AW: To earth, or not to earth...

[Martin Czech]

> If I would do it again, I'd use 1/4" *symmetrical* connections, and
> nothing else.

This is very interesting, since I'm right now before this step of
designing the first pcb and front panel. After that I can't change any
more. I'd like to discuss this further. For this, we need some information.

To all you modular owners out there, please respond:  

-What kind of ugly effects do you find  when patching your modular? 

-Please declare if you belong to the "banana" or the unsymmetrical
shielded party or whatever. 

-And the ground scheme of your modules. Ground bar, or star like?

-Is the cable shielding interrupted at one side? 

-How many modules do you have?

-How long is your longest patchcord?

-Are you very sensible to noise or is this not very important for you? 



Some arguments for going symmetrical:

PRO:

-Unshielded cable act as antenna, they pick up all kind of hf dirt.
Even if you live in a "clean" environment you should keep in mind that
we are all getting more and more "digital". I.e. computers and their
crt's, hard disk recording systems, all kind of sampling systems, like
reverbs, effekts etc. etc. The frequency of those devices can be quite
low, about 20-100kHz, and they will sure find a way to "mix" down to
audio.  This applys also for "control" inputs. hf into those inputs may
also cause noise in the audio path. I had to notice this recently, a
nasty squeeking creeping into my multiplier and LXP1 reverb. No way to
get rid of it. The squeeking could be modulated by programm and mouse
activity. Hey, I really can record with 93 dB SNR now, but at the same
time I have to fight this hf stuff (which can be up to -50 dB, 0 dB
beeing maximum amplitude for signals, it's digital).  This is shit,
really. That's why I'm going for symmetry for all my inter-device
connections at least.

-Shielded unsymmetrical cable can still pick up currents by induction.
The only way to cancel this is symmetrical connections, the shield tied
to metal housing at BOTH ends, and both housings properly grounded.
My observation, too.

-Just by changing the cable wiring (or phase switch) you would get 180
DEG phase inversion, for (allmost) nothing.

-It is not recommended to open the shield at one side of the cable,
since this arrangement would act as antenna.  So ground loops seem
unavoidable with shielded cable. My observation, too.

-Well, the extra cost for the 1/4" stereo plugs (instead of mono) and
sockets and patchcords and also the additional output stage are
neglegible.

-You could even "upgrade" older systems with a little pcb, and changing
mono sockets to stereo. There is still hope for you J.H. ! A high end
circuit was proposed in some elektor "audio" collection. I have it
somewhere.

-Doing it all symmetrical is good practise in all studios and radio
stations. They know why.

CON:

-As you don't know what is control and what is audio signal before
actually patching, this (symmetry) would also apply for "control"
inputs and outputs.

-What would you use as input diff amp? And what about offset problems
at this stage? One solution could be a ssm2017, but I've only seen ac
app notes so far.

-I have now finally found coloured patch cables, but they are of course
unsymmetric. Are there symmetric cables others then this thick
microphone stuff?

-This symmetry stuff is all paranoia (;->).

m.c.