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Save batteries with this handy dual-rail PSU.

The E&MM Twinpak.

  • Twin regulated DC supplies
  • +15v outputs at 120mA each
  • LED status indicator
  • Designed for long life & reliability
  • Easy construction

When you are all set up with your guitar or keyboards to record a track, there is nothing more annoying than the battery in one of your effects units deciding to give up the ghost. Even more frustrating and embarrassing if it happens when you are doing a gig. The specifications of battery operated units usually have to be compromised to allow for the unregulated, and possibly non-ideal voltage delivered by batteries. Today's operational amplifier based circuits invariably work better, and can be designed more elegantly using twin supply rails. This means that either two batteries have to be provided, or a single battery voltage has to be split using resistors to form two low voltage rails, increasing the battery drain, and lowering an already marginal headroom.

One obvious solution is to provide each unit with a twin regulated mains operated supply. The problem then is that if you have ten mains operated units, there will be ten mains leads and ten plugs requiring ten mains outlets. The danger of an earth hum loop will be increased ten fold, and think of the cost of duplication of transformers, capacitors and regulators!

The E&MM Twinpak provides the ideal solution. It delivers a twin regulated supply centrally, intended to be distributed to audio processing and effects units by 'daisy - chaining' from unit to unit, eliminating the need for a 'birds nest' of supply cables.

Figure 1. Twinpak circuit diagram.
(Click image for higher resolution version)


The circuit shown in Figure 1 is quite conventional in most respects. The full-wave bridge rectifier, BR1 rectifies current from the transformer, TX1 in to the reservoir capacitors, C1 and C2. Regulation is carried out by two monolithic regulators, Capacitors C3 and C4 cater for any transient current requirements on the output. The LED, D2 obviously illuminates when the supply is switched on. The zener diode D1, however, will cause the LED to rapidly dim if either supply voltage drops due to current limiting or thermal overload, giving an early warning of any trouble.

Resistor R1 provides the signal 0V to mains earth connection, but with a high enough resistance to prevent earth hum loop currents circulating. The components used have been deliberately over-rated so that the unit runs cool for a long, reliable life.

Figure 2. PCB component overlay (not to scale)
(Click image for higher resolution version)


All the parts, except the transformer are mounted directly on the PCB as in the component overlay shown in Figure 2. Assemble the PCB as follows: Insert and solder the veropins and the fuse clips first. Next, insert the resistors then solder and crop the leads. Now locate D1 and the capacitors, bending the leads out to secure before soldering and cropping. Locate and solder the regulator IC's and the bridge rectifier in a similar manner. Before soldering the switch in position, cut a piece of 1/16" SRBP (veroboard will do) about 6 x 12mm and glue it using an epoxy resin type adhesive to the underside of the switch. Apply the adhesive also to the other side of the SRBP, then hold the switch firmly in its position on the PCB until the adhesive sets (watch your fingers though!).

Figure 4. Panel drilling detail.
(Click image for higher resolution version)

If you are constructing the unit using the panels shown in Figure 5, fit a solder tag to the back face of the front panel using a 6BA countersunk screw, nut and a serrated washer. With the switch button fitted, hold the front panel in position over the front of the PCB assembly. To fit the LED, bend its leads down at 90° about 5mm from the body. Locate the leads in the PCB holes, noting correct polarity. Mount the LED to the panel using the LED clip and solder the LED leads in place, making sure the assembly is square. Check the assembly thoroughly at this stage, paying particular attention to the orientation of polarised components. Mount the transformer on the right hand pair of moulded bosses using self-tapping screws, inserting a solder tag with a serrated washer under one of the screw heads. Fit a 6BA screw in the drilled hole and tighten with a nut. Screw another nut down to the same height as the moulded bosses. Slide the front panel into position and secure the PCB on the two left hand bosses, and the 6BA screw. Before positioning the rear panel, fit the DIN socket and clamp the mains cable in place using the grommet.

Figure 3. Inter-wiring diagram.
(Click image for higher resolution version)

Complete the inter-wiring as shown in Figure 3, taking extra care with the safety earth connections. They are at the transformer mounting lug, the 6BA screw on the front panel, and the DIN socket on the rear panel. Under no circumstances omit any of these connections. Double check all the connections for safety's sake.

Internal view of the Twinpak.


Connect the unit to the mains and switch on. If the LED does not light, switch off immediately and disconnect the mains. Do not switch the unit on again until the fault has been rectified. If all seems well, leave the unit on for a couple of minutes, then switch off and disconnect from the mains. If nothing has become unduly hot, then all should be well and the lid can be snapped into place. Connect a 120 ohm 1W resistor across each supply rail to 0v for a few seconds, making sure that the LED does not dim. If you have a voltmeter, check the output voltage of each rail which should be 15v ± 5%.

The Twinpak and Comp-Lim.

The unit is now ready for a long, reliable life. Connect it to your first audio processing unit, such as the E&MM Comp-Lim, using a DIN to DIN lead made from a short length of mains lead. The DC power can be extended from there to the next unit in a 'daisy-chain' fashion. All E&MM units designed specifically for use with the Twinpak will have two cross-linked DIN sockets for this purpose.

A complete set of parts for the Twinpak including components, PCB, case and hardware is available from E&MM at a cost of £24.95, including postage packing and VAT. Please order as: Twinpak Kit.

The PCB is also available from E&MM at a cost of £2.25, including postage, packing and VAT. Please order as: Twinpak PCB.


Resistors - all ⅓W 5% carbon.
R1 1k
R2 560

C1,2 2 off 2200uF 25v electrolytic
C3,4 2 off 100nF polyester

D1 BZY88C18
D2 LED red
BR1 W005
IC1 78M15 regulator
IC2 79M15 regulator

15 0 15v
TX1 6VA Transformer
SW1 Mains latchswitch
SK1 3 pin DIN socket
F1 100mA 20mm Fuse
2 off Fuseclip
Case. Vero 212
LED clip
Switch button
3 core cable
Mains plug
Nuts & bolts etc.

Previous Article in this issue


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Sep 1982

Feature by Paul Williams

Previous article in this issue:

> Comp-Lim

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