RFI Picked up by Amp

L

lukemh987

New member
Hi everybody, I have an amp-related conundrum that I've as of yet not found a solution for: when my HT-5R's effects loop is engaged, I get a very audible radio frequency interference (RFI) from an AM station here in Maryland. This happens even when I take the lead plugged into the FX loop "send" into the "return". The RFI is present at the same level whether or not a guitar is plugged in.

I have tested all the pedals in the loop, the power source for the pedals, and replaced all patch cables and longer 1/4 inch cables running from the amp to the pedal board and back to the amp. I have purchased a Furman power conditioner, which did nothing to squelch the RFI. I have replaced the tubes. I have even brought it to an amp tech at a Blackstar approved shop. He examined the amp, found nothing wrong with it, and even demoed it for me--it was pristine and quiet even with all the pedals plugged in and the overdrive channel engaged.

I'm at my wit's end, having tried seemingly everything. It could be a problem with the mains in my apartment, though the recommended remedy--the power conditioner-- didn't solve it, and I'm not sure what else I could try. Alternatively, it could be the amp, despite it performing normally at the repair shop. Is it at all possible that something in the amp is awry and makes it susceptible to picking up RFI? Would love some input! Thanks everyone.
 
Re: RFI Picked up by Amp

How close are you to the nearest AM transmitters (radio-masts)?


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Re: RFI Picked up by Amp

VinceT said:
How close are you to the nearest AM transmitters (radio-masts)?


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Click to expand...

A quick Google Maps search shows there's a tower only 2.5 miles away. Is there a way to shield the amp from picking up this signal? Also, why would it only be audible when the FX loop is engaged? Just seems so strange.
 
Re: RFI Picked up by Amp

What cables are you using for your effects loop send and return? How long are they? Does the issue disappear with nothing connected to the loop? (It seems you are saying a patch cable connected has the same issue, what happens with no patch cable?)

I’d try using good, short, shielded cables to see what happens. I hope it’s not an issue with the amp itself.
 
Re: RFI Picked up by Amp

Another option is to build a setup without using the effects loop. This might involve getting another source for your distortion in the front of the amp.
 
Re: RFI Picked up by Amp

use a shielded guitar cable (separate shield)-not a regular guitar cable -but lift the shield from the TS 1/4" -that will fix it.

to be clear -this is 2 conductor 24AWG to 18AWG sized PLUS separate shield/drain cable that you only connect to the 1/4" connector on ONE side.
 
Last edited:
Re: RFI Picked up by Amp

You can try connecting a 150 pF capacitor between the signal and ground on the loop return. That should shut the RF to ground.

This was happening with my amp for a while at our rehearsal space. When I switched to a different AC outlet it stopped. I don’t use the FX loop, and it seemed to be from my pedal board.


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Re: RFI Picked up by Amp

NegativeEase said:
use a shielded guitar cable (separate shield)-not a regular guitar cable -but lift the shield from the TS 1/4" -that will fix it.

to be clear -this is 2 conductor 24AWG to 18AWG sized PLUS separate shield/drain cable that you only connect to the 1/4" connector on ONE side.
Click to expand...

Okay, so just put the Faraday cage on the cable, but where is the fun in that?
 
Re: RFI Picked up by Amp

NegativeEase said:
use a shielded guitar cable (separate shield)-not a regular guitar cable -but lift the shield from the TS 1/4" -that will fix it.

to be clear -this is 2 conductor 24AWG to 18AWG sized PLUS separate shield/drain cable that you only connect to the 1/4" connector on ONE side.
Click to expand...

Not sure I follow exactly. How many components would I need to purchase to make this happen?
 
Re: RFI Picked up by Amp

devastone said:
Okay, so just put the Faraday cage on the cable, but where is the fun in that?
Click to expand...

ha.... well I have to solve this issue in the studio professionally all the time -so we don't beat around the bush -we just murder the RF and move on :lmao:
 
Re: RFI Picked up by Amp

NegativeEase said:
use a shielded guitar cable (separate shield)-not a regular guitar cable -but lift the shield from the TS 1/4" -that will fix it.

to be clear -this is 2 conductor 24AWG to 18AWG sized PLUS separate shield/drain cable that you only connect to the 1/4" connector on ONE side.
Click to expand...
So basically balanced cable, tip to tip, sleeve to sleeve and drain to sleeve on one side?
 
Re: RFI Picked up by Amp

An update, after a little more tinkering, I've found that the interference is also happening when the FX loop is not engaged. Not sure if this calls for a different approach.
 
Re: RFI Picked up by Amp

Does this happen with the effects loop connected by just a (good) cable...no pedals?
 
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