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Upgrade the good old PRO4AA

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I recently got an older type of KOSS PRO4AA. The driver units seemed in good shape, so I upgraded the headphones. I first checked the driver units' polarity and fixed it. Older versions were reversed, and they don't sound better. It seemed KOSS realised that at some point in the past, so they flipped it at the latest version. The current PRO4AA reproduces much better sound than the older ones. This is the newer one:
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Contrary, this is an older one:
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Next, I modified the internal wiring to the balanced one and newly introduced a mini xlr connector with four pins to make cables detachable. Now, I even can enjoy re-cabling the headphones.
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I didn't replace the driver units, just tiny mods, but it significantly affected their sound production. I would say that those modifications totally refreshed the headphones' sound. Originally, those specs are good enough to cover my audible range, and I'm using them with a powerful headphone amplifier and a good DAC, so my KOSS PRO4AA still sounds brilliant.
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(Edited)
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Lowie
0
Dec 7, 2022
Cool work, How difficult is it to wire in the XLR 4-pin mini plug? Because I can't seem to find any tutorials online, neither the plug itself. And can you put in a mini XLR 4-pin male to 3,5mm jack in it as well?
LowieIf you are good at soldering, I don't think the wiring isn't too easy. Instead, it may be a bit more challenging to modify the back cover well to embed the 4-pin connector. That plastic is thick and hard. To say about the connector, I chose REAN RT4MP as a male plug: https://www.rean-connectors.com/en/product/rt4mp-1 If you also want to create its cable, you can use RT4FC-B for the tiny xlr female plug and a 3.5mm stereo mini jack 3 pole male for the opposite terminal: https://www.rean-connectors.com/en/product/rt4fc-b-1 In this case, the 3.5mm plug should share the single pole for GND with both LR sides. I don't think it can be a balanced connection, but you may improve the sound because you may be able to reduce the crosstalk. Good Luck!
Leodude
3
Nov 6, 2022
Could you do that work on mine???
Leodude
3
Nov 7, 2022
LOL!!! But you're more experienced than ME!!!! I probably should start by listening to mine before doing anything else. They are from the early 70's. The cord was replaced decades ago, but that's it. I would never mess with them, but if there were a simple and certain way to improve them, I'd be interested in that. Still, oughta at least listen to the again first. I see Koss will send a new pair if there's something wrong with them. But I am kinda skeptical about the workmanship of today versus the 70's. Thanks for the reply. Is there a way to continue this privately?
LeodudeActually, it depends on the condition of the driver units and whether they can revive. Old PRO4AA drivers have some buffer that seems rubber just under the top cover, which is usually fragile over time. If that has been collapsing, we should give up using the headphones. That mod I introduced is simple, but if the drivers aren't good enough, we should abandon that. Anyway, I live outside of the US, and I feel that isn't convenient for you. My account is @tomo_l_miamoto, though.
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