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Union Tube & Transistor LAB

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Page last updated Mar 14, 2026

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Description

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The LAB is an optical compressor. It uses custom optocouplers to achieve up to 30dB of musical-sounding compression.

We loved how in the recording studio even with tons of compression the old optical compressors such as the LA-2A, the LA-3, the LA-4 sounded smooth and natural.

The essence of the LAB's sound comes from the fast attack and slow release times of the custom optocoupler. The optocoupler's behaviour is quite complex, and program-dependent. It can't be modelled with simple resistor-capacitor networks. We wanted the pure response of the optocoupler to define the LAB's dynamics, and not supplement that with RC filtering.

To that end, we tried out just about every off-the-shelf optocoupler on the market, but ended up designing and manufacturing our own custom units. There's an interplay between dynamics, noise floor, distortion, and available compression on the one hand, and the opto's electrical parameters on the other. Since nothing commercially available fit the bill, we built our own.

With just two controls – compression and gain make-up – the LAB is fast and simple to use. It sounds great on bass and electric guitar, and the noise floor is impressive – quiet enough for studio use. It really shines on acoustic guitar, vocals and drums.

LAB Optical Compressor Specifications

Power Requirements +9VDC @40mA

Input Impedance 1MΩ

Output Impedance 100Ω

Frequency Response 20Hz–20KHz ±0.1dB

Maximum Input Level +7.5dBu

Maximum Output Level +10.0dBu

Compression/Gain-makeup 30dB

Noise Figure (Rs=10K) 3dB

For studio use you will need to attenuate going in and boost coming out of the LAB.

Product specs

Analog/Digital

analog

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BASSED.

Our FAVORITE Bass Guitar COMPRESSOR PEDAL, EVER! Union Tube & Transistor LAB Compressor

BASSED.

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Owner Insights

We analyzed real musician discussions from forums and Reddit to find what players love, question, and tweak about Union Tube & Transistor LAB.

Comparisons

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Union Tube & Transistor LAB is noted as an analog version of the LA2A, blending qualities of LA-3A and LA-2A designs, offering a unique tonal response.
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The LAB's digital competitors, like Source Audio Atlas, offer more features for a similar price, including multi-band EQs and USB connectivity for enhanced control.
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Users mention the Union Tube & Transistor LAB alongside other optical compressors like the Walrus Mira and PRS Mary Cries for its distinctive tone.
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Features and functionality

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The LAB pedal leverages light-dependent resistors, causing the VU panel light to correlate with the compression amount, mimicking the original LA-2A's visual and functional characteristics.
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Use cases and applications

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Some users find the LAB pedal intriguing for bass players, offering extra polish for live sets, although it's not traditionally used as a limiter.
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The LAB is favored for its placement at the end of a signal chain, where its optical compression enhances the overall sound without overshadowing other effects.
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User experience

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It's highlighted that the LAB pedal's perceived simplicity might not match its price point, with users comparing it to cheaper digital compressors with more functionality.
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It's reported that the LAB can replace other highly-regarded optical compressors like the Kongpressor, indicating satisfaction with its performance and versatility.
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Anthony Da Costa

Verified via Photo

Never posted a photo of this... my new rad pedalboard built by @xacttone in Nashville! Some of my favorite sounds on here...@uniontubeandtransistor @chaseblissaudio @wrenandcuff @bossfx_us @strymonengineering