'Good enough' is rarely good enough for SPL, as this thoroughly over-engineered preamp testifies!
Sound Performance Lab, better known simply as SPL, have made high-quality audio equipment for three decades now, and they've always prioritised sound quality above all else — but they've often pioneered innovative technologies too. On review here is the company's latest eight-channel mic preamp, the Crescendo, which is a substantial piece of kit, housed in a 3U rackmounting chassis which extends 310mm behind the rack ears and weighs over 10kg. From the user's point of view, it's very straightforward, just as a preamp should be, but it features some interesting technology that sets it apart from most mainstream preamp designs. In particular, the Crescendo uses bespoke, discrete transistor op-amp gain stages and output drivers, which SPL refer to as SUPRA op-amps.
Other manufacturers use discrete op-amp stages, of course, but SPL's approach differs dramatically in their use of unusually high power-rail voltages: the circuitry operates on symmetrical ±60V power rails, an arrangement which might be expected in a power amp but is very unusual within a preamp. The Crescendo is not the only SPL product to work with such elevated power rails; the same engineering approach is used in several of the manufacturer's high-end headphone amps, their phono preamp, several power amps, a D-A converter, the Neos console (https://sosm.ag/spl-neos) and, of course, all of their mastering console product range. So the technique is well-established, and the SUPRA op-amp modules have now been developed into a fifth-generation design.
The significance of the power-rail voltage is that it effectively defines the largest audio signal the circuitry can accommodate: the higher the voltage, the greater the audio headroom. In the days of valve equipment very high anode (plate) voltages were commonplace, so valve equipment has always operated with plenty of internal headroom — that may well be one of the appealing factors of the 'valve sound'.
However, transistors, and then op-amps, brought with them a general reduction in power-rail voltages. For example, the classic (transistorised) Neve 1073 preamp was designed to run on a single-sided 24V rail, while the 500-series modular system operates with ±16V rails, and most IC-based audio equipment rarely runs on anything higher than ±18V. So, bearing in mind that the maximum audio signal level will almost always be a Volt or two lower than the power rails, the highest signal level that can be accommodated in the 1073 is about 22V peak-to-peak, which translates to +20dBu, in round numbers. (In the published specs the 1073's maximum output is given as +26dBu — the extra 6dB comes courtesy of the...
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