Inova LA-2A manual Side-ChainCircuit, Output Circuit, Metering Circuit, A-24

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

							R16
	VOLTAGE						68K	C2
				CATHODE
	AMPLIFIER
				FOLLOWER
			R16
	C7C	R13	68K					.1
	30uF	220K
R9				R17	10K	R21
	C1
220K			C2			100K	C5	A-24
V1A	.02		V1B .1	V2A			10 uF	1
FROM						C3
R1						.1
	R11	12AX7
	68K

			R15	R18
R10	R12	R14	470K	1K
1.5K	470K	2.7K

V2B

2

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5

	12BH7
R20	R19	8
1K	470K

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

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