ByPass Caps on audio lines...... what's the "best" value?

[KF1Z]

Ok, so, 'dumb' question time.....

When bypassing audio cables in an RF environment.... what's the best value to use?

Or, multiple values?


Have an equalizer that seems particularly vulnerable to rf...
No, it doesn't squeal horribly (just a little), and does tend to add a bit of distortion...

At this point I'm assuming that I need to bypass the in/out lines on each end.

Haven't opened the box yet......

The cables are fairly good quality
And yes, it is a metal rackmount box.


Just generally wondering what values are the most effective........

Thanks

[k4kyv]

I remember once I was getting into a guy's stereo.  I told him to try placing a .001 mfd bypass cap from each of the four speaker terminals on his amplifier to chassis ground.  I had seen that work many times, as the two stereo speakers and their connecting cables often made good receiving dipoles.

He insisted that it would kill the high frequency response of his system.  I told him that I had put .005's on each side of my 500-ohm audio line to ground and that it had absolutely no effect on the frequency response beyond the limits of human hearing, so with .001's grounding an 8-ohm line he would have nothing to worry about, but all he wanted to do was argue with me.  So I let him solve the problem himself.

[N3DRB The Derb]

you wanted to kill his "tessitura", Don.

he paid 700 bucks for his speaker cables, and he needs every ounce of "tessitura" he can get.

[AB2EZ]

Bruce

The good news is that it is usually easy to fix this kind of problem because the rf frequency is roughly 1000x larger than the audio frequencies...

The key is to match the filtering method/cure to the specific situation.

In your case, it sounds like you are talking about line-level interconnect cables running between pieces of pro audio equipment. In that situation the output impedance of the source is very low (roughly 100 ohms or less), and the input impedance of the sink is fairly high (10,000 ohms or more). If line-level audio interconnects are, in fact, the source of the problem, I would suggest that you do the following:

0. Make sure the power cord is plugged into an rf-filtered outlet strip.

1. If you have balanced inputs and outputs available, then use them. Ground the shield at only one end of the cable (typically the source end). XLR cables (if needed) are readily available, and about $5.00 each from places like www.SpeakerRepair.com (on eBay)

2. If you have any of the ferrite cores that are used to build class E transmitter transformers (1 inch diameter, 1/4 inch wall thickness, ~1 inch long, type 43)... place one over each cable, at the end that has the grounded shield (for a balanced interconnect), or at both ends for an unbalanced interconnect. This will reduce the "common mode" rf current flowing on the shield of the cable. Typically, a phone plug will fit through the hole in this type of core. For an XLR cable, you will have to remove the XLR connector to slip the core over the cable... and then re-attach the XLR connector.

If the above isn't enough to fix the problems

3. Place a filter at the sink end (the end marked "input") of each cable. The filter would consist of a 1000 ohm series resistor on each side of a balanced input, or a 1000 ohm series resistor on both the hot side and the ground side of an unbalanced input.... and it would include a .001uF capacitor across the two sides (after the series resistors... i.e. closer to the sink end). Any rf voltage between the two sides (differential mode), or, for the unbalanced case, on both sides (common mode) will be attenuated by the filter by at least 30 dB, while there will be no effect on audio frequencies.

Best regards
Stu