Rebuilding dads old Koss PRO4AA
Growing up, my father fostered my enthusiasm for all things audio by gifting me his obsolete equipment. He had a friend in the hi-fi business and subsequently had new gear on a regular basis. I found myself rebuilding speakers, enclosures, cleaning components and enjoying the hobby. He had bought these headphones new and, having kept them through the years (a rarity for his equipment), I think they appreciated in sentimental value. I am generally terribly at buying birthday gifts, so I thought I would bring these back to their former glory.
Unfortunately I can’t begin by sharing a classic ‘before’ picture. Per usual, my eager hands dove into tearing the unit down before I could think of capturing the starting point… or really the process as a whole. I wish I could say that was a unique instance, but I’ve been disassembling things since I was old enough to be left unsupervised. I can tell you this however: only one side worked, the pads on both sides were shedding material, the cord was partially exposed and the unit as a whole was filthy.
Above The near complete disassembly. The old pads are disintegrated, the cable is severed, and one of the drivers is blown. Furthermore, glue was fracturing and old screws long since seized had begun to strip. The work was cut out for me.
Below Regrettably, my understanding of headphone audio was minimal at the time and I’ve realized I was probably trying to add features in that they weren’t designed for. Noise cancellation was all the rage as Bose had just released their now well-known QuietComfort unit. Hi-Fi stereo is best capture with an open back design, but I wanted to create a muted, dedicated chamber through dampening. Seen below, I added a dense memory foam as insulation inside the polymer casing. This definitely reduced exterior noise, but at the expense of the sound quality coming from the new drivers.
--> The orignal headband pad was old and had hardened considerably. It had also began to pit and flake away. I chose a closed-cell EVA material that is firm enough to hold its position but soft enough for extended use.
Above The plastic driver chassis was a bit too large for the newer replacements. I used a rotary tool to open the diameter and laser cut a mount to custom fit the new drivers into the older chassis. This added a couple millimeters in height (which made for a tight fit) but we got there.
Below To the right you can see the old driver next to the new mounted driver.
Just like every other bit of expanded foam, the pads were in poor shape… stretched out and disintegrating. Many newer headphone units use memory foam as pad cushions, and for good reason. As a material it is much more forgiving for long periods of use. I found some scrap in the appropriate thickness from a local foam supplier and was able to cut to the correct form.
Final wiring and assembly is seen above. I upgraded the original wiring (that was 18+ gauge) with some monster ‘audiophile’ insulated cable. In retrospect I should have put some form of cable cover on the exposed portions over the cups, but I had already soldered and pulled the cables into place. I suppose the exposed look isn’t the end of the world, but it is a departure from the oem look. Acknowledging that, it is exciting to see the final assembly come together.
Stereo sound returned! The new drivers sound great and the whole unit is comfortable to wear for hours (although heavy). Check out Koss’s microsite to compare to the original. They also compliment my dad’s growing vinyl collection and hifi equipment he’s collected over the years. Happy listening dad :)
