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

34

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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Image 16
Inova LA-2A manual Side-Chain Circuit, Output Circuit, Metering Circuit
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LA-2A specifications

The Inova LA-2A is a sophisticated optical compressor renowned for its classic design and exceptional sound quality. Hailing from the lineage of legendary optical compressors, the LA-2A combines vintage warmth with modern reliability, making it a staple in professional recording studios globally.

One of the main features of the LA-2A is its opto-electronic gain reduction circuit, which utilizes a light-dependent resistor (LDR) and a sealed optical element to achieve compression. This unique mechanism provides a smooth and transparent compression, allowing the audio signal to maintain its natural dynamics while controlling peaks effectively. The optical design contributes to a musical response, widely regarded as one of its defining characteristics.

The LA-2A also features a simple and intuitive interface. It includes essential controls such as Peak Reduction, which adjusts the level of gain reduction, and Make-Up Gain, which compensates for any lost level after compression. The device typically has two modes: the Compress mode for moderate compression and the Limit mode for more aggressive limiting. This flexibility makes it suitable for a variety of audio sources, from vocals to instruments.

Another noteworthy characteristic is its true tube circuitry. The LA-2A employs a vacuum tube in its audio path, contributing to its warm tonal character. This tube coloration enhances the richness of audio signals, making the device particularly popular for vocals and bass, where presence and depth are paramount.

Additionally, the build quality of the LA-2A is exceptional. The chassis is constructed from durable materials, ensuring longevity and reliability in demanding studio environments. Its vintage aesthetic features analog VU meters, adding to its charm while providing visual feedback of the gain reduction.

Inova has modernized the LA-2A with contemporary features while preserving its classic sound. Enhanced technologies such as improved signal-to-noise ratio and updated component designs ensure that the LA-2A remains competitive in today's digital landscape.

Ultimately, the Inova LA-2A is a beautifully crafted compressor that delivers a unique sonic signature. It combines vintage appeal with modern functionality, making it an essential tool for audio engineers and music producers seeking to achieve professional-grade mixes. Its smooth compression characteristics, intuitive controls, and durable design are testaments to its lasting legacy in the world of audio processing.
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