Monday, August 26th, 2019

Review of the Warm Audio WA-2A Compressor

By editorOctober 7, 2017


I had been working for a while with a singer-songwriter whose style ranges from more traditional R&B ‘up to’ soft rock.  Whilst his writing style was varied, the one consistent issue when recording, was the vast dynamic range of his voice.  I already owned a wide variety of VST plug-ins, including emulations of some of the classic compressors, including the Teletronix LA-2A.  However I was still struggling to get the vocal to bed well into most of the tracks, and as we were working with an album’s worth of material, I was beginning to get pretty worried.

I took a chance and bought, not one, but two Warm Audio WA-2As on spec.  The first time I mated the WA-2A with my troublesome vocals, I could feel all my worries simply melt away.  I had concern that a relatively slow attack compressor like the WA-2A would ever be able to ‘grab’ such a dynamic vocal style adequately but I was entirely wrong.  Not only was the WA-2A able to bring the lead vocal into the mix beautifully, but the previous need to ride the lead vocal fader throughout most of the tracks was minimised to occasional.  At times I was hitting GRs of 10dB on some of the choruses but the WA-2A never sounded anything other than natural.

Let’s have a look at the circuitry and front panel controls, and understand why this design (both the original and Warm Audio’s recreation) is rightly regarded as one of the world’s greatest compressors.

Audio inputs and outputs are transformer balanced using Cinemags which provide excellent signal handling and add a nice touch of colour.  The main audio paths use a total of four valves, two 12AX7’s, one 12BH7 and one 6P1 (instead of the 6AQ5 which is getting hard to get).  If you do prefer to hunt down an original 6AQ5 then Warm Audio have thoughtfully provided a socket so you can swap.

Warm Audio also helpfully provide a list of tube substitutes in the WA-2A user manual and were very helpful by e-mail when I enquired further about the best sources of spare values.

Before we even get into gain reduction, I have to say I now leave the WA-2A in my main vocal chain at all the times during mixdown, irrespective of whether gain reduction is required.  The WA-2A is particularly good at reducing mid frequency harshness in vocals without losing any vitality, in many cases much better than having to use EQ.

Okay so it’s a compressor; let’s get to that aspect.  The WA-2A uses a combination of an electroluminescent panel (driven by the side-chain audio) which illuminates on to a pair of photo-resistors (whose resistance varies depending on the amount of light falling on them) all housed in a light proof enclosure with the grand name of the ‘T4B optical gain-reduction module’

This type of gain reduction element has been around since the late 1950s but the T4B emerged through time as one of the strongest contenders to use in opto compressor designs and so is still popular to this day.

Although slow compared to some FET and VCA based compressors, the attack time of the WA-2A is respectfully fast (for an opto design) at 10ms whilst the release time is what’s sometimes called multi-stage programme dependant.  Warm Audio spec the release time at 60ms for 50% release and between 500ms to 5 seconds for complete release.  In practice, using the WA-2A as a vocal compressor, you do indeed see gain reduction ‘lingering’ for much longer than on say a typical VCA based compressor as the electroluminescent panel has a long ‘memory’.  At first I found this counterintuitive as I was so used to VCA compressors coming out of GR entirely between vocal phrases.  All I can say is that the end result sound entirely natural and you have a easy method, using only the Peak Reduction knob (there are no attack, release or ratio controls) to decide how you want to sit the vocal into the mix.

Part of the unique sound of the WA-2A derives from the complex compression curves.  First to say that the compressor slope is frequency dependant both in the shape of the knee (the soft transition from no compression to ‘full’ compression’) and the actual amount of compression at any specific peak compression setting.

The second compressor slope characteristic is that (at a specific setting) the rate of compression first rises with increased signal input (the knee), then enters a slightly more linear phase (corresponding to a fixed ratio) then backs off again (a kind of reverse knee) as you continue to increase the input level.  This is incredibly important to the sound as it prevents the WA-2A from over compressing when getting into heavy compression territory.  I suspect that none of this was intentional when the original ‘2A was designed, much more likely it was simply a characteristic of the T4 gain reduction module.

I’ve tried out the WA-2A on vocals on everything from ballads to out and out rockers and it works beautifully.

There are only two other operator controls on the front panel.  A rotary gain control with a massive 40db of make-up/output gain and a switch to select between compress or limit modes.  I nearly always use the compress mode on vocals if I am looking for a natural sounding vocal.  I only switch to the limit mode on vocals if I’m looking to affect the vocal sound in that direction.  It’s not a brickwall limiter as you would fine on some soft limiters but it does tighten up the audio to a much larger degree than the compress mode.

The limit setting works very well with bass and with all genres of acoustic guitar if you are looking for tighter control.

To complete the front panel tour, there is a nice large illuminated VU meter which can be switched to read either gain reduction or output level (either ref. +4 or +10).  I found that unity gain through the WA-2A was achieved by setting the output gain control at around 9 o’clock (the ‘20’ marking on scale) so I tend to use that as my starting point when getting into a mix session.

There is also a front mounted power switch which is good as it makes sense to leave the device powered down when not in use to preserve valve life.

Let’s take a look around the back …

Audio input and output are on both XLR’s and balanced ¼” jacks.  With the unit being fully transformer balanced, you can feed and load unbalanced with no issues.  Be aware that the input and output impedances are 600 ohms, so the unit will present a fairly low impedance load which some lower spec D-A converters might struggle to feed at high audio levels whilst preserving best audio quality.  Not a big point but be just to be aware.

There is a rotary knob to set the VU meter 0dB gain reduction reference and one further rotary knob to dial in a variable frequency high pass filter into the side-chain.  This was originally an FM radio broadcast requirement but can be used as a creative effect if you want to make the compressor less sensitive to high frequency transients.

I bought a pair of WA-2As with the intention of putting them across my stereo vocal subgroup, so the ability to link them for stereo operation is useful.  This is done using a standard ¼” stereo linking jack which is simply plugged across both units.  A rotary control is used to set up gain reduction matching between the two units.  The use manual explains how to do this and it takes only a few minutes to do and I’ve found it fairly stable in operation.

The unit is mains powered via a standard fused IEC socket with a 115/230VAC voltage selector.

As it advises in the user manual, this valve unit does get quite warm in operation so I took Warm Audio’s advice and left a 1RU rack space between the units as you can see above.

I know there will be many people who say you can’t tell the difference between an analogue hardware version of the ‘2A and a ‘good’ software emulation.  I suspect many of these people have never owned the hardware version.  To me, there is no comparison and I think you can guess which one I prefer!

The WA-2A is very good value version of the original Teletronix design.  The quality of construction internally is excellent and the unit uses a low noise torroidal mains transformer so hum levels are low.

Personally I would have limited use for a pair of WA-2As across the two buss but for most instruments especially acoustic stringed instruments, electric bass, and of course vocals, the WA-2A is really easy to use and gives superb results.

Highly recommended.

Robert Campbell
October 2017

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