Hero or Zero?

After years of producing high quality output transformerless valve amplifiers, Glenn Croft, to a zero feedback hybrid design. Haden Boardman listens to Croft's new Twinstar power amplifier.

From deepest darkest Birmingham hails Glenn Croft. He is something of an audio maverick, and about as 'anti commercial' as you can get. Never ask for sales brochures - they do not exist! For the past twenty years or so Glenn has hand-crafted his wares, manufacturing some of the most exquisite valve gear from the Micro series, through a range of output transformerless valve power amplifiers to conventional 'EI' and 'C' core transformer designs with complex regulated power supplies.

Although Glenn never went down the Single Ended triode route, he has stuck fairly close to his valve roots. But - shock horror - what is this? Not a new valve amp, but a hybrid design using 'Transvalve' circuit topology, to give the official title. Two octal based dual triodes drive the four MOSFET output and, erm, that is it! I have seldom seen such a simple circuit, no feedback, two power supplies (separate left and right power supplies for the output stage) and nothing but a handful of very high quality components.

The valve and MOSFET sections have their own individual power supply transformers, the valve circuits utilising a conventional EI type, with the output stage using a toroidal type. A small circuit board holds a relay circuit to mute the output while the two valves catch up with MOSFETS on switch on. The power supply is located towards the front of the chassis, no regulators are present on this model: simplicity coupled with high quality components is the order of the day here. Internal build is to Croft's usual standard with neat assemblies. Mains input is via IEC, gold phono sockets for input and gold 5 way binding posts for speaker terminals. Output power is a quoted 45W into 8 ohms - see our measurements.

Croft offers 3 additional upgrade to the cooking £1750 Twin Star, ranging from £2500 up to a whopping £4000, via £3500. The extra money buys you superior components, faster MOSFETS, bigger stabilised power supplies, and a 'super reg' 6C33 triode regulator on the two signal triodes of the top model. For the past few years most Croft products have been delivered in a large standard black case, but various levels of knob and fascia trims can be specified to buyers tastes. Paint far better than Croft products from yore, with a good quality crackle finish resting nicely with the sweet little power indicator, an illuminated Croft logo. Is this Glenn Croft paying homage to the old QUAD and Leak valve preamps?

SOUND QUALITY

From the moment it was turned on, it was clear this amplifier sounds superb. Hybrid designs usually leave me cold. They're either too warm in the bass with no real control, or uninvolving (i.e. the worst of both worlds), but the Twinstar is neither. Fast, delicate, powerful (within its remit), it gave better clarity than I have heard from any Croft power amp to date, and most rivals for that matter.

Starting with Jimmy Smith, the Croft was fast and dynamic. Willie Nelson's 'Stardust' on SACD (a glorious 1970's recording, produced by no less than Booker T.Jones) had oodles of definition and detail - again the Twin Star proved addictive. Nora Jones on CD was sweet light and open. On the more demanding vocals on Dinah Washington, the Croft turned in a knock-out performance, with great midband transparency.

Essentially an open, and clean yet lucid and engaging performer, it was able to do both the valve and transistor tricks with equal aplomb. The result that it's a flexible machine, happy playing anything from Café Del Mar through to classical jazz and even heavy rock. It gives a surprisingly open and uncluttered acoustic, with decent depth perspective and strong imaging, that lets the music flood out in a deliciously natural, organic way.

However, put against traditional transistor fare, it appeared ever so slightly veiled on some tracks, the last gasps of atmosphere and breath being held back. Could the MOSFET output devices be responsible - remember this amp has no feedback? Still this is still in comparison to amps which can't match the Croft's talents in the musicality stakes. Compared to the (admittedly cheaper) Quad 909, Ry Cooder and VM Bhatt's 'A Meeting by the River' did soften the upper treble somewhat, losing that last nth degree of incision and atmosphere.

Of greater concern with the review unit was the level of mechanical hum emitted from the casework. In my quiet listening room it was clearly audible, and a product of this class and calibre should really be a little quieter than this. The best fix I could find was to mount it on some isolation feet and stick it on the floor. To be fair, suppliers Eminent Audio seem to think mine is a rogue example, but if this does not prove to be the case then Croft should either redesign the metalwork, or fit better mains transformers.

This power amp was more than sensitive enough to run with a passive preamplifier, and with all the room in the case, perhaps Croft could turn it into a line level integrated? Indeed inserting a Vitale preamp in the chain confirmed the passive route to be the best with digital-only sources - the Vitale colouring the sound. Hooked up to my trusty Garrard 401(with Audio Technica arm, Denon DL 103 and Denon MC step up tranny), the combo sounded great - vinyl clearly suiting the Croft. It sounded both open and revealing, without a single hard edge to be heard. Sweet!

Obviously, the Twin Star makes no claims to be a muscle amp. Compared to the Quad 909, for example, it is obviously down on motive force, and has a looser and less imposing bottom end. It had enough drive not to get all upset with a pair of large Dynaudio speakers, but had the finesse to work well into a high efficiency design. The amp worked successfully with a large selection of ancillary equipment; every time offering a great performance. While it doesn't do as well as the Quad on large scale dynamics, it's so much better on 'microdynamics' - it can get loud and quite seemingly without effort, able to follow the flow of the music with great lucidity. By contrast, the Quad sounds stilted, despite its extra power.

Past Croft power amplifiers have left me underwhelmed. A couple of the OTL variants have been okay, but on the whole I found them too soft and lacking any kind of serious drive. Not so the Twin Star - it has neither a traditional valve or transistor sound, preferring the neutral route. With a good amount of power, an open and even sound yet most of the traditional valve attributes of musicality and cohesion, it's an impressive bit of kit. At £1,750 it is no bargain, but really can do things most other rivals can't. An interesting, charismatic, and able product that's well worth seeking out.

Haden Boardman

Verdict: Four Globes
With a taste of the best of both tube and transistor, this is a truly endearing power amplifier.


TECHNOLOGY

The Twinstar amplifier inhabits a different world to most amplifiers. It produces a lot of distortion, at all frequencies and output levels - no less than 0.7% with around 0.1% being the limit of audibility the Twinstar breaches this limit by some margin, so its distortion will be audible. With an 8 ohm load second harmonic dominated, but there was a lot of third too at higher levels. With a 4ohm load distortion rose to 1.6% with clearly waveform triangulation from third harmonic. Although distortion levels were fairly constant, suggesting low or perhaps negligible feedback, the spectrum analyser clearly showed upper harmonics. The Twinstar's appeal will be mainly subjective. Listen to it carefully against other amplifiers, using material you know. This amplifier does not measure well.

KEY FEATURES

  • Power 45Watts
  • Frequency response 5Hz-60KHz
  • Separation 82dB
  • Noise 110dB
  • Distortion 0.7%
  • Sensitivity 560mV

 


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