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Old 8th June 2007, 08:10 AM   #1
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Default Stray capacitance in audio circuit ?

Hello

I read few time in the forum that there is stray capacitance in audio circuit.

I was assuming that you have that problem mostly in RF circuit.

Wen I do a pc board I leave large space between lines, is it enough to avoid stray capacitance ?

Thank

Gaetan
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Old 8th June 2007, 08:32 AM   #2
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All your assumtions are correct
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Old 8th June 2007, 08:46 AM   #3
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Hi,
using a ground or power plane on double sided PCB will ADD to the stray capacitance.
Experts know how to avoid or use to advantage this problem. eg. a single trace above a ground plane has a 110ohm characteristic impedance at high frequencies when correctly proportioned.
Amateurs, like us, wade in without our galloshes (wellingtons) on.
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Old 8th June 2007, 06:40 PM   #4
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Stray capacitance can effect just about any kind of circuit.
Moire as frequency increases, so, it's more a problem for RF circuits and fast computer circuits.
But many audio circuits work at rf frequencies as far as feedmack and phase margin issues.
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Old 8th June 2007, 06:56 PM   #5
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Default Re: Stray capacitance in audio circuit ?

Quote:
Originally posted by gaetan8888
Hello

I read few time in the forum that there is stray capacitance in audio circuit.

I was assuming that you have that problem mostly in RF circuit.

Wen I do a pc board I leave large space between lines, is it enough to avoid stray capacitance ?

Thank

Gaetan

One square cm = 3 pF so it's very seldom this has a really big influence. A trace over a groundplane has a fraction of a pF. Did you have any special circuit in mind?
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Old 8th June 2007, 07:28 PM   #6
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It does depend on the circuit though. If you are working at high gain (even at 'low' frequencies), or using FET-input opamps then a handful of pF stray at the inputs of opamps can lead to unwanted interactions, even oscillation.

You can even get oscillation in unity-gain buffers (ie non-inverting pamp with output linked to -ve input) if, for some reason, you use a resistor instead of a 'xero ohm link/track) if that resistance, and that tiny stray, forms enough phase-shift at HF. This is really easy to achieve if you are also using video opamps or other high-bandwidth parts because diy audio lore 'demands' it

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Old 8th June 2007, 07:31 PM   #7
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Quote:
Originally posted by martin clark
Does dpened on the circut though. If you are working at high gain , even if low frequency, or using FEP-input opams then a handful of pF stray at the input sof opamps can lead to unwanted interactions, even oscillation.
I'd say in low noise high gain designs with low signal impdenaces, this is not a big problem if the pcb is good.
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Old 8th June 2007, 07:35 PM   #8
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I agree completely, it's a long way down the list of things worth worrying about.

But it's not always easy to convey what 'good' means. A few 10s of pF stray shouldn't be a problem in a preamp; but it could be , very easily, in a badly-thought-out line stage, or a DIY MC phono stage
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Old 10th June 2007, 05:38 AM   #9
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Default Stray capacitance in audio circuit

In reading this thread I saw mention of distributed vs lumped element representations for audio interconnect. At audio frequencies, over the distances encountered in home audio equipment, it is safe to say that a lumped element representation is good enough. Distributed (transmission line) modeling becomes necessary when the propagation delay through a channel becomes a significant percentage of the reciprocal of the highest frequency. Even at 20 KHz, this does not occur until the interconnect reaches lengths of ~1000 feet.

There was also mention that use of a ground plane increases stray capacitance. This statement partially true. Adding a ground plane may increase the capacitance between a signal path and ground. There may be instances where one wishes to maintain a very high imput impedance, for example, where this additional capacitance poses a problem.

However, a ground plane also serves to decrease the effective coupling between two signal traces, and it this coupling that is a potential problem in audio circuits. It is fairly easy to see how a ground plane works. Consider a pair of signal traces with a per unit length coupling capacitance of C1. Now consider the same pair of lines with a ground plane. There will be a per unit length capacitance between each trace and ground of C2. Typically C2 >> C1, so the addition of the ground plane introduces a capacitive voltage divider between the two signal traces.
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