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I recently took apart a Mackie 220W power supply for an analog sound board. Much to my surprise I found the following scheme:

Earth grounded: 48V -18V 18V

Seperate, isolated ground: 12V 5V

I am not sure if this has something to do with it being a supply for a sound board, but why would there be 2 isolated grounds?

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I'm wondering if the extra is for phantom power to the mics – Scott Seidman Mar 2 '14 at 15:57
@ScottSeidman That would be the 48V rail. – Matt Young Mar 2 '14 at 20:21
What is with the close votes? The question here is perfectly clear. – Matt Young Mar 2 '14 at 20:23
@MattYoung So far, I do like your answer the best. You were the only one to actually show an understanding of analog sound boards. I am not sure what the SE etiquette is, but I will probably wait a day or two to select an answer. – Napthali Mar 2 '14 at 21:50
up vote 2 down vote accepted

First we need to realize that this power supply for a large format mixing console, probably with about 40 channels, and 16 mix buses. As a result, there is a need for multiple supply rails. This breakdown may not be exact for the console that goes with the power supply, but in general:

  • 48V is always the phantom power bus
  • +/- 18V is for all the audio circuitry, and contains a +24dBu signal with 1.2V headroom at each rail.
  • 12V powers any cooling fans and goose neck lights
  • 5V powers any digital logic for mute groups, scene memory, etc.

The +/-18V rails need to be earth referenced by convention, otherwise ground loops will be a problem when the console is interfaced with other equipment downstream. The 48V bus also interfaces directly with audio circuitry, so must be referenced to the same ground. The 12V and 5V buses are powering electrically noisy things. To keep that noise out of the audio signals, they need to be kept isolated from the audio ground.

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It really depends on the power rating, application of device and noise immunity requirements for your ICs in your device. Sometimes, designers like to have separate grounds for digital and analog signals or power and the small-voltage signals to keep the controls and data signals away from the Ground noise errors. This is also implemented on the PCBs as well. i.e No designers would like to ground 10 A power signal and 100 MHz signals sharing the same plane. It may or may not help, but definitely a safe side for the design.

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Are you sure that tere is no any DSP in your device? Usually they are supposed to be there to provide noise immunity. Actually the digital part of any circuit is very much noisy due to switching. To avoide couple digital noise onto sensitive analog signals it is separated from analog part by using separate GND. It is common not only in PCB to have two separate ground planes, but also inside the ADC/DAC or DSP chips. You will see in schematic diagrams multiple ground symbols, that's usually a good place to start looking for the source of the problems.

EDIT If it is all analogue, may be the separate power used to supply the independend protections circuitry.

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This is an older unit; it's all analog. – Napthali Mar 2 '14 at 5:07
hmmm, i guess you're right, but just out of curiosity, isn't the digital switching at too high frequencies to disturb an audio system? could you expand a little more on that – Camilo Mar 2 '14 at 5:09
@ Camilo Bermúdez A medium-speed of a 12-bit successive-approximation (SAR) ADC may operate from a 10-MHz internal clock, while the sampling rate is only 500 kSPS. Even Sigma-delta (Σ-Δ) ADCs also require high-speed clocks because of their high oversampling ratios. 5-MHz or higher-frequency clocks needs for a resolutions of 24 bits – GR Tech Mar 2 '14 at 5:20
@GRTech thanks! I meant analog systems though. – Camilo Mar 2 '14 at 5:35

It's hard to tell without knowing what the exact specifications of the sound board are, however, one of the main reasons to have isolated grounds is to guarantee precise and stable power supply to a part of the system that might need it.

Providing a very stable and precise power supply can be quite expensive and at high voltage or current conditions it gets harder and trickier and more expensive, and most times all that precision and stability are only necessary in very specific parts of the system (usually at low voltage and current conditions), and not in the high power ones, under those circumstances it makes sense to have isolated grounds (a very stable one and a not-that-stable one) since a shared ground can carry all sorts of undesirable noise and mess up the stable supply.

The perfect example is a digital power amplifier, where you don't want your fragile digital circuits or pre-amplifiers to get disturbed by the wild transients (and noise in general) that external sources and the power amplifier might induce in its own power supply.

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