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Screened power cables for RFI ???

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Screened power cables for RFI ???

Postby Matthew Seed » Sat Jun 23, 2018 6:31 pm

Hi guys, I am in the middle of buying some new kit for my studio. In this process I am doing all I can to reduce some serious RFI issues I have from nearby Ministry Of Defence ariels transmitting. I’m having my guitar screened fully and bought some good quality Klotz double screened guitar cables etc.

Just bought a new amp and my question is. Is there any merit in these RFI screened power cables on an amp. Is that a place where RFI would seep in ? I don’t mean these ones for £500, I just mean one for about £20 or £30.

Thanks guys
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Re: Screened power cables for RFI ???

Postby Wonks » Sat Jun 23, 2018 6:49 pm

You've got a houseful of unscreened power cables that will pick up far more noise that a short IEC mains cable will do, and screening won't remove any of that noise. Probably the only thing worth doing is getting a clip-on ferrite collar and attaching that to the cable near the amp and any other audio equipment. Something like these (just picked as an example). https://www.ebay.co.uk/itm/Clip-on-Ferr ... SwCJxZ8jOC
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Re: Screened power cables for RFI ???

Postby Matthew Seed » Sat Jun 23, 2018 7:15 pm

Thanks Wonks, is there any benefit on having those on the guitar cables going in. Also the cables going out of and back to the effects loop?

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Re: Screened power cables for RFI ???

Postby James Perrett » Sat Jun 23, 2018 7:34 pm

This is one of those times when a valve amp is useful - the input stages aren't as susceptible to RFI as solid state. However, if you are using effects pedals, they're still going to be susceptible unless they're well designed from an RFI point of view.

At one place that I used to work the whole building had been built as a Faraday cage with metal in the walls to stop RF going in or out and I know that some people use chicken wire in studio walls in an attempt to achieve the same thing.

Good grounding is also important - keep mains cables short and straight.
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Re: Screened power cables for RFI ???

Postby Matthew Seed » Sat Jun 23, 2018 8:52 pm

Thanks James, that’s really interesting. The guy who is screening my amp is using some special graphite paint on the guitars cavity. I wonder would there be any advantage to using this paint in the inside of the amp and pedals. I only mean the cases of course. For example take our the amp chassie and paint the insides of the amp cabinet?

How dies RFI work, is some protection like this Better than none or is it case of, if there is a gap it floods in like water ?

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Re: Screened power cables for RFI ???

Postby Hugh Robjohns » Sat Jun 23, 2018 9:53 pm

Dealing with radio frequency interference is complicated because if you're hearing the effects there is something fundamentally wrong with the design of the equipment. To solve the problem you first need to know how the interference is getting in. I could be via the mains lead (although that would be spectacularly bad design), or it could be via the input or output cables,mor it could be directly into the electronics if they are not encased in an earthed chassis.

Obviously, the closer the equipment is to a strong RF source the worse an interference is likely to be. In a former career we used to take new TV Outside Broadcast trucks up to the Crystal Palace TV transmitter, run out mic multicores, and see if we picked up an interference (we didn't!).

Clamp on ferrites on the input, output, or mains cables might help if the interference is getting in via one or more of those routes, but are of more practical use in identifying the problem entry route. The better solution would be to modify the circuitry to cure the problem properly, which migh involve ferrites on the connector wiring, or RF shunt capacitors, or controlling the amp bandwidth, decoupling the power supply rails etc etc. none of which is trivial...

The graphite paint idea is appropriate for the wiring cavity of a wooden guitar (although I prefer self-adhesive copper sheet) , but an amplifier should be housed in a grounded metal case. Some valve amps are not fully enclosed, and I guess graphite paint might possibly help to reduce RF interference entry, but I would expect much success!

But this is all speculative... Are you sure you are actually suffering RF interference directly into the amp?

H
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Re: Screened power cables for RFI ???

Postby ef37a » Sun Jun 24, 2018 4:26 am

Hi Mathew, as Wonks said, just screening the IEC mains cable is pointless unless you run a dedicated screened cable all the way back to the consumer unit and even then that would only help if your mains came in underground in armour but almost all does these days.

Palliative, external measures can only help so far, as Hugh says, equipment must be designed to reject RF in the first place.

The valve amplifier is naturally better at keeping RF at bay but beware! Some designs use an IC front end and whilst the designs I am familiar with take great pains to stop RF not all do! (I am thinking of "copycat" designs here) . Similarly with pedals, it IS possible to proof them but not all manfctrs spend the extra few bob on the components that do so. Note as well that pedal output lines are almost as vulnerable as their inputs. The transformer at the 'end' of a valve amp stops a lot of junk getting back in that way.

But! Hugh again, look on the bright side, might not happen? If you do get trouble see if there is a "Ham" radio club in the area? If so there is likely some 'good ole boys' still around with expertise in this area.

Dave.
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Re: Screened power cables for RFI ???

Postby Matthew Seed » Sun Jun 24, 2018 8:27 am

Thanks Hugh and Dave. The rest of my new kit arrives this Wednesday so I will see how much is getting through then and make a call.

Thanks for the help
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