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Old 29th September 2012, 04:13 AM   #1
awedio is offline awedio  United States
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Default Capacitive crosstalk between input/output wiring

I have a preamplifier design (tube) in which I would like to reduce L-R channel crosstalk (ie. I would like to improve channel separation).

The space requirements and layout had me running unshielded left and right conductors very near each other, with input wires running down the right side of the chassis, and output wires running up the left side (tube gain stages are up front). Measured channel separation was poor as a result.

I want to use the same wiring paths, but if I separate the wiring into L/R paths I will be forced to run the input and output wires close to each other. Capacitive coupling at higher frequencies would theoretically cause some form of high frequency global feedback, I think.

What issues might arise?

Thanks for any tips!
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Old 29th September 2012, 07:16 AM   #2
Mooly is online now Mooly  United Kingdom
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Interesting question. Shielded cable should be pretty immune to being near the output wiring.

One often overlooked problem is that of unequal crosstalk between channels which can cause image imbalance. That really is something to try and get right but you need to be able to do measurements to see what is actually happening.

Crosstalk per se isn't necessarily a problem. Remember vinyl is only around 20 to 30db and I would imagine most amps improve on that by orders of magnitude.
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Old 29th September 2012, 09:25 AM   #3
DF96 is offline DF96  England
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L/R crosstalk is a lot less harmful than in/out crosstalk!

Use screened cable for the input, or twisted pairs with a bit of distance between them.

Forgot to mention: if the output impedance of the source is sufficiently low then capacitive L/R crosstalk will be small anyway. Could the crosstalk actually be coming from grounding rather than stray capacitance?

Last edited by DF96; 29th September 2012 at 09:27 AM. Reason: mention source impedance etc.
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Old 29th September 2012, 11:04 AM   #4
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Surely the easiest thing to do is just replace the unscreened wire with screened?
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Old 29th September 2012, 01:22 PM   #5
AndrewT is offline AndrewT  Scotland
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I find that cross channel talk to be a bigger problem.
It's when a selected source goes very quiet and a non selected source interferes with the wanted sound.
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Old 29th September 2012, 02:19 PM   #6
awedio is offline awedio  United States
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Thanks for the tips so far, everyone. I am aware that there are workarounds like using shielded wire, etc.

But, like many of us here, I am on a quest for knowledge! I really got curious about the effects of capacitive coupling between input and output leads and want to find out more about the possible results.

It seems like adding a frequency-dependent feedback loop to the system, low in level to be sure but rising with freq...?
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Old 2nd October 2012, 07:35 AM   #7
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Quote:
Originally Posted by awedio View Post
.....I am on a quest for knowledge! I really got curious about the effects of capacitive coupling between input and output leads and want to find out more about the possible results.....It seems like adding a frequency-dependent feedback loop to the system, low in level to be sure but rising with freq...?
"Motor boating" is one spectacular effect of LF in/out coupling, HF oscillation reflects strong capacitive coupling, but otherwise you have tone filters or controls (which is basically what frequency dependent coupling around an amplifier implies).

The strength of boost or cut depends on phase difference too but you won't get a lot of useful benefit without properly designed circuits.

Did you have something else in mind?
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Old 2nd October 2012, 11:01 AM   #8
DF96 is offline DF96  England
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In/out capacitive coupling is likely to lead to treble lift or cut, perhaps combined with ringing or oscillation. Best avoided, unless wanted and therefore designed in.
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