Handbook for Sound Engineers

Consoles 835

while across 10 k: it would be minus 12.22 dBm. But
it’s still 0.775 Vrms.

The reference level for zero impedance thinking is a
voltage, and the one chosen is the familiar 0.775 Vrms
with which everyone was historically used to dealing.
That voltage is distinguished as 0 dBu. Some have tried
to impose a universal reference based around a voltage
level of 1 V called the dBV for audio, which is easily
divided by 10 but has proved sufficiently confusing to
anyone brought up on the dBm that it is now all but
dead.

But wait! There’s more! The ubiquitous VU meter
when implemented as intended imposes a nominal
system level of +4 dBm (0 VU = +4 dBm at 600:),
and in territories and market segments where the VU
reigned, +4 dBm (and latterly +4 dBu) is still a common
reference. And try as one might, it is impossible to
ignore that there is more semipro recording and audio
gear in use than real audio equipment and that generally
uses the domestic level of 10 dBu as a nominal refer-
ence. Glad that’s all cleared up, then.

25.7 Operational Amplifiers in Consoles

Consoles utilizing integrated circuit operational ampli-
fiers (IC op-amps) have suffered from a curious
syndrome, collecting in earlier days a (sometimes
deserved) dreadful reputation, which has stuck. This
section is an attempt to explain the history, shortcom-
ings, and attributes of IC op-amps from conception to
present day, to point out how some shortcomings are
overcome and to provide reassurance that they are the
future of consoles. It is also an example that this, along
with most other technology, is well understood and
quantified, the concepts if not the details having been
defined many years ago, Fig. 25-16.

When ICs first came out (the Fairchild μA709, e.g.,
they were expensive, prone to oscillate, and had no
short-circuit output protection.

At this stage in the game, discrete transistor circuitry
ruled supreme in pro-audio while considerable
vacuum-tube gear was still in use. Techniques expanded
and ICs were tamed sufficiently to remain operationally
stable, but little high-frequency loop gain remained to
guarantee enough feedback to adequately reduce
high-frequency distortion. Also, they were very noisy.
Although their parameters could be set up to be accept-
able for any set application and gain setting, the very
nature of control in consoles is variable, so the devices
almost inevitably ended up operating away from their
optimum. Figure 25-16. Basic op-amp configurations.

F. Basic half-wave precision rectifier.

A. Voltage follower.

B. Noninverting amplifier.

C. Inverting amplifier.

D. Virtual-earth mixer amplifier.

E. Differential amplifier.

Input

voltage

Vin

Output

voltage

Vo

Vin R^2

R 1

Vo

R 1 R 2

Vo

Vin

1 2 3 4 5 6

Vo

R 1 R 2

Inverting

Vin

Floating Vm

Noninverting

Vin R^3

Vo

R 1 = R (^2) R 4
R 3 = R 4
Input
Half-wave
Resistor output
Vin