Consoles 889filter input out of two capacitors such that the input
signal is attenuated by the needed amount yet the
combined capacitance value is the correct value for the
filter—this can be a bit of a nightmare to drive
adequately, though. For many applications the free 4 dB
or so isn’t a problem—it can simply be assimilated as
part of the system-level architecture.
The 4 dB thing can be a nuisance. Where it is, or in
particular where an inverting filter stage is either conve-
nient or necessary, the multifeedback configuration
works well; indeed, lacking the problems of a
near-follower as in the case of the Sallen-Key it uses the
op-amp well. At high values of Q or extremes of
frequency, some component values can get far from the
ordinary midimpedance values seen elsewhere in the
EQs and filters described here, and one should be aware
of possible noise or op-amp current-drive limitation
issues as a consequence. Unlike the Sallen-Key
described, it is not readily possible to continuously vary
the turnover frequency, and it uses three capacitors as
frequency-determining components rather than two.
Otherwise, for fixed frequency filters this is a very
friendly topology.25.11.23 Second or Third or More Order?Without delving too deeply into psychoacoustics, the ear
notices easily third or more order filters being introduced
for much the same reasons as a high-Q bandpass filter is
obvious. There are severe modifications to the transient
response of the signal path and ringing-type time-related
components are introduced into the signal spectrum.
An application where this effect is not overly objec-
tionable is where the filters are defining bandwidth at
high and low audible extremes. Within the audible band
though, the ear is quite merciless toward such artifacts.
The transient response modification and time-
domain effects are not the end of the story; the relation-
ships between instrument fundamentals and their
harmonics in the turnover area of the filter are likely to
be interpreted as unnatural, especially should the funda-
mental be attenuated with respect to the harmonics.Figure 25-66. High-pass filter development.A. A simple inductance-capacitance high-pass filter. B. High-pass filter, with critical damping.C. Substituting a synthesized inductance.D. Circuit C redrawn revealing classic
Sallen-key high-pass filter.E. Introducing gain, enabling conveniently equal values of
resistance and capacitance to be used.F. Simple high-pass resistance-inductance filter.G. Simple high-pass resistance-capacitance filter."Bootstrap""Bootstrap"4 dB gain
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