Analogue systems

It sounds like the ground is daisy-chained on the power buss board.
Doing this opens the potential for the noise caused by one module to
bleed down any module to it's right on the rack, and if severe enough,
to every module in the system - even those powered by different PSUs 
 It's especially prevalent with systems with a lot of LEDs unless a
separate return line is run specifically for those grounds (ala
Buchla, which is why he has a dedicated 'noisy' ground buss and
partially why he went with an altogether different patchcord system
for control voltages - it keeps the noise out fo the audio path). 

There are a few things you can try, two of which won't require
modification. Please understand making electrical mods will void your
warranty...so think twice before you do them!):

1. Try mounting the VCOs of the far left of the rack.  This keeps the
daisy chain short as possible

2.  Move the PWM pot on the Doepfer VCOs so they fall silent (full
positive or full negative) then have a listen again to see if it's
still there.  PWM circuits are hard on power supplies, if you don't do
it right they will bleed into the return line which can be picked ff
by other modules.  Dieter's an excellent engineer, I'm sure he handled
this correctly.

3) On the Doepfer VCOs yo will see two 10uf caps somewhere close to
the power connector.  Replace those with a larger value - something
like 33 uf.  Make sure the caps you put in there have 16 volt or
higher voltage rating above or they'll blow about 10 seconds after
power is applied.  You;ll know when they do - it sonds like a
firecracker and leaves paperpulp all over the PCBA and insdie of the
rack.  The module won't suffer any damage if this happens, but it's a
pain to deal with.

4) Are you monitoring from the same module when you're hearing the
noise?  Is the same module going to the speakers?  Have you trie
monitoring form a different module?  If you have one module you use fr
your outputs, you may need to do the same cap replacmen on that one.

5) Another mod which will fix this is running the power to the module
through a lowpass filter BEFORE that power goes the module itself. 
Not so easy retrofit however as you've got to insert this mod in right
at the point the power comes into the board from the buss, before it
goes anywhere int the module.  What you do it calculate the maximum
power consumption of the module and divide that by the voltage (in
this instance, 12).  This gives you the resistor value (this is OHMs
law, btw).  What you'll need to do it cut the line where the power
leaves the connector and insert a series resistor there that's just a
bit under the value you came up with in your calculation.  I
understand this is a pain - but it will fx the problem.

5) Although not at all practical, another thing you may want to try is
running separate wires from the ground terminals on each of the module
power connectors on the rack that's powering the VCOs and run all
those lines directly to the ground tap of the rack's power supply. 
You won't have to cut the ground lines already there.  If they are
daisy chained together, this wil take take of it.

hope this helps???


- P



--- In analogue_systems@yahoogroups.com, "TacocaT" <zpardos@...> wrote:
>
> Hello,
> 
> I just recieved my rs15 case and I put a couple of doepfer style VCOs
> into th case and I can hear one VCO's output (faintly) through the
> other's output (no patches). This shouldn't be happening. Does it
> happen for anyone else? Is there also crosstalk when using the ASys
> power connectors?
> 
> I wonder if the doepfer racks have this problem at all.
>