Valve Compressor

Signal Processors > Compressors

Published June 2023

While it’s primarily a compressor‑limiter, the RS660 also features a switchable THD mode, which allows you to add more grit and warmth to a signal.

Can’t decide if you’d rather blow your vari‑mu budget on a Fairchild or an RS124? Chandler’s RS660 puts a little of both in one box...

Chandler Limited’s exclusive agreement with EMI/Abbey Road Studios has enabled founder Wade Goeke to bring to market not only accurate recreations of some of the most revered studio equipment of the 1960s, but also to develop new products that are inspired by those classics. Alongside the various legendary EMI preamps, EQs and compressors, for example, Chandler’s range includes original and EMI‑inspired rackmount designs and 500‑series modules, as well as two innovative takes on the classic large‑diaphragm capacitor microphone, in the form of the valve‑based REDD and the solid‑state TG.

Design & Construction

Their latest product, the RS660 compressor, is a hand‑built, transformer‑balanced, single‑channel, variable‑mu valve compressor‑limiter. Despite its decidedly retro appearance, though, it’s arguably as much a child of the 2020s as it is of the 1960s. Their first valve compressor, the RS124, was essentially a painstaking recreation (albeit a lightly enhanced one) of EMI’s ‘reworked’ Altec 436B, whereas this new model is an elegantly conceived fusion, combining qualities of the RS124 with others from its famous contemporary the Fairchild Model 660. It also tips its hat slightly in the direction of the EMI TG1 diode‑based limiter, which was itself originally designed to emulate the operation of a Model 660.

The dark brown fascia is bounded by a black trim plate that carries the unit’s official designation plus the Chandler, EMI and Abbey Road Studios logos, and also defines the limits of the control area. Within those confines are a momentary push switch and associated potentiometer that enable you to compensate for any asymmetry in the unit’s push‑pull output. There’s also a large, square, mechanical meter that displays compression on a scale of 0‑30 dB, an input level control (scaled simply 1‑10), an output level control (‑10 to 0), a three‑position switch to set the operating mode, and a switch to choose between the seven preset attack and release time combinations.

The control knobs are made of plastic rather than ‘period correct’ Bakelite, but nonetheless combine with the power switch and its red‑jewelled power‑on light to give the fascia a distinctively vintage vibe very. A slightly more modern‑looking toggle switch activates the side‑chain link between two connected RS660s.

The rear panel carries the XLR connectors for the transformer‑balanced input and output, a TS quarter‑inch jack socket that carries the side‑chain link for stereo operation, and a fused IEC mains input. A less familiar sight, unless you happen to own an RS124, is the switch that selects between either 200Ω (Abbey Road’s standard back in the day) or 600Ω output impedances. The input impedance is fixed at 15kΩ.

Taking the top off the RS660 reveals an internal transformer, valve and circuit board layout that, at first glance, bears an uncanny resemblance to that of the RS124. But closer inspection reveals that the circuit and power supply boards are somewhat more complex. Component quality and the level of attention to detail in the hand‑wired assembly are all of the exemplary quality you’d expect to find in a unit costing what the RS660 does. As with the RS124, this compressor is configured around a simple three‑valve, transformer‑balanced, differential signal path derived from the Altec 436B. In the RS660, the variable gain stage comes in the shape of a single 6386, rather than utilising the 6BC8 found in both the Altec and the RS124. It’s the same type as that used (in a series‑wired multiple of four) in the Fairchild Model 660. The substitution of the 6BC8 by the 6386 is the only valve change, with the RS660’s side‑chain and push‑pull output stage being built, respectively, around the same combination of 6AL5 dual‑diode and 6CG7 dual‑triode as found in the 436B and RS124.

In order to allow the correction of any drift in the balance of its push‑pull output stage and the audible low‑level modulation that resulted, the RS660 features EMI engineer Len Page’s ingenious method of balancing the two sides of the output stage. It’s something he originally developed for the Altec 436, to avoid having to stop a session while the unit went back to Abbey Road’s service department for realignment. In this elegant solution, which inhabits the RS660’s front‑panel balance control section, depressing the momentary button switch feeds a series of clicks through the unit, and these are nulled out using a slot‑headed potentiometer. Once nulled out as far as possible (the clicks never fully disappear) the output stage is balanced. In a nice homage that’s carried over from Chandler’s RS124, the white‑lettered front panel ‘BAL’ legend for this control is in the same typeface as the label stuck onto the front panel of the original EMI‑modified Altecs, and it will be familiar to any user of Dymo labels!

The RS660 features a switchable output impedance, and stereo linking courtesy of a TS jack on the rear panel.

Controls & Modes

The input and output controls’ potentiometers on the standard version of this compressor are both undetented, and this, in combination with the rather coarse 0‑10 front panel scaling, means that you’ll have to resort either to photographs or an old‑school Chinagraph pencil for accurate recall. However, as a cost option, the unit can be bought fitted with two stepped switches, which is obviously more convenient in that respect.

There are three switch‑selectable operating modes, called Limit, Comp and, somewhat unusually, THD (Total Harmonic Distortion). The last is a signal artefact that manufacturers have traditionally worked hard to minimise, but in recent years it’s become desirable to have more control over the amount of ‘warmth’ imparted by studio processors. The THD mode is inspired by a circuit used in Chandler’s TG1 limiter, which employs a diode network. When the RS660’s mode selector switch is set to THD, the unit’s compression and limiting function is disabled (as in the TG1), which means that, since all its transformers and valves remain in circuit, the RS660 has the ability (again, like the TG1) to add overdrive and harmonic distortion to any signal passing through it.

Just as you’d expect, selecting Limit or Comp switches the RS660 into its limiter or compressor modes, respectively. As a limiter, the RS660 has been deliberately designed to be “fast, coloured and lively, with the ability to add excitement to any source”. Switching from limiter to compressor mode raises the threshold, lowers the onset of compression, makes the knee smoother and less aggressive and modifies the gain structure, which makes for a more controlled, less aggressive response overall.

The seven‑position Time Constant switch sets the RS660’s attack and release time presets, with position 1 being the fastest and 7 the slowest. The first three positions are designed to replicate the coloured, faster settings of the Fairchild Model 660, while the fourth position has a more moderate, less coloured speed — it’s a halfway house between the faster settings and “purposely quite slow, and influenced by the gentle, uncoloured and low‑artefact sound of many vintage valve compressors, including the EMI RS124.” Chandler don’t provide more detailed specifications for any of the RS660’s attack/release settings, but although my inner geek might well feel better for knowing them, that lack of knowledge really doesn’t matter in the real world. It’s incredibly easy to get the sound that you want from the RS660 — provided, of course, that you enjoy its vintage vibe.

Wade Goeke has invested time and effort in ensuring that the new 6386 valve sounds like the vintage version.

As I’ve mentioned in a previous review, the current reissues of the 6386 valve do not perform quite like their venerable forefathers; they exhibit lower gain, less compression and significant tonal differences. This has led some designers to replace the reissue twin‑triode 6386 with a pair of 6BA6 pentodes wired as triodes. Instead, Wade Goeke has invested time and effort in ensuring that the new 6386 valve sounds like the vintage version, and judged on the sonic character of the RS660 he has succeeded in that.

In Use

The RS660 produces superb results across a wide range of compression, from the subtlety of a few decibels of gentle compression to needle‑pinning heavy limiting. With only an input level, seven time constants and an output level to play with, it’s much like the Fairlight 660, in the sense that it’s very much an ‘experiment and get to know it’ sort of processor. Once you’ve developed a sense of how the RS660 responds, though, you’ll find that dialling in the feel that you want in a track is a quick, intuitive and natural‑feeling process. Having said that, while its sound and compression characteristics mean it can excel across a channel or bus insert, it does lack a hard bypass facility, making in/out comparisons tricky if the insert point itself can’t be bypassed easily. Using it as a parallel compressor makes such comparison easier and opens up the opportunity to high‑pass or otherwise EQ the input to the compressor.

As with many other high‑end, valve‑based compressor‑limiters, the RS660 has that uncanny ability to add warmth, character and presence to any sources running through it, and it does that particularly well for vocals, acoustic guitars, bass and drums. But I also found the separate THD function (which is more about warmth and character than the sort of full‑on distortion you’d look for in a pedal) very useful for adding a bit of grit and harmonic colour to rocky vocals, drum tracks and DI’ed bass in particular.

With only a single RS660 available to me during the review period, I wasn’t able to fully evaluate the performance of a linked pair for stereo use, but I have absolutely no reason to think that there would be any problem in that regard, providing that you have the two units set to identical input levels and time constants. On that note, if its performance in a stereo bus/mastering role is important to you, it’s worth pointing out that there’s an option to purchase a matched pair with stepped input and output controls.

Verdict

With the RS660, Wade Goeke has successfully combined the essential qualities of the Fairchild 660 and the EMI RS124. The result hints at a development approach that, given the opportunity, EMI’s engineers might well have taken back in the day. This is a marvellous compressor‑limiter whose audio and operational performance and versatility make it an extremely attractive alternative to the usual suspects at the premium end of the compressor‑limiter market — it more than justifies its professional price tag, and I’m going to be very sad to see this review unit leave my studio!

Pros

A wonderful sound, full of vintage tube character.
Can impart warmth, saturation and presence.
A very usable range of attack/release time combinations.
Extremely intuitive.

Cons

No bypass function.
Side‑chain filter would be nice.

Summary

A valve‑based compressor that builds on the heritage of the two compressors made famous by Abbey Road Studios in the 1960s to deliver a superb audio performance full of vintage character, warmth and tone.