operational amplifier capable of a little more power
output than most of the integrated circuit ones.
Headset impedance varies from 600 ohms down to
24 ohms — possibly beyond this range. The volume
control at the input will allow almost any signal source to
drive the amplifier to overload, so turn it up from zero
until the headset signal is sufficiently loud.
An amplifier on perfboard laying on a bench is an
accident waiting to happen. Mount your board in a
project box on some standoffs. Of
course, you will want the box to be
large enough for two channels.
Place the power transformer as
far from the input connections as
I've tested this amplifier with
the following headphones: a
Sennheiser HD 202 ( 32 ohms,
approx. $30); a Sennheiser HD 650
(330 ohms, $400-$500); and with a
AKG 702 ( 60 ohms, $350-$539).
The AKG seems the least sensitive.
I prefer the HD 202 to the AKG.
The HD 650 is great!
Audio folks discuss the
damping factor as important to a
loudspeaker. Damping factor is the
ratio of the speaker or headset
impedance to the amplifier output
References online indicate that
for a headset, a damping factor of
eight or more is adequate. This
amplifier's output impedance is less
than 0.05 ohms, so the damping
factors are quite high.
I've measured the impedance
vs. frequency of the three headsets
I just listed. All have a slight peak at
about 50 Hz. One set measured
about 15% higher impedance at the
peak, and a better one was about
10%. A low damping factor would
result in the bass response being up
about 1.5 dB at 50 Hz in one case,
and 1 dB in the other.
This is a small difference but
probably detectable by someone
with good ears. That is, with a low
damping factor this might be
A high damping factor greatly
reduces the peak in the response.
In this design, I agonized over damping factor vs.
protection of the amplifier and headset. Initially, I had the
100 ohm resistor in series with the output which provides
current limiting, but a terrible damping factor for low
impedance headsets. Putting the resistor in the collector
circuit of the output follower gives the same current-limiting protection but the negative feedback from the
output decreases the output impedance (as long as the
amplifier is not overloaded) to a fraction of an ohm.
Hopefully, this discussion will help when you design your
own projects. NV
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October 2013 47