Making a Single Rail Linear Power Supply!

For a good clean sound it requires a good clean power supply. A while back I built the Whammy Headphone Amp by Wayne Colburn and was immediately impressed with how clean it sounded. Even with no audio playing, no matter what position the volume was at it was dead silent. From minimum volume level to maximum, there was no audible noise whatsoever. This is thanks to the simple yet effective power supply built into its design.

The board layout is fairly simple and straightforward as there are not that many components overall.

Design Parameters

The source current is supplied to the rectifier bridge via a PCB mounted toroidal transformer with dual primaries to accommodate 120v and 240v input and dual secondaries to allow parallel connection if required for more current. The rectifier bridge is equipped with an X2 rated polypropylene snubber capacitor to prevent QRR (the charge held internally to the diode) from creating a high-frequency spike in the power supply. Because the QRR is dumped rapidly, it creates high-frequency noise that passes through the filter caps which we don’t want happening here.

For the power supply to be effective, it needs to be silent. This means the current needs to be filtered so that the ripple is greatly reduced. As you can see in the image above there are 3 large capacitors which when coupled with series resistors forms a very effective filter for the power supply. It is also worth noting that the filter used is in CRC configuration which while effective it still has some ripple in the current. This is where the extra RC comes into play. The extra addition of the resistor and capacitor to make the filter CRCRC is extremely effective at nearly eliminating the ripple altogether.

I designed the board so that it provides a single rail output of positive current which is useful for powering most pre-amps and headphone amplifiers. The good thing with this design is that we can make use of the extra secondary output if needed by bridging the J7 and J8 jumpers to configure the transformer in parallel depending on the transformer and voltage regulator used. This makes the board verstile to provide for different voltage outputs. The D6 LED near the output pads is used to provide a dummy load for the regulator and I also added a header connected to a 2.8k series resistor to allow for connecting an external LED power indicator so you can build this into an enclosure.

Building and Testing

The board is fairly small for a through hole design measuring approx. 170mm x 71mm. The silkscreen is very clear and all components and values are labelled clearly. There are instructions printed on the board to set the configuration of the primaries depending on your country’s mains voltage. As with all builds we always start with the smallest components first.

I find it easier to solder the diodes and resistors first followed by the D6 LED and the output smoothing capacitor and then the snubber capacitor but of course it’s up to you. You may notice the oversized brown resistors used in this build. These are RN60D Vishay resistors which are rated slightly higher than the RN55D which seem to be difficult to get hold of at the moment.

Next I configured the transformer primary by setting the jumper wire to its relevant location which in my case sets the input voltage to 240v. I then soldered the transformer to the board and secured it with an M3 screw underneath for extra support. I mounted the regulator on the heatsink with a thermal pad and added a plastic insulator into the hole before securing it all with a nut and bolt. This allows for easy soldering when placing in the PCB as you can solder the legs of the regulator which then holds the heatsink in place to solder without it falling out. Lastly the large capacitors were soldered into place followed by a power inlet for testing. As the mount holes are all connected to ground this is where we connect the mains earth wire.

Measurements

This is the measurement showing the ripple after the first large capacitor.

This is the measurement after the second large capacitor showing the ripple is reduced.

This is the measurement after the third large capacitor showing the effectiveness of the extra RC in the filter.

This is the 5v version I made and it measures nicely on the multimeter. The other one I built is for 24v output as I plan to build both of these into a single enclosure as a dual output power supply with different voltage outputs for 2 of my audio devices.

Bill of Materials

The following components can be ordered from DigiKey:

399-5464-ND x1 Kemet R46KN322000M1M 0.22uf/250V Snubber Capacitor (X2 Rated Polypropylene)
493-1883-ND x3 Nichicon (UPW) UPW1V332MHD 3300uf/35v 20% Capacitor
1N4004-TPMSCT-ND x5 Micro Commercial Co 1N4004-TP Rectifier Diode 400v/1A
5.1ZCT-ND x2 Yageo FMP200JR-52-5R1 5.1 ohm 2W 5% Resistor
HS352-ND x1 Aavid 531102B02500G Black Extruded Radial Fin Heatsinks
2.80KXBK-ND x1 Yageo 2.8k 1/4W 1% Resistor – MFR-25FBF52-2K8
1.00KXBK-ND x1 Yageo 1.0k 1/4W 1% Resistor – MFR-25FBF52-1K
516-1792-1-ND x1 Broadcom HLMP-1790-A0002 3mm / 2.3mcd / 1.9v / 2mA Green LED
493-1864-ND x1 Nichicon (UPW) UPW1V221MPD6 220uf/35v 20% Capacitor

MC78xxCTGOS-ND x1 ON Semi. MC78xxCTG 78xx Positive Regulator
(Replace xx with the required regulator voltage. Example: 24v – 7824, 15v – 7815, 5v – 7805)

If you want better resistors then you can order the Vishay ones below if they are available:

RN55D2801FB14-ND x1 Vishay Dale RN55D2801FB14 2.8k 1/8W 1% Resistor (Military, MIL-R-10509/7)
RN55D1001FB14-ND x1 Vishay Dale RN55D1001FB14 1.0k 1/8W 1% Resistor (Military, MIL-R-10509/7)
PPC5.1W-2CT-ND x2 Vishay BC Components PR02000205108JR500 5.1 ohm 2W 5% Resistor

The following transformers are good if using 15v to 24v Regulators:

TE2261-ND x1 Amgis L01-6365 25VA/22v Transformer (568mA per output)
1295-1079-ND x1 Talema 70065K 25VA/22v Transformer (568mA per output)

The Talema Transformer Datasheet can be found HERE.

Remember: DC current is approx 1.414 times AC current.
*** Be careful when choosing transformer! DO NOT EXCEED 1A on the PSU! ***

Board Availability

Drop an email to contact@audiosy.net if you are interested in obtaining one or more boards for your use.

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