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.
Remember next month, we will discuss the theory of
this headset amplifier design. Happy listening! NV
September 2013 33