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Roger Mayer RM58

Feed-forward FET Limiter & Tape Simulator
By Bob Thomas

Roger Mayer RM58

Re-live the sound of the '70s with this classy combination of limiter and tape emulator.

Roger Mayer is probably best known herein the UK as the electronics engineer who made effects pedals for Jimmy Page, Jeff Beck and Jimi Hendrix back in the 1960s. But his move to the USA in 1969 to form his own recording-studio equipment company means that, in America, he is at least as well known for the limiters, equalisers and noise gates that found their way into major recording studios during the 1970s and 1980s. In 1989, having successfully reissued the Octavia (famously associated with Hendrix) and launched the Rocket series of guitar pedals, Mayer returned to England and has since specialised in developing and manufacturing an impressive range of guitar effects that now numbers over 20 models.

Alongside the guitar-oriented products, though, in recent years Mayer has returned his attention to studio electronics, after the interest in his 456 Stereo Analogue Tape Simulator — which itself was based on a circuit Mayer had developed and issued initially as a mono guitar pedal. The 456 tape-emulation circuitry then also found its way onto each channel of his two- and four-channel rackmount microphone preamplifiers, and these have now been joined by the Roger Mayer RM58 Classic Stereo Limiter, reviewed here. As the name implies, this is an update of his legendary RM58 from the early 1970s, and it incorporates Mayer's 456HD tape‑simulation circuit too.

New Clothes

The updated RM58 Classic Stereo Limiter is unusually handsome. Its stainless-steel chassis carries a brushed stainless-steel fascia, with control cut‑outs that are lined with a clear, resin-based material which has been used on Roger Mayer products for over 15 years and protects the controls' legends and scales. Whatever this material is, it seems to be extremely resistant to physical damage, and wax pencil markings leave no trace when wiped off!

As with the original limiter, two large, mechanical VU meters dominate proceedings, and these have been given the ability to display input and output levels as well as gain reduction. Unusually, the output level displayed on these meters is not the RM58's actual output level, but instead the level hitting the unit's 456HD tape‑simulation processor — this mimics the gain staging in pre-digital days, when a console's VU meters would be used to judge how hard an analogue tape recorder was being driven. (Needless to say, we were pretty obsessive back then about keeping the tape machine and console meters and signal levels correctly lined up, and that has left me with the ingrained habit of checking that all my meters — whether hardware or software — line up where they should before I start working.)

Being a two-channel unit that can be linked for use with stereo material, each side of the panel possesses identical controls, and these are laid out as mirror images of each other. Personally speaking, I found this layout a little disconcerting at first, but that probably says more about me and my preferences than about the layout Mayer has chosen for the RM58. The upper control recess on each channel carries 10-position rotary switches for attack and release, next to a three-position meter-mode switch. The lower control section carries detented (21-position) threshold and output control knobs. The mirror imaging means the attack and threshold controls sit at the outsides of their respective recesses.

A centrally positioned, horizontal cut-out sits in the space below the twin VU meters. Its mirror-image layout occupies the space on either side of the central stereo link button and its pair of blue indicator LEDs, where you'll find the buttons and indicator LEDs of the channel and 456HD bypass switches. Outside these sit the switched, 21-position 456HD trim pots that control the RM58's channel output levels.

There's little to see on the rear, though the screw-locking connector for the PSU provides a reassuring indication of quality.There's little to see on the rear, though the screw-locking connector for the PSU provides a reassuring indication of quality.On the rather Spartan rear panel, all you'll find are two pairs of male and female XLR connectors, which obviously cater for the RM58's transformer-balanced inputs and outputs, plus a locking connector for the external 48V DC universal-voltage power supply. The inputs and outputs are balanced by custom ferrite-core transformers, and rather than buy these in, they're manufactured in-house by Mayer to ensure that they deliver a wide bandwidth and a highly linear phase response. The input transformers can accept levels in excess of +35dBu, whilst the output transformers are designed to interface with a DAW's A-D converters, and have a headroom of at least 24dB above nominal operating levels. All audio circuitry is made up of discrete components, runs in Class‑A and has a frequency response that runs goes up to 100kHz.

Take It To The Limit

The RM58 features the proprietary FET-based feed-forward gain-reduction circuit that Roger Mayer originally developed back in the late 1960s. A FET (Field Effect Transistor) is a type of transistor that behaves in a similar way to a triode tube, and it enables circuit designers to achieve attack times that are, in general, much faster than those that can be obtained from vari-mu or optical compressors, making FETs particularly suitable for use in limiting applications. Add in the feed-forward design — in which the gain-reduction control signal is derived from a point in the signal path prior to the gain cell (as opposed to the feed‑back topology, in which the control signal is tapped post the gain cell) and you end up with a limiter that can react to excessive peaks in the signal level extremely quickly if desired.

In the RM58, Mayer developed what he describes as a "dynamically controlled gain reduction" method that, because it can control both positive and negative wave fronts, is able to maintain the centre of a stereo image whilst applying significant amounts of gain reduction. Mayer's FET-based design gives the RM58 the benefit not only of fast attack times but also wide-ranging threshold levels, compression ratios and release times. The eagle-eyed will have noticed the absence of a dedicated ratio control; Mayer explains that, since the RM58's dynamic gain reduction reduces signal levels above the set threshold exponentially in real time, the compression ratio changes continuously, thereby producing a very natural and musical-sounding result. Another benefit of the RM58's limiter acting independently on both negative and positive wave fronts is that there's no latency in the event of a negative peak arriving first at the detector...

All of which brings us neatly on to the attack and release times. The 10 switched attack times are designed to cover a very wide range of recording situations, both in tracking and on the mix bus, and I'm told that they were selected after considering feedback from many famous producers. The 10 release times have been selected to provide, in conjunction with the attack control, many different options, depending on the effect desired, even down to mimicking the behaviour of an optical compressor.

Even the twin VU meters are worthy of mention, since the gain-reduction measurement uses an independent side-chain that is identical to that in the audio gain-reduction path. This ensures real-time accuracy in the display of both the gain reduction and the effects of the attack and release time constants on it. In displaying the input and output levels, the RM58's meters employ custom-designed VU meter drivers and wide‑band meter interfaces to deliver a ballistic response that is accurate to ±0.2dB from 20Hz to 80kHz.

Tape On Me

Roger Mayer's 456HD Analogue Dynamics Process is a essentially a proprietary high-speed, non-linear, wave-shaping amplifier that's been designed to mimic the wave-shaping produced by a perfectly aligned Studer A80 24-track analogue multitrack open-reel tape machine, recording onto two-inch Ampex 456 tape — minus all the alignment issues, tape hiss, and wow and flutter. Everyone who engineered back in the days when analogue multitrack tape ruled the roost quickly learned to manipulate the levels going to tape in order to produce tape saturation (compression) and low-order harmonic distortion, both of which, when used with care, can enhance the sound of a track.

Roger Mayer's inspiration for the 456HD process apparently came from listening to early CDs that had been transferred from analogue masters (remember the AAD — analogue recording, analogue mastering, digital conversion — designation on early CDs?) and recognising the enhanced fidelity that this approach can deliver... provided that the transfer isn't done from the vinyl cutting master! Mayer has a theory that the enemy of ultimate audio fidelity when recording digitally is the anti-aliasing filter that sits in the signal chain before the A-D converter, and that using the 456HD process to control and shape each positive or negative peak gives the engineer the confidence to use the maximum possible digital range to obtain the best resolution and quality, as the process can limit the inter‑sample peaks that DAW peak meters are unable to display.

The controls for the left and right channels are laid out in a mirror image of each other.The controls for the left and right channels are laid out in a mirror image of each other.

In Use

Setting up the RM58 isn't a difficult exercise. The manual recommends starting out with the unit set for unity gain, with the 456HD process bypassed, attack and release set to positions 5 or 6, threshold adjusted so that the needles of the gain-reduction metering just moves, and output set so that the output signal level is equal to the level when the unit is in bypass — and going from there

To set up the 456HD processing, the manual recommends setting the output level (input to the 456HD processing) to +8dBVU and then adjusting the 456HD trim control so that peaks reach -3dBFS at your DAW interface's input. Set up like this, the 456HD process will act as a tape machine, reach saturation point at +9dBVU, which will result in recordings that will not go into digital distortion, but will deliver the desired positive qualities of tape emulation. The beauty of the 456HD trim control is that you can adjust the levels going to your DAW without having to change your limiter setup.

The limiter itself is, to me, entirely intuitive in operation and, starting from the manual's recommended unity gain setting, adjusting the threshold and balancing the attack and release timings enabled me to dial in exactly the amount of gain reduction that I was looking for every time. Once I'd got what I wanted in the limiter, bringing in the 456HD processing allowed me to fine‑tune the overall dynamic picture and, when sonically appropriate, to drive its virtual tape machine into saturation in order to add some second‑ and third‑harmonic distortion.

Conclusion

The sound and effect of the RM58 limiter and its 456HD processing will hold genuine appeal for those who are looking to achieve a distinctly analogue character in the music that they produce. The limiter can be extremely transparent in action, and its fast attack times make it ideal for use during tracking to protect a DAW interface's A-D converter inputs from transient overload. Sound-wise the RM58 limiter is a bit special and I can't think of any compressor or limiter that I've come across that can deliver the same level of unobtrusive control, overall clarity and low-end width.

To my ears, the sound of the 456HD processing is stunning. Although it doesn't do exactly what the Studer A80 that I cut my engineering teeth on did, when used judiciously it can certainly add a similar sense of depth and richness to vocals, bass, piano and acoustic guitar. This process is also rather effective in taking any nasty edge away from a heavily distorted guitar track — and without removing the all-important drive and excitement. But where I feel that the 456HD truly excels is sitting across a drum bus or on individual drums, a role in which it successfully mimics the effect that was once obtained by recording drums to multitrack tape and bouncing the results into a DAW.

While you can hardly call the price 'cheap', vintage RM58 units have sold for astronomical prices, and it's arguable therefore that this new Roger Mayer RM58, with the added tape-emulation circuitry, is a bit of a bargain. I certainly have no hesitation in recommending an immediate audition to anyone looking to add some of that 1970s analogue magic to their music. In fact, I'm sorely tempted to hang onto the one that I have here!

Alternatives

If you're looking for a limiter at around the RM58 price point, you've a wide choice available to you both from major manufacturers and from boutique builders. But despite the name, the RM58 Stereo Classic Stereo Limiter is not just a limiter — its integrated 456HD processing means there really isn't a direct equivalent.

Pros

  • Superb, wide-bandwidth audio performance.
  • Transparent, super‑fast limiting action.
  • Great-sounding tape saturation emulation.
  • Well priced for the results it achieves.

Cons

  • None I can think of!

Summary

An update on the legendary original limiter, the RM58 now combines a feed-forward FET limiter, Class‑A audio electronics and the 456HD tape simulation to produce a superb-sounding unit that brings with it some of the analogue magic of the 1970s.

information

£2999 including VAT.

KMR Audio +44 (0)20 8445 2446

sales@kmraudio.com

www.kmraudio.com

www.roger-mayer.co.uk

Published May 2019