|Elliott Sound Products||Project 05, Rev-B|
Power Supply for Preamplifiers (Revision B)
Rod Elliott - ESP
The original version of P05 has been around for a very long time now (around 4 years before it was retired), and the successor (Rev-A) is over two years old as well. Although the performance of the original or the Rev-A was not lacking in any way, Rev-A saw the change to adjustable regulators. This allows greater flexibility (one can easily make a small variable lab supply with the new version), and the adjustable regulators actually have lower noise.
The Rev-A PCB had a loss of AC detector, but this is now upgraded to a full muting circuit. Muting is now even easier (the AUX output can simply drive a relay), or it provides a useful signal for any other circuits that can benefit from a muting signal.
For anyone who feels a burning desire to upgrade their original P05 or P05A to the new P05B, the board is the same size, and has the same mounting hole positions. The new PCB is a couple of millimetres shorter than the original P05, and will fit into the same location.
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. The supply voltage will be too high for 3-terminal regulator ICs, and they will fail. This will also be a problem if the main amp is under warranty or you just don't want to fiddle with it.
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). Plug 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 regulator resistor networks will have to be changed accordingly to reduce the DC to ±12V or so. In fact, the supply may be configured for any voltage from ±2.5V up to ±25V (although 15V is the most practical for opamps).
Alternatively, the supply can be run from a conventional split voltage transformer (e.g. 15-0-15V AC). It is designed to be as flexible as possible, and to this end, an auxiliary supply is also provided, complete with a 'loss of AC' detector for the muting circuit. This can be used to power a muting relay, with no additional circuitry needed ... other than the relay and a suitable voltage dropping resistor for the coil. Even the diode is on the PCB.
As always, 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 will be left on permanently in most 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 (if you use the single winding transformer option) - there is no need to break both leads with the switch. Naturally, if you use a standard transformer it is better to switch the mains to conserve power.
Figure 1 - Preamplifier Power Supply
All component values, bill of materials and comprehensive instructions are made available when you purchase the PCB. If you want to build a supply without purchasing the board, then use the circuit shown in the original P05 article.
If a single AC supply is connected between GND and AC2, the rectifier is a "full-wave voltage doubler" type, and with an input of 16VAC will provide about +/-20V DC at a current of up to at least 100mA - this should be enough for the most power-hungry preamp. All diodes are 1N4004 or similar (400V / 1A rating for all). (Note that the supply will work, but if you use GND and AC1 for a single AC supply, the loss of AC detector cannot function, as it gets its signal from the AC2 terminal.
If a split AC supply is used (such as 15-0-15V AC), then the transformer centre tap connects to GND, and the two 15V winding ends connect to AC1 and AC2. Although virtually any transformer of 0.5A or more will work (provided the voltage is correct), there is very little to be gained by using anything more than 30VA (and even that is likely to be overkill).
The 3-terminal regulators specified are TO-220 types, and unless your preamp requires lots of current, they will not require a heatsink.
The diodes around the regulators prevent reverse voltages being applied to the regulator chips under any condition. They are not strictly necessary, but are considered a good idea. The bypass caps are as close to the IC power leads as possible to prevent oscillation.
Photo of Completed Unit
The photo shows the completed PCB, and has no heatsinks for the regulators. Heatsinks will not be needed in most cases, but using them will do no harm, either. Make sure that they are well insulated from each other, or are insulated from the regulators with mica or silicone washers.
The PCB can be wired to use a single 16V AC supply, or a 15-0-15 AC supply from a conventional power transformer. Or, if you need to, it may be powered directly from an existing source of DC - make sure that the input voltage is below +/-30V under all operating conditions - this is important. For this connection, the rectifier diodes must not be used, and the loss of AC detector won't function unless it is connected separately to a source of AC. This option is recommended for experienced hobbyists only, although the construction guide does have some additional information.
It will be noted that there are no component values shown, other than for the semiconductors. This information (plus quite a bit more) is available in the construction guide - when (and only when) you purchase the PCB.
As an added bonus, the PCB can be used to implement the little lab supply described in Project 44. The voltage pots are connected in place of R4 (A & B) and R6 (A & B). The only thing that you will need to do is add a decent sized heatsink, and in this case a suitable bracket is recommended. If you only ever plan to use the supply for preamps, the heatsink can even be omitted, although I don't recommend this.
This function needs a bit of explanation. There are quite a few circuits (both opamp based and discrete) that insist on making stupid noises, especially as the supply voltage falls away to zero. The most common are squeaks and whistles, or sometimes rather disconcerting clicks and pops.
Adding a muting relay solves this (and there are a few described in the project pages), but there are no boards available, and they can be irksome to wire up. Using just the Auxiliary output from the P05B connected to a relay, you have a muting system - note that you will almost certainly need a resistor in series with the relay coil.
The circuit activates after about 0.5 second when power is applied. This plenty of time to prevent switch-on noises, and would typically be used to power a relay where the normally closed contacts short the signal to earth. When the relay activates, the short is removed and you have normal operation. When AC power is removed, the Aux output will fall to zero within a few AC cycles, the relay will release, and muting will again be activated. All of this happens well before the voltage has fallen far enough for the attached circuits to make a sound, so any of the silly/annoying noises you used to get will be muted, and will not get through to the power amp.
When power is first applied, Q3 is turned on by the charging current of C13. This shorts the base of Q2 and prevents it from turning on, so there is no output. After C13 is charged, Q3 turns off, Q1 and Q2 turn on, and DC is available at the aux output. As soon as AC is removed, C11 discharges rapidly, removing base current from Q1, so Q1 and Q2 turn off, removing the DC. An attached relay will promptly drop out, activating the mute function.
The Aux output requires a load - any relay will be more than enough, and if it is used to power some other circuit (such as the P110 remote control), no additional load is needed. The minimum load should be about 10mA. If the aux current is less than 10mA, you may need to add a resistor to draw a few extra milliamps to ensure the voltage drops quickly after AC is removed.
Note that the Aux output is not regulated! Taking any switched current from the regulated supply is not a good idea, as it is possible to induce noise into the regulated supply. This rather negates the whole idea of using a low noise regulated supply in the first place.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2007. 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.|