You really need to see the original article – Project 36 – to see all the design details for this project. The project presented here is simply a modification of the original design, with much lower power dissipation and adapted specifically as a headphone amplifier.
Class-A is ideal for this application, since headphones are such an intimate way of listening. An amplifier for ‘phones should be as clean and free from crossover distortion as possible, and must also be quiet. A background of hiss and hum does nothing to enhance the listening experience.
Headphone amps are somewhat misunderstood, but in reality there are few points that need to be made. Most ‘phones are designed to be operated with a source resistance of 120 ohms, and damping factor (as applied to conventional loudspeakers) is largely irrelevant. The actual source impedance should have very little (if any) effect on the frequency response or dynamic behaviour, since there is no cavernous enclosure and no heavy cones to try to control.
The IEC 61938 international standard recommends that headphones should expect a 120 ohm source (5V RMS maximum) – regardless of the headphone’s own impedance. If the manufacturer followed this standard, the 120 ohm resistor used in this circuit will not affect sound.
Power requirements are usually in the 10 to 100mW range, and this is quite sufficient to cause permanent hearing damage. With the current set for 330 mA as suggested, this amp will be able to drive a minimum of 2 (but probably 3) sets of headphones at once. With 40 Ohm ‘phones, it can give a maximum power of over 150 mW, so caution is needed to prevent hearing (and headphone) damage. Even with 8 ohm ‘phones, power will be about 110mW – more than enough to have you asking people to repeat everything they say.
The final circuit for the DoZ headphone amp is shown in Figure 1. It is almost identical to the original (well, apart from the output transistors and size of C3, it is identical), and there is no longer the need for massive heatsinks and TO-3 output transistors. As shown, there are outputs for 2 sets of headphones. Needless to say, only one channel is shown – the other is identical.
For final testing you will need a multimeter. As shown in the power supply circuit below, use a 10 Ohm resistor in series with the power supply positive lead. When you measure 1 volt across this resistor, this means that the amplifier is drawing 100 mA. The resistor remains in circuit, providing a useful reduction in supply ripple. You will lose about 3.3 V at operating current, and a 5W resistor is sufficient – it will get slightly warm. The output resistors (120 Ohm) should be rated at at least 2 Watts – a pair of 220 ohm 1W resistors in parallel will do just fine (the absolute value is not critical).
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