Mac Mini Power Supply

"I have always found switchmode supplies to be VERY noisy and mechanical sounding."

I don't question your findings but I hope you realize that this voltage (is it 18V?) goes inside of Mac Mini thru internal switching regulators to deliver voltages for micro and rest of digital circuitry.

Linear powers supply is also a switcher operating at 120Hz. It takes current from mains in very narrow current spikes of big amplitude. Width of the spikes depends on the load.

Switchers got bad rap from cheap computer power supplies but realized properly are not only very quiet but also line and load regulated (class D is switching mode supply). High switching frequency is not audible and easy to filter out. For that reason Jeff Rowland uses switchers in Capri preamp.

Where would one find plans to build this psu and a parts list? I'm intrigued by this idea. Thanks Mintzar.

You are correct, Kijanki.

Switchmode supplies in many products on the market DO NOT have their switching frequency above the audible spectrum, however. Mac Mini included.

My amplifier is a Class D amplifier with a Linear PSU. The switching aspect is well above the audible spectrum. EVERY switchmode supply I have heard was noisy compared to a linear supply. Companies like Bel Canto, W4S, Nuforce, etc all use switching supplies and are incredibly noisy and dry sounding. Granted, most companies don't put expensive power supplies in their gear, it's just too expensive to manufacture for a large company.

The power coming from the wall is the power used to create every signal in your audio system. Better in = better out. And if you build a better supply for your source you start with better. If you start with an amplifier, but have a crappy source the signal can only be as good as the source.

Switchmode supplies with a switching frequency in the audible spectrum have a moment where zero current flows, the lower the switching frequency the more noticeable those periods of off time become. 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. The off periods are why switchmode supplies sound noisy and often mechanical.

As for the Mac Mini PSU. There are no plans or parts lists. This is a design that I did the research for and constructed myself. The only tidbit of "plans" are the pinout for the iSense DC connector for the Mini. What I can tell you is that you need to build an 18v5 PSU capable of outputting 6A. What I do is build a 22V PSU and regulate it down to 18v5. That means the regulator, resistors, capacitors, and diodes all have to be able to handle 6A of current (preferably something like 10-12A MINIMUM for safety).

Send me an email and I can potentially get you started. It took some trial and error.

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.