I recently scored a pair of rare Sony APM-22ES speakers at an auction for $6. For anyone unfamiliar with Sony’s late-1980s APM (Accurate Pistonic Motion) line, they feature iconic, flat-square aluminum honeycomb woofers driven at four points to eliminate cone breakup.
The original woofers were intact, but like most 40-year-old APMs, both factory tweeters were completely dead. Rather than tossing these pieces of Japanese audio history, I decided to do a clean, reversible resto-mod.
The Tweeter "Stealth" Retrofit:
Sourced modern Peerless OC25SC65-04 1" textile dome capsules.
Rather than cutting up the front baffles or fabricating external round adapter plates, I mounted the round Peerless capsules entirely behind the original square factory trim pieces to preserve the stock 1980s laboratory aesthetic.
Used a friction-fit gasket and a reversible structural hot-glue bead around the rear housing to achieve an airtight seal against the woofer's back-wave pressure without permanently altering the vintage plastic.
Crossover Re-Voicing & Rebuild:
The stock crossover pushed the factory tweeters down near an aggressive ~1.3–2 kHz, which would quickly distort and overheat modern compact textile capsules.
Re-voiced the high-pass path by replacing the stock parallel film blocks with a single Dayton Audio PMPC 8.2 \mu\text{F} Precision (1%) film capacitor. This shifted the crossover hand-off up to ~3,200 Hz, keeping the Peerless domes in their safe operating window while letting the ultra-rigid, distortion-free flat APM woofers carry more of the upper midrange.
Replaced the degraded, 40-year-old black Elna bipolar electrolytic capacitor on the woofer low-pass leg with a modern Dayton Audio DMPC 15 \mu\text{F} film capacitor to clean up bass damping and ESR.
Kept the original 2.2 mH iron-core inductors, parallel tweeter coils, and factory circuit breakers intact.
Cosmetics & Clean-Up:
Cleaned and conditioned the faux-wood vinyl wrap cabinets to bring back their original satin luster.
Hand-brushed the oxidized cast-aluminum woofer frames to restore the factory metal finish, sealed them to prevent future corrosion, and gently cleaned the original rubber surrounds.
The Custom Stands:
To elevate and properly decouple the ~35 lb monitors, I built a set of bespoke stands using solid Black Walnut plates and 3" brushed aluminum columns. They utilize a hidden internal continuous tension-rod spine and a heavy multi-layered ballast core to keep the center of gravity locked near the floor.
The Sound:
The combo of modern Scandinavian textile domes with the lightning-fast transient response of Sony's flat-square honeycomb woofers is incredible. Bass is fast, tight, and punchy, and the high end is silky smooth with zero ear fatigue.
For a minimal investment, these perform right up there with modern four-figure standmounts. Happy to answer any questions about the APM platform or the crossover tweaks!