Although Compact Cassette was launched back in 1963, it wasn’t until the nineteen seventies that the format truly came of age. Indeed at the turn of this decade it was still very low on the evolutionary scale, being the province of small mono cassette portables that found their way into cars, bathrooms, childrens’ bedrooms and gardens.
All the more amazing to think then, that by the end of the seventies, you could buy products like this. Aiwa’s AD-6900 was – at the time of its launch – one of the most expensive and sophisticated hi-fi stereo cassette decks in the world. It’s hard to understate the significance it had to any tape fan, back then. It was more than just a machine to record music with, it was a veritable ‘declaration of intent’ about what the Compact Cassette format could do. Hugely sophisticated, brilliantly engineered and built, it had open reel enthusiasts wondering how long they could hold out before cassette took over.
Aiwa already had a great reputation for cassette machines, and the AD-6900 showed how the company was at the top of its game. Nakamichi was widely thought to make the best cassette decks, but they were hugely expensive – its high end designs costing about the same as a small car – but Aiwa set out to equal, or better, these for far less money. Even so, this flagship deck cost around £400 in the UK, the price of a new motorcycle.
The engineering inside this deck is staggering. Take the top off the 450x120x327mm case and you’ll see it is jam-packed with electronics, including 13 integrated circuits, 160 transistors, 85 diodes and 12 FETs. The quality of components is very good indeed, as are the two motors – one for the drive reel, one for the capstan. Its three heads are excellent too, comprising a ferrite erase head and a siamesed record/playback one. This is high precision design running a four micron gap on the record head and a one micron gap on the playback – optimal sizes for each. Aiwa claimed this was the reason for the unit’s stellar frequency response of 20Hz to 19kHz [at -3dB]. The V-cut profile of the head was said to reduce low frequency irregularities caused by the physical interaction of the tape and the playback head. Both heads sat on the same 4-point stabilised diecast alloy base, to keep azimuth between the two perfect.
Along with the special head arrangement, the other standout feature of the AD-6900 is the Flat Response Tuning System, working in conjunction with a pair of novel, dual-needle meters. At the time that this deck came out, FRTS was the most advanced bias and record calibration adjustment system of its type, by a long way. It allowed users to set the deck up accurately for any type of tape on sale – be it Ferric, CrO2 or FeCr. It had both low and high frequency oscillators feeding test tones onto the tape at -15 VU at 400Hz and 8kHz; this made for ultra-precise bias calibration. Three adjusters were fitted to the front panel, meaning that users could effectively have preset bias and record calibration ‘presets’ for three types of tape. The dual meters were unique at the time, showing both VU and peak recording and playback levels, as well as being used by the FRTS for tape set-up.
Elsewhere, the high attention to detail continues. There’s a 38-pulse frequency-generated servo motor; the complex discrete transistor logic control circuitry allows one-touch cue/review without leaving play mode. The fitment of robust Ferrite heads means that the user head wear associated with this, is not an issue. The power supply uses a toroidal transformer instead of the then-standard frame type. Overall build quality was excellent, the Aiwa being one of the heaviest cassette decks made at 9.8kg. Wow and flutter was quoted at an extremely low 0.04%, and signal to noise ratio a superb 68dB (with Dolby B). Total Harmonic Distortion was said to be just 0.9%. This was the absolute state-of-the-art when launched in 1978.
Just one year later, the AD-6900 II was launched, with small but significant differences. Cassette aficionados are divided about whether this is the more desirable of the two, because it was better in some respects and worse in others. The key improvement was its new Metal tape compatibility; this gave a staggering 20Hz to 20kHz [at -3dB] frequency response. The biggest loss was the twin-needle meters, the peak metering being replaced by a small LED bargraph between the two analogue meters. This might have seemed more futuristic at the time of its launch in 1979 but lacks the novelty and visual drama of the original twin needle system. In most other respects the decks were identical to use.
Sonically the AD-6900 is quite superb. With a good Chrome tape, you can record high levels on to it and there’s an eerie lack of the usual nasties you associate with cassette. Pitch is very stable, treble is crisp and open, and the bass is strong and gutsy. Midband is surprisingly detailed and transparent, and sound staging is excellent. It is one of the few cassette decks I have heard that genuinely sounds near-peer to a serious open reel tape recorder, it really is that good. For example, you can record from a high quality turntable, play it back and find yourself assuming that you’re listening to the turntable, if you don’t consciously remind yourself. It’s all the more staggering to think that this was just eight or so years after the hi-fi cassette deck started as a breed – and at that time the word ‘hi-fi’ was a very loose description.
These machines are still reasonably plentiful compared to some other high end cassette decks of the time, and surface on eBay and in classified advertisements every couple of months – although the usual caveats about buying a forty year old tape deck apply. Price depends on condition and condition varies a lot, which is why you need to buy the best you can afford and/or expect to service it. It’s a difficult deck to work on, and will need the attention of a good specialist cassette deck service technician. Head wear is not an issue but rubber parts and solder joints are things that degrade over time. Expect to pay £400 for a good working example that doesn’t need work.