[sdiy] Signals leaking into the PSU?

[brianw]

On Feb 19, 2023, at 12:10 PM, Mike Bryant <mbryant at futurehorizons.com> wrote:
> Sorry Brian, and it may just be the way I've read what you've written, but that's not quite true.

As I suggested: Read Bill Whitlock's papers. He says it better than I can in an email.

> There are numerous papers, often with opposing views, but the ones I read as being more interested in safety, as opposed to a best audio performance, are here : 
> https://www.ranecommercial.com/legacy/note110.html
> https://www.ranecommercial.com/legacy/note151.html
> and then expanded in some of their other papers.

Those papers point out the difference between chassis ground and signal ground. The optimum wiring diagrams show three wires for cables: signal, signal ground, and shield. Shield is yet another consideration that is distinct from safety ground.

Whitlock points out that airplanes have no connection to Earth, and still have properly grounded audio (and other) signals. This is his way of diverting attention from the wall socket.

I'll also point out that some folks with deep wallets run balanced power, with 60 VAC on each of the two wall socket blades (separate from safety ground on the third prong), opposite in polarity to produce 120 VAC in the power supply transformer. In these studios, nothing has signal ground coming from the wall socket.

Basically, if you review all of the papers, none of them suggest using the wall socket *directly* as a signal ground reference.

In all fairness, Peter never said which of the three wires at the wall socket were ground. I'm just pointing out that none of them are signal ground.

Brian

p.s. Behringer probably gets around safety regulations by moving the 120 VAC conversion to a wall wart, where they can just use a third-party product that's already approved. Nothing but low voltage enters the product, and thus the product doesn't need a metal chassis or safety ground.

> One key sentence is "First let's examine why they must be tied together. We'll cover where and how in a moment." along with numerous references to "grounded".  
> 
> Also in the UK, and some other countries, the wiring regulations require a mains ground connected to the frame of any 19" rack mount as it's currently classified as IT equipment.  Yes I know Behringer and others are selling their racks with a 2-pin class II PSU and this is in the queue for discussion in the relevant sub-committee when time permits as obviously the regulations may require change, but currently the official line is the frame should always be grounded (in the UK).
> 
> That's not to say how mains and audio grounds are connected, it's the big question mark in the Rane paper and they cover various scenarios.  
> 
> There is also the case that the most popular PSUs used in DIY synths is class I and so mains ground has to be tied to signal ground.  Lifting that ground is illegal in most countries.
> 
> But as you say, how things are grounded must always be considered properly from the beginning, not tacked on at the end.
> 
> From: brianw
> Sent: 19 February 2023 19:32
>  
> As a public service announcement, I would like to stress that the "wall socket ground" is for safety. It should never affect the signal noise, and circuit designers should never create anything that ties signal to the wall socket ground. I'm talking about the third pin on three-prong wall sockets.
> 
> I looked at the page in the provided link, which is a huge text, and found that it references Bill Whitlock and his papers on signals, grounding, and safety. I stress that folks should read Bill Whitlock's papers before designing anything that connects to wall socket ground.
> 
> Brian Willoughby
> 
> 
> On Feb 19, 2023, at 8:45 AM, Peter Knoot wrote:
> > This is actually quite a common issue.  The advice given so far is sound; bypass capacitors are certainly a must, but not sufficient solution.  A comprehensive approach must be taken for any Eurorack case (which tends to be far noisier than standalone equipment) to become noise-free.
> >  
> > Proper filtering requires a good, low impedance ground path.  This includes the module circuitry (proper PCB layout practices), power distribution bus boards, power supply and of course the wall socket ground!  Many folks don't even consider the latter.
> >  
> > Here's a great white paper by Genus Modu to give you some idea:
> >  
> > http://www.genusmodu.com/products/libb-wp-01.html
> >