power supply

The PSU consists of

the transformer to get you from 110/220V near the low voltage required
a rectifier to convert AC to DC
and a capacitor to provide some smoothing

A few considerations and findings to these elements:

Transformer

In almost all configurations we used 2x9V 80VA toroidal transformers by Talema or Noratel. We didn't really get to look into any differences of various brands. To check the output voltage we calculate e.g.:

- we want 5V for AVDD
- 5V + the minimum voltage drop for the selected regulator is 3V = 8V
- the smoothing capacitor (the big one we like to use) multiplies the transformer's output (above mentioned large 9V) by √2 –> 9V*√2 = 12V7

but the rectifier will drop it by approx. 3V
12V7 - 3V = 9V7 > 8V ok

80VA seem a lot. I'm sure smaller transformers will do. We stick to "bigger is better" and what we have seen in similar high end products.

Multiple Windings ?

The idea came up if a galvanic separation of each of the power supplies units + an exact Vin for each regulator (thus less heat) would result in audible improvement. That's what one can see here. Therefore a 93 VA custom made transformer was used for the X-5 prototype. It has two 5V5-0-5V5 one 4V2-0-4V2 and one 8-0-8 secondary winding.

Using a Schottky diode, a 3300 uF cap and a single regulator for each section didn't provide an audible improvement. To the contrary the single cap PSU setups worked better. The reason for that is the minor ripple rejection of the the small caps compared to one large 10.000uF Kendeil.

Center Tap

In the beginning we were not aware of the different ways for using the secondary windings. It was mentioned somewhere (maybe here) that a center tap configuration would "relax" the transformer. Also most Naim Audio PSUs work like that. So we stick to a full-wave center tap layout with shottky diodes. Most transformers come with two secondary windings anyway. When the idea of the low impedance copper ground plate took form, the center tap leading to the ground plate after the caps seemed to be a natural option.

Star Ground

The copper plate design has one massive ground. On prototypes without the copper plate we sticked to a star ground near the reservoir cap.

On the IQaudIO Pi-DAC+ there is a hole in the middle of the PCB labeled 0V. Somewhere it was mentioned to put the star ground next to the output so we tried to use this spot as our star ground. That prototype had a couple of other issues so no result on that for now. In later models we connect the separated grounds of pi and dac to one point even at the copperplate.

Main Cap as CRC Filter

Here's the input to that. We built one but didn't test it for some time.

Then, we opened a thread at pinkfish media, presenting the X8 mark I prototype. People there suggested we could improve the PSU with a CRC filter using PSUD, a power supply simulation software to calculate values. Implementing a second cap with a resistor in series immediately improved musical reproduction and joy of listening!

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