Handbook for Sound Engineers

934 Chapter 25

level by 6 dB or 10 dB helps matters tremendously, and
the gain is made up either in the mix-amps or the post-
fader buffer amps (the latter being normal). This does
compromise bus noise (quiescent console output noise),
but since the main justification for doing it is the high
level of signals present, the pluses outweigh the
minuses. This depressed channel system is worthwhile
in any circumstance, regardless of metering type, where
there is likely to be a great unknown lurking on the end
of an input line.
Some of the disadvantages are that all the channel
insert points operate at the depressed (10 dBu) level,
which may or may not give problems in some less than
versatile outboard devices. The more immediate
concern is that other internal channel circuits will need
adjusting.
Machine line-in feeds from the Ac and Bc input
differential amplifiers will need to be dropped by 10 dB.
This drop is easily accomplished by altering the values
of the resistors around electronic switches to scale down
a factor of 3.16 (10 dB), as shown by Fig. 25-104. The
PFL bus mix-amp gains are required to increase 10 dB
(the extra bus noise here is no great crime), and an extra
10 dB of gain is put into the prefader auxiliary feed

buffer amplifiers. Reestablishing main path gain to

unity is simply achieved by upping the gain of the

post-fader buffer amplifier in Fig. 25-104 and by

changing the feedback bottom leg resistors in Fig. 25-96

from 1.8 k: to 430:. This provides for 10 dB of fader

back-off and the necessary 10 dB reinstatement.

If all that sounds complicated, just bear in mind that

it’s achieved with gain changes—in this case just with

resistor changes. It doesn’t matter that the machine

monitor differential input amplifiers are still operating

at a normal undepressed level. The Ac check is directly

monitoring a console output, which is at normal level

anyway, so there is no head room problem. As for the Bc

check, if we have more level coming back from the

machine than we’re putting in (A' check), then it’s time

for realignment.

It is entirely possible to recalculate the values around

the differential amplifier to drop 10 dB and still maintain

input balance, but that would greatly increase the

number of component changes necessary to alter channel

system level. This is no mean consideration should you

choose to do so on a console of 32 or 48 channels.

Ultimately, it is up to the designer to make the

product—this mixer—as transparent and free for the

Figure 25-103. System level architecture.

Input

amp

Input

A.

B.

C.

Pre

insert

EQ

Post

insert

Fader Bus

Mix/line

amp Output

A. Flat – 0 dBu referred throughout.

B. Depressed – 4 dBu throughout, level made up passively by output transformer.

C. Channel Depressed – 10 dBu in channel, 0 dBu output.

Post

fader

buffer

amp