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McDSP Royal Mu, Royal Q & MC-3

APB Plug-ins By Sam Inglis
Published April 2021

McDSP MC‑3 multiband compressor.The MC‑3 multiband compressor from McDSP.

McDSP’s unique analogue plug‑in platform continues to grow, with three impressive new processors joining the collection.

Hybrid is hot right now, and lots of people are finding novel ways to integrate analogue processing into computer‑based mixing. The most radical and, to my mind, the most successful is McDSP’s Analog Processing Box or APB platform. To the user, APB processors look and behave almost exactly like conventional plug‑ins. Behind the scenes, however, these plug‑ins are sending your audio down a Thunderbolt cable to be manipulated by a highly sophisticated, configurable array of analogue gain and saturation processors.

Two APB units are currently available, respectively supporting eight or 16 mono channels of processing. These represent a significant investment, but unlike UA’s UAD2 and other digital co‑processing systems, there’s no additional charge for plug‑ins. In other words, all APB plug‑ins are free to all users. That includes the superb Moo X valve‑style mixer that was added to the range last year, and it now includes three new plug‑ins which bring the total number available to 10.

Rebel MC

As already mentioned, the APB’s analogue circuitry is designed for gain control and saturation rather than frequency‑based processing. This has always seemed a sensible decision to me, since compression and ‘analogue warmth’ are widely felt to be areas where digital plug‑ins struggle to compete. However, one of the clever things about the APB format is that the plug‑ins can also incorporate host‑based digital processing, not only at the start and end of the signal path but also in control signals such as compressor side‑chain feeds.

The fruits of this flexibility are obvious in the MC‑3 multiband compressor (shown above), which uses digital filtering to divide the input signal into three frequency bands. These are then sent to separate channels of analogue APB processing to be compressed and saturated, before being recombined on re‑entry to ‘the box’. Each band offers the usual Threshold, Attack, Ratio and Release controls, plus its own Saturation dial. Bands can be soloed and muted, and an external side‑chain input is freely selectable by any band if required. Each band also has its own ladder‑style gain‑reduction meter in addition to the global VU meters.

As McDSP’s documentation points out, true analogue multiband compressors are rare beasts, and command eye‑watering prices. By contrast, there are many purely digital plug‑in multiband compressors, but these are mostly designed to be transparent rather than colourful. I’m sure I’m not alone in that I usually reach for multiband compression when there’s a problem to be solved, not when I want to add character and grit to a source.

All of which makes MC‑3 refreshingly different. Its retro ’70s interface chimes well with the sound, which employs fairly aggressive and ‘grabby’ FET‑style compression. Unlike some of the APB plug‑ins, the saturation here is not especially subtle, and by cranking up both the Gain and Saturation knobs you can drive a band into really blown‑out grunge. It’s brilliant for adding attitude to underwhelming drum loops, DI’d bass guitar, rock & roll vocals and more. And, of course, there’s nothing to stop you using it in more conservative ways on the master bus or for more subtle tone‑shaping. The only thing that stopped me using it more was that a stereo instance takes up six APB processing channels, and I wanted to use them for other things!

Royal Icing

Hot off the presses at the time of writing are a rhyming pair of new plug‑ins called Royal Mu and Royal Q. Whereas MC‑3 is perhaps happiest titillating individual sources, this Royal duo are most at home on the master bus.

As the name implies, Royal Mu is inspired by classic vari‑mu compressors. These, of course, operate on the principle whereby a control voltage is used to adjust the gain of a remote‑cutoff valve. There are no valves in the APB, so in effect, solid‑state analogue circuitry is being used to emulate valve behaviour.

McDSP's Royal Mu plug-in.McDSP's Royal Mu plug-in.

Unlike the MC‑3 plug‑in, Royal Mu uses only one APB channel per audio channel (so one channel for mono or two for stereo), even though it implements several stages of analogue processing. As well as a compressor with the usual Threshold, Ratio and time‑constant controls, Royal Mu also offers a peak limiter and an output saturation circuit. Stereo instances can operate in dual‑mono, left/right or mid/sides modes, but possibly the most interesting feature is the Bias control.

This is perhaps a confusing name, since it has nothing to do with the bias of any emulated valves; rather, it shapes the frequency response of the compressor, in a subtle but noticeable way. Positive bias values tend to allow more low‑frequency content through, while negative values emphasise the upper midrange. Conceptually, it’s a little like the Thrust control found in some API compressors. There’s also a gentle high‑frequency shelving EQ with a fixed cutoff and slope; this is hard‑wired before the compressor and can be useful to counteract the tendency of some compression settings to dull the top end.

[Royal Mu] Even if you push it into obvious compression, it still sounds smooth and rich, and although you can use the EQ and the saturation stage to make the mix significantly brighter, it never gets harsh.

Royal Mu is one of those processors that sounds great immediately, but also offers a wider range of sounds than you realise at first. People often talk about a good bus compressor having the ability to ‘glue’ the mix together, but in my experience, they seem to mean different things by this; at any rate, the ‘glueing’ action of an SSL‑style compressor is rather different from that of a Manley Variable Mu! As you’d expect, Royal Mu falls more into the latter category. Even if you push it into obvious compression, it still sounds smooth and rich, and although you can use the EQ and the saturation stage to make the mix significantly brighter, it never gets harsh. There’s a lot of sonic territory to explore in the interaction of the Gain and Peak (limiting) controls. It would be nice if the meters could be made to display gain reduction from the limiter as well as the compressor, but that’s a minor criticism, and certainly doesn’t stand in the way of Royal Mu being my new favourite mix compressor.

Join The Q

As I mentioned earlier, one of the clever things about the APB system is that it allows analogue compression and saturation to be combined with digital equalisation and other processes. In the Royal Q plug‑in, it’s the digital EQ that takes centre stage. Classic passive equaliser designs such as the Pultec EQP‑1A use an active gain stage to make up the level lost in the EQ circuitry, and some of their distinctive character comes from the valves and transformers used in that gain stage. You could think of Royal Q as applying the same concept, except that here the analogue gain and saturation follows a digital EQ algorithm.

McDSP Royal Q plug-in.McDSP Royal Q plug-in.

An unusual feature of Royal Q is its ‘double shelving’ architecture. Each of its four bands can operate in fully parametric mode, with continuously variable control over frequency, gain and bandwidth; but each can also be switched to shelving mode, so if you like, you can have one high shelving band turning over in the midrange and a second adding high‑frequency air, or a low‑frequency shelf adjusting the ‘upper bass’ while another controls the low end proper.

Once again, the analogue saturation that is applied to the equalised signal is subtle, but it’s definitely there! You can easily confirm this by setting both channels identically and switching from left/right to mid/sides mode; there’s often a noticeable change in the tonality or the stereo image as saturation is applied separately to the sum and difference signals rather than to the left and right channels. There’s no control over the degree of saturation, but to my mind it’s perfectly judged, adding just that hint of warmth that you only notice when it’s not there.

Many of McDSP’s plug‑ins are inspired by classic hardware, but they don’t seek to emulate specific processors. In the case of Royal Q, there are obvious similarities with the Manley Massive Passive, which also implements the ‘double shelving’ approach. It’d be interesting to do a direct comparison, but the Massive Passive retails for well over £5k, and the SOS review budget doesn’t quite stretch that far! What I can say is that Royal Q does exactly what I’d expect and want a processor of this type to do.

Of particular note is the upper band, which offers frequency settings that go right up to 25kHz. That, of course, is outside the range of human hearing, but the effects of a wide‑band boost centred on 25kHz certainly aren’t, and they sound glorious on the right material. The effects of ‘double shelving’, meanwhile, are particularly apparent at the other end of the frequency spectrum, where a shelving cut at 250Hz combined with a boost at 80Hz or so can be magic.

One of the clever things about the APB system is that it allows analogue compression and saturation to be combined with digital equalisation and other processes.

Full Steam Ahead

There’s no denying that McDSP’s APB hardware units are costly investments. However, as I pointed out in my original review of the APB16, the perception of value depends what you compare them to. High‑end analogue gear never comes cheap, and it would be a rich studio indeed that could contemplate buying eight or 16 channels of outboard compression as an alternative. Even if you did, it wouldn’t be configurable, and couldn’t offer the same level of DAW integration, recallability and support for automation.

These new plug‑ins make the case for the APB even stronger, offering something genuinely different from the existing plug‑ins whilst maintaining the same very high quality level. Royal Mu and Royal Q, in particular, will put the APB system onto the radar of mastering engineers, potentially opening up a new market. And with every new APB plug‑in being free to existing owners, the benefits of buying into this platform will only grow over time.


  • Great‑sounding processors that really demonstrate the versatility of the APB platform.
  • Royal Mu and Royal Q will be very tempting for mastering engineers as well as for mixers.


  • A stereo instance of MC‑3 uses six APB processing channels.


If we needed additional proof of McDSP’s Analog Processing Box concept, these new plug‑ins provide it in spades.


Free to existing APB owners.

SX Pro +44 (0)800 6522 320

Free to existing APB owners.