To any nineteen-eighties audiophile, the Sony WM-D6C – the 'Walkie Pro' to its friends – was one of that decade's most iconic products. Not only was it the ultimate Walkman, but by virtue of its superb engineering happened to be one of the finest sounding cassette recorders that money could buy, full stop. That's right, in some respects it beat even the top Nakamichis of the time – despite being smaller than a paperback book. Although it was based on technology which preceded any Walkman, it was never bettered and remained in Sony's personal cassette line-up almost to the end. In the fast-moving world of Japanese hi-fi, that was quite an achievement.

One of the reasons for the success of the WM-D6C was its impeccable pedigree. The machine's origins lay in the TCM-600 (TCM-100 in some markets) 'Pressman' model of 1978, a pocket-sized mono model intended for newspaper reporters and business people. Despite being the smallest cassette recorder on the market at the time, the Pressman is remembered only for one thing – it formed the basis of the 1979's TPS-L2. This was the world's first personal stereo cassette player, the machine which would achieve global fame as the Walkman a year later.

At the heart of both models was an excellent transport mechanism driven by a servo-controlled ironless core motor, via a crossed belt. A precision-turned brass flywheel was used to keep the speed stable, aided by a smaller but faster rotating flywheel turning in the opposite direction to cancel the inertial effects of the machine being carried. Further refinements included automatic stop in playback mode and an effective fast forward/rewind arrangement with cue and review.

It was this mechanism which formed the backbone of the WM-D6 of 1982, the original Walkman Professional. Good though it was, some improvements were nevertheless necessary to elevate the performance of the TCM-600 transport to proper 'professional' standards. The most important of these was to replace the belt drive to the capstan with an arrangement known as 'disc drive', taken from the TC-D5. Disc drive took as its basis the layout used in Uher's iconic 4000 Report open-reel portables of the early nineteen sixties.

The capstan flywheel was driven directly by a bevelled pulley attached to the motor shaft, friction being provided by a precision rubber tyre set into the rim. As a further refinement, the Sony version included a magnetic ring and sensor coil built into the bearing hub so that the capstan speed could be read out directly. What made the WM-D6 unique was that this signal was then compared to a quartz crystal reference so that absolutely accurate tape speed could be guaranteed under all conditions. Additionally, an automatic stop system was added to the winding modes. It was not possible to do this mechanically without a significant redesign. Instead, an electronic system was adopted, stopping the motor a short while after the rotation of the spools was blocked. The original mechanical arrangement was retained for releasing the keys at the end of playback and recording.

1984's WM-D6C added two more important features that would make the Walkman Professional an absolute classic masterpiece. The first of these were line-level inputs and outputs for direct connection to a hi-fi system; the second was the inclusion of Dolby C noise reduction alongside the Dolby B that the original WM-D6 had. The combination record/replay head also received an update; the WM-D6 was fitted with Sony's 'S&F' (Sendust and Ferrite) head from the TC-D5M, the best Metal tape compatible head available at the time. The new head featured an amorphous core, a material with excellent magnetic properties which enabled maximum signal transfer to the tape with minimal noise and distortion. Together these changes made the WM-D6C the complete cassette machine, small enough to be easily portable, stable enough to be used as a car player and good enough to use instead of a conventional full-sized cassette deck.

So useful was the WM-D6C that it remained in production largely unchanged until 2003, despite the advances made in portable digital recorders during this time. Revisions were made inside, most notably the re-drafting of the printed circuit board to use modern surface-mounted components so that a large amount of hand assembly that was still required for the original version could be avoided. The record/replay head was also replaced, the original amorphous type was substituted by a more conventional unit which was used elsewhere in the Sony range. It was still a quality component but proved less wear-resistant than the original. It took a while longer for the 'amorphous head' script to be deleted from the badge on the machine's front, however!

THE LISTENING

The passage of time has meant that the Walkman Pro no longer appears to be especially small, there are many portable recorders available now which are a fraction of its size. What remains surprising is that no cassette deck built on anything like this scale comes anywhere near to matching the WM-D6C's performance. Indeed, it takes a full-sized machine of the highest calibre to convincingly better it in sound quality terms, for which you have to sacrifice any thoughts of portability. Rock-solid speed stability is the key, along with a decent head that really can punch the signal onto the tape. The level meter is small and oddly calibrated but as a rough rule of thumb setting the recording control so that three of the five LEDs light (four if Metal tape is being used) results in impressively transparent recordings which retain the full character of the original source – useful for making tapes to save wear on precious LPs for example. Too much level results in compression at the top end, something made all the more evident if Dolby is used.

The Dolby C circuit is clearly well implemented, and recordings made using it don't exhibit hiss to any troubling degree. However, the selector switches for both this function and the tape type selector are small and fiddly; manual tape type selection is, in any case, an anachronism for a machine of this era and it is easy to leave the switches wrongly set. On the subject of switches, the one for the meter/battery check function is also very small, but a worthwhile saving in battery power can be made if the meter is turned off when not needed.

The way that recordings made on the WM-D6C retain a real sense of space and timing – things invariably blurred by lesser recorders – make it really stand out. Compared to an original programme taped from CD for example, and the WM-D6C's recording made on Chrome (TDK SA-X) or Metal (TDK MA-R) tape differs mainly by a subtle softening of the top end, this being only really detectable in a direct A/B comparison. The broadness and depth of the sound picture contracts by a similarly small amount too, but they are not entirely lost in the familiar cassette muddle routinely demonstrated by lesser decks. It is easy to forget after a short while which format you are listening to, which is always a good sign.

Yet for all its strengths, the WM-D6C isn't perfect. In common with many cassette recorders based on seventies designs, the frequency response on playback rolls down in the upper treble, roughly from 12kHz upwards. This can make pre-recorded cassettes lose some of their sparkle if Dolby B is used, but is less evident if it's switched off. The recording circuits compensate for this, and when playing back tapes made on the deck itself, the response is flat and extended, indicating that the head is not the limiting factor. In the factory the Walkman Pro was set up for Sony's own cassettes which are known to be marginally more sensitive than the standard types sold in some regions, notably BASF CR II and European-spec TDK SA. Perfect Dolby tracking, therefore, calls for a slight adjustment to be made to the recording current, not a difficult job for a well-equipped workshop.

Finally, while the quartz-locked capstan gives impeccable figures for speed stability and wow, it cannot fully mitigate the effects of a small motor and a comparatively lightweight flywheel. The result is a slight but occasionally audible degree of flutter, making some instruments sound ever so slightly coarsened. Violins sometimes appear strained, although by usual cassette standards the problem is tiny. None of this keeps the WM-D6C out of the very top tier of cassette recorders though – its overall performance is astonishing.

ON THE ROAD

From a practical system integration point of view, the Walkie Pro's line-level input is well scaled, so it's easy to make connections directly to any modern hi-fi system. The line output is slightly lower than one would expect from a full-sized deck though, so it is sometimes necessary to set the amplifier's volume control marginally higher than usual to obtain a normal listening level. One way around this is to use the headphone output, but this adds a small extra hiss in the background which is outside the sphere of the Dolby circuit to correct. When used with headphones, the output works very well; there's plenty of level available which lets you use far better cans than those originally supplied. It's also much stronger than the headphone stages of many anaemic sounding modern digital audio portables.

The subject of power units is a potentially thorny issue, as there wasn't one supplied with the unit from new and owners often made their own arrangements. The best suitable power unit that Sony made was the AC-4A which provides an electronically smoothed and stabilised 6V source. There are numerous others, but whatever you choose, it's essential to ensure that the polarity is correct and the output voltage never rises to an excessive level. If in doubt, it is best to stick to batteries, they give the cleanest source of power, and rechargeable types can be economical to use if managed correctly. While on the subject of keeping your WM-D6C in good condition, it is wise to also avoid excessively thick and heavy signal cables with needlessly bulky plugs, since the weight and drag these put on the delicate sockets at the back of the machine can damage the connections to the printed circuit beneath.

While early WM-D6s are rare, the WM-D6C remains ubiquitous and is always easy to find – so there's no need to settle for anything even tatty or compromised, unless you're on a very tight budget or fancy a challenge. Despite being a tough and dependable model, there are things which can go wrong which the potential buyer should be aware of.

Firstly, be sure that the battery carrier isn't missing (as many are), it is a separate part and batteries cannot be loaded if it isn't there. Rechargeable packs that fit in its place were sold as an accessory, but these are all depleted now - do not accept one as a substitute. Leakage from old batteries can ruin both the carrier and the densely packed circuit board below it, so look carefully at both parts before buying. Some owners don't like paying for batteries and use an external power unit instead. This is okay only when the correct Sony ones are employed – connecting an adaptor of the wrong voltage or polarity will instantly destroy the servo IC or the transistors inside the DC-DC converter. If the motor runs too fast (early models) or not at all (later types), then this is the likely cause. The required replacement parts are still sporadically available, but they are tiny, and fitting them without causing even more damage is a task only for the most skilled of hands.

If the automatic stop does not work in playback or record, then the play clutch belt is probably worn out. These are easy to obtain but again fitting is no task for a beginner. No rewind is another stock fault which usually resolves to seized linkages around the gears which drive the spools. Again, the parts involved are extremely small and cannot be accessed without a fair amount of other dismantling, so the work is probably best entrusted to an experienced workshop. Finally, an intermittent loss of output in either channel of the headphone output is a prevalent issue with all personal stereos. It usually is as the result of cracked soldering or damaged track work around the socket itself. A simple soldering job will generally effect a repair.

THE VERDICT

Sony's WM-D6C is simply one of the finest analogue tape recorders that sensible money can buy, and has the additional – and for some, huge – benefit of being portable. Whether you want to enjoy pure analogue music on a long-haul flight, tape your vinyl collection for home use or bootleg live gigs in style, it's an essential tool. Prices are climbing but still reasonable – £500 buys you a really excellent one, half that for good usable example. That's a lot of classic audio for the money.