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Mixing linear and switching PSUs
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Mixing linear and switching PSUs
pix
Is there an advantage in getting a linear PSU for the analog modules in the signal path, and keep all other modules in a separate switching PSU?

Since they’ll ultimately share the same ground is there an advantage in this noise wise?
ersatzplanet
The optimum way is the way used in the Doepfer PS3 supplies - a switcher followed by a linear regulator. The power efficiently made by the switcher and the filtering out of the switcher's oscillator done by the linear regulators is the best combo. No large heat inducing transformers and the ability to work on varying wall voltages is also a big plus. Doepfer has made the decision that most +5 requirements are in the digital parts of modules so has the +5V as a straight switcher output, and that is good thinking too.
Graham Hinton
pix wrote:
Is there an advantage in getting a linear PSU for the analog modules in the signal path, and keep all other modules in a separate switching PSU?

Since they’ll ultimately share the same ground is there an advantage in this noise wise?


The amount of noise is not determined by the PSU type, but by the type of loads, the current drawn, and the impedance of the common 0V wiring.

Digital circuits are more tolerant of power supply ripple, but they can put a lot of switching noise in to the 0V, particularly if they have a lot of modules with LED displays. Ideally the 5V PSU should be isolated, its return should be a separate path from the PSU to the distribution and the distribution 0V a very low resistance, less than 1 milliohm. This is not the case in most pcb based distribution systems, especially with multiple cards and is only achieved with a single busbar right across the system.
pix
got it, thanks! That was my question, whether there would be potential problems caused by the common 0V. Also I think all digital modules I have have internal regulators and use the same -/+12V.

I'm definitely getting a higher noise floor now that my system is increasing in size. And it sounds like digital noise for sure. So I was wondering it it would be worth it to have a dedicated linear PSU just for my analog "audio path" modules.

But ultimately the 0V has to be shared right? Even with different PSUs the ground path of the patch cables will eventually do it. Is this right?
Graham Hinton
pix wrote:
That was my question, whether there would be potential problems caused by the common 0V.


You have to have a common 0V. The resistance and layout are the issue.

Quote:

Also I think all digital modules I have have internal regulators and use the same -/+12V.


It does not matter whether digital modules use +5V or +12V for their internal power, the current still returns via the 0V. Analogue modules tend to be about even on the +/-12V rails so they don't put much in to the 0V. They put some noise in though, every comparator switching in every VCO contributes something, but nothing like, say, a 7 segment display switching between digits or a DSP chundering away.

Think of each module as a black box and look at its power spec. The difference between the positive currents and the negative current is going to the 0V and this has both a DC and an AC component.

Quote:

I'm definitely getting a higher noise floor now that my system is increasing in size. And it sounds like digital noise for sure. So I was wondering it it would be worth it to have a dedicated linear PSU just for my analog "audio path" modules.


It will help, but it depends more on if those modules are particularly sensitive.
Interference is transmitted in the power rails by common impedance coupling which occurs mainly in the cables between the PSU and the distribution. If modules are on different PSUs they can't couple this way.

Quote:

But ultimately the 0V has to be shared right? Even with different PSUs the ground path of the patch cables will eventually do it. Is this right?


Yes and no, respectively. An unbalanced system has to have a common 0V however large it is, this includes other equipment that is connected to it unbalanced, like mixers and FX.
When you patch two modules you are adding a resistance in parallel with the path between those modules through the 0V. 3.5mm patch cables have quite a high resistance screen, something like 100 milliohms per metre. If the 0V distribution is of the same order, which it will be if it is pcbs and ribbon cables, then every patch will change the 0V. It the 0V resistance is much lower then the cables have a much smaller effect.

Of course, cable screens should not be carrying signal currents (because what screens the screens?). If modules were designed to correct audio practise by connecting screens to a grounded equipotential chassis, then all the signal return currents go through the 0V.
The irony is that the more you spend on a modular synthesizer the worse it gets and you don't find out until you have spent a lot and thus have a reluctance to change. This is due to most of them being based on dubious and outmoded design practises. If you want to go large look at how large studios are built: separately powered units and balanced interconnections. None of this should be a surprise, the principles were well established before the first Moog, and can't be ignored.
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