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Gem Audio Labs Sculptor

Discrete Preamp With EQ & Peak Limiter
Published March 2014
By Hugh Robjohns

Gem Audio Labs Sculptor

We check out Gem Audio’s classy and charismatic interpretation of the recording channel strip.

I first came across Polish manufacturers Gem Audio Labs in 2011, when the company was called Generic Audio. Its first products were the intriguing and impressive Preceptor and Compactor dynamics processors (reviewed in SOS October 2011). The company were restructured and renamed in September 2012, but still manufacture the original Preceptor (in both A and T forms) and Compactor, and have now expanded the range with the Sculptor, which is essentially a mono channel strip, combining a solid-state mic preamp with high- and low-pass filters, a semi-parametric four-band equaliser, and a fast ‘brickwall’ peak limiter.

Overview

The Sculptor is housed in a 2U rack-mounting case, featuring a gloss black faceplate that’s detailed with crisp white control legends. The rear panel carries the usual IEC mains inlet with integral fuse holder and voltage selector (115/230V AC), and a pair of quarter-inch TS jacks provide side-chain access to the peak limiter in the form of master-out/slave-in connections. These allow stereo linking with a second Sculptor to maintain stereo imaging accuracy. A removable plate in the centre of the back panel conceals an accessory slot for a planned A-D card, which was not yet available at the time of writing. On the right side of the rear panel, XLRs provide the balanced audio input and output, both via Carnhill transformers, while two quarter-inch TRS sockets provide a balanced insert send and return, which break into the signal path immediately after the preamp stage.

The front panel is arranged neatly and logically, with fourteen rotary controls and seven toggle switches. Helpfully, there are three different knob sizes to aid function separation and navigation, and everything is very clearly labelled. Mains power is activated with a very chunky toggle switch on the right-hand side of the panel, illuminating an orange jewelled lamp above the Gem company logo.

A four-way rotary switch in the top left-hand corner selects the input mode, starting with a high-impedance, DI input (an unbalanced TS input socket is provided on the bottom left of the front panel). The other options all apply to the rear-panel XLR, with a line-input mode and two mic-input options (with and without 48V phantom power). A larger knob directly below this input selector adjusts the preamp gain in 5dB switched increments, spanning -5 to +65 dBAn optional A-D converter card is planned, as hinted at by the blanking panel on the rear of the Sculptor.An optional A-D converter card is planned, as hinted at by the blanking panel on the rear of the Sculptor. in mic mode and a smaller -25 to +45 dB range between the nine o’clock and one o’clock positions for the line mode. A toggle switch on the left-hand side flips the input polarity.

The largest knob separates the preamp stage controls from the EQ controls, and provides a 21-step output fader, calibrated simply from 0-10. With the control fully counter-clockwise it provides -74dB of attenuation, with the increasing positions offering -26, -21, -18, and -16 dB, increasing by roughly 1dB steps thereafter, up to 0dB. This allows the preamp stage to be driven hard for effect, while the output level is backed off to prevent overloads in subsequent equipment.

Equaliser

All but one of the remaining control knobs relate to the EQ, which is laid out generally with the frequency controls on the top row, and the gain controls on the bottom row. Usefully, the fully counter-clockwise position of each gain control is labelled ‘Out’ and provides a section bypass, which is in addition to the full EQ In/Out toggle switch (with associated warning LED) over on the left-hand side of the front panel, which bypasses the entire EQ stage including the filters.

The first vertical pair of EQ knobs are for the high- and low-pass filters. These are active whenever the EQ is switched in, and both provide 12dB/octave slopes. The low end can be rolled off from 20 (the effective bypass position), 40, 60, 90, 150 or 300 Hz, while the high-end options are 4.7, 7.5, 9.1, 12, 16 and 20 kHz (bypass).

Unusually, the low-EQ section is boost-only, but with both peaking and shelving response options. The upper knob controls the frequency selection, with the first five switch positions providing peaking responses at centre frequencies of 30, 50, 70, 100, and 200 Hz. The last two switch positions offer shelving boosts from 100 or 200 Hz. The gain is adjusted by the lower, 11-step switch, with the first three positions offering Out, +1, +2, +4 and then incrementing in 2dB steps up to +18dB. LF cut can be achieved, if necessary, by using a modest LF boost in concertwith a high frequency setting on the high-pass filter.

The lower-mid section is configured as cut only and is useful for helping to reduce mid-range muddiness. The centre frequency options are 200, 250, 350, 500, 600 and 850 Hz, and the gain is adjustable from Out to -6dB in 1dB increments, followed by 2dB steps to -15dB. A toggle offers ‘wide’ or ‘sharp’ bandwidth options. The upper-mid section is equipped with similar features, but has a boost-only response centred at 1.2, 1.8, 2.2, 3.9, 5.8, or 8.1 kHz. Gain is adjustable from Out to +6dB in 1dB steps, with 2dB increments thereafter to +15dB, and the bandwidth can be switched between sharp and wide again.

The internal layout of the Sculptor, revealing the large Carnhill output transformer bolted to the rear panel next to the XLRs, and the smaller white-topped input transformer towards the front. The diode valve used in the limiter is on the right-hand side of the PCB, near the centre, and the stacked toroidal mains transformers are in the top right-hand corner, along with an RFI filter. The case lid has its own safety ground tag.The internal layout of the Sculptor, revealing the large Carnhill output transformer bolted to the rear panel next to the XLRs, and the smaller white-topped input transformer towards the front. The diode valve used in the limiter is on the right-hand side of the PCB, near the centre, and the stacked toroidal mains transformers are in the top right-hand corner, along with an RFI filter. The case lid has its own safety ground tag.

Finally, the high section provides a shelf boost, with turnover frequencies selectable from 6.8, 8.1, 12, 16, 20 and 27 kHz. Gain is adjustable from Out to +4dB in 1dB increments, and then on in 2dB steps to +16dB.

Brick Walls

The third element of the Sculptor is a peak limiter, which is designed to provide protection against clipping an A-D converter, rather than for more creative manipulation of a signal’s dynamics — although I’m sure it will be used creatively by some too! With only two controls, the limiter is very easy to set up. An uncalibrated rotary control provides continuous threshold adjustment ranging from +10 to +20 dBu, while a toggle switch selects either a hard or soft knee mode. The former uses a circuit based around ‘vintage silicon diodes’, apparently, while the latter uses a diode valve (see Internals box). An LED gain-reduction meter indicates how hard the limiter is working, with 0.5dB resolution over the top half of the display followed by 1dB increments up to 7dB.

Internals

The Sculptor is a genuine hybrid device, employing discrete transistors, high-quality op-amps, and even a valve in its construction. A classic Carnhill VTB9046 input transformer appears to feed a discrete transistor mic preamp stage. Burr Brown OPA604 op-amps are used throughout the signal path — all socketed — along with several discrete transistor amplifier stages. The balanced insert send and return, which follow the mic preamp stage, are handled by a THAT 1646 balanced line driver and THAT 1200 receiver chips.

The limiter actions are shown here for minimum, mid and maximum thresholds, with the red traces showing the hard responses, and green the corresponding soft responses.The limiter actions are shown here for minimum, mid and maximum thresholds, with the red traces showing the hard responses, and green the corresponding soft responses.

Second-order high- and low-pass filters are based on fairly conventional active electronics, the Q of which is tuned to provide 1dB of boostjust before the roll-off. The LF section of the EQ employs a Carnhill inductor in an LC configuration, while the two mid sections use parallel active gyrators (active inductors, essentially). The high shelf employs a simple RC topology and the electronic design of each section was chosen specifically to deliver the desired sound character: ‘big and fat’ for the low end, ‘transparent and fast’ for the mid-range, and ‘silky and smooth’ for the high end.

Two completely separate limiter circuits are provided for the hard- and soft-knee responses. The soft path uses a MOSFET in combination with an EAA91 dual-signal diode valve, while the hard path uses ‘vintage’ silicon diodes — but I can’t tell you much more about the limiter circuitry as the ICs in this area of the circuit board have had their identification markings deliberately removed and obscured!

The complete set of high- and low-pass filter settings, exhibiting the deliberate 1dB ‘kick-up’ just before the start of the second order slope. Interestingly, the responses appear to rise at the bandwidth extremities, rather than fall off steadily.The complete set of high- and low-pass filter settings, exhibiting the deliberate 1dB ‘kick-up’ just before the start of the second order slope. Interestingly, the responses appear to rise at the bandwidth extremities, rather than fall off steadily.

The linear power supply is very unusual in having a pair of stacked toroidal transformers — one large, and one small — presumably the combination being the most cost-effective solution to generating the plethora of different power rails required by this complex hybrid circuitry. The back panel of the metal case is used as a heatsink for a large number of regulators (11) and a pair of transistors,which appear to be the drivers feeding the Carnhill VTB9070 output transformer.

Build quality is to a very high standard throughout, with most electronics implemented in standard components and mounted on a large PCB, which covers almost the whole floor area of the case. A second board behind the front panel carries a lot of the EQ components and switches, as well as the gain-reduction metering circuitry. A small multi-pin connector towards the centre-rear of the main PCB appears to provide the signal and power access point for the planned optional A-D module.

In Use

Like its siblings, the Gem Sculptor performs well and sounds very musical. I wouldn’t call the Sculptor a highly transparent preamp, though — it definitely imposes an audible character. On the test bench, the Sculptor’s distortion figure, even for line inputs, is around 0.1 percent,The full set of HF shelf frequency settings at maximum gain.The full set of HF shelf frequency settings at maximum gain. and the frequency response exhibits a distinct 2dB HF boost above 5kHz. These contribute a degree of richness and warmth through the low-mid, and some airiness at the high end, both being musically flattering in a way which will undoubtedly be appreciated by many potential users.

I did notice, however, that although the LF response is pretty flat down to below 20Hz throughout the majority of gain settings, the Sculptor can sound noticeably lean at maximum gain, and my bench tests revealed that there is progressively more roll-off below 300Hz through the top three mic gain positions, amounting to -12dB by 20Hz at the maximum 65dB gain setting.

Including a balanced insert point is a very useful feature, since it not only allows an external processor — typically a compressor — to be patched into the signal path, but it also enables the direct preamp signal to be recorded independently of any EQ and limiting treatments, and it allows line-level signals to be passed through the EQ and limiter stages bypassing the preamp and input transformer — all valuable options.

The complete set of LF EQ responses, with the green traces showing the bell modes at maximum gain, and the red traces the two shelf responses.The complete set of LF EQ responses, with the green traces showing the bell modes at maximum gain, and the red traces the two shelf responses.

There’s plenty of mic gain on hand, as well as the ability to cope with high output mics in front of loud sources. The preamp was designed to take being overdriven fairly gracefully.This is all the more usable with the inclusion of the output level control which provides a high degree of precision over a 20dB range spread across most of its rotation.

The EQ section is unusual in the way different bands offer only boost or gain, but not both — but this actually reflects the way most people use EQ most of the time, boosting the high end for air, the low end for warmth, the upper mids for presence, and cutting the lower mids for clarity. The provision of six frequency options plus wide or narrow bandwidths for mid bands makes it easy to find the optimum settings, and having bell and shelf modes for the low end is very practical too. I liked using the EQ a lot — it is a very musical design — and the fact that everything The full collection of mid-cut EQ responses at maximum gain and all frequency settings in wide (green) and sharp (orange) bandwidth modes.The full collection of mid-cut EQ responses at maximum gain and all frequency settings in wide (green) and sharp (orange) bandwidth modes.is switched makes it easy to get back to favoured settings.

If tracking with sensible headroom margins, the peak limiter is superfluous and I found that I didn’t make much use of it. But for anyone wanting to work with minimal headroom, or needing to trap occasional large transients, the limiter does what it is claimed to do with an almost tape-like action. The limiter action is relatively soft-kneed and acts over a roughly 3dB range leading up to the limit threshold, with the Soft mode starting lower and more gently than the Hard mode, especially at the lowest threshold settings. At the lowest threshold the limiter held the level just below the claimed +10dBu, while the highest threshold appeared to be just above +18dBu rather than the claimed +20dBu — not that anyone would be too bothered.

As above, but for the Mid Boost EQ, with wide (purple) and sharp (red) settings at maximum gain and all frequency options.As above, but for the Mid Boost EQ, with wide (purple) and sharp (red) settings at maximum gain and all frequency options.

Overall,the Sculptor is a very nice-sounding channel strip, oozing charisma and with very well thought-out features and facilities. This is a very worthy partner to the Preceptor and Compactor, and I imagine many will combine one or other of those dynamics processors with the Sculptor to form a very characterful vocal recording chain.

Alternatives

There are lots of ‘channel strips’ on the market, but few with similar features and at a similar price to the Sculptor. The Cartec PRE Q1A is the closest I can think of, combining a transformer-coupled preamp with a nice and inductor-based passive EQ stage based partly on the Pultec design.

Published March 2014