Photos: Mike Cameron
For anyone recording real instruments, the idea of a rack full of vintage outboard preamps is an appealing one, but few of us have the necessary budget. One solution is to buy modern clones instead of the real thing, and there are many companies providing a wide range of suitable products, some claiming to be identical, some with modest enhancements — but all with greater reliability! However, even the costs of these clones can be prohibitive to the home-studio user.
When I was first getting into music recording, DIY electronics skills were de rigeur: if you couldn't afford to buy something you needed, you made it instead. There were more electronics magazines than home-recording magazines back then, all crammed full of interesting audio projects. For better or worse, the interest in DIY electronics has waned as computer music has waxed — partly because modern electronics is so very sophisticated and requires advanced techniques and equipment. Of course, classic recording equipment uses classic electronics, and that remains within the grasp of any averagely competent DIY enthusiast.
A trawl of the Internet will reveal plenty of plans and diagrams for various classic outboard equipment, and many enthusiasts enjoy tracking down the components and building them — and get great results. But if you're after something slightly more convenient, the Sound Skulptor products reviewed here could be just what you're looking for.
The Sound Skulptor range is designed, produced and sold direct by Synchronia, a small two-man company in the south-west of France. Their main business is as a recording studio, but a home-grown interest in vintage preamp clones led to requests for more and the launch of the modular product range reviewed here. Currently there are four preamp modules, two power supplies and a rack housing to choose from, and the preamps can be mixed and matched to suit needs or budgets. In some cases there are also customisable options.
The system can be bought either as a fully constructed and tested product — like any other commercial design — or as a kit of component parts to be fully assembled as a DIY project. The majority of the circuitry is discrete, using full-sized components, and ICs are only used for things like voltage regulation and clip level metering.
Very detailed constructional notes and schematics are provided, complete with pictures and step-by-step build and test guides, so that anyone who can solder neatly and who owns a digital multimeter should be able to build and calibrate these modules quite easily. However, should it all go pear-shaped, the company offer a repair service to fix completed but non-functioning modules, with the work charged at an hourly rate.
For this review, I was sent a ready-built rack so that I could listen to the quality of the finished modules in a reasonable publishing time scale. The 1U rack was pre-fitted with one MP73, two MP12s (using different transformer and amp designs) and one MP32 module. The whole thing was powered by a PSL1 free-standing power supply. A larger option, the PSL2, can power up to three fully loaded racks simultaneously. Synchronia also mentioned that they will be releasing a new preamp module later this year (the MP66 tube microphone preamplifier, which is 'inspired' by designs from the '60s, will have the same form factor as previous modules, except that it will occupy two slots).
The completed unit looks very professional and well put together on the outside. The various input modules have different coloured front panels to aid recognition, with professionally etched and filled control legends. It all looks very high quality and solid. In fact, the only hint of DIY about it is the coloured paper sticky dots used to identify which DI input socket relates to which preamp module — and it would be easy to label these in a more up-market way, if required, during construction.
Inside, the unit is just as well built, with minimal but neat wiring between the I/O sockets and the preamp cards. I was pleased to see that proper attention had been paid to earthing arrangements — there is no 'pin-1 problem' here. The DI input card acts as the connection hub for everything, with flat ribbon cables radiating from it to carry power and the DI input signals (and automatic input switching) to each module. Again, it's all very neat and easy to configure and service. The modules themselves (and the DI card) are constructed from high-quality components on nice fibreglass PCBs, with clear component identification throughout. The board layouts are neat and show careful design and attention to detail.
The Sound Skulptor web site provides all of the necessary schematics, layouts, assembly guides and test procedures, as well as lots of pictures of each stage of the work, component identification, general DIY instructions and a complete user manual for the current modules and features — all as PDF files. So you can easily study exactly what is on offer for yourself, and see the quality and simplicity of these designs. I was very impressed.
DI Module & Power Supply
The SOund Skulptor DI02 module is an integral part of the system, as it distributes power to all the other modules. It provides two front-panel DI inputs that can be forwarded to any two of the four module slots in the rack (although to keep the cable routing neat it makes sense to link them to the first pair of modules).
The DI input circuitry is a simple FET-based design, which reduces the level and balances the signal to suit the preamp modules' mic inputs (so it goes through the preamp's input transformer, if fitted). The DI input is selected in place of the normal mic input automatically (via a relay) as soon as a plug is inserted. The maximum input level is +17dBu and the gain is fixed at -20dB.
There are two power supply options: a smaller one to drive a single rack and a larger version that can power up to three racks. Both are linear designs that provide balanced and regulated power rails (±27V DC) plus a separately regulated +48V phantom supply. The IEC mains inlet can be switched for 230 or 115V AC operation. The circuitry also includes short and over-voltage protection, as well as temperature sensing.
The first module I tried was the MP73, which is closely based on the Neve 1073 design. Although there are some changes to the way the gain switching is performed (replacing Neve's multi-position gain switch), these only simplify the design slightly and improve its operational characteristics. The gain stages are essentially the same. Most signal switching is performed by relays between the two power rails — no switching currents are returned directly to ground, and there are separate audio reference and power grounds throughout.
The input and output are via Carnhill transformers, separated by three discrete gain stages running off a single-sided 24V supply rail with its own local regulation. Most of the gain transistors are BC184s, with a 2N6488 to drive the output transformer. The input-signal source (rear-panel socket or DI stage) is selected via a transformer activated when a plug is inserted in the DI socket. An internal jumper link allows the input impedance to be switched between 300(omega) and 1200(omega).
The controls are fairly simple, with a three-way gain switch that offers 40, 55 or 70 dB. Maximum gain is with all three stages in circuit, but the lower two are with the first amplifier bypassed and the gain of the second stage adjusted as necessary. A conductive plastic rotary input-attenuator sits between the first and second gain stages to allow fine level setting, and another conductive plastic control between the second and third gain stages adjusts the output level.
There is also an output polarity-reversal (switched via another relay) and a phantom power switch (with a red toggle to aid identification). A bi-colour LED glows green for signals above -40dBu and red when the signal peaks within 3dB of clipping. It is driven from an unusually sophisticated circuit that monitors all three amp stages.
The technical specifications are all very respectable, with a maximum input level of more than +17dBu (from 50Hz up — transformer saturation limits the headroom below that), and a generous +27dBu maximum output level. Effective input noise is -124dBu and the bandwidth stretches between 10Hz and more than 30kHz (-1dB points). Distortion is better than 0.0035 percent and common-mode rejection better than 74dB at 10kHz.
The MP73 module certainly has a vintage sound character to it, thanks to all that lovely transformer iron and discrete class-A circuitry. The sound is best described as full bodied, rich and silky — the last a term often used in association with early Neve designs. It has that 'larger than life' quality, too, that sets the best preamps apart from the rest. The sound is detailed but not hyped, and has an open character that seems unstrained and easy. I found it worked well on pretty much everything, adding body and character to close vocals, or smoothness and integration to other instruments. It may not be the fastest preamp for cracking drums, but it certainly has the headroom to cope, and plenty of line drive to peak even the most insensitive of A-D converters.
The MP12 is a simpler design, but more advanced in many ways than the MP73. There's an input transformer, single gain stage, and output transformer — and all three elements can be mixed and matched to give radically different sound characters and qualities.
The input and output transformers can be selected from a long list of models by Cinemag, Llundahl, Jensen, Sowter, Edcor and others. Any discrete op-amp (DOA) gain stage that is pin-compatible with the API 2520 format can be used, and the circuit board has provision to adjust the balanced power rails (between ±12 and ±24V) to suit different designs.
A 'discrete op-amp' is exactly that — a collection of discrete transistors arranged in the same kinds of configurations as used in integrated circuit op-amp designs, but with a range of benefits that are mainly due to the larger amounts of silicon involved. Clearly, different DOA circuit designs have different attributes and sound characters. They also have different DC characteristics and offsets, so the MP12 PCB design also includes various options for DC or capacitive coupling to the output transformer, and manual or DC-servo nulling.
Sound Skulptor offers five different DOA options to accommodate different input transformer impedances, but other devices from the likes of API, Millennia, Forssell, SCA, John Hardy, JML and others can also be used.
Two different variations of the MP12 were included in the rack for comparative purposes, the first fitted with Cinemag input and output transformers and an SK25 gain stage. The second version was fitted with a Lundahl input transformer and a different Cinemag output transformer, plus an SK99A discrete op-amp module.
A total of 70db of gain is available, controlled by a rotary gain control along with switched input pad and stage gain. The output polarity can also be inverted and a third toggle switches on phantom power. An output control adjusts a balanced 18dB attenuator that follows the output transformer, to enable the output levels to be reduced if necessary. A bi-colour LED indicates signal level and clipping, as before.
The specifications for the all-Cinemag transformer version are generally better than the MP73, but not by much. The maximum input level with the pad engaged is +20dBu, maximum output level is +29dBu, and the gain range can be varied between 28 and 69dB (without the pad). Effective input noise is -127dBu, and the frequency response extends between 7Hz and 46kHz (-1dB).
The first version of the MP12 fitted with the Cinemag transformers behaved as I imagined it would, providing a tight, punchy 'American' character with a slightly forward mid-range. The manual refers to this as the 'LA' sound (typical of the big API consoles of the mid 1970s). It would be perfect for rock drums and tight, controlled vocals.
The second version surprised me, simply because it had no real character at all — it was extremely clean, neutral and transparent, and would definitely be my choice of configurations for serious classical work with distant stereo pairs. It still had a full sound and that slightly larger-than-life quality, but it didn't colour the sound in an artificial way. The first version definitely had a character to impart — very different to the MP73, but still musical and much more funky.
Synchronia ship the Sound Skulptor products from their base in France and all pricing is in Euros (so pricing outside the Euro zone will fluctuate according to the exchange rate). The full Sound Skulptor rack in this review is available ready assembled, or as a complete DIY kit. You can also order individual components to build your own custom system:
Complete mic preamp DIY kit: 1576.13 Euros.
Pre-assembled mic preamp kit: 2316.65 Euros.
MP73 and MP12 mic-pre Modules: 262 Euros each; MP32 285.84 Euros.
DI02 Dual Direct Instrument input: 106.44 Euros.
SKMP case: 169.83 Euros.
PSL1 power supply: 185.38 Euros.
PSL2 power supply: 247.57 Euros.
All prices include VAT and exclude shipping costs, which can be calculated using the Sound Skulptor web site.
The MP32 design uses two DOA gain stages in a symmetrical, balanced configuration. As with the MP12, any 2520-compatible low input impedance DOA stage can be used, with two options from Sound Skulptor. The SK99B is very clean-sounding and was supplied in the review unit, while the SK47B is said to introduce some interesting musical distortion characteristics.
There's no input transformer in this design — in order to improve the transient response — but there's an output transformer, to retain a degree of electromagnetic 'warmth'. Again, various transformers can be used with different amounts of 'iron' character and signal bandwidths. A Llundahl model was fitted to the review model, which gave a very wide bandwidth and low distortion.
The maximum gain of the MP32 design is only 62dB, which is lower than the other designs — essentially because of the absence of the input transformer's voltage gain. The technical specifications make impressive reading, with a maximum input level of +22dBu, maximum output of +29.5dBu, and an EIN figure of -127dBu. The bandwidth (with the Llundahl transformer) is an astonishing 3Hz to 220kHz (-1dB), distortion is below 0.0045 percent and common-mode rejection is better than 90dB.
The controls panel differs from the other preamp modules in that it doesn't have an output level control. There are just a rotary input-level knob and three toggle switches to set the gain range (high-low), output polarity and phantom power. A bi-colour LED indicates signal level and clipping, as with the other designs.
Of all the modules, this one sounded the fastest and cleanest to me, with the most transparent and open sound of all. Given the styles of music I mainly tend to work with, this is the preamp module that I would be tempted to use because it just amplifies the signal it is given without adding its own DNA along the way. For many this might be too clean and clinical, and the MP12 or MP73 versions would hold far more appeal. But that's the beauty of this system — you can mix and match preamps to create your perfect combination.
I was very impressed with the Sound Skulptor range. The mechanical and electronic design is first rate, with excellent attention to small but important design details. The range of transformer and DOA options in the MP12 and MP32 affords a huge degree of sonic customisation and flexibility, while the MP73 is about as authentic as you can get for the Neve sound.
As a fully built commercial product this system has a lot going for it, but as a modular kit for the DIY electronics enthusiast it is difficult to fault. Everything is very well documented (although there are some amusing bits of 'franglais' now and again), and the metalwork, circuit boards and components are all of the highest quality. If I was going to do something like this kit, Synchronia's way is exactly how I would do it — and that's about the highest praise I can give it! Each kit may appear to be fairly expensive, but you have to bear in mind that the finished product is very high-end stuff, with sound quality and looks to match — and additional preamp modules are actually surprisingly affordable.
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Test plots to accompany the article.
Audio files to accompany the article.
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