You are here

Generic Audio Preceptor & Compactor

Analogue Dynamics Processors
Published October 2011
By Hugh Robjohns

From a new and little-known Polish company come two compressors made with a "fanatical attention to detail”. Sounds intriguing...

Generic Audio Preceptor & Compactor

Generic Audio are relatively new Polish manufacturers who have launched a small range of high-quality hardware, all designed with a passion for professional audio, and with fanatical attention to detail. The current range includes the Preceptor Models A and T (see the box elsewhere in this article), and the Compactor — and all are compressor‑limiters with interesting design features and superb build quality.

Overview

The Preceptor Model A is an unusually large (3U rack‑mounting) two‑channel dynamics processor that uses mostly discrete, class‑A, solid‑state circuitry. The black-painted steel case, which has a glossy front panel, stretches 245mm behind the rack ears and weighs a fairly substantial 6.4kg. Part of the reason for the 3U height is to accommodate the chunky custom 70VA torroidal mains transformer (with both electrostatic and electromagnetic shielding), which is mounted edge‑on in a central, steel‑partitioned section of the case. A useful benefit of this construction is that there's plenty of front‑panel real-estate available, allowing nicely spaced and generously proportioned controls and metering.

On the rear panel, an IEC mains inlet socket (with integral fuse holder) occupies the centre, with a voltage selector switch (230/115V AC) immediately above. To the left and right are pairs of XLRs for the balanced line inputs and outputs, along with quarter‑inch TRS sockets to accept balanced side‑chain input signals. Pin 1 of each XLR is bonded directly to the chassis in the recommended way and, not surprisingly, the Preceptor passed the 'Wendt Hummer' test with flying colours: there's no risk of ground‑loop hums here!

Internally, this device is constructed to a very impressively high standard. The partitioned central section contains all the mains wiring between the IEC inlet, an EMI filter block, the on‑off switch and the custom torroidal transformer. Separate secondary windings are provided for each of the two channels, the connections for which pass through rubber‑grommetted holes in the steel partition panels. Even the centrally mounted stereo link switch is screened from the mains section by a steel box section! Each audio channel's main PCB carries its own rectifiers, generously specified reservoir and smoothing capacitors, and five separate voltage-stabilising circuits for the different amplifier stages, with the regulating power-transistors mounted on a shared heat-sink fin running across the centre of each board. Most audio stages operate with ±20V supply rails, but the make‑up gain stage operates on ±28V to provide additional headroom.

The Preceptor includes a number of useful controls, including a Blend knob that allows parallel compression and harmonic blending to be achieved without complex routing.The Preceptor includes a number of useful controls, including a Blend knob that allows parallel compression and harmonic blending to be achieved without complex routing.The channel electronics PCBs are constructed using conventionally sized, high‑specification and low‑tolerance components, with a lot of discrete low-noise transistors and WIMA capacitors in evidence. Each board also carries five op‑amps (all but one are socketed), but I can't tell you what these devices are because they've all had their identification markings sanded off! Apparently, these op‑amps are 'transparent' types, though, and are employed only in the blending circuitry (see below). The front‑panel controls support three vertical daughterboards, and all the internal wiring is very neat and carefully laid out. The input and output level switches are constructed with an array of resistors around their peripheries, and the large, vintage‑style VU meters are back‑lit. The attention to detail in the mechanical and PCB design, and the sub‑assemblies is very clear, and can't be faulted at all.

The Preceptor manual is well written and detailed, and contains a useful system block diagram. This reveals some interesting design aspects, starting with a relay‑switched hard bypass facility, and a Carnhill input transformer which provides high/low input gain and impedance options, as well as unbalancing the signal for the gain control section — or 'adjustment amplifier'. The latter is built around a discrete differential amplifier stage, stabilised with 16 carefully matched Zener diodes. The output from the adjustment amplifier feeds through a line amplifier into a 'blender' circuit, which allows the processed signal to be mixed with the original input to provide a variable parallel-compression configuration. Strangely, the dry signal is derived directly from the balanced input before the transformer, and the balanced to unbalanced conversion is performed by another active stage. The output from the blender section is converted back to a balanced signal with another active stage and passed to the output XLR.

The dynamic control side‑chain is configured as a feed‑back topology, deriving the control signal from the output of the gain‑adjustment amplifier. This signal is high‑pass filtered and rectified to provide a DC control voltage, which is conditioned via the attack and release controls before being passed to the control port of the adjustment amplifier. This DC control signal is displayed on the gain-reduction VU meter, and can be connected with that of the second channel via the stereo link switch. This mode ensures that both sides apply exactly the same amount of gain reduction, thus preventing image shifts. The external side‑chain input can be selected as an alternative input source for each channel's side‑chain processing.

Operational Controls

External side‑chain inputs are included alongside the main audio I/O on the Preceptor's rear panel.External side‑chain inputs are included alongside the main audio I/O on the Preceptor's rear panel.

All of the operational controls are switches — either rotary or toggle types — so that settings are both accurate and repeatable. In fact, the level difference between channels for matched settings is claimed to be within ±0.1dB (in link mode). The controls are laid out identically for the two channels, and are all pretty standard and intuitive. Either side of the large, kidney‑shaped VU meter are separate input and output level controls. These are both 21‑step, gold‑contact rotary switches, scaled simply from 1 to 21. The input level essentially acts as the threshold control, adjusting the input signal level relative to a fixed internal threshold, although the very first position mutes the input completely. The output level switch provides a ±10dB range, and determines the level of the compressed signal feeding the blender stage.

The first in a group of three toggle switches above and to the left of the meter activates the relay hard‑bypass function. When the unit is active, the meter glows yellow, changing to orange when in bypass mode. The second toggle alters the input impedance, and on the review model (serial number 22), it was labelled as switching between 300Ω and 1200Ω and had little effect on the signal level. From serial number 24, the design was changed to switch between 10kΩ and 2.5kΩ, providing low- and high-gain options, respectively.

The last toggle here switches the side‑chain source between the input signal and the external input. The external input signal can be used as a 'ducking' control, forcing the Preceptor to reduce the level of the main input signal in response to the external signal's volume — dipping a guitar part in response to a vocal line, for example. Alternatively, you could feed in an equalised version of the main input signal to use the Preceptor as a de‑esser.

Rather unusally, if the unit is switched to external side‑chain mode but without an external input connected, it generates quite pronounced harmonic distortion, because of the way the external input stage is connected within a feedback loop in the side-chain circuitry. This is a deliberate feature, and the strength and character of the distortion can be altered with the input level and impedance switches. The effect can always be tamed, of course, by adjusting the blend control to favour the dry source signal, which makes this an unusual but musically very useful facility!

Above and to the right of the meter, another pair of toggle switches configure the unit in compression or limiting modes, and adjust the overall 'speed' range of the attack and release controls. When set to compression mode, the Preceptor has a fixed 2:1 ratio, while in limit mode it behaves in a similar way to the ultra‑fast Fairchild 660 with something exceeding a 15:1 ratio. The speed switch offers three alternatives of 'fast', 'medium' and 'slow', effectively setting the boundaries for the variable attack and release control rotary switches, which are placed directly below the VU meter. These two gold‑contact switches each offer 11 positions, scaled simply from 1 (fastest) to 10 (slowest), with the eleventh setting on each providing an automatic (logarithmic) attack or release response.

In 'fast' mode, the attack control ranges from 0.5ms to 20ms, with the medium setting doubling those times and the slow setting multiplying them by a factor of three. The release control spans 25ms to 2s in the fast mode, with doubled and trebled time constants for the medium and slow settings.

The penultimate front-panel controls are another two rotary switches, located either side of the attack and release controls. To the left is a five‑position switch adjusting the side‑chain high‑pass filter's turnover frequency. The options are off, 40, 80, 120, 200 and 320Hz. To the right is the output blend control, which is another 11‑position switch. Each step position adjusts the blend or ratio between the source (dry, fully counter‑clockwise) and compressed (wet, fully clockwise) signals by 10 percent. As has been mentioned, this provides built‑in parallel compression operation, or can be used for blending in the required amount of harmonic distortion when the unit is switched to external side‑chain but without an external input (see above).

Finally, a simple toggle switch in the centre of the unit's front panel, below the mains on‑off rocker switch and orange, jewelled power light, provides side‑chain linking between the two channels, to permit stereo operation free from unwanted image shifts. When in linked mode, the controls on each channel remain independent and should be matched to ensure equal and consistent reaction to dynamic changes in each channel.

In Use

My Audio Precision test measurements agreed closely with the published specifications. Signal‑to‑noise ratio was in excess of 85dB (A‑weighted), falling to 73dB unweighted. Crosstalk between channels was below ‑100dB. Frequency response extended between about 10Hz and 40kHz at the ‑3dB points, and maximum input level was a fraction above +21dBu. These numbers give a good indication of the quality of the design, but as always, it's the ears that really tell you what you need to know.

The Preceptor is an impressive machine. The fundamental character is one of transparency and subtlety: it isn't hard to encourage it to get pumpy and aggressive if that's what you need, but it never seems to lose control. The fact that everything is switched makes it easy to match settings between channels, and to log and come back to preferred settings when required. Stereo imaging in the linked‑channel mode was absolutely rock solid, and the automatic attack and recovery modes seemed well judged and effective. Tuning the attack and release settings manually creates a wide range of effects, from smooth and articulate to seriously hard-hitting and dynamic. The Preceptor can make drums sound tight and punchy — the high‑pass side‑chain filter helping to prevent the kick drum from taking over — or make a string section sound smooth and melodious. It is equally at home with, and equally useful for, subduing individual instruments while tracking, helping everything gel together across a stereo mix bus, or even adding the final polish in a mastering suite.

This is a remarkably versatile unit with very well judged parameters, and the built‑in blend control adds the cherry to the top of the cream bun, by allowing you to squash things to death and then bring back the detail and edge by mixing in some of the source signal again. Parallel compression is rarely so easy! I was rather dubious about the 'harmonic distortion generator mode' obtained by switching to the external side‑chain input without feeding in an input… but I have to say it works rather well. Again, it is the blend control that really makes it usable and controllable, but it adds a nice rich edge to most sources that I'm sure many users will find extremely useful.

Verdict

The bottom line is that this is a seriously impressive musical tool. Few compressor‑limiters can claim to be a master of all trades — most have isolated strengths as a tracking device, a bus compressor, or a mastering unit. The Preceptor really can claim to be able to earn its keep in all three, and hold its head up high while doing so. So that's the good news. The inevitable bad news is that something this well built, and sounding this good, is going to be expensive. The asking price puts it amongst the likes of the Elysia Mpressor and my long‑standing favourite, the Neve 33609JD, amongst a few others (see the 'Alternatives' box). But it deserves to sit in that company, and if I had to pick just one unit to cover all bases. I think I might well end up walking away with the Preceptor. It's well worth auditioning.  

Alternatives

The Preceptor Model A (and T) are priced to compete directly with the likes of the Elysia Mpressor (reviewed August 2008), Crane Song's STC8, AnaMod's AM670, Shadow Hills' Octograph, and my long‑standing favourite compressor‑limiter, the venerable Neve 33609JD. The Elysia is probably the closest in terms of versatility. The slightly less expensive Chiswick Reach stereo valve compressor and Thermionic Culture Phoenix Side Chain bring valve sonics to the party, but the handy distortion mode in the Preceptor more or less ticks that box anyway!

The Compactor

The Compactor offers many of the controls of its larger sibling, though it lacks, for example, the Blend control.The Compactor offers many of the controls of its larger sibling, though it lacks, for example, the Blend control.Generic Audio Preceptor & Compactor

The Compactor is, in essence, a more affordable version of the Preceptor. Housed in a more compact 2U chassis, and with a slightly smaller 60VA‑rated custom mains transformer, it shares exactly the same circuitry throughout the signal path, and is still manufactured to the same exacting standards. However, the facilities and operational controls have been scaled back slightly, to reduce cost without sacrificing overall performance or quality. Again, all the controls are switches — either rotary or toggle — but there are fewer of them and they have fewer settings. For example, the Input and Output level controls have 11 positions instead of the Preceptor's 21, and the input‑impedance/gain and side‑chain input toggle switches have been omitted (the hard bypass toggle is retained). There are no TRS sockets on the rear panel for an external side‑chain input, either. The input control is scaled from ‑5 to +10dB, with 1dB attenuation steps and 2dB gain increments. The output control is scaled ±10dB, which is the same as the Preceptor, but it has 2dB steps instead of 1dB. Two further toggle switches on the Compactor provide the limit/comp mode selection and provide hard/soft‑knee options (a useful feature borrowed from the Model T Preceptor). Another, shared, toggle switch provides the stereo‑link functionality.

The side chain high‑pass filter has been retained, but offers only four options (off, 80, 200 and 320 Hz), omitting the 40 and 120 Hz options, and the attack and release controls provide six settings each instead of the Preceptor´s 11. The slow‑medium‑fast range toggle is also missing. However, the attack and release controls span almost the full ranges offered on the Preceptor, albeit with coarser steps. The Compactor's attack control offers 1, 2, 5, 8, 12, and 30 ms modes, while the release control has 0.05, 0.1, 0.25, 0.5, 1, and 2 seconds. Neither control has an auto mode.

The same vintage‑style VU meters are retained to show the gain reduction for each channel, and a chunky toggle and jewelled lamp are provided on the right hand side to switch the mains power on and off. The major omission, compared with the Preceptor's facilities, is the Blend control — but while that is a great shame, it's something that can be replicated fairly easily by connecting the Compactor in a send‑return signal path with the console or DAW, to allow parallel path processing (albeit with a compensating converter delay if working directly with a DAW).

The Compactor is really designed to appeal to those looking for a mix‑bus compressor that can also serve as a tracking compressor. The ultimate precision and versatility of the Preceptor, while very nice, is arguably more relevant to high‑end mastering, and for most other applications the modest reduction in setting options and facilities provided by the Compactor will have a negligible effect. The good news is that it sounds every bit as good as the Preceptor, because it uses identical circuitry throughout the signal path, and it is considerably more affordable, since it costs some 35 percent less than its bigger siblings. This is still serious money, of course — but then this is still a very serious product.

Preceptor Models A & T

The review model of the Preceptor is the type A, but the Model T is very similar, looks almost identical and costs the same. However, the Model T has an extended set of attack times, and there is only one set of release times (spanning 50ms to 2s — a merging of the fast and medium settings of the Model A). The attack times no longer overlap across the three modes, with the fast setting spanning 0.5 to 12ms. The medium range covers 13 to 25ms and the slow option extends between 26 and 38ms. Also, an additional 'Action' switch provides two distinctly different threshold and compression knee modes. The Soft mode is, as its name suggests, a more subtle and gentle configuration suited to mix bus and mastering applications, while Hard is more aggressive and better suited to tracking.

Published October 2011