|Elliott Sound Products||Project 05|
Power Supply for Preamplifiers
Rod Elliott - ESP
Please note that this project is now superseded by P05 Rev-C. Original PCBs are no longer available.
Preamps may in some cases use a simple regulator, with the supplies taken from the main amp power supply. This can be a problem if the main amp is of very high power, as the supply voltage will often be too high for 3-terminal regulator ICs. This will also be a problem if the main amp is under warranty or you just don't want to fiddle with it.
For these occasions, a simple, high performance supply can be built using an external AC power pack (no mains to worry about, and you don't even need a power lead). Power packs (wall warts, wall transformers) are available in a variety of voltages, and if you can find a 16V AC version, this is ideal. With 16V, you can easily get +/-15V DC regulated, using the circuit shown below. If you cannot find a 16V unit, you can use a 12V version instead, but the regulators will have to be changed to 7812 and 7912 (12V) types.
Another alternative is to mount a suitable transformer in a plastic or metal box, and just bring the secondary out on a pair of leads with a female line XLR on the end. The mains input can be a fixed lead or an IEC power connector. Remember to ground the chassis of the transformer (if a conventional type) and any metal on the box used. The disadvantage of this is that you will not have the safety factor afforded by the Double Insulation rating of an external AC power packs.
Inclusion of a fuse suitable for the transformer used is highly recommended, and a thermal fuse is a good idea too, since the power transformer may be left on permanently in some installations. If a power switch is incorporated in the preamp, this can be a simple low voltage type since no mains voltages are present, and can be in either AC input lead - there is no need to break both leads with the switch. Naturally, if you use a centre-tapped transformer, you will need to break any two of the AC leads with the switch.
Figure 1 - +/-15V Preamplifier Power Supply
The rectifier can be operated with a single winding, where the AC is connected between AC1 (or AC2) and GND. This is a 'full-wave voltage doubler' type, and with an input of 16VAC will provide about +/-20V DC at a current of 100mA - this should be enough for the most power-hungry preamp. All diodes are 1N4002 or similar (100V / 1A minimum rating for all). If you have a transformer with a centre-tapped winding (typically 15-0-15 V), the centre tap connects to GND, and the winding join to AC1 and AC2.
The 3-terminal regulators should be the TO-220 types, and unless your preamp requires lots of current, they will not require a heatsink.
The diodes around the 3-terminal regulators prevent reverse voltages being applied to the regulator chips under any condition. They are not strictly necessary, but are considered a good idea, and should not be omitted. Keep the 100nF caps close to the IC power leads to prevent oscillation.
Use of an XLR connector is but one suggested possibility for the AC inputs, because these devices are extremely rugged, provide very low contact resistance and cannot fall out. They are rather large however, and may be difficult to mount if space is a problem. Right angle plugs are available which can reduce the depth behind the preamp somewhat. Feel free to use the connector you prefer, or the regulator can be located in the same enclosure as the transformer and possibly the remainder of your circuit.
Photo of Completed Unit
The photo shows the completed PCB, and has small clip-on heatsinks for the regulators. These will not be needed in many cases, but will do no harm, either. Make sure that they are well insulated from each other! In the photo, you will see that the diodes (left hand side) are not used - this unit was powered from a DC supply in the same case, so they weren't needed.
The PCB can be wired to use a single 16V AC supply, or a 15-0-15 AC supply from a conventional power transformer (as shown above) - make sure that the input voltage is below +/-30V under all operating conditions - this is important). Another feature of the board is much better filtering than the schematic shown above. High frequency noise in particular is attenuated massively, and the output DC is very quiet indeed.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.|