[sdiy] keeping signals seperate..avoiding noise

[Laurie Biddulph]

The problem with this method is that it assumes that you will get a `zero 
ohm' contact between each panel and the module rail. In reality there will 
be layers of dirt and oxidation on the rail surface, the panel surface, the 
fixing screw surface and no guarantee that inserting the screw will provide 
good contact between the module and the panel.

Grounding each panel through its attached pcb means that the panel will be 
at true ground potential (assuming no losses etc through the boards and 
power cables) and that all panels will be at the same potential. This means, 
ideally, that the panel would be grounded through either a dedicated ground 
power rail (PANEL GND = PGND) or as close to the power connector on the 
attached board. I believe Oakley modules do the PGND approach although they 
also assume that you do not have a 100% MOTM compatible module in the system 
(MOTM do not split the grounds on the power connector so can't offer a true 
PGND connection). Oakley assumes the 2nd option where he gnd is made at the 
power connector (I think).

In my Panther modules there is a ground strap provided on the pcb that is 
used to ground the associated panel. I don't have PGND (not an option in 
Doepfer compatible systems) and make my panel connection as close to the 
front edge of the pcb as possible for manufacturing convenience.

Best Regards

(Mr) Laurie Biddulph
Phone: +61 (0)2 4340 0938
Mobile: 0400 257 645

Elby Designs
ABN: 70 022 727 605
http://www.elby-designs.com

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----- Original Message ----- 
From: "Simon Brouwer" <simon.oo.o at xs4all.nl>
To: "sdiy" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, November 16, 2008 8:57 AM
Subject: Re: [sdiy] keeping signals seperate..avoiding noise


> Seb Francis schreef:
>> Justin Owen wrote:
>>> -----Original Message-----
>>> From: Laurie Biddulph [elby_designs at ozemail.com.au]
>>> Subject: Re: [sdiy] keeping signals seperate..avoiding noise
>>>
>>>
>>>> As a matter of good construction, the power supplies for each board 
>>>> should be run direct to the main power supply in
>>>> what is sometimes referred to as `star' fashion. Daisy-chaining from 
>>>> one board to the next will allow noise induced on
>>>> to the lines from one board to be radiated to the others.
>>>>
>>>
>>> Would the same apply to pots wired as volume controls - i.e. where one 
>>> pin goes to Earth/Ground?
>>>
>>
>> No, it only applies when you are drawing current, especially current 
>> spikes, as then the resistance and inductance of the power line causes 
>> the voltage along it to fluctuate.  It different circuits share the same 
>> power line, the spikes will then couple between them.
>>
>> For panel controls it may be better not to use star-earth if they are 
>> also earth-connected through a metal panel as you could get ground loops 
>> (=hum).  When using shielded cable it is normal to connect the earth at 1 
>> end only to avoid ground loops.
> IMO in synthesizer modules the ground of a module should never be 
> connected to its metal panel. The panel should be isolated from the 
> module. The panels should be grounded by connecting one point of the 
> mounting rails to the star point.
>
> -- 
> Vriendelijke groet, Simon Brouwer.
> | http://nl.openoffice.org | http://www.opentaal.org |
>
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