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general Q: 0V reference, connecting PSUs & Crosstalk
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author general Q: 0V reference, connecting PSUs & Crosstalk
Noiseconformist
Hi Tony,
on fb you mentioned crosstalk in conjunction with 0V-reference and connecting PSUs via weak connectors:

Quote:

The problem is not so much of maximum current capability but of resistance, which is essentially determined by the available contact area.
The larger the resistance the greater the voltage drop,
and even a few milliohms on the 0V lines can cause noticeable crosstalk between modules.


I'm not an EE and have only a vague idea why that is, so I couldn't explain it.
I can't see, why a few milliohms would cause significant crosstalk, given the relatively low currents flowing.

Could you PLS elaborate?
Thanks a lot, Michael.
Synthbuilder
Hi Michael,

Noiseconformist wrote:
I can't see, why a few milliohms would cause significant crosstalk, given the relatively low currents flowing.

There was a couple of situations that came up in that thread on Facebook. Now that particular quote was to do with the original poster's desire to distribute 0V from a single PSU to multiple locations and cases. The current travelling along 0V connections can be quite large in this instance. There are several reasons for this but where you have one PSU and multiple cases the main reason for this is usually the current imbalance between the negative and positive rails going through that one connection. If the +15V is taking say 1A and the -15V taking 750mA, then 250mA will have to flow through the 0V back to the power supply.

250mA travelling through, say, 10 milliohm would give a voltage drop of 2.5mV. That might not sound a lot but that's a noticeable 3 cents of pitch deviation on a VCO. And 10 milliohm is probably considerably less resistance than the poster's DIN plugs and the lengths of inevitably small diameter wire he'll be using to fit into the tiny DIN plug's housing. The bigger the resistance the greater the voltage drop across that connection.

The other situation mentioned in my posts on that Facebook thread referred to multiple modular cases each with their own PSU. This is less susceptible to 0V errors but they can still occur. The power supply transformers often have a little leakage by which the mains voltage is capacitively coupled into the secondary winding. This can create a small 50Hz/60Hz (and more often its harmonics) current on the 0V connections if the 0V isn't earthed at the PSU which it wouldn't be with a double insulated line lump like the Yamaha PA-30. Thankfully, in a well designed line lump these leakages are really small and can be safely taken away with the audio connections. But they are still there and will still create a small amount of noise on the audio output as soon as you connect your modular up to an earthed item like an amp or mixer.

Return currents from audio signals will also disturb the 0V. These are quite small, but each 10V output signal into 50Kohm input will create a current of 200uA that needs to be returned along the 0V connections. These can add up quickly in a large patch.

The upshot to all of this is to reduce the differences in voltage across the whole 0V connection system. And the best way to do this in an unbalanced audio system is to reduce the resistance within the 0V network. That is, big wires, chunky connections and small distances.

Tony
Noiseconformist
Hi Tony, thanks a lot for that!
Potential issues are clearer now.
Only that I went down the single-psu-road a long time ago.
But will rethink for the new cabinets. hmmm.....

Michael.
Noiseconformist
BTW, this power bus bars are literally massive!

http://www.martinjankoehler.com/synthesizers/5u-cabinets/

(close ups)
Synthbuilder
Noiseconformist wrote:
BTW, this power bus bars are literally massive!

Yeah, the bigger the better. Although, you'll run into the law of diminishing returns once you go even larger than that. I guess it all depends on what modules are in the cabinet and how big it is. Bigger cabinets require careful thought and a very low resistance solution. I've never had a problem in my 10U cabinet with using a couple of Dizzy* PCBs. The key though with using my Dizzies is that you need to make sure they are very close to the power supply, or connected with some very thick cables.

* Dizzy issue 1 was poorly thought out... way too long, both for the shipping and the overall resistance. That was a very long time ago though and I've learnt my lesson.

Tony
Noiseconformist
IMHO this particular cabinet seems saturated in many respects.
I like the "heavy industries" look somehow but it certainly wouldn't suit me for gigging. I wouldn't want to lift it.
And the long Dizzys in my 20U cases are doing fine.

m.
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