Handbook for Sound Engineers

890 Chapter 25

Second-order filters, assuming moderate filter Qs
associated with Bessel or Butterworth characteristics,
score well in both respects. There is less transient
response disturbance and less tonal characteristic modi-
fication. There are few who would dispute that they
sound more natural and musical than tighter filters. A
small wrinkle is to leave a small controlled amount of
underdamped bump in the filter frequency response.
This has two consequences: one is the slightly more
rapid out-of-band roll-off, but the other, a subjective
effect, is that the extra program energy introduced by
the hump serves to help offset the loss of energy below
the turnover frequency. The perceived effect on intro-
ducing the filter is more of a slight change in sound
rather than a direct drop in low-frequency response and
strikes a better subjective compromise than
techno-striving for the ultimately flat, perfectly
measuring filter.

25.11.24 Equalization Control

Achieving bare response shapes of whatever
nature—high-pass, low-pass, bell-shaped bandpass, or
notch—does not really constitute a usable EQ system.
The shape, even if variable in frequency and bandwidth,
is either there or not, in or out, no subtleties or shades;
some means of achieving control over the strength of
effect is vital to the cause. By far the most common (but
certainly not the only) control requirement and one
easily understood by operators is lift and cut, where the
frequency areas relevant to the various filters are
required to be boosted or attenuated by any variable
amount within given limits. Determining these limits
alone is good for an argument or two, dependent on
such disparate considerations as system head room,
operator maturity, and, obviously, application. An EQ
created specifically for wild effects is not a stable
device. An adjustment of 20dB is not unknown (and
not, unfortunately, unheard); a 6 dB adjustment, in
contrast, is often far more than enough particularly for
spoken voice. A general median accepted by most
manufacturers is to provide between ±12 and ±15 dB
level adjustment on channel-type EQs.

25.11.25 The Baxandall

Hi-fi-type tone controls needed similar basic opera-
tional high-frequency and low-frequency boost-and- cut
facilities, and a design for this dating from the 1950s by
Peter Baxandall has since been an industry standard in
assorted and updated forms. A development of the
Baxandall idea is represented in Fig. 25-67 based Figure 25-67. Development of Baxandall-style equalizer.

Unity at center

RS RF

Vin Vo

(Low

impedance

source)

Max Min Vo = Vin × RF/RS

A. Infinitely variable gain-inverting amplifier.

Vin

Capacitors

form high-pass

filter with

potentiometer

B. Continuous variable high-frequency boost and cut.

100 k 7 2.2 NF

HF

LF

150 7 Vo

TL071

C. A belt-and-braces low-pass arrangement.

Vo

HF HF

Vin

D. An op-amp-based Baxandall-type high-frequency and

low-frequency shelving equalizer.

39 k 7

Pots = 100 k7Linear

Vin 470 7 4.7 k 7

2.2 nF

100 k 7 4.7 k^7

5.6 k 7 470 7

39 pF

100 nF

LF LF

220 pF 470 k 7

4 K 7

220 pF

220 k 7 2 M 7 220 k 7

180 k 7

330 nF

100 nF

EF86

6.3 V

1 M 7

+200 Vdc

3.3 M 7

4 K 7 47 MF

220 K (^7) 220 pF
E. A slightly more authentic Baxandall.