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11
Side-Chain Circuit
The previously described gain reduction circuit is controlled by the control voltage which is
supplied by the side-chain circuit. The LA-2A is a feed-back style compressor. This is due
to the fact that the signal that is used to drive the side-chain circuit is affected by the gain
reduced signal. This signal is first fed into the “Peak Reduction” potentiometer (R2), which
controls the amount of side-chain drive and in turn controls the compression threshold and
amount of gain reduction. A 12AX7 is then used as a voltage amplifier to increase the
signal level. A pre-emphasis circuit is provided on the output of the 12AX7. Originally
designed for broadcast, the LA-2A allowed for side-chain equalization, which allowed the
operator to make the compression more or less sensitive to the voice frequency bands.
For musical applications, this equalization is usually set to a flat frequency response.
Subsequent to the filter, a 6AQ5 provides the signal necessary to drive the electro-
luminescent panel. EL panels were often used for night-lights and hence are usually
designed to be driven with 120 volts, 60 Hz AC. They were not designed for audio, and
applying the wide-bandwidth signals that arise in audio applications results in a shortened
lifetime of the part.
Output Circuit
Figure 7 - Schematic diagram of the LA-2A output circuit.
The output circuit is comprised of a 12AX7 which operates as a voltage amplifier followed
by a 12BH7A which operates as a cathode-follower. This is followed by the output
transformer, which provides impedance matching and a balanced output.
Metering Circuit
The metering circuit in the LA-2A has 3 modes selected by a front-panel switch, allowing
for output level monitoring at +4 and +10 dB as well as gain reduction. As mentioned
R9
220K
R10
1.5K
R12
470K
R14
2.7K
R11
68K
C1
.02
V1A
C7C
30uF
R13
220K
C2
.1
R16
68K
R17
10K
R15
470K
R18
1K
R20
1K
R19
470K
C3
.1
R21
100K
C2
.1
R16
68K
C5
10 uF
1
3
4
8
A-24
2
5
V1B
12AX7
12BH7
V2B
V2A
CATHODE
FOLLOWER
VOLTAGE
AMPLIFIER
FROM
R1