The Award Nominated, CharterOak SCL-1 Discrete Compressor Limiter will provide a constant output level regardless of input level or frequency, without the familiar pumping and gasping effect that is inherent in most compressors and limiters. This can be attributed to the design of the circuit, which combines the features of waveform differentiation and integration. This allows for fast attack times without overcompensation and a release characteristic that is a function of dynamic range and average program level. The device achieves this by employing a rectifier circuit that possesses a parabolic average charge curve. The end result of the circuit is a quick release that releases to a continually changing average level, which is determined by the parabolic charge curve of the storage capacitors. The CharterOak SCL-1 circuit design is straight forward, and employs NO MAGIC BLACK BOXES OR OTHER WALLET SHRINKING COMPONENTS. All of the components used in the design of the CharterOak SCL-1 are readily available items and are employed in a manner so as to provide plenty of headroom for years of reliable operation. The CharterOak SCL-1 is designed to fit any standard 19″ rack. The unit may be used as a stereo unit, or as two independent limiters in the dual mode. This feature enables the end user to employ the device in one or two track production applications. Do not be afraid to drive the limiter, even with classical music formats. It WILL NOT pump or gasp. The processor is delivered with a lifetime warranty on all parts and labor and easy access to factory technical support.

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.

Specifications:

Frequency Response: 20Hz-22kHz

Harmonic Distortion: Less then 1% with 20dB of gain reduction @ +20dBm output

Maximum Output: +22 dBm

Signal to Noise Ration: -85dB referenced to +10dBm

Attack Time: Adjustable from 100us to 5ms

Hold Time: Less than 2ms. Unit provides a soft clip. (Less than 3% total harmonic distortion @ 3dB clip)

Release Time: Adjustable from 20ms to 2 seconds

Cross Talk: -90dB

Input Impedance: 600 Ohms balanced

Output Impedance: 600 Ohms balanced

Sync Tracking Control circuits are matched to within less than .25dB over the entire gain reduction range Power Requirements 117VAC or 230 VAC