Audio Engineering

Audio Amplifi er Performance 329

(4700μ F/63 V) gave Figure 10.8 , where the midband distortion is gone, but

the LF distortion rise remains. What special audio properties this component

is supposed to have are unknown; as far as I know, electrolytics are never

advertised as low midband THD, but that seems to be the case here. The volume

of the capacitor case is about twice as great as conventional electrolytics of the

same value, so it is possible the crucial difference may be a thicker dielectric

fi lm than is usual for this voltage rating.

Either of these special capacitors costs more than the rest of the amplifi er
electronics put together. Their physical size is large. A DC-coupled amplifi er
with protective output relay will be a more economical option.

A little-known complication with output capacitors is that their series reactance
increases the power dissipation in the output stage at low frequencies. This
is counterintuitive as it would seem that any impedance added in series must
reduce the current drawn and hence the power dissipation. In fact, it is the load
phase shift that increases the amplifi er dissipation.

10 100 1 K 10 K 50 K

0.050

0.010

0.001

0.0005

Audio precision aplast$$ THD  N(%) vs Freq.(Hz) 14 Aug. 96 19:43:35

Ap

Figure 10.8 : Distortion with and without an “ audiophile ” Cerafi ne 4700- μ F/63-V capacitor.

Midband distortion is eliminated but LF rise is much the same as the standard electrolytic.