Variable + Phantom Power Supply

Some notes on my variable + phantom power supply.
Please do not copy and repost these photographs or link directly to them from my server.


Many older preamp designs require a single positive DC supply of around 24 Volts. For a modern preamp we also require a low current phantom supply of double that voltage. This is true of the 1108 preamp as well as Neve and others.

After seeing other attempts and reading numerous posts I kept coming back to PRR's post and in particular the last paragraph where he outlines why it's a good idea to keep the phantom on a separate winding.

Circuit Description

Running with his suggestion I went with a full-wave voltage doubler fed from one 25 Volt winding of a dual secondary transformer. The doubler that runs the phantom supply is then fed through an R-C filter to reduce the ripple. A capacitor across the voltage reference further reduces noise and ripple at the output.

The other secondary feeds a straight 317 adjustable regulator. With full wave rectification this circuit can supply roughly twice the current of the phantom circuit. This makes the two supplies a better match for each other than some solutions such as regulating a high voltage phantom supply down to run a preamp circuit where much of the power is wasted as heat and the phantom supplies more current that could possibly be used for the number of preamps the supply can handle.

While it is possible to add a capacitor and link to run both circuits off a single transformer secondary this is not recommended, though the pads are there on the board. DIY does not have to mean using the cheapest solution to a problem. Why try to use a single secondary winding to produce all the needed voltages for your circuit when it can be done in a more robust way with fewer performance compromises?


The BOM contained in the PDF linked from the left column describes the recommended values. You will need a suitable power transformer for your local line voltage with two separate 24 to 30 Volt secondary windings. A modest 15VA toroidal will be more than adequate to run two 1108 circuits which are specified at 45mA per preamp. Calculate a suitable transformer based on your needs taking care not to exceed the capabilities of the regulators themselves (refer to the manufacturer's datasheets).

Use a heatsink for both regulators or you may use the side or bottom of the cabinet (with insulators, of course) as the heatsink. It is possible to bend the leads of the tall capacitor C3 to mount it parallel to the PCB so the entire assembly will fit in a 1U rack with the regulators mounted on the bottom as shown in the photograph above.

Mount the 1 Watt resistor clear of the board to promote cooling. While common multiturn trimmers with inline pins may be used, "Y" series trimmers with an offset middle pin are recommended as they provide better mechanical stability when mounted on the PCB.