A handy line level headphone amplifier for audio monitoring - design by Adrian Barnes

Many musicians will remember coming home of an evening feeling creative but being restricted as to the amount of noise they can make in the process by those immediately about them. This is particularly frustrating for drummers, who can only turn the volume down if they have a synthesized kit in the first place. To minimize the sound made the ultimate would be to use headphones, which allow the drummer to hear exactly what the audience would hear (rather than a lot of sticks tapping on pads due to the low volume).

Of course there are a lot of other applications for headphones, in practising guitar, in monitoring mixers without upsetting the PA sound balance and for quick tune-ups before going on stage. Unfortunately in many applications the equipment involved just won't have the ability to power a pair of headphones, often having only a line level output (for instance the two drum machines reviewed this month). The obvious solution is a small headphone amplifier having a line input and headphone level output, and the design presented here is small enough to be used as a stand-alone unit or to be built into existing equipment such as mixers.

Since the project was designed to be run from its own battery the output level had to be chosen so that the battery would not be flattened in a very short time, so the output level is comfortable and not a strain on the ears. At this level, if the input is not too high then distortion will be quite low. Distortion can be reduced even further when running at almost full volume by increasing the supply voltage from 9V to 18V, giving more headroom and less clipping. However, two PP3 batteries used in series would not fit in the case used to house the prototype.

Input Levels

A variable gain stage on the input allows inputs of between 100mV and just over 1V to be accommodated, which encompasses line level. The gain stage also acts as a buffer before the volume control, giving the unit a high input impedance which is useful when the output impedance of a mixer is uncertain.

Design Considerations

The main factor to be taken into account in the project was efficiency, since amplifiers do tend to take a lot of current and only a limited amount of power is available. For this reason the design was kept fairly simply, using a class B amplifier IC, the TDA 2002. The headphone socket connects the two phones in parallel, giving a total impedance of half of one of the phones. Since most phones are 8ohms, this gives 4ohms, at which impedance both reasonable battery life and volume are produced. A lower impedance than this will drastically decrease battery life while a higher impedance will increase sound quality. Personal hi-fi headphones at around 32ohms are ideal too and can be reasonably cheap.

Supply voltage plays a part too. At 9V the available power is just under 2W, increasing to more than double that at 18V with a corresponding decrease in battery life.

Finally, heatsinking. A small heatsink on IC2, even though the case is closed, will help to keep things cool and not absorb too much power. A clip or bolt-on type will do fine, if not a simple strip of metal with a hole in it. The TDA2002 does have a thermal shutdown feature, the thermal overload circuit reducing the drive to the output stage when the junction temperature exceeds a certain level. The result is a reduced supply current and power output, consistent with maintaining the junction temperature constant.

Circuit Description

Incoming signals are referenced to 0V and AC coupled through C1 to the inverting input of IC1, an LF351 via R1.

R2 and R3 provide a false earth to the non-inverting input of IC1 by potential dividing the supply rails. This is smoothed by C2 and decoupled by C10.

The output from pin 6 of the IC in this configuration will therefore be the signal at pin 3 minus the signal at pin 2 multiplied by VR1/R1, VR1 setting the overall gain. In other words, since pin 3 is constant, the output is the input inverted and multiplied by VR1/R1. IC1 has a low output impedance which can drive the volume control VR2. C11 decouples the supply to IC1. C3 then AC couples the signal to IC2, the non-inverting input of which is pin 1.

Feedback is via R5 and C4 to the inverting input pin 2. Gain is therefore set by the ratio of R6/R5 which in this case is 20 log (2.2 x 103/220) = 46dB. C5 and R4 are provided at this stage to give an upper frequency cutoff so that power is not wasted on amplifying inaudible signals.

This leaves the signal to be passed through output capacitor C7 to the headphones and frequency stability components C6 and R7. C8 stores charge for spurious peaks and C9 decouples IC2's supply.

Construction

Construction is straightforward although the PCB should be used because tracking around IC2 is critical and a Vero layout may oscillate. Insert the PC pins and link first then proceed as seems logical. Fig 2 shows the overlay and switch wiring. The phones socket is connected up seemingly backwards, but isn't as IC1 has inverted the signal. A flying lead through a hole in the case is the input and the case used here has an integral battery compartment.

The front panel should be drilled at 2cm from either end (the holes being 2cm apart) giving just enough clearance for a knob.

Setting Up

Very little setting up is needed except to ensure that the headphones are of the correct impedance and that IC2 is heatsinked (heatsunk??). Then, using the intended source, adjust the gain control so as to make the output at full volume as loud as possible without being distorted (unless that's what you want). If the output is distorted even with the gain right down, then reduce the input by soldering a resistor between pins 2 and 4 on IC1. Start with a 4k7 and work on to smaller values until a correct value is obtained.