910 Chapter 25
to some op-amps for glue. A simple, very low parts
count compressor with all the active elements contained
within this part is shown in Fig. 25-85. As can be seen,
it relates strongly to the block diagram of Fig. 25-84.
25.13 Mixing
25.13.1 Virtual-Earth Mixers
The circuit diagram of Fig. 25-86 in its simplicity belies
the hidden design that is in the relationship of the
circuitry to its mechanical and electrical environment.
This is where the care and feeding of op-amps
(Section 25.7) and grounding paths (Section 25.8) really
pay dividends. Mix-amp stages, with large numbers of
permanently assigned sources such as in the main mix
buses, are as crucial to the overall well-being of a
console as any front-end stage could be. In a typical
situation, as a unity-gain virtual-earth mixing stage with
33 sources (channels plus access), the amplifier is being
asked for about 30 dB of broadband gain, as much as
any other stage in the chain including both the micro-
phone preamp and/or secondary input stage.
25.13.2 Noise SourcesAll the following about mix devices assumes that
system grounding is impeccable. Jolly good. That said:
That mix-amp gain is sometimes referred to as noise
gain is not accidental. Unless care is taken to balance
fader-back channel noise contributions against this
self-generated mix-amp noise, the latter could well
predominate and arbitrarily determine the noise floor
for the entire console. Similarly, channel noise contribu-
tion should equal or outstrip mix-amp noise, but not
excessively so: ideally they should equally contribute,
to the extent that channel-off noise contribution should
not necessarily impact the overall bus noise, while bus
noise should not significantly impact channel-on noise.
Self-noise generation in the mix-amp is predominantly
the amplified thermal noise of the paralleled source and
feedback resistances, device input current noise, and
surface generation and recombination noise. The last
two can be minimized by device choice. Thermal noise
is physics and is here to stay. Common sense on first
glance says to make the mix resistors as low in value as
possible but this has the downside that too low a value
would cause quite large signal (hence, ground) currentsFigure 25-85. A feed-forward VCA compressor using THAT 4301.10 MF10 MF10 MF10 MF10 MF+15 V++15 V++(^1) IN
2
4
7
5
6
8
0
- 012
2 M
15 V
+15 V
1Ve
Ve
10 k
Precision rectifier
rms detector averager
Indicates 3300 ppm temperature
compensating resistors
Thresholder
Threshold
Ratio attenuator Summer
IT RMS
5.1 k
28.8 K
OUT
CT
15V
IN 4148
IN 4148
22 pF 100 nF
300 k
392 k
20 k
1.43 k
1.2 M
"Fader" input
18
19
20
0
1.65 M
50 7
15
14
169
11
10
20 k (^17) VCA 13
150 R
20 k
20 k
5 K1*
100 nF
100 pF
100 nF
10 k
10 k
Ratio
+15 V
15 V