Nuts and Volts - September 2013

high to accommodate high impedance headsets. Thanks to a friend, I’ve tested the amplifier with 32 ohm, 60 ohm, and 300 ohm headsets.

This design is capable of considerable power in terms of what a headset needs. It was made this way so it can accommodate a wide variety of headset impedances. Set the volume at a reasonable level. Audio is not much of a hobby if you damage your hearing! If you have done everything correctly, you should hear clean audio. The amplifier clips at an output of around 35 mA peak. This is no problem in terms of the signal required to drive a headset to high volume. Now, all you have to do is build a second channel, and mount the amplifiers and power supply in a suitable project box. Once securely mounted, it will be very reliable.

Measured distortion at three volts out at 20 Hz on up to about 1 kHz is 0.0012%. It changes very little from no load to a couple hundred ohms. I have a Hewlett-Packard 339A distortion test set that includes a very low distortion signal generator and the analyzer. It measures total harmonic distortion plus noise (THD+N). The 339A measures 0.0012% when the signal generator output is connected directly to the analyzer input; that is, the reading is the same with the amplifier as it is without it.

Distortion at 20 kHz measures 0.0021% with a 1K load, rising to about 0.008% with a 100 ohm load. Distortion will depend on the impedance of your headset and the sensitivity. More sensitive headsets require less input power and will have less distortion.

Readings are obviously very near the limit of the capability of the analyzer. The normal listening level will be about 0.3 to one volt AC at the headset. Response is down 3 DB at around 600 kHz. Response must go far beyond audio frequencies in order to insure that there is enough negative feedback at 20 kHz to reduce the distortion to a low level. (This topic deserves an article by itself.)

I think it is fairly safe to say that distortion will be less than 0.01% over the audio frequency range at normal to loud listening levels with nearly any reasonable headset. Distortion depends somewhat on construction techniques and on individual transistor characteristics.