884 Chapter 25
the way humans react to audible stimuli, and also from
the way nature deals with things acoustic.
If something is acoustically resonant, it will need a
similar electrical resonance response shape to compen-
sate for, extract, or imitate it in the console. Acoustics
are defined by exact analogs of the first- and
second-order filters and the time-domain effects that
we’ve been delving into here in EQ.
Differing wall coverings have absorption coeffi-
cients paralleled very closely with shelving-type EQ
curves. Apertures and partly enclosed spaces big and
small act like second-order resonances identical to elec-
trically resonant circuits. The physical size of a room
determines the lowest frequency it can support just as a
high-pass filter would. Initial and other major room
reflections effect precisely the same changes on audio
as deliberately introduced electronic delays; the
frequency-dependent propagation characteristics of air
are emulatable with slope filters.
25.11.16 Bandpass Filter Development
Methods of filtering come thick and fast once the basics
are established. The development of a popular bandpass
filter arrangement is shown in Fig. 25-64. It starts as two
variable passive single-order filters of a common cross-
over frequency point, ganged so that they track. Recon-
figured slightly, Fig. 25-64B, to minimize interaction,
they are shown with their drive and sense amplifiers.
Wrapping the two networks around an inverting amp
isolates them completely from each other, improving the
filter shape. The bandpass Q is rather low, well under
one, leaving it rather limited in scope for practical appli-
cations. Positive feedback from the amplifier output
back to the noninverting input sharpens the Q.
Yes, it does look rather like a Wein Bridge oscillator.
Attempting to get the Q too high proves the point
unquestionably!
25.11.17 Listening to Q
This raises the problems of excessive Qs. Fortunately,
extremely high Qs (greater than ten) are unnecessary or
unusable for EQ purposes. The higher the Q becomes,
the less actual spectral content of the signal it modifies,
so despite the fact that its peak gain or attenuation is the
same as a lower Q filter, it seems to do subjectively less.
Judicious care is required in setting up the filter to
enhance or trim exactly what is required. Accidental
overkill is easy. Figure 25-64. Bandpass filter development.
VinVinVinVoVoVoVoR 1R 1R 1R 1R 1 R 2R 2R 2R 2R 2R 12R 11RF 1RF 2RF 3
RF 4S 1C 1C 1C 1C 1C 1C 2C 2C 2C 2C 2A. Very basic two-pole filter.B. Reconfigured with source and sense amplifiers.C. Elements isolated around inverting amplifiers.E. Switchable Q with ganged switched
compensating attenuator.D. Positive feedback introduced to increase Q.VinVin