A flexible design suitable for mono or stereo monitoring, featuring independent level controls for each, channel and adequate power to drive high or low impedance headphones.

In spite of the popularity of home recording and the proliferation of small commercial studios, there are very few commercial headphone amplifiers on the market and those that are tend to be expensive. The design described here is built around the LM380, a readily available audio amplifier IC which will provide a maximum of two watts of audio power into an eight ohm load.

Each PCB is a self-contained stereo headphone amplifier in order to retain maximum flexibility and indeed, the board can be used as two mono amplifiers if required. The prototype, which features two channel operation was built into a four inch deep rack-mounting case incorporating six amplifier PCBs and a simple series regulated power supply.

When an input signal (from a foldback auxiliary, say) is fed into jack A only, it is routed to all six headphone amps but if a different signal is plugged into socket B, then A is subsequently routed to phone amps 1-3 and B goes to phone amps 4-6.

Additionally, if a mono jack input is used, the rear panel switch must be set to 'mono' so that the signal is routed to both left and right sides of the headphones.

Circuitry

Each PCB contains two LM380 ICs configured to provide a maximum voltage gain of around ten. Each board is locally decoupled via R1 and C9 and C3 prevents RF instability which can be a problem with this type of IC. To further encourage stability, the board is laid out with the input and output circuitry kept as separate as possible, and R6 limits the maximum current into the headphones to prevent overloading whilst also ensuring a large tolerance of different load impedances.

R4 and C5 form a Zobel network to prevent any instability caused by reactive loads (aren't they all?). Figure 1 also shows the power regulator circuit which is a series regulator based around a 2N3055 transistor. 18V is applied to the base via R8, ZD1 and a voltage of around 19 to 20 volts will be available at the emitter.

Construction

If the rack-mounting format is to be employed, drill the metalwork first. The only important dimensions are the ones relating to the spacing of the PCB pot and socket holes, and those relating to the power transistor on the rear panel. As different makes of rack box differ in size slightly, no specific measurements are given but we suggest that you use the PCB as a guide to the front panel hole spacings, and the transistor mica insulating washer for the rear panel holes.

The power transistor must be insulated from the rear panel, so use the proper mounting kit provided with it, not forgetting the heat sink compound, and make sure that there are no burrs to damage the mica washer.

Assemble the PCB as shown in Figure 2 and check that the ICs and electrolytics are correctly oriented. When mounting these PCBs, to ensure correct spacing place the serrated washer on the pot, behind the panel, and then fit the nuts. It should be mentioned at this point that the ICs should be soldered directly to the PCB and not socket-mounted as they utilise the PCB track as a heatsink.

Figure 3 shows the power supply (PSU) layout which is so simple that a PCB is not required; the components are simply fixed to the solder tags of REC1.

Figure 4 shows the wiring of sockets A and B for dual channel operation, S2 being the mono/stereo switch. Also shown is the bus wiring for the PCBs and the power lead connections.

Testing

Unless you have some project building experience under your belt, construct the PSU first and test it before connecting it to the PCBs.

Temporarily solder a 1K resistor between the 20V line and 0V when testing the power unit, otherwise C10 will stay charged up all day when you switch off, resulting in pretty unamusing sparks when you try to continue work!

Next assemble the PCBs and, if you are of faint heart, check each one individually before connecting them all up. Pin 8 on each IC should give a measured reading of around 9 or 10 volts without any input. If they read 0V or 20V, then there's a real problem - either an IC is damaged or there's a fault on the PCB.

Finally, install all the PCBs and connect the input sockets using screened cable and then wire the 20V supply to the bus as in Figure 4b. Fit the lid and the unit should be ready for use.

In Use

It is envisaged that this headphone system would be driven from one of the auxiliary sends of the studio mixing desk and its gain is structured so that it will work well from a line level input.

Normally, all the headphones will receive the same signal but, of course, some musicians (namely drummers) will need more level than others. If, however, you enjoy the luxury of two or more pre-fade auxiliary sends, the system allows the amplifiers to operate as two independent groups of three, so that each group may be fed with a different headphone mix if required.

Mono or stereo operation is possible and the design is such that if you need only one or two headphone amps for a particular job, you can make up just as many as you require. Remember also that, one board will drive two pairs of headphones in mono with independent gain control - providing the ganged pot is replaced with two single ones.

This modification is very simple, but it is recommended that the stereo jack socket is replaced by a mono one, and a second mono socket hardwired in to carry the output for the second pair of headphones.

The PCBs are available at a cost of £2.95 each (including VAT/P&P). Please order as Headphone Amp PCB and send to: HSR Mail Order, (Contact Details).

All cheques/PO to be made in pounds sterling. Also please allow 28 days minimum for delivery.