Explanation of how modules "putting noise on the ground" bus?

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The Real MC
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Re: Explanation of how modules "putting noise on the ground" bus?

Post by The Real MC » Sun Sep 06, 2020 11:40 pm

This is a pretty good resource on Eurorack power distribution that discusses ground noise GenusMod.com

One of the worst sources of digital noise is scanned LED switching - higher currents than most ICs. You have to decouple the supply rail to the LEDs. If that isn't included in the design of a module then a decoupling circuit isn't hard to add. A simple one is to insert a 4.7ohm resistor (wattage of resistor depends on LED supply rail current draw) between LED voltage rail and system voltage rail, then place a 10uf bypass cap on the system side and a 470uf bypass cap on the LED side. There are more sophisticated decoupling circuits, never hurts to start simple.
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Re: Explanation of how modules "putting noise on the ground" bus?

Post by Mungo » Mon Sep 07, 2020 12:33 am

ricko wrote:
Sun Sep 06, 2020 8:46 pm
A related question: one issue that comes up from time to time is where VCOs sync with each other because of signals on the power lines between them, preventing the desired chorus effect of slightly detuned oscillators.

It seems something more likely when you have a pcb with multiple vcos, but also sometimes with seperate vco modules. The usual advice is to make sure there is enough decoupling at the integrator reset and comparator, and to keep 0v lines between the VCOs as distinct and long as possible: a star at the plug.

But what is the best advice for VCO modules, to reduce synch between modules on the same bus/Power supply?
..it depends! that noise could be coming from the common/0v as discussed in this thread or it could be coming from the power rails. Reducing the impedance between the power rails and the common/0v as discussed in the post above helps as they "support" each other but its only ever reductions and never a 100% fix. Another approach used by other manufacturers is to have the regulators as close to the modules as possible, they have low impedance outputs which can keep the oscillators supply from seeing that noise. But all that is getting away from (and potentially making worse!) the noise in the common/0v which appears in all the unbalanced signals, eurorack is an unbalanced format, so you can't escape it.

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Re: Explanation of how modules "putting noise on the ground" bus?

Post by YashN » Mon Sep 07, 2020 3:50 pm

The Real MC wrote:
Sun Sep 06, 2020 11:40 pm
This is a pretty good resource...
... insert a 4.7ohm resistor (wattage of resistor depends on LED supply rail current draw)
Cool info.

I have noticed another thing with LEDs in gear:

It seems everyone is hell-bent on checking the max current in the datasheet and powering them with the highest current possible.

I've done a set of experiments with increasing the resistor to them to check how high I can go and still get a decent visibility on the light, and you can go pretty high.

I'd rather get them the minimum current for them to light up and still be visible.

If possible, modulate the current so that they go right around that low visiblity threshold without turning them off completely (just above threshold = ON, and a little below threshold = pseudo-OFF as current still flows but the visibility isn't there)- should help with less noise too.

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Re: Explanation of how modules "putting noise on the ground" bus?

Post by nigel » Mon Sep 07, 2020 7:53 pm

YashN wrote:
Mon Sep 07, 2020 3:50 pm
I've done a set of experiments with increasing the resistor to them to check how high I can go and still get a decent visibility on the light, and you can go pretty high.

I'd rather get them the minimum current for them to light up and still be visible.
I almost commented on the thread earlier with a similar comment. If you use high-intensity LEDs, you can drop the current below 1mA and they're still bright enough. (Unless you're playing under spotlights or something. And let's face it, I'm never going to be playing under spotlights :) )

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Re: Explanation of how modules "putting noise on the ground" bus?

Post by EATyourGUITAR » Mon Sep 07, 2020 9:15 pm

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Re: Explanation of how modules "putting noise on the ground" bus?

Post by KSS » Mon Sep 07, 2020 9:47 pm

re: LEDs
It's also possible to remove them from the 0v and power instead between the rails. Or as a less effective but still useful practical alternative choice, power them between the more lightly used negative rail and 0v. <--"light"ly used. I'll get my coat.

Along with other balancing acts you might employ to reduce their effect on 0v and rails. like switching between them and a non-lit equivalent current draw.

Graham's brought this up in a few threads and it doesn't seem to be noticed much.
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Re: Explanation of how modules "putting noise on the ground" bus?

Post by KSS » Mon Sep 07, 2020 9:55 pm

EATyourGUITAR wrote:
Mon Sep 07, 2020 9:15 pm
the older I get the more I realize
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Treat utility modules as stars instead of backup singers.

Treat power supplies like Rockstars instead of roadies!

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Re: Explanation of how modules "putting noise on the ground" bus?

Post by YashN » Tue Sep 15, 2020 8:49 am

Would be a shame that this thread is derailed and stopped because of some specific issues with a member, best solved by staying away from the forum and perhaps consulting a professional. Maybe these posts could be removed or moved elsewhere?

Now, to get back on track, I have a sneaking suspicion:

None of the manufacturers implement the LEDs cleanly in their gear, along the whole chain of music production.

That starts with the synths, the sequencers (all those flashing LEDs, all necessary though), the mixers, to the outboard gear, etc...

Am I right?

This bugs me.

We should find a practical way of fixing those pieces of equipment. Separate transformer and power rails for them?

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