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Warm Audio WA-2A Tube Opto Compressor

Affordable Unit Revisits a Gain Reduction Classic

The WA-2A Tube Opto Compressor/Leveling Amp is Warm Audio’s rendition of the classic Teletronix LA-2A Leveling Amp, developed in the early 1960s by engineer Jim Lawrence who wanted a way to automatically “level out” widely varying audio levels at the broadcast station where he worked.

Under a single control knob, a leveling amplifier is a compressor with a medium attack time, a medium to slow release time, and a high compression ratio. For program broadcast audio sources, a leveling amp’s threshold is set low enough so that it is in a constant state of varying amounts of gain reduction.

The WA-2A copies the Teletronix LA-2A with the same attack time of 10ms and a release time of 60ms for the first 50 percent of recovery and then, depending on the depth of compression, 500ms to 5 seconds or longer for complete recovery back to unity gain. Total release time relies heavily on the duration of compression and level of the incoming signal.

Like the LA-2A, the “engine” for controlling gain in the WA-2A is an optical attenuator. Warm Audio has chosen the Kenetek T4B module that’s capable of up to 40 dB of gain reduction. The T4B module plugs into an octal tube socket and is a pin-for-pin direct replacement for the original LA-2A’s module.

The nonlinear release timing and gentle, almost lazy operation of the WA-2A are caused by the “memory effect” of the photocells inside the T4B. It takes time for them to recover depending how bright the panel glows and for how long it has glowed.

Simple Front, Back Panels

The WA-2A is built on one main circuit board and packaged in an all-steel 2U cabinet with the built-in 120/ 240-volt power supply that uses a large toroidal power transformer in a shielded enclosure. It features a regulated B+ power supply and uses DC on the filaments—two things the LA-2A never had.

The front panel has a center-mounted lighted VU meter and large toggle switches for power and Limit or Compress—different compression ratios. There are both Output Gain and Peak Reduction controls and a VU meter switch for reading +4 or +10dB output levels and gain reduction. I liked the +10dB scale for measuring the unit’s hot output levels and for calibrating it.

The back panel has controls for Stereo Link calibration for chaining two WA-2As via a short TRS cable, VU meter zero-adjust, and a variable Pre-Emphasis control for boosting high frequencies in the sidechain signal.

Pre-emphasis dates back to the LA-2A’s use as a limiter at broadcast stations. For noise reduction purposes, FM broadcast audio uses a 75-microsecond pre-emphasis curve—a +6dB/octave boost starting at 1.5 kHz and reaching up to +17 dB or more by 15 kHz. FM home receivers reverse this curve or de-emphasize it. But, if not well controlled with a limiter, this massive high-frequency boost may over-modulate the transmitter, causing side-band “splatter” interference to adjacent radio channels.

The WA-2A’s Pre-Emphasis control, at full clockwise, offers a flat sidechain response; turning it CCW starts to compress more in the high frequencies.

However, with the WA-2A installed in my outboard rack, meter zeroing, adjusting stereo linking and using pre-emphasis for creative purposes are impossible to do easily. I plan to move all three of these controls to the front panel on both of my WA-2As.

Leveling With the WA-2A

I used the hardware inserts in Pro Tools for each of my two WA-2As, as I wanted different settings for the verse and chorus lead vocals on separate tracks. Inserted after a touch of gentle midrange EQ from a plug-in, the WA-2As provided a finishing touch with a hard-to-describe presence and color only a quality tube opto-compressor gives to vocals. Both WA-2As were set to no more than 3 to 5 dB of indicated gain reduction, and both were in Limit mode. I found these well-recorded vocals to sit in the track perfectly, always heard but never too loud.

Next, I inserted the WA-2A on the song’s fretless bass guitar track, where the lyrical and solo-like up/down octave playing style required major leveling out to work in this pop song. Using a single WA-2A, I compressed heavily with gain reductions of over 10 dB. I did not pick up any additional distortion, and I noticed and liked the loud attacks at re-entry moments after long note sustains finished and when the bass came back in on downbeats. I had to edit out a few fret pops and other noises that became much more audible, but it was worth it for the cool, organically smooth and musical bass sound.

Using pre-emphasis fully CCW on two WA-2As on a parallel stereo drum bus (without the kick drum) opened up the overall drum sound by compressing less on the snare hits and tom fills. Compressing all of the top kit tracks usually brings out the cymbals excessively, but using maximum Pre-Emphasis caused the cymbals to level out and the snare and toms to come forward.

When setting stereo-linked compression, I used 1kHz tones and pink noise to set the Stereo Adjust control for matching and calibration. Once fully warmed up, I found little drift in this setup throughout the session—something I cannot say for some of the LA-2As I’ve used in the past.

I often found that using the Pre-Emphasis control is a great way to refine the leveling action—particularly on bright sounds or bass-heavy sources. Like the LA-2A, the WA-2A is an archaic electronic design that uses tubes and optical attenuators that both age and change over time. So while the WA-2A is a vast improvement in reliability and stable operation over the LA-2A, you won’t find the super-precision, exactitude and repeatability of modern processors or DAW plug-ins. What I found was a lovely organic, smooth color that sounded awesome on vocals, bass and stereo drum overheads. There is nothing in my outboard rack or plug-in folder like this. I own two, and I highly recommend that any engineer get one or two of them!


COMPANY: Warm Audio Inc.

PRODUCT: WA-2A Leveling Amp



PROS: Classic tube leveling amp sound.

CONS: Important controls are on the rear panel.

Try This

Warm Audio has thoughtfully provided an extra 7-pin ceramic socket wired in parallel with the 6P1J; it will accept the no longer reliably available 6AQ5/6005 as originally used in the LA-2A. I substituted a 6AQ5/6005 in one of my WA-2As and compared the sound difference using the same source fed to both units at the same time. After re-zeroing the VU meter, I had to turn the Peak Reduction knob three detents higher to achieve the same amount of gain reduction (indicated) as the other stock WA-2A. I preferred both units with the 6AQ5/6005. I found the sound closer to an original LA-2A, plus I found the units to operate at cooler temperatures.

Barry Rudolph is an L.A.-based engineer and educator. Visit him at