[sdiy] Signals leaking into the PSU?

[Neil Johnson]

Hi,

Ben Bradleywrote:
> Roman Sowa wrote:
> ...
> > BTW, throw away that old LED requiring 330 ohms to light up, and buy new
> > one which shines up brightly at 0.5mA. 30mA square curent jumps in power
> > rails are troublemakers.
>
> This is true, modules should not cause jumps in current pulled from
> the common power supply. Likewise, modules should be immune from
> voltage changes in the (allegedly regulated) common power supply.

Also, the problem with unbalanced signalling in most modular
synthesizers (well, pretty much all of them really) is that the common
ground serves two purposes: a return path for power supply current,
AND as the signal common reference shared by modules.  So when LED
current surges flow in the common ground line and causes small
voltages due to I x R, with nice sharp edges with lots of HF content
that zips right through three-terminal regulators, that gets seen by
every opamp in the system.  And opamp PSRR falls with frequency, so HF
noise on the power rails just goes into the power pins and onto their
outputs.

> A problem with the current multiplier (whether single BJT or op-amp)
> is any current change in the output will still appear instantly on the
> current pulled from the power supply. It may "regulate" the output,
> but at the expense of what's powering it.

Think of power rails as dirty rivers.  Yes, there's clean water there,
but it's mixed in with lots of dirt.  Filtering removes (most of) the
dirt.  But it's not ideal, so anything you can do to minimise pushing
noise out onto the power rails is a good thing.  For example, modules
with DSPs or ARMs should put ferrite beads on the power entry to keep
the HF noise local and not leak out onto the power rails.

> The best way (IMO) to keep constant current pulled from the power
> supply is by using parallel regulators. I haven't used the LM4040 that
> was just mentioned, but I've used quite a few LM431/TL431 for
> regulation.

The correct term is "shunt regulator".

> This also keeps the current returned to the power supply through the
> ground (or the negative supply rail) the same as well. This leeps
> small voltage changes between modules from happening, which would
> otherwise change VCO pitch and such.

In a bipolar system the only current that flows in the ground is the
balance current, which is the difference between the currents in the
positive and negative rails.  Ideally this balance current is 0.
As Roman suggested earlier IIRC, a much better way to drive LEDs is a
constant current chain between the two supply rails, with switches
(electronic or mechanical) across the LEDs to short them out when you
want them off.  This has two distinct advantages:
1) it keeps the current draw constant, so no sharp edges of current on
the power lines;
2) it keeps noise out of the common ground line.

> I of course agree with reducing current jumps by using a more
> efficient LED. It doesn't seem efficient to have an LED powered from a
> 12V source. Eurorack has a 5V supply for such things, but there's
> still a common ground (a non-zero resistance back to the power supply,
> and in common with the other power sources) that can get polluted by
> varying currents.

The Eurorack 5V rail is a bad idea in an audio environment.  It sets
up an imbalance and thus injects noisy digital current into the power
ground line, which in a modular synth is also the signal common.  And
considering how much current modern DSP/ARM-based Eurorack modules
pull then the weak wires on the ribbon won't survive long.  It's not
unusual to see ground currents of several amps in large Eurorack
systems (usually from the positive rail to ground).

There are ways to deal with this.  And many books written on this
subject.  It's not rocket science, but cargo-cultism is rife and
difficult to counteract.  Some years ago I archived this amusing - yet
so true - post (I think from modwiggler):
https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=13

There is a veritable treasure trove of design knowledge in the form of
analogue mixing desk schematics and service manuals.  Hundreds of
them, from Mackie, AHB, Soundcraft, Neve, SSL, Studer, DDA, and so on.
Many of them can be found with a little bit of googling.  And you'll
see how to switch LEDs quietly, how to power opamps for low noise, and
how to distribute power in a large bussed audio system.

Neil