Audio Amplifi er Performance 32710.5.2 Advantages of DC Coupling
- No large and expensive DC-blocking capacitor is required. However, the dual
supply will need at least one more equally expensive reservoir capacitor and a
few extra components such as fuses. - In principle, there should be no turn-on thump, as the symmetrical supply rails
mean the output voltage does not have to move through half the supply voltage
to reach its bias point—it can just stay where it is. In practice, the various
fi ltering time constants used to keep the bias voltages free from ripple are likely
to make various sections of the amplifi er turn on at different times, and the
resulting thump can be substantial. This can be dealt with almost for free, when
a protection relay is fi tted, by delaying the relay pull-in until any transients are
over. The delay required is usually less than a second. - Audio is a fi eld where almost any technical eccentricity is permissible, so it
is remarkable that AC coupling appears to be the one technique that is widely
regarded as unfashionable and unacceptable. DC coupling avoids any marketing
diffi culties. - Some potential customers will be convinced that DC-coupled amplifi ers give
better speaker damping due to the absence of output capacitor impedance. They
will be wrong, as explained later, but this misconception has lasted at least 40
years and shows no sign of fading away. - Distortion generated by an output capacitor is avoided. This is a serious
problem, as it is not confi ned to low frequencies, as is the case in small-signal
circuitry. For a 6800- μ F output capacitor driving 4 W into an 8- Ω load, there
is signifi cant midband third harmonic distortion at 0.0025%, as shown in
Figure 10.6. This is at least fi ve times more than the amplifi er generates in this
part of the frequency range. In addition, the THD rise at the LF end is much
steeper than in the small-signal case, for reasons that are not yet clear. There
are two cures for output capacitor distortion. The straightforward approach
uses a huge output capacitor, far larger in value than required for a good
low-frequency response. A 100,000- μ F/40-V Aerovox from BHC eliminated
all distortion, as shown in Figure 10.7. An allegedly “ audiophile ” capacitor
gives some interesting results; a Cerafi ne Supercap of only moderate size