844 Chapter 25
So much for the expected result of improved tran-
sient response through having a wide-open frequency
response. As is now obvious with hindsight, deliberately
limiting the input frequency response of the mixer to a
little more than the audio band results in an amazing
cleanup of the sound. By removing a lot of the inaudible
signals that cross modulate within themselves and with
in-band signals, the cause of much of the lack of trans-
parency and mush that had become the trademark of
early-generation IC op-amp consoles is eliminated.
Despite improved devices with greater open loop
gains at far greater bandwidths, this approach remains
valid today. By band limiting the program signal to
reduce inaudible signals as early in the chain as
possible, there is far less chance of their generating
unwanted audible products. A front-end low-pass filter,
operating in conjunction with all the other low-pass
effects of feedback compensation arrangements
throughout the console, should provide adequate mini-
mization of these products in modern devices.
Purist arguments about the undesirability of any
deliberate filtering seem rather futile in a world of real
devices—all but a very few transducers and what they
hear/reproduce are an embarrassment above 20 kHz,
and final signal destinations—like anything digital, or
otherwise inherently band limited. However, with
96 kHz digital sampling threatening to become main-
stream, widening the window to at least utilize some of
the fabulously hard-won bandwidth may be in order in
systems where such is likely. Band limiting, to whatever
sane degree, is a particularly powerful tool for obviating
funny noises and lack of sonic transparency, and its use
shouldn’t be abdicated without a fight.
25.7.13 Slew-Rate Effects
Slew-rate limiting occurs when the fastest signal rise
time the amplifier is expected to pass exceeds the speed
of the fastest stage in the amplifier; the input transient
becomes slurred to as fast (or slow) as the amplifier’s
capability. It is a level-dependent effect; at low levels
the input signal’s transient may be well within the
amplifier slew envelope and escape unmutilated, but as
the input gets larger the transient’s slope can equal or
exceed that of the amplifier.
Slewing gives rise to intermodulation effects that are
dependent upon both frequency and signal level. The
louder and faster the input transient, the worse the
damage. A common subjective result of this limiting is
for the high end of a drum kit to change in character of
sound with differing levels of the lower-frequency
instruments on which it is riding. Another favorite is the
“disappearing snare drum” in which, again, the sound
radically alters with changing level.25.7.14 Device Idiosyncrasies and the FutureMany circuits rely somewhat on the extremely high
input impedances of the BiFET devices and their very
low required input bias currents. Using bipolars every-
where may result in unavoidably generated output offset
voltages that could manifest themselves in extreme
instances as switch clunks and scratchy pots. Also, the
feedback phase-leading compensation may or may not
be appropriate for devices other than BiFETs, especially
with some bipolars with less than ideal internal poles. If
there’s a temptation to use more conventional bipolar
devices, particularly those in multiple packages, it is
also worthwhile examining their characteristics when
inputs or outputs are taken above or below the power
supply potentials. If the device structure under such
circumstances is unprotected and turns into a
silicon-controlled rectifier that deftly shorts the power
supply with a bang, you are possibly better off using
something else. In short, if a device is chosen specifi-
cally for an application and support circuitry designed
for it, adding another device for the sake of adding it is
usually nonproductive and often a step back. Op-amps
and their surrounding components should be regarded
holistically.
The proliferation of amplifier elements in modern
console design has mushroomed further in recent years
with the availability of compact and extremely low-cost
IC op-amps. Increasingly complex functional blocks are
becoming increasingly commonplace. If, in order to
improve their electrical and sonic characteristics, it
would mean an increase in size and cost of well over an
order of magnitude, would they still be quite as
popular? In the good old days of tubes, it was not
through any lack of expertise that equalizers even of
today’s complexity did not exist; it was just the size and
cost would have made even the reckless shudder. Also,
it is to be noted, they were not really thought necessary.
By way of history repeating itself, though, the
astounding complexity of many digital audio algorithms
(e.g., the use of as many as nine biquads to achieve not
a whole EQ, just a single section, which alone would
require a mere 27 op-amps to emulate) makes concerns
about analog technology overkill seem a touch quaint.25.8 GroundingA human working visualization of anything electronic
soon becomes impossible without a mental image of the