Back in the mid nineteen seventies, Hitachi’s development and productionising of MOSFETs (Metal Oxide Silicon Field Effect Transistor) played a massive part in the hi-fi world that was then just around the corner. The world wasn’t even fifteen years into solid-state, and still on a long and steep learning curve. But Hitachi’s new power MOSFET output devices removed – at a stroke – many of the problems associated with standard power transistors. They delivered cheaper and more reliable power than ever, and arguably better sounding too. It was such a step change that most amplifiers of the nineteen eighties and nineties – and even many today – use the direct descendants of these Hitachi devices. It was nothing short of an epoch-making technology that made power cheaper for the masses.
What was so special about the technology? Well, conventional transistors work by permitting a small flow of current to control a larger one, but MOSFETs switch by voltage control – which requires far less energy, and in turn produces less heat. The result is that the same amount of silicon gives a higher output than standard bipolar power transistors, with lower noise and distortion too. In turn, you can have simpler, shorter signal paths to get the same result. By way of example, the Hitachi 1979 HMA-6500 power amplifier you see here takes only eight transistors per channel to reach a claimed 55W RMS, with total harmonic distortion of under 0.02% –remarkable for a product of this period.
As per Hitachi tradition, the company made pretty much every single component inside this amplifier – with the 2SK133 and 2SJ48 power MOSFETs at its heart. Yet it didn’t keep the technology all for itself, and soon the rest of the industry wanted a piece of the action. Rival Japanese manufacturers like Sony didn’t bite – as a matter of corporate pride – but many other companies did. You need only to look inside a Myst TMA3 or an Inca Tech Claymore – specialist British amplifiers which came out just a year or two after this Hitachi amplifier – to see the self-same power output devices. It made sense because Hitachi supplied the application notes to other designers freely, as well as the parts, and it was a great way of getting a robust and powerful amplifier onto the market in a relatively short space of time.
The HMA-6500 was launched as part of Hitachi’s new range of high end preamplifiers and power amps, selling for £120. The flagship HMA-9500 cost three times this, and was a massive design putting out 120W RMS per channel – which seemed a staggering amount of power at the time. Below this was the HMA-7500 which delivered 80W, and the HMA-6500 just 50W. Even so, the baby of the range was still very powerful for its day; only really high end amplifiers did better, such as Naim’s NAP-250 which was way more expensive. The range also received matching HCA preamplifiers, which were far less technically interesting, and the HA series of integrateds.
The build quality of the HMA-6500 is quite something. Of course, it ticks the right boxes for superficial styling of that era – a big slab of brushed aluminium for a fascia and the obligatory power meters, but it was also beautifully done inside too. Component quality was very good throughout, with no capacitors in the signal path, a sturdy chassis and all wiring and circuit board population done to a high standard. Being a Japanese amplifier expected to sell in high quantities, reliability had to be tip-top, and nor was the amplifier to damage its new owner’s equipment. That’s why it sported an integrated circuit that cut the main speaker relay if it appeared that anything was going wrong, as well as muting the speakers on switch-on and off. Because of its high standing current – 30W even at idle – a large heatsink was fitted to keep things cool.
Unusually, the Hitachi offers a choice of either AC- or DC-coupling, via a small rear mounted slider switch. The latter mode of operation means it theoretically has the same gain at 0Hz (DC) as it does elsewhere. This isn’t necessarily a good thing, so is best used only when you’re sure your preamplifier is AC-coupled, as is the matching Hitachi HCA-6500. Measured performance was absolutely state-of-the-art, with a claimed frequency response of 5Hz to 100kHz, a signal-to-noise ratio of 115dB, a damping factor of 50, and speaker load impedance of 4 to 16 ohms.
In use the largish – 435x153x306mm – and heavyish 9.4kg – power amplifier is very nice indeed. It has the feel of a high quality product without going over the top in the way that some Japanese hi-fi can do. Indeed, it feels in some way akin to a Japanese Quad 405 – compact, beautifully made and purposeful, with no nonsense or messing around. It’s true that the edges of the brushed aluminium are slightly sharper than you might wish for, but I still see worse in some new products today. Also, the ballistics of the power meters are pretty slow, but really they are there just for show.
At the time of its introduction, one of the biggest limiting factors in amplifiers was power output. Of course, you can have a fine sounding amplifier without much welly – such as the Nytech CTA252XD II receiver which was very popular with audiophiles back then – but there is no substitute for watts per channel. This was even more the case back in 1979, when most speakers struggled to achieve sensitivity figures of over 85dB or so. However fine sounding an average 25W amplifier may have been, it would be way out of its comfort zone producing serious levels of sound. This is an instant win for the HMA-6500, which sounds clean and gutsy with less efficient loudspeakers, with headroom to spare.
There’s also a special character to these Hitachi MOSFET amps of that era; they seemed remorselessly energetic all the time. Whatever music you play, they bluster through it like they’re in a hurry. This makes for an exciting, uplifting and enjoyable experience; there’s a lot of foot-tapping going on with Earth, Wind and Fire’s Fantasy for example, a period disco track that the HMA-6500 practically sets fire to. Transients are super-fast, the bassline pogos up and down and the hi-hat cymbal work seems so crisp and propulsive. Driving a mid-price speaker of that exact era like my Wharfedale Dovedale SP2, this power amp really ratchets up the pace. This speaker is pretty warm and slow by modern standards, yet you won’t notice this via the Hitachi.
It’s not all good news however. It’s certainly fair to say that this power amp has quite a ‘technical’ sound; it’s not tonally warm and rich, and doesn’t sugar the musical pill so to speak. Tonally it’s quite grey, very much following the stereotype of a solid-state amplifier. The opulent sound of London Town by Wings for example, is stripped bare of its sweetness and bright tonal colours. Instead, it’s as if the Hitachi says “this is how things are going to be when I’m in command”, and then goes on to impress the listener with its gusto and power, especially on dynamic crescendos which it handles with relish. It’s quite a macho sounding amplifier, one might say, and not one to get all romantic and smoochy.
This of course makes it great for electronic music; give it the pulsing beat of Cerrone’s Supernature and it’s very much up for some fun. Here it sounds controlled, measured and accurate, yet at the same time rhythmically adept and very much in a hurry to crack on through the song. Production effects like the gated synth noises panning from left to right shine out of the mix, meaning that it’s a great amp to impress your friends. Bass isn’t quite as thick and powerful as you might expect, but is certainly taut and sinewy and doesn’t go absent without leave when you really crank up the volume. At the other end of the audio band, treble is clean and explicit, albeit lacking some subtlety and air.
Classical music is maybe its biggest failing, although it’s all a matter of taste of course. Basically it gives the opposite presentation to what you’d expect from a good valve amplifier. Everything is tidy, ordered, in its place and clearly delineated from everything else, but it lacks any sort of organic tonal warmth. Although image location proved good on the first movement of Beethoven’s Pastoral Symphony, it’s not especially expansive sounding from left to right, and nor does it have great stage depth. Instead the recorded acoustic is laid out before you on a plate, rather than the listener feeling immersed. The HMA-6500 does redeem itself on dynamic peaks however – basically whenever it gets the chance to show its loudspeaker driving ability.
Hitachi’s power MOSFET HMA-6500 is not the world’s greatest power amplifier, but it’s still an interesting moment in hi-fi design, and therefore something of a collector’s item. It is literally true to say that it – alongside the higher end amplifiers in its 1979 range – changed hi-fi history in their own small way.
The first generations of Hitachi power MOSFET amplifiers have an excellent reputation for withstanding enormous abuse. They’re the rugged infantrymen of that period of hi-fi, amplifiers that just keep on keeping on. This is partially due to the output devices themselves – which really do take some breaking – and also the generally excellent quality of the other components. Interestingly, even British-made ‘cottage industry’ amplifiers such as the Myst TMA3 are highly reliable thanks to their use of these Hitachi internals. Quality will out, as they say…
The biggest issue with the HMA-6500 is actually the power meters; the mechanisms can stick, giving even more inaccurate readings than when they working as the manufacturer intended. Also, the bulbs inside them inevitably fail, but can be easily replaced with miniature 12V 60mA wire-ended lamps. It is worth getting the amplifier professionally serviced because there are some internal adjustments which can go out over time, so do try to acquire an original service manual if you can; eBay is your friend here. Indeed it’s also the best place to look for actual HMA-6500s; there were a good few sold in the UK but many more in Germany, so the amp is readily available for under £200 there. This is a crazily low price for a piece of ‘affordable high end’ that is significant in the great scheme of hi-fi things.