Figure 19. Driver stage with
long-tailed pair input.
Figure 20. 50 Hz rumble or
Figure 21. 10 kHz scratch or
power amplifier designs.
A common annoyance when playing old records
(discs) through audio power amplifiers is that of scratch
and/or rumble sounds. The scratch noises are mainly
high-frequency (greater than 10 kHz) sounds picked up
from the disc surface, and the rumbles are low-frequency
(less than 50 Hz) sounds that are mostly caused by slow
variations in motor-drive speed.
Each of these noises can
be greatly reduced or eliminated by coupling the play-er's audio signals into the
audio power amplifier input
via a filter that rejects the
troublesome parts of the
audio spectrum. Figures 20
and 21 show suitable circuits.
The high-pass rumble filter of Figure 20 gives unity
voltage gain to signals above 50 Hz, but gives 12dB per
octave rejection to those below this value, i.e., it gives
40dB of attenuation at 5 Hz, etc. Emitter-follower Q1 is
biased at half-supply volts from the R1-R2-C3 low-impedance point, but has negative feedback applied via the R3-
C2-C1-R4 filter network. The circuit's frequency turn-over
point can be altered by changing the C1-C2 values
(which must be equal); thus, if the C1-C2 values are
halved (to 110nF), the turn-over frequency doubles (to
100 Hz), etc.
The low-pass scratch filter of Figure 21 gives unity voltage gain to signals below 10
kHz, but gives 12dB per
octave rejection to those
above this value.
This circuit is similar to
that of Figure 20, except that
the positions of the resistors
and capacitors are transposed in the C2-R4-C4-R5
filter network. The circuit's
turn-over frequency can be
altered by changing the C2-C4
values, e.g., values of 3.3nF give a
frequency of 7. 5 kHz.
The Figure 20 and 21 circuits
can be combined to make a composite scratch and rumble filter, by connecting the output of the high-pass filter to the input of the low-pass filter; if
desired, the filters can be provided
with bypass switches, enabling them
to be easily switched in and out of circuit, by using the connections of
Note that if the Figure 20 and 21
designs are to be built as a single
unit, a few components can be
saved by making the R1-R2-C3
biasing network common to both
Next month's final installment will
describe a miscellaneous collection of
useful transistor circuits and gadgets.
Figure 22. Complete scratch/rumble filter, with switching.
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