Making a Dual Rail Linear Power Supply!

Following on from the single rail power supply I wrote about a while back I actually created this dual rail version first which was inspired by the power supply on board the Whammy headphone amp.

I was that impressed with the performance of the Whammy I wanted to create a separate board which was solely just the power supply and nothing more. When designing the dual rail power supply layout I wanted to simplify it as much as possible to make it easy to build so I removed the configuration methods which means the output voltage is set by the regulator installed in the PCB.

I obviously kept the CRCRC filter aspect as this is where the magic happens. If you refer back to the single rail power supply article you will see how effective the filter is which also performs the same way in the dual rail version.

Design Parameters

As with the single rail version this power supply uses the same type of input transformer which supports 120v or 220v mains input depending how its configured but the secondary outputs on this power supply are connected in series instead through the same rectifier bridge and centre tap connected to ground so we can split the current into positive and negative supplies. As mentioned earlier the same CRCRC filter is used before the current is passed to the regulators. The LEDs you see near the output are not used for a reference voltage like on the Whammy’s power supply but instead to provide a small dummy load on the regulators as they can act weird without a load attached. They also of course provide a visual indicator that the regulator is working and providing an output current.

Building and Testing

As this board has nearly twice the amount of components as the single rail version this actually resulted in a much bigger PCB. Think half the size of the Whammy PCB its actually the same length but in a future revision I plan to try and redesign the layout and maybe use some surface mount components to make the overall size smaller so it is more friendly to use for different audio projects. I built this board to show you the steps but I actually plan to use it with one of Wayne’s BA 2018 Linestage amplifier boards when I get one.

As usual it is easier to populate the board by soldering the smallest components first followed by the largest and lastly the transformer. This make the whole soldering process much easier. First we start by soldering the resistors and diodes as shown in the picture below.

Next we add the capacitors starting with the smallest ones first. I also added the regulator voltage load LEDs too and the cable for the external power LED ready for when the board is mounted in a chassis for my amp build. The only thing left to do at this stage is to configure the input supply voltage and add the transformer remembering to add the M4 securing screw to the bottom to provide additional support when its mounted onto the PCB.

Measurements

I won’t post oscilloscope measurements again as they will just show the same results as the single power supply I wrote about but I have tested and the measured ripple is a nice flat line across the screen. I measured both of the regulator outputs and they are outputting as they should with the positive and negative only having a very tiny 90mV variance between them. I will probably add trimpots in the next board revision so we are able to tweak and balance the two outputs without hassle.

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 x6 Nichicon (UPW) UPW1V332MHD 3300uf/35v 20% Capacitor
1N4004-TPMSCT-ND x6 Micro Commercial Co 1N4004-TP Rectifier Diode 400v/1A
5.1ZCT-ND x4 Yageo FMP200JR-52-5R1 5.1 ohm 2W 5% Resistor
HS352-ND x2 Aavid 531102B02500G Black Extruded Radial Fin Heatsinks
2.80KXBK-ND x1 Yageo 2.8k 1/4W 1% Resistor – MFR-25FBF52-2K8
1.00KXBK-ND x2 Yageo 1.0k 1/4W 1% Resistor – MFR-25FBF52-1K
516-1792-1-ND x2 Broadcom HLMP-1790-A0002 3mm / 2.3mcd / 1.9v / 2mA Green LED
493-1864-ND x2 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)

MC79xxCTGOS-ND x1 ON Semi. MC79xxCTG 79xx Negative Regulator
(Replace xx with the required regulator voltage. Example: -24v – 7924, -15v – 7915, -5v – 7905)

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 x2 Vishay Dale RN55D1001FB14 1.0k 1/8W 1% Resistor (Military, MIL-R-10509/7)
PPC5.1W-2CT-ND x4 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)

You can also use 18v transformer for 15v regulators if you want a slightly higher current.

The Talema Transformer Datasheet can be found HERE.

Remember: DC voltage is approx 1.414 times AC voltage.
*** Be careful when choosing transformer! ALSO 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.

Disclaimer:
The boards are provided as is without warranty of any kind. They are intended solely for personal use and are supplied as a bare board only which is classed as an unfinished product. While every effort has been made to ensure the board has been designed with industry standards in mind, it may not depict an exact representation of these standards and as such there may be minor inconsistencies or oversights in the design process. While these boards have been extensively tested, there can be no guarantees whatsoever that the board is completely free of defects and as such caution and due diligence should be taken at all times when assembling and using the board.