[sdiy] Powering a second bit of kit from same PSU(wiringoptions)

[Richie Burnett]

I'm not familiar with the bit of kit, so I would power them from separate 
power supplies to be on the safe side.

-Richie,

-----Original Message----- 
From: random variate
Sent: Monday, February 23, 2015 11:02 PM
To: Richie Burnett ; Tom Wiltshire
Cc: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Powering a second bit of kit from same 
PSU(wiringoptions)


Richie, Tom and others - thanks foe your input. Very much appreciated.

So, I have a Synth Driver from SIGT, which is based on the IP that they 
bought from a company called DCE Inc. My second Synth Driver is one of these 
DCE units.

What I am gratefully hearing, is that I should check if the negative rail of 
either Sybth Driver unit interacts with any Audio Ground.

I am pretty sure that I wired up the PSU cable such that negative rail was 
completely independent of audio ground, and any Audio ground was connected 
to earth via the third cable we have in UK 3pin power cabling.

From: Richie Burnett
Sent: ‎18/‎02/‎2015 21:06
To: Tom Wiltshire
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Powering a second bit of kit from same 
PSU(wiringoptions)

Hi Tom,

> From: Tom Wiltshire
> Subject: Re: [sdiy] Powering a second bit of kit from same PSU(wiring
> options)
>
> Can you explain a bit more about this ground loop situation, Richie?
> I'm not seeing what's going on, and I don't want to be encouraging someone
> to try something if it's genuinely risky.

Taking the TB-303 as an example, in a momentary stroke of insanity Kikumoto
put the voltage regulator (pass transistor Q45) for the 5V logic in the
negative side of the supply from the external power connector, instead of
the more common (and more sensible) positive rail.  See top right corner of
schematic here:

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/rolandtb303.gif

This means that the negative terminal of the power jack and the sleeve of
the audio output jack are not at the same potential!  And this is the
dangerous part, if the same power supply is also used to power a piece of
equipment that does have it's negative power rail connected to the ground of
its audio jack, this will effectively *bypass* the voltage regulator inside
the TB-303 when their audio outputs are connected to the inputs of a mixer
for example.  As you can imagine the 5V logic chips don't like 9 to 12V
across them.

The same "trick" appears here in the TR-606:

http://machines.hyperreal.org/manufacturers/Roland/TR-606/schematics/roland.TR-606.schem-6.gif

And I've seen guitar pedals that generate a split-rail supply from a
single-rail DC supply using a virtual ground at half the DC supply voltage
for op-amps.

My point was, don't assume that the supply negative rail and the audio
ground are at the same potential.  They could be at different potentials for
different pieces of equipment, and trying to use one power supply to power
lots of things in parallel could attempt to force "supply negative" and
"audio ground" to be at the same potential for all pieces of equipment.
Possibly with disastrous consequences.  I'm sure anyone who blew up the CPU
in their TB-303 by trying to power it from the same adapter as a distortion
pedal, really wishes they'd just shelled out for another wall wart :-(

In an ideal world, there would be no electrical connection between power
jacks and audio jacks, and they would be galvanically isolated from each
other, like MIDI ports.  This would eliminate ground loops, USB power
"whine", and regulation issues like the one we've just been discussing.  But
DC-to-DC converters are expensive so designers compromise.  Kikumoto made
many other compromises! ;-)

-Richie,

_______________________________________________
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl
http://dropmix.xs4all.nl/mailman/listinfo/synth-diy


No virus found in this message.
Checked by AVG - www.avg.com
Version: 2014.0.4800 / Virus Database: 4257/9168 - Release Date: 02/23/15