Mac Mini Power Supply

Mintzar - I don't know of a single switching mode power supply that operates in audible band. The lowest I know to operate is about 50kHz - non audible. Modern switching power supplies are switching at zero voltage/zero current while linear power supply switches at max voltage.

As I understand it you need about 6A for this supply while LM338 is rated only 5A (continuous). Linear regulators are noisy, require huge transformers are most often unregulated. They are also not as easy to design as it seems. Older regulators like LM338 are sensitive to output capacitors' ESR - might start oscillating when ESR is too high but also oscillate when it is too low.

"Ideally you would want the switching frequency to be so fast that it would be impossible to tell when it turned off and back on." - Only for efficiency sake. Fast switching brings more noise pollution.

As for designing with LM338 - I did it over 20 years ago and would not do it again. LM338 is really old stuff. I requires almost 3V input output differential making it total of 8.5V drop across regulator if you provide for +10/-20% line variation (3V +18.5Vx0.3). This means 8.5V x 6A = 51W (big heatsink). In addition LM338 has very poor ripple rejection equal 0dB (none) at 100kHz. You can do much better with modern regulators. Transformer is also a problem because 110 W supply requires almost twice rating because of the nature of load. Current spikes from bridge/capacitors have much higher RMS value than average value heating copper (windings) while high frequency content is heating the core. If you need to deliver 110W with 50W lost in regulator I would pick at least 250W transformer.

I use a 300W transformer. The LM338 can handle up to 12A. Since the Mini uses 6A MAX it works just fine. The Mini actually idles around 2-4A 99% of the time.

I am not using the LM338 as a standalone regulator. My next project will be to use series regulators to get more current output. But that will be a VERY serious PSU.

At any rate, the ESR of the Muse caps is perfectly fine for the LM338. The PSU has been running perfectly fine for the past four days. If it was going to fail it would have done so already. The first power supply I built failed within five minutes.

Old doesn't mean bad. We are still using the same math developed by western electric YEARS ago for everything audio. Many vintage pieces of gear STILL sound better than modern gear.

Is there better? Of course there is. There's always better.

However, I wouldn't shoot down an option without giving a better alternative.

Congrats on building your power supply. I for one am convinced that proper power supplies can make a huge difference. For those not mechanically or technically inclined, Bolder Cable makes a nice ps for the mini. It's a bit pricey ($625.00).

I have the ECM and I would say it rolls off the highs. It makes digital sound less harsh and biting. With the ECM the highs sound more organic/analog. Can anyone explain how this is done with the transformers in the unit?

I don't think it really is a power supply.

I do have an adapter hooked up the the power supply of the Mimi that allowes me to use a high end power cable. Needless to say the PC does makes a huge difference from the stock cable.

What kind of $$$ are we into with such a power supply?

I borrowed the boulder supply from a friend of mine, one of the reasons I wanted to build one myself. My supply had a much larger impact on the sound, but both are very nice pieces of gear. I was offering to build my supply to a few people for $400.

As I understand it, the ECM uses Balanced power transformers. That means rather than having 120v on the hot conductor and 0 volts to ground on the negative, you have +60 and -60, which are out of phase. In theory that cancels out common mode noise in the same way balanced analog circuitry works.

While I can't say for certain, almost ALL power conditioners that boast a "more analog" sound are using suppression capacitors and ferrite cores. Ferrite tends to work fairly well on digital gear for rolling off what people consider to be digititis. Ultimately it's a cheap trick, but has poor tradeoffs. I'm yet to find a power conditioner that can hold it's own against a Topaz Isolation Transformer with 146dB common mode noise rejection.

People may say a better mac mini power supply makes the sound more analog-sounding. I personally just think that the power supply, and more accurately the non-linearity and ripple caused by crappy power supplies is what causes a piece of gear to sound digital.

Analog is just digital that's been pressed to LP.