AudioScape ASA-Variable‑mu Compressor

AudioScape’s take on the classic RCA valve compressor is much more than just a clone...

By Nick Mavridis
Published November 2025

Measuring a staggering five rack units tall, this behemoth, single-channel valve compressor undeniably has one of the poorest height-to-channel-count ratios! And with nine valves and five transformers employed in the design, the AudioScape ASA-6A will probably not win any medals when it comes to efficiency either. So you’d hope that what might appear to be an absurdly over the top design can cast some sort of magic on your audio. Does it manage to do that? Let’s find out...

Design & Improvements

Of course, it wasn’t AudioScape’s idea to claim so much rackspace! From the overall form factor, the RCA ‘umber grey’ chosen for the meticulously crafted housing, the big VU meter and both the bulky RCA-style knobs and the two red pull-knobs at the top, it’s pretty clear that the ASA-6A is based on the vintage RCA BA-6A variable mu compressor. Developed in the early 1950s, in that famously ‘bigger-is-better’ country near Greenland, the BA-6A found use mainly in radio stations, but over time it also became highly valued in music recording studios. Today, original units are very sought after and, you guessed it, awfully pricey. Naturally, then, there’s a demand for more reasonably priced recreations.

Florida-based AudioScape have quickly earned a reputation for building high-quality but relatively affordable US-made replicas of classic processors such as the UREI 1176, the Pultec EQP-1A and the Gates Sta-Level. It won’t be surprising to those familiar with the brand, then, that their ASA-6A manages to remain very true to the original RCA design, to the extent that it employs NOS valves wherever available, custom-wound transformers, carbon composition resistors and paper-in-oil capacitors.

Yet, AudioScape have also chosen to enhance the parameter set, to address some of the limitations of the original (for example, its fixed attack time). All of the original BA-6A’s values remain selectable (they’re clearly labelled ‘Stock’) but the number of front-panel controls has doubled. The user can control both input and output level and separately set the compression threshold, and can independently set the attack and release times too. There’s also a switchable 120Hz side-chain filter, as well as an attenuator that can be switched into the negative feedback (NFB) circuit, to increase both the level and harmonics. And while it’s a mono unit, the side-chains of two ASA-6As can be combined for stereo use. On the subject of controls, the chunky RCA-style knobs all look great, though I did feel they might benefit from a slightly higher turning resistance. The red knobs at the top are effectively drawer-handles: pull these, and the upper half of the front plate tilts forward to reveal all the transformers, valves, calibration pots and the power switch.

With so many valves used, both for gain reduction and amplification, it’s good that the valve types employed in the ASA-6A don’t, at the time of writing at least, cost a fortune to replace. This colossal compressor’s most important function is performed by one of its two 6SJ7 pentodes. It’s a classic variable mu design, in which gain reduction is achieved by shifting the valve’s bias; the hotter the signal, the more the grid voltage becomes negative, and thus the lower the amplification. With the classic BA-6A, the operator achieves compression by juggling the input and output levels, but the level inevitably influences the sound character too. You can work that way with the ASA-6A if you wish, but its separate threshold control means you have independent control over the harmonics and the gain reduction. In fact, you can even switch the Bal-Limit knob to its off position to disengage the compression entirely, and use the whole heap of valves and transformers as a wonderfully over-engineered saturation processor! Overall, there’s up to around 40dB of gain available, and up to about 30dB of gain reduction is achievable.

The labels Single and Dual refer not to any stereo-pairing configuration, but rather to the timing curves. Single means fast attack, slow release. Dual gives you the opposite, as well as affecting the range of the additional controls. The minimum attack time for Single is approximately 1ms, while the maximum release may reach up to 1s. Getting used to the interaction between the various parameters might at first seem a little challenging, but its not too complex and, as always, you’ll get to grips with it before too long.

There being so many valves in the ASA-6A, it’s no surprise that it runs quite hot — it could easily double as a coffee warmer or control-room heater! — so all that space in the massive box isn’t just for show; it means there’s room for air to circulate and for heat to dissipate. And speaking of warm valves, allowing half an hour for the ASA-6A to warm up before use is definitely a good idea, since in the first minutes after switching on it sounds somewhat glassy and flat to my ears.

Big Box, Even Bigger Sound!

Once up to temperature, though, the AudioScape ASA-6A definitely sounds the way it looks! Even without compression and without driving it too hard, this is a processor that enriches and slightly softens every signal in a pleasing way. Indeed, almost all signals seem to benefit from the classic sonic enhancements that we attribute to American valve equipment: everything immediately sounds bigger and more full-bodied. It makes me wish I could patch one of these in every one of my console’s channel inserts.

That said, there will obviously be times that you aren’t looking to add this sort of character. The transformers in the ASA-6A can make signals sound a tad ‘grainy’, and a little detail gets lost. Driving the amps too hard may result in wooliness and lead to a fuzzy and ‘wobbly’ bass sound. If you notice any of that, though, you can just dial things back a little, and you’re set. And in general, it sounds so velvety, soft, round and warm, that it always puts a smile on my face instantly. Everything run through it just sounds more... expensive!

Still, I doubt that anybody would buy such a bulky and expensive valve processor for saturation purposes alone. The gain-reduction behaviour is every bit as important, and the advantages of the variable mu topology are exemplary in the ASA-6A. Whereas FET and VCA compressors can seem to grab your audio with a firm hand, the ASA-6A eases into its compression, with a smooth transition and a non-linear, natural recovery, that leaves you with the impression of a ‘thoughtful’ and unobtrusive narrowing of the dynamic range. With Bal-Limit set to Dual and the timing parameters set to Stock, the compression seems to be ‘intelligent’, by which I mean that it rides the level in a very natural-sounding way that makes me want to reach — somewhat reluctantly, since I feel it’s overused in audio gear reviews, but it’s very much accurate here — for the term ‘musical’. That’s definitely the case with vocals, electric bass guitar and overdriven electric guitar, but it can also tame pianos, acoustic and clean electric guitars perfectly.

Many other compressors, some variable mu models among them, are prone to ‘ironing out’ the signal. In other words, although they do offer control over the dynamic range, they can often seem to render the signal flat, narrow and rather lifeless. That’s not the case with the ASA-6A: the more it reduces the audio, the fatter the sound seems to become, but it always remains ‘natural’, even with hefty gain reduction, and doesn’t get excessively noisy either. To my ears, this is the standout feature of this compressor.

Set to Single mode, the ASA-6A becomes more vivid, and more ‘three-dimensional’. It never loses its main characteristics and doesn’t get ‘fidgety’, even when allowed to pump. Even with a slower attack time, it’s hard work trying to make it sound ‘ploppy’ and ‘thwacky’. I find Single mode better for beefing up bass and snare drums, or making brass instruments step forward in the mix. Loosening the leash a bit by flipping the NFB toggle switch makes the ASA-6A show some feral fangs.

I have to offer praise to AudioScape for implementing the added parameters, particularly the attack, release, side-chain HPF and separate control of the threshold and input gain. I realise that they’re controls found on lots of compressors today, but it would have been easy to make a ‘lazy clone’ of the RCA device, and their inclusion here really adds to the versatility and appeal of a wonderful-sounding classic compressor. Generally, I prefer to keep the side-chain high-pass filter engaged almost always, and I find being able to change the attack time particularly helpful on a compressor like this.

I thought it might help readers if I were to offer some points of reference, so I compared the ASA-6A with some other variable mu compressors in the studio: the Amtec Model 099 and Electric & Company’s EC5B. The 099, which is descendant of the a Fairchild 660, generally comes across as sounding more refined and controlled, with a slightly tighter bass response. The EC5B, on the other hand, is modelled broadly after the UA 175, a valve-based predecessor of the more famous 1176, and which seems less ‘rounded’, and both punchier and more aggressive-sounding. But words can only tell you so much, so I’ve also created some audio examples that you can find on the SOS website at https://sosm. ag/audioscape-asa-6a.

Verdict

So, just how helpful is a single-channel compressor that occupies more rack space than some consoles, and looks like a component warehouse had to be stripped half-empty just to get it built? A good analogy would be something like a Coles 4038 ribbon mic: the design may seem anachronistic, but the sonic results speak for themselves. The ASA-6A is amazing. It makes literally everything sound better, even when it’s not compressing at all. And, equipped with very useful added and enhanced parameters, this beast of a valve processor is far more versatile than the RCA device it’s based on. Its best job is controlling the dynamics of any type of signal in a very natural way, while simultaneously making it appear ‘bigger’. To address my initial question, then, yes, there’s some magic in this unit, and yes, your signals will benefit. But is it worth the cost? I realise that not everyone can afford or justify blowing this much cash on a compressor. So let me put it this way: you might not need one, but try it and you’ll certainly want one!