Ok, this was an impulse buy when I was at Walmart. Not only an impulse buy, but also an overpriced buy b/c Walmart charges $21 for the UR21 and the suggested list price is only $20! If I had to do it again, I would spend my money elsewhere. At the time, I needed a cheap closed headphone to use with my TV, and this was the only cheap closed headphone Walmart sold. Given Koss' reputation for great cheap headphones (the Koss PortaPro remains one of my favorite portable open headphones), I decided to take a chance on the UR21. I made a mistake. If only Walmart sold the Koss UR20, which has the same list price, but better reviews!
UR21 Summary: Out of the box, the UR21 is anemic in the bass region (partially caused by the poor fit b/c the earcups don't seal tightly at the back of the ears), has a vanishing midrange (which makes it difficult sometimes to understand speech) and suffers from severe enclosure resonances that can manifest as faint echos or buzzing. If I adjust the headband while wearing the headphones, the clicking noise is loud to the point of being painful.
In short, I did not like the UR21. Instead of returning it, I thought that I might be able to improve the sound by modifying it.
The Mod:
The goal of this mod is to rebalance the UR21's sound, by tilting upward the bass and midrange response. The enclosure resonances have to be dampened as well. The easiest way to do this is to stuff the earcups with a sound absorbent filling.
The only materials needed are a screwdriver and a small amount of polyester batting. The batting can be from an old pillow. Just take out 3 or 4 large wads of batting and sew the pillow closed. The screwdriver is for small Philips head screws. Many "dollar" stores sell "pocket" screwdrivers that will work for this project.
It's a good thing that Koss designed the UR-21 enclosure for easy disassembly. The ear cushions are mounted on frames. First, remove the ear cushions by twisting them off. On the right earcup, twist the cushion clockwise and then GENTLY pull it off. On the left earcup, twist counter-clockwise.
Prepare 2 thick disks from the polyester batting - maybe about 3" in diameter and 2" to 3" thick. If the batting has clumps, tease the fibers until the density is uniform. The disks should have an equal amount of material. If the batting has too much air in it, pat it down when shaping the disks.
Work on one earcup at a time. Each earcup is held together by 6 tiny Philips head screws. Remove them and carefully open the earcup. Pulling the sides too far apart could break the wiring inside. Insert one disk of batting. Seat, prod and poke the batting so that it evenly fills the enclosure. Check that no batting blocks the screw holes, but a tiny bit of batting in the channel of the headband slider will help muffle that awful clicking noise. Close the earcup (making sure that the headband slide is correctly inserted) and replace the screws. Stuff the other earcup in the same way.
Note: when replacing the screws, avoid applying too much torque or the screws could be stripped.
To test the mod, plug the headphones into a music player. For test music, choose tracks in mono or set the player to mono or use a stereo-to-mono adapter for the headphones. Listen for any imbalances in the sound. I've found that the initial amount batting increases the bass output and more batting tends to tighten the bass. More batting will help bring out the midrange too.
If one side sounds more "live," it could mean that side is still exhibiting resonances and needs more batting. If the bass on one side sounds too bloated, add more batting to control it. If the bass is too "dry" or tight, remove some batting. Tap the earcups. The sound should be a dull thud. When the balance of highs, mids and lows is pleasant and the mono signal is centered in the headphones, try listening to some stereo tracks and make any further adjustments to the batting. I ended up packing the earcups fairly full with almost double the amount of batting I started with.
This mod does NOT turn the UR21 into the Sony V6 or maybe the equally affordable AKG K26P (which I have not heard), but the overall sound quality is MUCH improved with deeper, louder and tighter bass and greater clarity in the midrange. The enclosure resonances are now better damped, and the headband noise has been reduced as well. The modded UR21 can still benefit from a little EQ boost in the midrange and/or treble.
If you already own the UR21, I highly recommend this mod. It costs next to nothing to implement and the results are very worthwhile. If you are thinking of buying the UR21, I suggest taking a close look at the UR20 first, which has received better reviews. I have not heard the UR20, but it looks like a better headphone design and the specs say it is more efficient as well. For $21 though (or as little as $13 from some discount places), a modded UR21 is a good buy.
Note: the UR21 is a medium efficiency (rated at 90dB/mW) headphone by modern standards. This mod will reduce the headphone loudness slightly.
[Edited by cmoy on 11-25-2006 at 04:02 PM.]
Attachment: A2300.jpg,A2301.jpg
posted 11-16-2006 08:45 PM CST (US) 
