Sony APM-4

The APM-4 is the penultimate ESPRIT component Sony launched (the TA-N902 was the ultimate), and, as such, inherited from the research made on APM-8 post-prototypes around 1981/82.

As a result, the enclosure was completely revised and rounded off everywhere : the front and back are two solid pieces of carved wood, held together with the multi-braced (and rounded) top and bottom.
As for the earlier APM-6, the entire enclosure is made of thick bended and carved wood - not cheap MDF with a cool veneer added.

The filter was built like that of the APM-6 and APM-8 (polypropylene caps potted in SBMC and OFC wiring) and is placed at the bottom & back of the enclosure, with a little tweeter level pot added outside above the terminals.

The drivers were however not a (re)new(ed) design as they were imported directly from the APM-77 !
The bass and high drivers are completely interchangeable between the "little" APM-77 and the APM-4 ; the mid driver seems to have a different surround but the diaphragm, magnet, structure and dimensions are the same.
Visually, the 4 / 77 versions differ by their finish : matte & slightly grey-ish aluminium plates for the 4 vs. bright anodization for the 77.

The production run should be of...
At least 600 pairs were made for the japanese market so one could be tempted to imagine 400 pairs made for the rest of the world to end up with a round figure (no pun). However, that seems high or optimistic - probably more like 200 pairs (tops) for all export markets so 800 pairs in toto, the overwhelming majority of which went the way of trash throughout the later 1980s and the 1990s.


As one Sony engineer once said, Accurate Pistonic Motion drivers weren't necessarily better, they were a different way to reach for the same goal as conventional dynamic loudspeakers.
Often tagged as "typical Sony gadgetry", it should be noted that respected brands such as Pioneer and Technics produced far more flat-driver 'speakers than Sony ever did.

However, Sony is the only one who made them really flat : the bass and low-mid drivers all have four coil/magnet/diaphragm pistons instead of one regular assembly pushing a flat (and round) honeycomb.
And that is why properly restored Sony APMs make truly stunning loudspeakers, with excellent linearity and dispersion, enormous power handling (input & output) and very little distortion.