English Acoustics

Stereo 21c Valve Amplifier

£5,000

In the space of just ten years or so, tube amplifiers went from hero to zero. Back in the late nineteen sixties, the hi-fi world – and indeed the wider world of consumer electronics – began moving from thermionic valves to solid-state transistors as its preferred amplifying device. Of course, it was always going to happen, as vacuum tubes – as they're also called – were expensive to make and ran very hot, creating a whole world of pain for circuit designers. Transistors produced far less heat and were tiny by comparison, so what was not to like?

I am just about old enough to remember valves in everyday devices, such as televisions, open reel tape recorders and radios. The world that I grew up into, in my early years, still had them. A decade later, however, when I reached my teen age at the end of the seventies, pretty much everything had become solid-state, and those earlier times of a world powered by valves seemed like another life. By the early eighties, valves had become terminally unfashionable – sad, decaying, dusty relics from a backward world, or so it seemed. That's certainly how it looked to thirteen year old me – who was now the proud owner of a digital watch and had his eye on a Commodore PET microcomputer. “The white heat of technology”, and all that jazz.

Peter Farrow was obviously much savvier than me. We're probably not too far off in terms of age, but he fell in love with the Leak Stereo 20 when one of his school friends obtained one. “I already had a strong self-taught understanding of electronics, and my fascination for valves has never gone away”, he says. “Once I saw and heard the Stereo 20, I wanted one for myself. I remember that first day of hearing one very well; it was playing Grace Jones' The Apple Stretching from a TEAC dbx reel-to-reel tape player on a pair of studio monitor speakers. I eventually bought this grey Stereo 20 from my friend when he obtained a pair of Quad IIs.”

It was this memory that lead him to create English Acoustics, based just down the road from me in deepest, darkest Wiltshire. A passionate electronics whizz, he always loved the Leak Stereo 20 and thought it special enough to build a business around. Coincidentally, as I reached my twenties, I began to get 'valve curious', having heard a friend's Stereo 20. The hi-fi magazines that I'd read since the late seventies told me that valve amps were rubbish, but I heard a well-restored example of the aforementioned Leak at the end of the eighties, and it blew me away. I began to doubt what I'd been told to think by magazines that were sustained by an almost exclusively solid-state industry…

THE STORY

The Stereo 21c is all about – one might say – 'traditional values in a modern setting'. Although Pete has restored Stereo 20s in the past, his product is manufactured from the ground up as a new amplifier and harnesses the original circuitry to some modern design flourishes. So it isn't a restored classic, but rather more of a reimagined one. He takes up the story…

“We decided that it would be great to recreate the Stereo 20 as a modern product and possibly make a TV show with my media company about the process. The TV show never got finished, but we pursued the amplifier rebirth idea, and the result was the Stereo 21c (21c for 21st Century). We chose the Stereo 20 because, even having had Quad IIs, we found the latter was problematic with valves, had very low sensitivity, lacked the smoothness of the Leak, and had less bass. The Stereo 20 is very well-known and almost universally loved when people start talking about valve amps. The Quad II just seemed like an also-ran by comparison.”

“The Stereo 20 was one of the first, if not the first, hi-fi stereo power amps on the market and launched in 1958 at the London Audio Fair when stereo records were also introduced. At the time, other manufacturers, like Quad, were using two mono amplifiers for stereo, so the Leak lead the pack in this regard. Indeed it was at or near the top for vintage valve amplifiers of the time. To my ears, nothing from that era sounds as good. This is partly due to its fantastic output transformers, which are bigger than those of the Quad II and larger than other amplifiers using EL34s for twice the power output. These large transformers gave superior performance, easily delivering full power below 20Hz and above 20kHz. The later higher power Leak amplifiers had undersized transformers by comparison, and never achieved full power at 20Hz…”

Pete says that the Quad II also had woefully low sensitivity for full output, and its output transformer only provided full power at 30Hz and above; some say this was to avoid overloading the Quad Electrostatic ESL57 speakers with too much bass. I would concur, and hearing a pair of these was probably what put me off valve amplifiers in my younger days. To me and many others of my generation, the breed was famous for sounding fat, sluggish and soft – with no bite or punch. This was very much how an ageing, unrestored pair of Quads sounded back in the day!

The Stereo 20 isn't perfect, though – it's too sensitive for modern equipment and has a rather basic, built down to a price, chassis design. Like the Quad II, it also has a tendency to run hot and burn output transformers. Yet still, it's such a well thought out thing, in general. “It closely follows the Mullard standard circuit for EL84 valves, and its simplicity and symmetry of the design is easy to understand”, Pete says.

NEW WAVE

“We knew from the start that reimagining the Stereo 20 was going to be a tall order”, Pete tells me. “The amp has an army of followers who worship the original sound, so we had to produce an amplifier that had the DNA of a Stereo 20 but was better in every way – and still deliver the smooth sound that everyone loved, but in a modern and more dramatic way. The golden rule was that the original audio circuit had to remain unchanged. However, the sensitivity was too high for modern equipment, so to fix this, we applied a passive attenuator at the input to lower the sensitivity to 450mV rms for full output.”

The chassis has been substantially – but subtly – upgraded. On the 21c, it's a 2mm thick laser cut affair with a 'joggle' on the bottom, so there are no metal edges anywhere on the outside of the chassis. This is recessed by 3mm and has a 3mm thick steel plate fitting flush into it. Hinchley-style transformer end caps are made from solid aluminium billets, whereas the original Stereo 20 were pressed, giving an inferior look. The preamp connector is now no longer on the front of the amp for a neater visual look.

The 21c's circuit board “matches exactly the size and component layout of the original Stereo 20”, says Pete – and it uses custom made paper in oil capacitors that match the original design values of 0.25uF, which is a capacitance value that isn't available off the shelf today. In addition, the amplifier has a medical-grade power input filter with dual fuses, which means even if the mains is reversed (Live/neutral swap), it is still fused on the hot side. The filter completely negates the requirement for upstream passive mains filters.

Other changes include the deletion of the Stereo 20's impedance and voltage selectors; an in-house designed transformer is used that lets the amp drive speakers from 4 ohms to 8 ohms. Pete says these output transformers have a much wider bandwidth than the originals and better current delivery. The improved mains transformer design means it runs approximately 20C cooler than the original. They're all made locally to the Wiltshire factory, whereas the custom capacitors are made in the USA; the connectors are made in the UK and Europe. “Every effort has been made to use parts from the UK and Europe where physically possible”, says Pete – so this is a long way from being a Shenzhen-made valve amplifier which has been given an English sounding name.

The Stereo 21c has a soft-start system that gently powers up the amp from cold and a fixed-logic controller inside that measures output tube current, temperature, HT current draw, and shuts off the power if it is excessive. Furthermore, it has an orientation sensor, so the amplifier won't power on if it's upside down or on its side, plus a vibration sensor which powers the amplifier down if it's knocked or there is an earthquake, and a temperature sensor inside the mains transformer that cuts the power if it gets too hot. “We are not aware of any other amplifier having such comprehensive protection circuitry on the market”, says Pete. The logic controller is even upgradeable, so users will be able to view remote telemetry on the workings of the amplifier via a web browser or phone app. Last but not least, there's even an hour meter, so users know when to change their valves – after approximately 5,000 hours.

The Stereo 21c is a thoroughly re-engineered version of the original Leak Stereo 20 then, whilst keeping that all-important audio circuitry completely intact and faithful to the original design. All of which results in a bit more power than the original and around half the distortion. Bandwidth at around 14W RMS output is 13Hz (0dB) to over 50kHz (-3dB); noise is quoted as below -90dB (ref 1W). The original Leak version puts out around 11W RMS per side, with a THD of 0.125% to the Stereo 21c's 0.06%.

All of this makes for a very polished and professional-looking 'reimagined' Leak Stereo 20, and one that works in a much more modern way too. However, the icing on the cake is the paint finish. This was always good on the original Leak design, but nowhere near as special as this. The paint job is available in any automotive colour that the customer wants and is performed by a highly respected car paint shop in Wiltshire. I've seen several examples, and they're strikingly good – almost mirror finish, like a lake of still water. It's better than the factory finish on my modern BMW and equal to that of my classic, forty-year-old Jaguar XJ-S.

THE LISTENING

Anyone who's heard the original Leak Stereo 20 will think the English Acoustics recreation to be spookily similar, yet dramatically better. The Leak was always a lovely sounding thing, with a silky, warm tonality – slightly sugary but not sickeningly so – allied to plenty of rhythmic get-up-and-go and a spacious soundstage that gave everything in the mix room to roam. The English Acoustics 'brother from another mother' takes all this and adds more detail, insight, openness and – best of all – grunt.

It's no muscle amp, that's for sure, but still, you end up wondering quite how it's able to drive awkward loudspeaker loads as loud as it does. It has a sense of ease that the original lacked and even more rhythmic urgency. This, in turn, expands the range of music that you can enjoy on this amplifier – I found myself really enjoying punk rock and new wave music, plus a healthy dose of electronica when the original Leak was more comfortable with solo female vocal and gentle jazz. It's a really clever product, then, retaining so much of the Stereo 20's charm and gentle euphony but adding a whole extra layer of muscle and stamina.

I did most of my listening via my Yamaha NS-1000M monitor loudspeakers, fed by a Chord Electronics Hugo TT2 DAC (with variable output). It turned out to be a peachy combination. The Yams were – as ever – very neutral and revealing, while the Chord did its Chord stuff with a clean, tonally quite dry sound that matched the gently sepia-tinged sound coming out of the Stereo 21c. For an (allegedly) 14W RMS per channel power amp, the latter made a big impression on my speakers; these don't take prisoners with low powered amplifiers but didn't really worry the Stereo 21c – until the volume was turned way up, then I heard just a hint of strain. Most people in small to medium-sized rooms and/or with neighbours with whom they need to stay friendly shouldn't find it underpowered in practice.

One of the loveliest facets of the original Leak was its beautiful tonality; it was never fat and full like a Quad II but still served up a distinctively 'nice' sound that flattered whatever it played. The Stereo 21c also does this but adds a good deal more detail and incision, making it more satisfying to listen to when you're really focusing in on the recording. You still get the 'feel good' vibe, but you can hear the lustre of brass instruments better and the grain of the human voice; cymbals have more of a shimmer, and bass guitar sounds chewier. All of this was clear to hear on Average White Band's Twilight Zone, which is a gorgeous early seventies analogue recording. Each strand of the mix sounded really tactile and earthy, yet still a little more luxurious than you'd ever get from solid-state, even a good Class A design.

Then there's that lovely, lilting rhythmic prowess. This is no Technics SU-R1000 bruiser; it doesn't pile-drive the music's natural force out at you; instead, it sort of sashays along in a relaxed but dextrous way. The notes just float by, and you find yourself in a state of unexpected relaxedness as things ebb and flow. It's a very seductive listening experience, one which is quite different to solid-state – and like good chocolate or champagne is rather 'moreish', you just can stop partaking! The soul/funk of Twilight Zone was lovely, yet the magic still worked on far more aggressive post-punk such as The Stranglers' seminal cover of Walk On By.

One reason for this is the Stereo 21c's bass performance. Traditionally a weak point of valve amplifiers – primarily due to poor output transformers – the English Acoustics amp sounded positively punchy. Although its bottom end lacks the grip of a well-designed solid-state design, it still makes a delightful, slightly soft and sumptuous sound that stays in perfect time and has impressive dynamic articulation, too. You have to push the amp rather loud before the bass begins to run out of puff, and before this happens, its presence is never forgotten. The great, almost virtuous bass guitar on this track really pushed the song along – giving it a steamrolling quality that all the other instruments, such as guitar and organ, seemed to play off.

This is a scungy old recording – no hi-fi show dem favourite this – but the musicianship is sublime, and the Stereo 21c showed this in spades. Its midband is lovely and lyrical, yet there's a lot of insight too. You can really hear into the recorded acoustic and pick out oodles of detail – unlike the original Leak amp, which always sounded a little misty to me. This is great for intricate jazz, such as Herbie Hancock's I Have a Dream, with its vast, walk-around soundstage. This classic late sixties Blue Note recording can sound huge and enveloping on the right system, and so it proved here. I loved how I could hear the trumpet placed far left, the trombones hard right and the flute and flugelhorn centre stage, just above the piano.

Even with hard-hitting techno, the Stereo 21c didn't disappoint. Kraftwerk's epic Techno Pop, with its crashing dynamics and rock-solid synthetic electronic bass drum sound, was carried impressively. The speed of the rhythmic elements in the song surprised me, as did the way they syncopated together so tightly, but on this track, it was the dynamics that really took the lead. This has crashing transients, followed by – what feels like – long spaces in between the beats, then more huge thumping. This amp showed how brilliantly agile it is by keeping a firm grip on everything and deftly capturing the music's emotion and articulation. It gives the lie to this who think valve amplifiers are slow and woolly sounding – they can be the diametric opposite.

Of course, it's not perfect, and better and/or more powerful valve amplifiers are available. Yet the trick of the Stereo 21c is its sheer across-the-board ability at its price, allied to buckets of good old fashioned charm. Often amplifiers have one or the other, but rarely both. For me, its party piece is to sound as alluring as an old school classic valve amplifier but not to behave like one – there's no sound of output transformers complaining, or mysterious dropouts in the bass, or indeed misty midbands and flattened dynamics. Instead, this sounds very modern in many respects but still has oodles of old school appeal.

THE VERDICT

I reckon you'll come away beguiled by this dinky little valve amplifier, whatever music you play. Yes, it's expensive, but it punches far above its weight sonically and is exquisitely made in a country where manufacturing isn't cheap. Better still, this isn't just a cynical rehash of the old Leak Stereo 20, but rather a seriously conscientious attempt to better it in all ways whilst retaining its charm and making it more suitable to modern life. As a result, the English Acoustics Stereo 21c is an absolutely stellar power amplifier at the price, with bags of character and a great deal of talent – a little piece of perfection that's good enough to make its solid-state rivals sweat.

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