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Audio shielding and grounding
trafo Posted: 1 Oct 2020, 07:10 PM
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Reading about your adventures with analog audio circuitry, such as the heaphone amps, and more recently, the rednoise generator (rumbler) instantly reminded me of a couple of old articles, published by audio gear manufacturer Rane (you may already be familiar with them):

Sound System Interconnection
https://www.ranecommercial.com/legacy/note110.html

and more importantly:
Grounding and Shielding Audio Devices
https://www.ranecommercial.com/legacy/note151.html

In which shielding and grounding practices are discussed in detail.

I'm reproducing here an excerpt I especially enjoy from the second article, under the subheading Chassis Ground vs. Signal Ground:

«Let us examine the distinction between chassis and signal ground in audio devices. Chassis ground is generally considered any conductor which is connected to a unit's metal box or chassis. […]

Signal ground is the internal conductor used as the 0 V reference potential for the internal electronics and is sometimes further split into digital and analog ground sections. Further signal ground splits are also possible, though it is important to remember that all "divisions" of signal ground connect together in one place. This is usually called a star grounding scheme.

It is easy to confuse chassis ground and signal ground since they are usually connected together -- either directly or through one of several passive schemes. […] The key to keeping an audio device immune from external noise sources is knowing where and how to connect signal ground to the chassis.

First let's examine why they must be tied together. We'll cover where and how in a moment. There are at least two reasons why one should connect signal ground and chassis ground together in a unit.

One reason is to decrease the effects of coupling electrostatic charge on the chassis and the internal circuitry. External noise sources can induce noise currents and electrostatic charge on a unit's chassis. Noise currents induced into the cable shields also flow through the chassis -- since the shields terminate (or should terminate) on the chassis. Since there is also coupling between the chassis and the internal circuitry, noise on the chassis can couple into the internal audio. This noise coupling can be minimized by connecting the signal ground to the chassis. This allows the entire grounding system to fluctuate with the noise, surprisingly providing a quiet system.
»

Perhaps this accurately describes your experience with shielding as well? Ones reaction, when connecting a simple ground clip to your shield immediately suppresses the noise, is probably a bit of astonishment, no?

Either way I think these articles can be quite interesting for those who are new to audio electronics such as myself, or those who only dabble with this subject occasionally.

Last edit by trafo at 1 Oct 2020, 07:10 PM

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tsprlng Posted: 1 Oct 2020, 09:10 PM
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hapless technoweenie

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Ahh I remember these pages from years ago! They are really good, cheers for the reminder. I wonder if we should be worried they are under "/legacy/" :\

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trafo Posted: 1 Oct 2020, 10:10 PM
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Yeah they were recently moved from rane.com to ranecommercial.com. Luckily they are all backed up by the wayback machine, though it looks like Rane is phasing out the old posts. The glossary is already offline which is too bad as they are referenced quite often in the notes.

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mit Posted: 2 Oct 2020, 03:10 PM
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yeah whatever

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Those pages are new to me, excellent stuff. I shall enjoy reading through them carefully.

One controversial point I used to argue with people about is sticking ferrite beads between analog ground and digital ground on PCBs. I have frequently seen boards designed with ferrite beads separating the ground planes, which in my mind is totally wrong. Both grounds should be at the same potential and have a minimum of resistance between them. The purpose of separating the grounds is to control the return path of the supply current. That should be confined to a small trace under or next to the power supply. The whole point is to minimize the loop area and hence the inductance.

Anyway, you'd be surprised at how aggressively people argue about this stuff.

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trafo Posted: 3 Oct 2020, 12:10 AM
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Wow, those arguments sound exhausting! Should perhaps be possible to determine a suggested solution using some good old science? I would hope so for the sanity of everyone involved.

Anyways, another note from Rane which you might enjoy is number 109, Why Not Wye?. It efficiently criticizes the use of Y-cables to sum two outputs together and provides schematics for how to do it properly. Very simple circuits indeed; just a small case, a few connectors, some wiring, and an assortment of resistors. Could be a nice project for beginners, to build and analyze. I know I'm considering making a few :-)

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