PSUs: What I Always Wanted to Know, But Was Afraid to Ask

For a quick simple answer the power supplies in any electronic circuit are another input to that circuit, but these inputs are an error signal we need to minimise.

Since we actually want no AC signal appearing here, a good PSU will effectively reduce AC content (noise, current modulations etc.) and allow the circuit to behave as designed.

Any signals that do appear on the PSU rails are, in effect, errors that can affect the circuit's operation and allow it to produce something slightly different from what is intended (e.g. audio signal + some noise).

A better PSU also reduces interaction between stages, again having the same effect. This is often enahanced by having additional regulators to feed different circuits (e.g. XPS, Supercap).

One can reduce susceptibility in the basic circuit to PSU imperections, but never eliminate it, meaning a better PSU can always provide an upgrade to almost any circuit. Naim's early preamp designs are particularly susceptible having quite poor basic power supply rejection, one of the reasons they can be so transformed by a better PSU.

There is a fundamental limit though to what a PSU in an external box can do. For ultimate results the PSU has to be inside the unit being powered, or at least regulating (measuring) at that point.

Andy.

Thanks Andrew,

for your response. However, indeed, your conclusion kind of puzzles me, since it would appear the better the CDP or preamp gets the more likely it is it needs an external PSU, wouldn;t it?

Herman

A power supply must supply a consistent low noise voltage to the circuit. In the case of the HiCap 2 24v supply are provided. So different parts of the circuit can be provided with independent(ish) supplys (i.e demands by one part of the circuit should not effect the other part of the circuit).

The supercap and XPS have 6 separate supplies. The idea of separating the digital and analogue supplies (as per in the 82/52) is so that any noise in the digital circuits (i.e. switching, lighting etc) is isolated and does not interfere with the analogue circuits that are supplied by a different supply.

The problem with external supplies is that the valotage is measured in the HiCap (for example) but is then passed down a 1 metre snaic that will mean a voltage drop and potential noise in introduced. This can be reduced by doing further voltage regulation on board (as per the CDX/CDS). However the best supply would always be to eliminate the snaic. BUT of course if the supply is inboard you have all the other problems of magnetic fields etc that the external supply reduce significantly. It's a balancing act that Naim have a very good solution to that works well. Other manufactures take dirrerent approaches.

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At the end of your post you say there are limitations as to waht an external PSU can do, and that the ultimate is for the PSU to be inside the pre-amp - can you enlighten me further as to why this is?

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Simply because output impedance is degraded significantly by the SNAIC cable, which has finite impedance and hence introduces voltage variations (i.e. noise) at the equipment, directly correlated to current flows within the cable.

This can be overcome by a technique called remote sensing, which would require two more wires in the SNAIC, to allow the external PSU to measure and regulate at the load.

LM317's can't do this to both rails though

A.

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However, indeed, your conclusion kind of puzzles me, since it would appear the better the CDP or preamp gets the more likely it is it needs an external PSU, wouldn;t it?

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Not sure how you came to thisd conclusion, but it would be more accurate to say "...the more likely it is it needs a better PSU.

There will always, I'm sure be benefits to having the transformer etc in a seperate box, but the regulators are better close to the circuit they are powering, i.e. in the CDP / preamp etc.

A.

Thanks Greeny, Andrew.

Why I came to the conclusion better CDPs need external PSUs? Simple observation: the CD5 is a stand-alone machine with optional Flatcap, the CDSII needs the XPS.

Anyone interested in saying a little more about what the digital and analogue circuits do in a cdplayer?

Herman