EMF Filter caps?

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EMF Filter caps?

Postby chris p » Sat Mar 21, 2009 3:26 pm

I was pulling apart an old piece of gear the other day and was reminded that it was common in the old days to have pf value ceramic caps from pins 2 and 3 of an XLR to pin 1, which I believe were intended as a simple EMF filter. Values between 20pf and 100pf seemed common.

What's the modern story on these? Are they still thought to be a good idea? I work in some EMF-unfriendly environments and was thinking about adding them to my gear inputs, but I really like to hear from our tech gurus first, in case they do more harm than good.
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Postby rob » Sun Mar 22, 2009 9:31 am

yes, EMF filtering can be useful.
but sometimes simple filtering like a pair of caps can increase problems depending on the circuitry on each side of the socket. As with earthing there science involved, but also a degree of suck it and see.
Common mode chokes are one of the best solutions and good earthing and true balancing with high common mode impedance is the go. ( as easily achieved with transformers, less easily with transformerless circuitry )

bear in mind that a lot of the older equipment you see with caps across the input, wether a neve console or an old tube limiter were designed for the broadcast industry. Broadcasters live in an environment where high levels of RF are present and it is good design practice to absolutely ensure that no RF enters the audio chain. For this reason you also find that the frequency response of a Neve 1073 module is totally slugged above 20kHz. This was specified by broadcasters and is quite contrary to Rupert's beliefs for some time now on the importance of an extended frequency response to say 100khz.

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Postby rick » Fri Apr 17, 2009 8:34 am

http://www.neutrik.com/us/en/audio/204_ ... tlist.aspx


you will have to wait and read the next AT article for my thoughts :)
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Postby Milo » Fri Apr 17, 2009 9:30 am

As Rob mentioned, things like removing the small cap between base and collector in the first pre stage of a 1073 improves the HF response. It was put there for RF suppression, apparently, and I'm glad I found an Old Guru site which describes the change, plus other small ones.
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Postby chris p » Fri Apr 17, 2009 10:24 am

Rick wrote:you will have to wait and read the next AT article for my thoughts :)


A pity I no longer subscribe so I substituted DIY frenzy for gear lust as my obsession de jour. At least with the DIY bug you end up with something.

Isn't there a hidden link somewhere to your articles on this site? I went looking for it half-heartedly a while back but could find it easily.
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