4 cable method / ground loops (?)

Branagh

Member
Dear community,

I have been a happy owner of a PBC10 for quite a while now and I'm using it in a 4 cable method setup with the FX loop of the amp while also switching channels on the amp. I have tried to use decent components like the GigRig modular power supply on my board to minimse noise but I'm still fighting with the occasional, random noise/hum issues in different locations and at the same locations at different times.

Because I can't find anything at fault with my cables, power supply, setup etc. I tried to do some research and it seems that ground loops seem to be a rather common problem with the 4 cable method. I'm not very good at electronics but it seems that using isolator boxes can fix this issue.

My question is if anyone here has had and solved similar issues and, in case an iso box is a good idea, where do I put it in the signal chain?

Thanks a lot already!
 
Yes, one or more isolation transformers is the typical solution. I recommend starting at the end of the signal chain and working backwards. Put one on the connection to the amp's effect loop return. If that isn't enough, add another to the effect loop send. You generally wouldn't want or need to put one on the amp's input.
 
Thanks for the quick reply!

Just a quick question to make sure I have understood correctly:

Option A means I put the isolation transformer just after "Out A" on the PBC, before the signal goes back to the amp's return.
Option B means I put it it between the Amp Send and "In 5-6" or "In 7-10", depending on how I route it.

And maybe another question concerning the choice of the isolation transformer: Some, like the GigRig Humdinger are 9V powered themselves while others like the Palmer PLI-05 (https://www.palmer-germany.com/en/products/line-isolators/5185/li-05) are not powered. Does this make a difference for my purposes? Also, the Palmer seems to have two channels, so am I correct that I could use it to realize both options as mentioned above?
 
Yes, you have it correct.

I've used neither of those transformers, but it looks like the GigRig one is buffered, which may result in better audio quality overall. The Palmer one could be used for both connections, that's true.
 
Great, thank you again!

I promise I'll stop asking question, but since you mentioned the buffer in the GigRig, there's one thing I've been wondering for a while now: Since the PBC already has three buffers built in, can I really expect better overall quality by adding additional buffers? This might also influence future additions to the board then.
 
Great, thank you again!

I promise I'll stop asking question, but since you mentioned the buffer in the GigRig, there's one thing I've been wondering for a while now: Since the PBC already has three buffers built in, can I really expect better overall quality by adding additional buffers? This might also influence future additions to the board then.
Additional buffers only help when a signal gets "unbuffered". For example, the PBC has a buffer on its input, but if your first pedal is a King of Tone, it has an unbuffered output. Depending on what's next in the signal chain, it may interact with the KoT's relatively high output impedance. In those cases, another buffer in between would help.

In your case, the PBC output is buffered, so if you're going straight into the isolation transformer, there's no need for any other buffer there. If you need a transformer on the amp's loop send, it depends on the design of the amp's effect loop. If it's a good design, it's also well buffered and you wouldn't need an additional buffer.
 
Well, thanks for the explanation! That means I probably won't need buffers in the isolation transformers since Friedman claims they have good FX loops. But I'll look a bit into the other pedals in line and see if I can still improve the situation elsewhere.
 
Well, thanks for the explanation! That means I probably won't need buffers in the isolation transformers since Friedman claims they have good FX loops. But I'll look a bit into the other pedals in line and see if I can still improve the situation elsewhere.
You should have no problem with a Friedman loop.
 
Hi (Ron),
Not to highjack the thread, but you said something in your response that I would like to know more about. You said:

"You generally wouldn't want or need to put one (iso-transformer) on the amp's input."

I always felt an iso-transformer on the amp's input changed the attack or transient response. Is this what you meant when you said you don't want to put one on the amp's input? What about something like the Mesa Clearlink Receive, which is designed to go between guitar and amp inout? I have found with my mesa amp's your suggestion of putting iso-transformers on the loop's return first and if needed on the send as well to work in best.
Thanks in advance.
 
Hi (Ron),
Not to highjack the thread, but you said something in your response that I would like to know more about. You said:

"You generally wouldn't want or need to put one (iso-transformer) on the amp's input."

I always felt an iso-transformer on the amp's input changed the attack or transient response. Is this what you meant when you said you don't want to put one on the amp's input? What about something like the Mesa Clearlink Receive, which is designed to go between guitar and amp inout? I have found with my mesa amp's your suggestion of putting iso-transformers on the loop's return first and if needed on the send as well to work in best.
Thanks in advance.
An unbuffered isolation transformer is going to load down the guitar's pickup for sure, causing tone problems to some degree or another. I don't know anything about Mesa's product, but if it's a buffer with a transformer, it will definitely be better than an unbuffered transformer.

EDIT: While technically correct, this is a bad answer. You'd never need to have an isolation transformer between a guitar and amp unless something was in between like the PBC, and the PBC will buffer the signal before hitting the isolation transformer. Sorry, answering too many questions at once can mess me up...
 
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Hey there again, I have just received my isolation transformer (went with this one from Lehle because of its size and alleged quality: https://lehle.com/EN/Lehle-P-ISO) and I have to admit I'm a bit confused.

Today was a day I had a very bad ground loop again, but apparently it had nothing to do with the FX loop since it didn't go away no matter where I put the iso transformer. Disconnecting everything from the loop didn't help either. However, once I put it between the PBC and the amp's input, it was gone.

[EDIT: My bad, I accidentally only disconnected the cables in the loop half way, pulling them completely obviously also got rid of the ground loop. This lifts some of my confusion, but it's still true that the only way to get rid of the ground loop is with the box in front]

Now, this directly relates to the question of buffers and cgscotto's question again. If I follow you correctly, you're saying it's a bad idea putting an unbuffered transformer in front of the amp. On its page Lehle claims the transformer is specifically designed for "low-strength-high impedance" signals from instruments, claiming that standard transformers impact the signal significantly. Irrespective of potential marketing talk, is that the problem you were referring to?
 
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What I was referring to was more about having a transformer right after the guitar. I wasn't thinking when answering, and didn't really account for the buffer in the PBC. It should work pretty well between PBC and amp input.
 
Alright, then that would definitely solve my problem. Well, thanks again for a great product and great support, appreciate it!
 
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