LA502T Tube Optical Compressor for 500 series

$719.00

$719.00 VAT incl.

$719.00 VAT excl.

The LA502T is a soft knee, electro-optical tube compressor based on the LA2A design, which we have adapted to the 500 series format.

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LA502T

The LA502T is a soft knee, electro-optical tube compressor based on the LA2A design, which we have adapted to the 500 series format.

Building on the legacy of our acclaimed LA502, it delivers the effortless dynamic control of an optical cell, followed by the rich harmonic warmth and organic depth that only tubes and transformers can provide.

It shines on vocals, bass, drums as well as in full mixes.

The LA502T adds a few features to the original such as a new ratio of 6, two high-pass filter in the side chain and two slower attacks of 10 and 20 ms.

Install it in your API* compatible Lunchbox^TM rack and start working.

Highlights

Based on the LA2A compressor,
High voltage powered tube makeup gain circuit,
Transformers on input and output,
Extremely simple to use, never sounds bad,
True RMS compression,
Program dependent attack, release and ratio makes it very transparent sounding,
3 ratios : 3 (LA2A Compress), 20 (LA2A Limit) and one additional ratio 6.
Optional High pass filter in the sidechain @100 & 200 Hz,
3 Attack times : 5mS (LA2A), 10mS and 20mS,
Can be linked for stereo,
Hardware relay bypass,
Available assembled or as a full kit.

Controls

1: PEAK Sets the amount of compression from zero to maximum gain reduction. It defines the threshold at which the compression appears.

2: RATIO 3 positions switch that set the compression ratio.

Ratio 3 (left): This is the COMPRESS setting in the LA2A. The actual ratio value is program dependent and changes with the level.
Ratio 20 (center): This is the LIMIT setting in the LA2A. With this setting the LA502 works like a limiter.
Ratio 6 (right): This is an additional intermediary setting.

3: HPF This switch inserts a high-pass filter in the sidechain which produces a fat sound and eliminates bass driven pumping.

0 (left): No filter, works like the original LA2A,
100Hz (center): 100Hz high pass,
200Hz (right): 200Hz high pass.

4: ATTK Sets the attack time of the compressor.

5mS (left): This is the attack time of the LA2A.
10mS (center): We have added a delay in the sidechain allowing percussive sound to pass through less affected.
20mS (right): The delay is further increased.

5: ON/OFF/STEREO

ON (left): Compressor active,
OFF (center): Hardware relay bypass,
STEREO (right): Compressor active and stereo link connected for stereo operations.

6: GAIN This potentiometer sets the output level of the compressor.

7: Meter Shows the gain reduction from -3dB to -20dB.

What's in the kit?

Absolutely everything!

Passive components (resistors, capacitors, inductors),
Transformers
Active components (Tube, diodes, transistors, LED's, Opto-coupler, IC's),
Control elements (switches, potentiometers, knobs),
Connectors, relay,
PCB's,
Front panel and side panel,
All hardware.

You bring the solder (use good quality solder, 1mm diameter max) and the Lunchbox.

Difficulty level

The LA502 kit is a medium difficulty easy kit. It can be undertaken by everyone who is able to make a clean solder joint.

The setup is simple. It only requires a decent quality AC/DC voltmeter.

Reference PK029

Specific References

Technical specifications

Measure	Conditions	Value
Idle supply current	No input signal	V+ : +155 mA V- : -145 mA
Input impedance	f=1kHz	14 kΩ
Maximum input level	defined by input transformer	> +25 dBu
Maximum output level before clip	f=1kHz	+28 dBu
Minimum gain		- ∞
Maximum gain		+35 dB
Frequency response	Deviation < +/-0.5dB	10 Hz - 20 kHz
Attack time		from 5 to 20 ms
Release time		60mS for 50% release 1s or more for 100% release

Documentation

DOCS Code	Documents
From DOCS-24	User manual Parts list Components layout Assembly guide Setup guide Schematic

The LA502T is a professional-grade tube optical compressor designed by Sound Skulptor. Its architecture combines a transformer-balanced signal path, vacuum tube gain stages, and a sophisticated solid-state sidechain for precise dynamics control.

1. Primary Signal Path

The audio signal follows a classic high-end topology intended for "warmth" and harmonic enrichment:

Input Section: The signal enters through a balanced input and is coupled via the T1 transformer.
Optical Attenuation: Gain reduction is achieved through an optical element (OC1). This component consists of an internal LED that shines on a photoresistor. As the sidechain increases current to the LED, the photoresistor's resistance changes, attenuating the signal.
Tube Gain Stage: To compensate for signal loss and provide "tube character," the circuit utilizes an ECC81 (12AT7) dual triode (V1.1 and V1.2). These stages provide the primary makeup gain.
Output Driver: The final output stage uses 4 paralleled OPA2134 high-performance op-amps (U1.1, U1.2, U2.1, U2.2) to drive the T2 output transformer.

2. Sidechain and Control

The sidechain determines how the compressor reacts to the incoming signal without being in the direct audio path:

Level & Filtering: The sidechain signal is controlled in level by P2, the PEAK potentiometer then amplified by U7.1 and sent through a high-pass filter (HPF) controlled by SW2. Users can select between Off, 100Hz, or 200Hz to prevent low-frequency content (like kick drums) from over-triggering the compression.
Detection: The sidechain signal is then rectified by the diodes D5 and D6 before going to the attack time circuit.
Timing Constants: Attack time is managed by SW3. It offers settings such as 10ms and 20ms to dictate how quickly the optical element responds.
Stereo Linking: The switch SW4.1 allows the two sidechains to be mixed together for stable stereo imaging.

3. Metering and Power

Gain Reduction Meter (GRM): A ladder of six LEDs (LD1 through LD6) provides a visual representation of gain reduction. This is driven by LM339 voltage comparators (U3 and U4).

Power Supply: The unit operates on standard ±16V rails. However, because it uses a vacuum tube, a specialized SKTPS2 DC-DC converter (U9) is used to generate the high-voltage B+ rail and the heater current (H+/H−) required for the ECC81. This module includes two synchronized (anti phase) voltage regulators, working at a very high frequency (500kHz), generating very low noise.

Room mic (SK49) dry

Room mic (SK49) compressed with LA502T

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LA502T Tube Optical Compressor for 500 series

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