CharterOak Products | > Model SCL-1 Reviews & Testimonials

Charter Oak SCL1

Published in SOS August 2010
Reviews : Processor
Hugh Robjohns

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 600? input and output impedances. The maximum output level is a healthy +22dBm (it’s correct to use dBm rather than dBu, given the 600? 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.

Verdict
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!

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

PROS
Extraordinarily transparent level control.
Automatic gain make-up is inaudible and extremely effective.

CONS
Controls and markings may initially confuse some users.
Lacks audio level metering.
Too subtle for heavy dynamic-range control.

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

Published in SOS August 2010

------end------

Music Tech Mastering Vol 2
9 out of 10 star review
SCL-1 DISCRETE COMPRESSOR LIMITER

CharterOak claim this to be one of the most transparent buss compressors available. Mark Cousins takes a closer look.

You only need to look at the variety of different designs of compressor to realise that there’s more than one way of controlling the dynamic range of a recording. While some engineers chase after ‘character’ compression, others realise the significant benefits of a transparent compressor – a sonic tool able to massage the dynamics of a recording without imposing a noticeable sonic fingerprint. However, achieving true transparency – without unwanted pumping artefacts, HF reduction, etc – isn’t an easy task, and in many cases demands a pricey compressor.

CharterOak’s SCL-1 Discrete Compressor Limiter is primarily designed as a buss compressor, making it highly suitable for both mixing and mastering applications. Throughout the SCL-1’s design CharterOak have placed a real emphasis on the unit’s transparency, using a range of innovative techniques to ensure the programme material retains as much of its precompressed sound as possible, yet still delivers plenty of efficient gain control.

On the busses
The SCL-1 works as both a two-channel stereo compressor and, using the dual switch, as two mono compressors should you want to use it on individual tracks in the mix. On first impression the controls seem to be largely familiar, with input and output levels, dynamic threshold, slope (or ratio, in other words), and attack and release time. However, one unique feature of the SCL-1 is the static threshold, which in essence, seems to enable you to ‘calibrate’ the SCL-1 to various different knee settings so as to change the response of the compressor.

As you’d expect, the immediate quality of the SCL-1 is one of absolute transparency. As hard as you push it (even using as much as 20dB of gain reduction!), it’s difficult to make the output sound in any way ‘distressed’. All the characteristics of over-compression are conspicuous by their absence – the high-end remains clean and bright, transient details preserved, and any unwanted pumping artefacts kept to an absolute minimum. Indeed, to the untrained ear you could almost be forgiven for thinking that the SCL-1 had been left in its bypass mode, despite the needle hovering around 5-7dB of gain reduction, or more.

An invisible touch
Looked at more closely, you start to see how the SCL-1 achieves such impressive results. Firstly, the release seems to intelligently respond to the averaged signal levels, reducing any unwanted returns to unity gain (0dB, in other words) in-between notes, and hence keeping any breathing artefacts to a minimum. This subtle response works wonders at controlling an overarching dynamic – gently reducing levels in louder passages of music while slowly ‘opening up’ during quieter segments of the track. Secondly, the SCL-1 features an auto gain-makeup, so that the output level stays consistent even as you apply greater amounts of gain reduction. As a result, you get to hear how the sound changes in response to altering the threshold and ratio rather than having to juggle with the output levels to assess the compression in context.

Despite its transparency, the SCL-1 is still a highly effective gain control device. On delicate acoustic instruments it can perform a marvellous job at holding an instrument’s place in the mix, making it sound much more like a controlled performance rather than a deliberately squashed recording. Across the mix, the SCL-1 adds body without being unduly dictated by elements such as a kick drum, and when pushed harder it can still deliver just enough ‘mojo’ to make a trip through the SCL-1 a worthwhile excursion.

Subtle squash
The SCL-1 is undoubtedly one of the most transparent buss compressors we’ve encountered. Of course, this sound won’t suit every application, and if you actively enjoy a little pumping or a more heavy-handed response you’ll want to look at an alternative design. However, for those looking to preserve signal integrity, the SCL-1 is a triumph of forward-thinking design – a versatile and sonic-effective compressor that only makes it presence felt in its ability to control gain, rather than destroy a good mix. MTF

Measuring Up
Good buss compressors aren’t cheap, so it would be fair to say that the SCL-1 represents good value for money. Other good transparent buss compressors include the GML 8900 (£4,700), and the Crane Song STC-8 (£3,290). Alternatively, for more character, try the Manley Variable Mu (£3,695) which provides a vintage-style result.

SUMMARY
WHY BUY
¦ A highly transparent sound and performance
¦ Automatic gain-makeup
¦ Intelligent release characteristics
¦ No HF pumping

WALK ON BY
¦ Too subtle for some people’s tastes
¦ Two thresholds can be confusing at first

VERDICT
A great choice for sensitive and refined gain control over the mix. But for some the results might be too subtle. 9 out of 10 stars Key Features ¦ Discrete Compressor Limiter ¦ Intelligent release characteristics ¦ Static and dynamic thresholds ¦ Auto gain makeup ¦ Dual and stereo operation 124

Mastering Volume 2 MusicTech Focus www.musictechmag.co.uk Review CharterOak SCL-1

------end------

Resolution Magazine March 2009

CharterOak SCL-1
Gain reduction has long been the healthiest, in terms of choice, of the outboard product
sectors but most variants concern themselves with subtleties of flavour rather than
dramatic differences in operating principle. GEORGE SHILLING is blown away by a new
and wonderfully unique compressor.

CharterOak has been making boutique microphones since 2002. Endearingly, rather than boosting his ego, founder Michael Deming (an engineer and producer of some note) named the company after a local Connecticut landmark. With a product catalogue comprising mainly exotic microphones, CharterOak has surprised everyone with this highly unusual stereo compressor (MSRP US$2899), an early example of which I was lucky enough to try.

Flipping the far left toggle to On makes the large VU light up. The two channels’ controls are arranged above each other and each channel has seven pleasantly tactile knobs. These machined aluminium knobs are etched around the rims with a zero to 10 scale and they are smoothly damped, although a few of the knobs were snagging very slightly on the front panel of the review unit. CharterOak is already aware of this early problem and now simply mounts them slightly further from the front panel when the hex nut is secured.

At the far right a pair of toggles select between Dual and Stereo mode, and Metering of Channel 1 or Channel 2 Gain Reduction. In Stereo mode the Channel 1 controls become master, with the control signal derived from both channels. Input and Output gains sensibly always remain independent, allowing for precise left-right calibration.

The manual suggests fairly extreme initial settings as a starting point to help you understand the concept. Of course, I initially didn’t read this(!) and wondered why the meter was so far off zero. First, the Input gain should be set at full tilt, which provides the lowest noise floor, with the furthest right Output gain knobs needing to be set at around 5 for 0vu output. The next knob is Static Threshold and this effectively calibrates the unit. It is recommended initially to set the unit to stereo mode and the metering to Channel 1, and tweak this knob until the meter settles on zero.

Lowering Static Threshold (clockwise) moves the meter past zero, providing a harder knee — as when pushing all the buttons in on an 1176 but less extreme, and continuously variable. Raising this (anticlockwise) gives a softer knee by creating less potential for swing in the control circuit, and simply setting it a dB or two below zero softens the knee. Next along is the more conventional Dynamic Threshold control. It is recommended to crank this to 7, i.e. a fairly low threshold. The review model needed a fairly hot signal to enable a suitable threshold to be set and a couple of resistor values have been changed in more recent examples allowing for 10dB lower threshold.

Attack and Release knobs are merely labelled 0 to 10 and work in opposite directions to each other — fast Attack is anticlockwise while fast Release is clockwise. These should initially be set to fairly fast and then you set the compression Ratio. This knob varies continuously between 1:1 and 1:20 so setting it halfway at 1:10 is recommended. Having made these adjustments, with a little tweaking it was possible to see average compression of -6dB or more on the meter, and hear, well, very little obvious effect! The transparency of the gain reduction and the effect of the gain make-up in the circuit results in an astonishing smoothness and clarity. Comparing a section of programme with and without the SCL-1, average perceived level was several dBs higher when using the processing of the SCL-1 (when normalised), and a delightful, subtle overall glow is revealed. The subtleties of different settings gradually become apparent, but large changes frequently sounded fairly subtle.

The design brief was to achieve complete transparency and Deming says it turned out even better than he expected. After 25 years of mixing without a bus compressor, he now has it hard wired across the mix. It is faster and cleaner than any other compressor I have ever encountered. There is always a measure of ‘auto’ recovery taking place, so with release set fast, dynamic material, like pop music, will make the meter waggle like crazy. The Auto circuit always releases to the constantly changing average level and this is what prevents any pumping or gasping.

The control circuit is effectively a discrete VCA circuit, with a control circuit governing the FET, which in turn goes to the gain cell. Cleverly, the circuit also makes up much of the gain automatically. However, this, and the lack of a bypass can make it difficult to tell what the SCL-1 is actually doing, such is the subtlety of the compression in some situations. There is no Bypass, but I understand that by request relay bypass is being planned as an option (as is a mastering version with 11-step potentiometers). Even with extreme compression, the stereo image stays remarkably true, and there is no discernable loss of top end, a by-product that you tend to expect with most compressors.

I initially had the unit for a classical/showtune vocal and piano session, and used it for piano recording and again subsequently for mix bus where it proved to be the perfect processor for the job — invisibly and subtly reducing dynamics a little, without any discernable pumping or artefacts. In fact, I was way too cautious with the settings; it really is rather difficult to overdo things with the SCL-1. However, despite the transparency and lack of apparent distortion, there is certainly some enhancement audible with heavy compression settings. Some impressive solidity was noticeable in the low frequencies of a pop-rock mix when the SCL-1 was driven fairly hard.

The unit offers a ‘soft symmetrical clip’, so the mix certainly cooks, while retaining much of the dynamic range. I did find the metering a little misleading as it doesn’t take into account the subsequent gain make-up, so even if the meter is off the scale past -20dB, total gain reduction is rarely more than about 6dB. CharterOak is considering switchable metering to show net gain reduction at output, which I think would be useful.

While there is some narrowing of the dynamic range, this kind of compression is generally too subtle for rock vocals. However, I did have remarkable success, making a dynamic vocal sit perfectly in a track by connecting the two channels in series using Dual mode, and using what would be fairly brutal settings on any other unit. The warmth, presence, size and microphone character shone through, and the vocal glowed rather than sounded squashed.

The SCL-1’s philosophy has been cleverly executed and this is a wonderfully unique processor.

PROS Uniquely transparent gain reduction; excellent auto-recovery; clever gain make up circuitry.

CONS A few early-model niggles — all promised to be sorted; too subtle for some applications.