This is a
design circuit of audio amplifiers with DC coupling to the
load are not often encountered these days, even though they offer definite
advantages. One advantage is that there is no need for the complication of a second
(symmetric) power supply; another is good frequency and phase response. Also,
no special electrolytic capacitors are needed for voltage stabilization, and
switch-on ‘thump’ is much reduced. This is the figure of the circuit;
It
consists of a voltage divider, a voltage follower and the loudspeaker in the
headphones, whose other side is connected to the junction of two electrolytic
capacitors, providing the virtual earth. The potential at this point is, of
course, half the supply voltage. All we need to do now is suitably couple in
the audio signal to be amplified. This figure shows a practical realization of
this idea in the form of a stereo headphone amplifier. The amplifier itself
consists of IC1 and P1, R3 and R4 (giving a gain of 11). This part of the
circuit demands no further explanation, and the same goes for the voltage
divider mentioned above, formed by R1a and R1b. The signal is coupled in via
the potentiometers. C2 and R2 have a special purpose: C2 connects the bottom
end of the potentiometers (ground for the input signal) to the virtual earth.
However, this capacitor creates a feedback path which can lead to oscillation
of the amplifier under some circumstances. R2 damps this tendency to oscillate.
It is possible to calculate suitable values for these components, but it is
better to determine them by experiment. C2 must be sufficiently large that
stray electric fields do not cause unacceptable hum at the output. R2 must be
sufficiently large that the voltage at the amplifier’s virtual earth stabilizes
quickly enough after switch-on. The polarity of the electrolytic is unimportant
as no significant voltage appears across the network. It is possible to try the
circuit out with the C2/R2 network shorted and observe the behavior of the
circuit at switch-on using an oscilloscope. Depending on the degree of
asymmetry in the circuit, the voltage at the virtual earth point can take a
considerable time to stabilize.
