Preamplifi ers and Input Signals 237inductor–capacitor passive circuit layout, which allows adjustment of the HF turnover
frequency by variation of the capacitor value.
Such frequency adjustments are, of course, also possible with active fi lter systems, but
require the simultaneous switching of a larger number of components. For such fi lters
to be effective in their intended application, the slope of the response curve, as defi ned
as the change in the rate of attenuation as a function of frequency, is normally chosen to
be high—at least 20 dB/octave—as shown in Figure 7.74 , and, in the case of the fi lters
operating in the treble region, a choice of operating frequencies is often required, as is
also, occasionally, the possibility of altering the attenuation rate.
This is of importance, as rates of attenuation in excess of 6 dB/octave lead to some
degree of coloration of the reproduced sound, and the greater the attenuation rate, the
more noticeable this coloration becomes. This problem becomes less important as the
turnover frequency approaches the limits of the range of human hearing, but very steep
rates of attenuation produce distortions in transient waveforms whose major frequency
components are much lower than notional cut-off frequency.
10 K10 K10 K
10 K10 K 10 K10 K(L)(R)10 K Outputs(LR) 10 K(LR)(RL)10 K0V0V0VIC 5IC 6IC 40V0V10 K22 KIC 30V10 K10 K(R)(L)
Inputs
10 K
(R)IC 2IC 122 KFigure 7.72 : Channel separation or blending using matrix addition or subtraction.