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Charter Oak SCL1

Compressor-Limiter By Hugh Robjohns
Published August 2010

Charter Oak SCL1

Charter Oak are not yet as well known for their outboard gear as they are for their mics, but this unusual compressor could change all that...

Charter Oak are best known as an American boutique microphone producer in business since 2002. However, for the last couple of years they have branched out into the outboard market too, producing the PEQ1 equaliser and the subject of this review, the SCL1 dual‑channel compressor-limiter.

Everything about the design and manufacture of the SCL1 is closely controlled by founder Michael Deming, who has a well established and respected track record as a recording engineer and producer — so you can be assured that he knows what is required of a good compressor‑limiter. First shown in prototype form in mid‑2008 at the AES convention in Amsterdam, this processor uses entirely discrete electronics to construct a FET‑based VCA, and features unusually fast attack times and programme‑dependent release curves.

Design & Construction

The SCL1 is housed in a black‑painted steel, 2U, rackmountable case, which extends about 305mm behind the rack ears. The internal construction involves a great deal of neat hand‑wiring between the front‑panel controls and the two separate channel circuit boards, which are populated with conventional discrete components — I counted 16 transistors (including FETs) on each board, and no ICs. Apparently, the basic design is derived from a pretty obscure broadcast dynamics processor, although it has been extensively refined to suit the recording and mixing environment. Cinemag transformers are employed for the balanced inputs and outputs, and a steel divider separates the audio circuitry (and transformers) at the front of the box from the linear mains power supply at the rear.

The audio I/O on the rear panel is all via XLRs, operating with the old standard of 600Ω input and output impedances. The maximum output level is a healthy +22dBm (it's correct to use dBm rather than dBu, given the 600Ω termination format), and the signal‑to‑noise ratio is given as either ‑80dB or ‑85dB relative to +10dBm, depending on which set of published specifications you read!

Channel crosstalk is specified as better than ‑90dB, and harmonic distortion is claimed to be less than one percent with 20dB of gain reduction and a +20dBm output level, which is a pretty impressive figure. The two channels are apparently matched to track within 0.25dB over the entire gain-reduction range when operating in the stereo mode, helping to ensure stable stereo images.

As well as the quartet of audio XLRs, the rear panel also sports the usual IEC mains-power inlet, along with a recessed voltage‑selection switch (117 or 230VAC) and a fuse holder. A toggle switch is provided on the rear panel to switch the unit on — although there are no markings to suggest which is the 'on' position. Fortunately, the large VU meter on the front panel illuminates when the unit is powered. Confusingly, there appears to be a second on‑off switch on the front panel, and this isn't mentioned in the Operating Manual at all. On some of the early units, this was a power switch, but on current models it provides a full relay bypass facility to switch the entire unit out of circuit, by linking the physical inputs directly to the outputs. Given that most people will use the SCL1 as a bus compressor, a single bypass switch affecting both channels is acceptable, but for those who want to process two independent channels simultaneously, it might become a little frustrating. I understand that future models will include more informative bypass-switch labelling.

The front panel is neatly laid out, with this bypass toggle on the left, followed by two rows of seven rotary controls, each with an elegant aluminium knob. The skirts of these knobs are scaled from 0 to 10, but other than a single marker dot on the panel at the 12 o'clock position, there are no other calibration marks; just the control function names and some very generic operational markings. The upper row of controls determines the settings for both channels when the unit is switched to stereo mode (except the input and output level knobs, which are always fully independent).

The first pair of controls adjusts the input gain, followed by controls for both Static and Dynamic Threshold. The Static Threshold control is rather unusual, and sets the initial control-voltage bias. The manual suggests adjusting this control to null the meter for a zero reading (ie. zero gain-reduction with no input signal) before adjusting the Dynamic Threshold. The latter control then sets the required compression or limiting threshold for the audio signal, with the control markings showing arrows to indicate the high and low directions. The threshold range is adequate, but hot signals are easier to work with, generally. Once the required dynamic threshold setting has been established, the Static Threshold control can then be adjusted further, if required — it basically determines the way the control voltage swings between the peak and average levels of the audio signal. Put into more practical and meaningful terms, increasing the control voltage by a decibel or two on the meter (turning the Static Threshold control anti-clockwise) essentially softens the compression knee curve, and vice versa.

Next up are the Attack and Release time‑constant controls, both being marked simply with arrows indicating the fast and slow directions. The attack-time range is from 100 microseconds (zero on the control knob) up to five milliseconds (10 on the knob's skirt). One hundred microseconds is unusually fast for a compressor (although not uncommon for a limiter) and would typically lead to transient distortion in some compressors. The release time range spans 20ms to two seconds, but with a programme‑dependent release curve that provides a faster recovery from brief high‑level transients, while maintaining a slower long‑term average level control for more gentle dynamic changes.

It's worth noting that whereas the release curve of most compressors dumps the attenuation, returning to a unity‑gain position, in the SCL1 the release curve essentially tracks between the peak amount of dynamic gain-reduction and the attenuation required to control the constantly changing average signal level — and that's the key factor in why this compressor sounds so transparent and clean. Attack and release times for typical mix situations might correspond to a setting of about 3 on the Attack control and 7 on the Release. The reason they aren't both 3 (or 7) is because these two controls operate in completely opposite directions to each other, which I found rather confusing, initially!

Next along is the Slope control, which adjusts the compression ratio from 1:1 up to 20:1. The mid-point on the control knob (5) equates to a 10:1 ratio. There isn't much practical difference between a 10:1 ratio and 20:1, so devoting half the control's rotary action to this region seems wasteful and reduces the resolution for the more creative lower ratios. The final rotary sets the output level, with appropriate make‑up gain provided automatically by the compressor circuitry. The single, large illuminated VU meter can be switched to show the gain reduction of either channel, using the adjacent toggle switch, but there is no facility to monitor the actual input or output signal levels. A second toggle switch configures the unit for dual‑channel or stereo operation.

On Test

The rear panel of the SCL1 includes balanced XLR ins and outs for each channel, as well as the main power switch. The latter isn't necessarily a problem, as you may always want it to be on!The rear panel of the SCL1 includes balanced XLR ins and outs for each channel, as well as the main power switch. The latter isn't necessarily a problem, as you may always want it to be on!

The SCL1 is built to high standards — just like Charter Oak's mics — but I was initally confused: the rotary controls seemed disappointingly scratchy. However, some investigation quickly revealed that the front panel of the review unit had been protected with a polythene membrane during manufacture, and that polythene had not been removed prior to fitting the controls. Bits of the membrane were evident under the fittings and switches, and stray pieces of plastic trapped around the fixing nuts of the rotary controls were rubbing on the underside of the knobs. Happily, though, only a few units were shipped like this, and the problem doesn't exist on more recent SCL1s. With the plastic removed, the true quality and smooth, nicely weighted action of the rotary controls (both the potentiometers and the aluminium control knobs themselves) was clearly revealed.

The arrangement of controls is more or less logical, although the inclusion of the Static Threshold control is unusual and will undoubtedly confuse some users, as will the reversed operation of the Attack and Release controls — not to mention the absence of control-parameter markings on the front panel and the inability to monitor the actual input and output levels. Apparently, the lack of control markings is a deliberate Charter Oak policy, designed to encourage engineers to use their ears rather than apply generic parameter settings that might not be appropriate given the way this device operates — an argument that does have its merits.

Having set the SCL1 up as a bus compressor, dialled in reasonably sensible starting positions for all the controls, and achieved gain reduction dipping healthily down to ‑8dB or so on the meter, I initially wondered what was wrong, because I couldn't hear any typical compression artifacts. I even wondered if I had mis‑plugged something, so that I was still hearing the original signal rather than the SCL1's output! However, further investigation revealed that I really was listening to the compressor's output, but that the compression is just incredibly clean and natural sounding — even with apparently extreme settings. There just is no indication of pumping or breathing, no loss of HF detail and no nasty transients, just a slightly higher average energy level and a more controlled output signal.

With most bus compressors, you end up having to filter the side chain to prevent the kick drum and bass from driving the compression all the time, and while auto‑recovery settings usually work adequately, it's not unusual to have problems with gain reduction being dumped unceremoniously at points when many elements of a track drop out at the same time, leaving, for example, an exposed vocal. Neither of those familiar issues ever surfaced with the SCL1, regardless of the musical genre or mix style I tried it with.

This is one situation where you really do have to switch regularly between the original input signal and the SCL1's output to hear the effect of the processing, because the processed signal sounds so natural all the time. In fact, I can't think of any other bus compressor that sounds as transparent and natural as the SCL1 — it really is quite extraordinary in the way it works so seamlessly and virtually inaudibly, even with extreme control settings that would be just plain silly on anything else!

This remarkable feat can be explained partially by the fact that the SCL1 is really a gentle automatic gain control system, with some dynamic compression added on top, rather than a conventional compressor. As a result, it doesn't generally apply as much total dynamic range reduction as other bus compressors might do with similar settings — but that doesn't mean it doesn't do a superb job, especially as a bus compressor.

When faced with a ludicrously dynamic vocal track, or the vagaries of my own beginner's bass playing dynamics, it fared less well. Not because it sounded nasty — I never managed to make it sound bad, no matter what I did — but simply because it just didn't seem to have the range to bring huge dynamic changes under control. Daisy‑chaining the signal through both channels, each providing half the total required dynamic-range reduction, did work to a more practical extent, and sounded very nice, too, with none of the obvious artifacts that a more conventional compressor would impose when applying the required dynamic control in one hit. But clearly the SCL1's strength is in bus compression or in containing more modestly dynamic sources. I had great success with some accomplished acoustic guitar tracks, for example, and on stereo drum kit stems.

I've already commented on the remarkable transparency of the SCL1, but it's not entirely neutral: there is a subtle character to the sound. I'm not talking about the usual compressor artifacts, because they really are barely audible, but rather about subtle tonal or harmonic changes, of the kind generally associated with high‑end analogue electronics. There's a slight extra weight at the bottom end, perhaps; a modest lower‑mid bloom that helps to enhance the scale of the source in a musically enhancing way. It's subtle, of course, but becomes more obvious as the unit is driven harder and with higher signal levels — and it can certainly handle generous signal levels without complaining.


The Charter Oak SCL1 is an impressive compressor that's very different from virtually everything else on the market at the moment. It genuinely excels as a bus compressor, polishing the mix in a way that nothing else comes close to doing. While probably much too subtle for extreme dynamic control on individual wild sources, the SCL1 does work admirably in applying a gentle controlling hand to reasonably well‑controlled sources that just need a little help to sit nicely in the mix. You could think of it more as an astonishingly capable level controller than a conventional compressor, and once you install it on your mix bus, you'll be very reluctant to unplug it!  


Other than some very specialist broadcast processors, I can't think of anything that comes close to offering this degree of transparent level control.


The SCL1 is a unique and very transparent‑sounding compressor/limiter ideally suited to bus‑compression duties.


£2450 including VAT.

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