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Wednesday, March 20, 2013

Monoprice 9927: Modified by Rin Choi

Disclaimer: This pair is owned by Inks.

After losing two units of 8320 & three units of 9927, now the modification technique has been fully finalized. The IEM shown above is the ultimate Monoprice 99RC, with enlarged & elongated nozzles and a pair of cyanoacrylate-coated drivers. 

By default, Monoprice 9927 has several fundamental issues:

1. An annoying peak @ 5 kHz range
2. Short bandwidth
3. Bad fitting

Thus, a cyanoacrylate adhesive has been coated on the diaphragm to solve #1. Also, nozzles on both channels have been replaced with a pair of drill-carved 5/16" long pvc pipes (3/16" ID) to correct #2 & #3. With the enlarged bore size, the entire acoustic output has been widened to 3/16" as well.

The ultimate goal is to knock off the problematic peak, while keeping all the original acoustic properties intact. In addition, with careful calibration, the balance of both channels is now equalized.

Although the result is still quite far from true hi-fidelity, the tonality is now much more neutral, easily overwhelming any budget-priced IEMs, such as Vsonic VC02 or HiFiMAN RE400. Currently omitted, adding a small amount of acoustic resistance at the output of the IEM will definitely help tame any remaining harshness.

A special thanx goes to my mentor, Soonja, as well as Inks, who provided me with a bunch of disposable spare pairs.


  1. Great Work!

    Looks like you went with instant adhesive in the end.
    Nail polish worked reasonably well making a hard layer, but corroded the plastic wrap I was testing on so I sort of put the idea to the side. Might have to watch the brand / solvent used in the polymer, or the material of the transducer diaphragm.

    Glad it worked out for you and really looking forward to more details on this mod!

    1. Thanx to you, Nereis, for providing me with a hint. It eventually turned out that the cheaper the better! Not only it dries super-fast, the adhesive also spreads extremely quick, removing the risk of clogging of 9927's front damper mesh. Had to triple-coat to achieve the optimal mass, but the effort was well worth it.

      The coating itself is easy, but the nozzle replacement is such a PITA to match the size/angle of the nozzles on both channels. One of the lost 9927 was disposed due to size mismatch :-/

  2. This actually pretty interesting work!

    If you could provide step by step will be greater!

    Will try it myself if there is clearer instruction!

    1. Not sure if I could go back to this IEM soon, but will do my best.