max temperature. If the temps are cooler, you get more hours. When looking for replacements, match the lead spacing, Farads, voltage, and same or higher temp rating.

For the isolation, are you wanting that wired in line with the guitar input?

Sent via Tapatalk

 

[Josh380]

For the isolation, are you wanting that wired in line with the guitar input?

Sent via Tapatalk

I think in order for this to work to eliminate AC hum, it would need to be wired inline with the power. I've done some research, and smaller isolation transformers would be more suitable for the guitar input.

Also, my amp draws 32w, so it seems the transformer in the previous link is perfect.