Getting back to dynamics with KOSS's flagship from the mid 1970's. They are a bit heavy, kind of ugly and unweildly, but these headphones provide a sound that while dated is surprisingly pleasant to listen to even now.
I'm not gonna lie, they leave a bit to be desired in stock form. Pretty dark sounding with decent transients and a great soundstage for a closed can. Damn insane bass impact and power, though a bit lacking in extension. Has some pretty heavy rolloff in the high frequencies as well. Nicely textured and tactitle especially on the lows, but a little bit cloudy and murky. Kind of uncontrolled, tizzy sounding treble that can be a bit harsh at times. Very smooth sounding and enjoyable to listen to.
Build wise... yeah these are almost as tank-like as the Sixty. Thick, dense plastic with brushed stainless steel yokes and a spring-steel headband core. Rock-solid yoke design with metal pins to ensure durability. Pretty brushed aluminum KOSS badges on the backs of the cups. The inside of the cup has latticing to control airflow and the outer headband is sleeved in nice tan leather. The earpads and inner headband are KOSS's proprietary air-filled Pneumalite vinyl material, which while a bit stiff from age and not the most comfortable thing in the world, get absolute perfect sealing and provide insane isolation and impact. Okay, maybe that's optimistic; those pads SUCK when they get hard.
However, this is one of those crazy vintage cans that can improve wildly with modifications, and I've spent over a year messing with them. Heh heh.
Before I get into modifications for this headphone I need to address the different driver revisions that exist. There are 3 dfferent KOSS drivers, one of which is a black sheep.
KOSS Pro Driver Guide (Click to show)
Note: Be cautious
It is very easy to break a vintage Koss Pro driver. You should never remove it from the baffle plate unless you absolutely have to. The diaphragm is simply clamped in place by a metal ring, and can end up left behind if you just pull the driver out. This will sever the voice coil connection irreparably. If you do need to remove it, you need to clip off the four anchors at the edges from inside the cup and BLOW INTO THE BAFFLE PLATE to get the driver out of its mount. Make sure you have your hand ready to catch the driver as it pops out.
Also, if you choose to do a recable, you <CANNOT> solder directly to the pins of the series 1/2 4AAAs. This will also sever the voice coil connection and melt the plastic frame. Salvage the clips from the old cable and solder your new wires to those (with them off the pins of course).
Most common of the three. White and black housing with a paper damper attached using a clear plastic bolt. A tiny bit grainy compared to the series 2 but otherwise sounds exactly as described above.
Similar to the first series but with a thinner diaphragm and larger magnet. All white or all black housing with a foam damper held on by a gray nut. Sounds very similar but the grain is removed and it has better detail and soundstage. Best of the three but unfortunately is also the rarest.
This is the black sheep. All white with a smaller magnet and voice coil, and a felt damper. Worst sounding of all three. Used in some 4AAAs and all "Plus" models of KOSS headphones. Absolutely gross sounding. Here's a guide to avoid it:
Oookay, now for the mods:
 (Click to show)
Works with both series 1 and 2 4AAAs. Convert the 4AAA to an open-back headphone by carving out a hole where the KOSS badges are. This removes a reflection in the earcup that causes an echo-like resonance. It also gives the soundstage a little more width. It does not change the character of the 4AAA very much, but perhaps increases the treble a bit.
Pry off the plastic backs of the earcups using a credit card or a small flat-head screwdriver. Remove the two brushed aluminum KOSS badges on the earcups by gently prying them up. Be careful not to damage the plastic of the earcup or the badge itself. They're usually glued down tight, so you might have to heat them with a blow-dryer to get them off without damaging them. Put them aside; you'll need them later.
There are a number of ways to cut out the hole. One option is to use a 1 1/8" wood drill bit to drill out the hole. Another option is using a pocket knife to "drill" a hole in the center of the earcups where the badges were. Continue cutting around in a circle until you reach the edges of the circles, and carve it to shape. Then, use some fine- grit sandpaper to even out the surface of the plastic where the cut is. This method is a little more involved, but is not difficult and gives cleaner results.
Before you put the earcups back on, grab the badges you set aside earlier. Put some double-sided tape on the backs, and stick them to the plastic bolt on the back of the driver. Make sure they're completely centered and straight. This is only for aesthetics, but considering how ugly the inside of the 4AAA is I strongly recommend it. Gently snap the earcups back onto the frame. If the wires are showing, push them down into the earcups using a toothpick or something similar.
 (Click to show)
The paper damping built into the drivers of the series 1 4AAA models deteriorates and crumbles over time, which brings the bass up and causes some boominess.
To fix this, first remove the backs of the earcups using a credit card or small flat-head screwdriver. Unscrew the plastic bolt on the back of the driver. Use a coin if necessary. Carefully remove the original damping. It will crumble to pieces as you do this, so do it over a plate or a garbage bin and with the headphone held upside down. Make sure you get as much of the old damper off as possible, and try to avoid getting any of the residue into the bass ports on the back of the magnet. Once you're done, wipe it clean with a damp paper towel. There may be spots where chunks of the paper have become cemented to the magnet, but as long as all 4 bass ports are uncovered it won't matter.
Use whatever damping material you like. Blu-tak isn't recommended, as it seemed to completely kill the bass. Felt caused bloom in the midrange and some congestion. Fleece seemed to sound the most similar to the original damping. Cut out a circle of the material for each side and cut two slits in the centers. Slide them onto the plastic bolt and screw them back into place. Gently snap the earcups back onto the frame. If it sounds off, adjust the plastic bolt until it sounds to your liking.
For Series 2 4AAAs, you can adjust the foam damping by adjusting the gray bolt on the magnet. Tighter is less bass, looser is more. I find that the stock KOSS tuning is a bit bassy for my tastes so I tuned it down a quarter-turn or so.
Baffle Plate Mod [Experimental]:
 (Click to show)
This involves cutting out the baffle plate to remove and/or replace the acoustic lens in front of the driver to reduce resonances. I have not done a lot with this one yet so use at your own risk. To cut out the baffle, first disassemble the cup and remove the driver to avoid damage. Remember to follow the rules I mentioned earlier about removing the driver to avoid breaking it. Remove the earpads as well. Take a pair of end nippers and cut the plastic at the four outermost notches of the acoustic lens; this will require a total of 8 small cuts. The plastic is pretty mmassd so you shouldn't have to worry about damaging your tools, but I still wouldn't use a nice tool for the job. Once the plate has been removed, take fine or mid grit sandpaper to the inside of the hole to get rid of any excess plastic that could interfere with the driver.
My initial impression is that the sounstage becomes wider and the treble becomes less tizzy while it also becomes more recessed and the isolation is sigificantly reduced. I have not done this to both sides of the headphone yet. Use at your own risk as this mod is highly destructive and I only did it because I have spare parts for mine.
Additional modifications I've done to my pair include completely rewiring the headphone internally with new wires from a spare AKG cable and adding a 3.5mm jack on both cups to make the cable removable. The rewiring did clean up the sound a bit because the old wires were pretty badly corroded but it was a very minor change and was done mostly for cosmetics. I also harvested a pair of pads from a mint condition KOSS K/6A. A better option would be buying new ones from KOSS, though you have to call them to do so.
Overall Ranking: #4 of 12
--Sound: 7/10 (a bit dark)
--Comfort: 4/10 (ow)
--Looks: 6/10 (utilitarian)
--Modding Potential? Yes, significant
Driver: Dynamic, 55mm, mylar cone
Implementation: Closed (modded to vented); Sealed baffle
Impedance: 250 ohms
Earpad type: Circum-Aural, Pneumalite® Vinyl
Weight: 530g (without cable)
Cable type: Coiled, Single-entry
Cable length: 2m
Edited by takato14 - 2/5/14 at 10:10am