Croft's home-grown valve amps meet US exotica head on

by Paul Boak.

'Valve amplification designed and built the proper way' is the slightly contentious advertising slogan in the Croft Acoustics brochure.

No, I hadn't a clue who Croft were either, until I stumbled upon a demonstration at the Wolverhampton show that was actually good enough to entice me into the room. Once I had fought my way through the crowds I was faced with an array of valve amplifiers that left me pinching myself to make sure I wasn't dreaming. We all know that the gentle orange glow of thoriated tungsten is synonymous with massive engineering, but the build quality and sheer size and complexity of the Croft amplifiers is enough to make even American exotica look puny by comparison.

Croft Acoustics is a small - OK, tiny - outfit that consists of two music lovers, Glenn Croft and Peter Bruty, who can be found in sunny Wolverhampton as I write this, but will probably have moved to larger premises in north Birmingham by the time this is published. The story is a familiar one: some enthusiasts unhappy with what is commercially available build their own equipment and then decide to produce it for general sale. The similarity with most other first effort esoteric products ends here, as not only are the Croft amplifiers constructed to the highest standards that I have ever seen in domestic hi-fi, but the fit and finish is as good as anything the rest of the world can offer, including Japan.

Viewing the interior of any of the products (an inexpensive preamp, a dual mono preamp with two power supply options, and 110 watt monoblock power amps) reveals some unusual design features, the most striking of which is the total absence of printed circuit boards.

It has always been a mystery to me that amplifier manufacturers promote the use of special connecting cables and plugs, but then slap all the innards onto a piece of resin impregnated wallpaper or glassfibre which often employs somewhat dubious quality printed wiring. I often wonder if a hard-wired amplifier would sound better than the machine-soldered variety. Is it only coincidence that the best available transistor amplifiers have a substantial proportion of the manufacturing costs devoted to building good PCBs with wide conductor tracks and component spacing, and eschew machine soldering?

In all the Croft products every component is either hardwired or tag board mounted, a very laborious operation which is necessarily reflected in the retail cost - the cheapest pre/power combination is, reach for the Aspirin, 2880.

Still, comparable US exotica is hardly bargain basement either, so don't be too hasty to scream rip off. This economy package uses the cheapest preamp, a valve design with an on-board solid state power supply, which is relatively affordable at 380 retail. The design follows established conventions in having no tone controls and minimum facilities.

Moving up the scale, a dual mono valve preamp is available with a choice of two power supplies, one massive, the other gargantuan. The actual preamp box itself is exactly the same in each case, and as may be expected at the high price level (125O, or 2650 with the Mega power supply) is immaculately constructed from exotic components. The only feature which I dislike is the use of two separate volume controls which I find rather fiddly to use.

If there is any one Croft component which should leave any self-respecting electronics engineer open-mouthed, it is the Mega preamp power supply. In transistor preamps, the power supply provides the voltage rail or rails, usually 20-3O volts, and earth. In valve designs, the required HT rail voltage is an order of magnitude greater, and an additional low voltage, high current supply is necessary for the Croft heaters. The Mega supply has no less than six separate HT sections, one for each stage of each channel, and two separate heater supplies. The really crazy thing is that it's all done with valves! Most modern valve designs now use a regulated transistor supply, but Croft maintain that valves are inherently superior for rectification and regulation as well as signal handling. I have never seen any irrefutable proof that this is so, but I must admit that I find the majority of transistor amplifiers sound horrible. While nearly all valve amplifiers sound relaxed and musical. (The address for protest letters and violent threats is on page five.)

In any case, there's no denying that the Mega supply is one hell of a piece of engineering. No less than four transformers and 30 valves are housed in a huge cabinet which needs a block and tackle to lift it. For the peasants a smaller regulated supply is available, which struggles by on a single supply for each channel. In both supplies the 12.6V heater supply is also split into two mono sections for good measure.

Croft's mono power amplifiers are even more unconventional in that they have no output transformer, bearing some slight resemblance to the famous Futterman design. In a conventional transformer-coupled valve design, the output valves drive a transformer which alters the voltage and current ratios to drive the loudspeakers. The Futterman and Croft designs dispense with the transformer, using multiple valves in a series/parallel network to drop the voltage and raise the current capability. A feedback loop is provided around the stage to reduce the output impedance.

The main difference between the American and Wolverhampton designs is their HT arrangements. The Futterman has a conventional single-ended supply, and is capacitor coupled to the speaker to block DC offset. The Croft has a split-rail HT (+ 330V and - 330V) which is set to be in equilibrium, resulting in zero offset and the elimination of the coupling capacitor. Both lines are fuse protected, so that if one fails the other should blow immediately. If it doesn't, reach for the fire extinguisher!

"One of the things these amplifiers really excel at is portraying the tonal quality of acoustic instruments, in such explicit detail as I've not heard before except at live performances."

One of the drawbacks of direct-coupled valve amplifiers is their load sensitivity. The Croft literature warns against the use of low impedance speakers, suggesting that optimum results will be obtained with the normal 8 ohm variety. Don't let that lead you into believing that the power amplifiers have been engineered with some cost expedience; far from it. The output stage comprises ten GE 6080 double triodes, and the HT lines are fed from an enormous 625 VA toroidal transformer. Separate supplies for the driver and heater stages are provided by transformers rated at 5OVA arid 225 VA respectively.

The cost? 2500 per 110 watt stereo pair.

At these sort of price levels the Croft amplifiers had better be good, and by golly they certainly are. At the beginning of the article I mentioned my introduction to them at the Wolverhampton show, where they were far and away the closest thing to live music. The same effect was apparent when I was able to listen to them at a later date, at Pete Bruty's flat.

One of the things these amplifiers really excel at is portraying the tonal quality of acoustic instruments, in such explicit detail as I've not heard before except at live performances. Naim and Audio Vois amplifiers and Linn Isobarik speakers have the same strength, nowadays usually referred to as tunefulness, but to my ears the Croft betters even these fine components, albeit at a price.

Very low listener fatigue is usually claimed for valve amplifiers, and here again the Croft set up is comparable with the best available. When I went along to listen at Pete Bruty's, I took my long-suffering wife, who is not at all keen on hi-fi equipment. In the car on the way home, I asked what she thought. 'Well at least it didn't give me a headache', came the reply; and believe me, that is some compliment by her standards.

I was quite surprised at this, as the listening room was quite small and the system absolutely dominated the furnishings. A huge Mentor turntable (more on that later) fed the glowing mass of valves, which were in turn dwarfed by a pair of Klipshorns. I was expecting the sound to be capable of shaking the walls, but the listening quality turned out to be excellent at all listening levels. The bass was much more controlled than I expected, and the Croft must now be classed along with the excellent EAR power amplifiers as having virtually all the best points of both valve and solid state designs, combining both midrange transparency and sweetness with tight, powerful bass.

How nice it is to report that British innovation is alive and well in the West Midlands.

Massive Mega power supply for Croft's preamp

~  H1-FI ANWERS JUNE 1984

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