Update: Latest mods/tweaks and measurements as of 4/2/15: http://www.changstar.com/index.php/topic,1841.msg61995.html#msg61995I thought the PM-2 had several disappointing aspects, so I decided to see if I could improve it with mods. Stock, it has good bass and mids, but treble has issues. Either too depressed in some spots or too peaky and rough in others. Bass was clean and fairly neutral but lacked balls. Not sure what it is, but some headphones that measure fairly flat in the bass sound powerful, and some do not, assuming both are well driven from big boy amps. The PM-2 falls into the latter category, IMO. So, it kind of had this soft, almost too laid-back, yet also slightly too rough sound. Yeah, no thanks. I thought the HD600/650, especially damped, made it sound silly. The PM-1 had a better response curve but was overall too dark and constricting sounding. I liked it overall despite this, but it was kinda boring.
You know what comes next. I replaced all the internal damping with my trusty open-cell foam I got from Foam Factory! And I mean I removed all internal damping, including the stuff glued to the back of the driver. I replaced it with a 3-layer damping stack sandwich party of the following stuff:
On bottom, closest to exterior cup grill, a layer of this (1/4"):
http://www.foambymail.com/CR/solid-charcoal-regular-foam.htmlOn top of that, 1/4" layer of this:
http://www.foambymail.com/P/poly-foam.htmlLastly, a layer of this on top of all that (i.e. closest to back of driver):
http://www.foambymail.com/CF0-125LF/1-8-inch-charcoal-regular-foam.htmlIt does take some force to get it closed with all this shit in it. If you do not have hands or arms, I do not recommend you attempt this mod.
I also closed up portions of the cup grill with Creatology adhesive foam squares. Basically, I'm just trying to reduce the amount of cup venting. I left the side of the cups pointing to my face more "open" inside and the back side more "closed" with the foam squares. This will make more sense with pictures. I also put a lot of drawer liner inside the cups.
To make this all work right, I purchased the OPPO velour ear pads. Nothing else done on the front side. Just pooped the velour pads all up on there. I'm not sure how the PM-2, with its slightly different internal damping vs. the PM-1, would sound with just swapping to velour pads. I don't think I like the stock PM-2 pads all that much.
How does it sound? Well, the first thing I noticed was extra bass. It's a touch more boomy and perhaps not
quite as clean as before (can't do a direct comparison), but still pretty damn good. Gives a better sense of power and kick. Not so much limp-dickage as before. PM-2 is no longer pre-pubescent sounding. I think I like a bit of a tasteful, clean bass-bump on most headphones. Treble also sounds considerably cleaner and smoother. Moar and better detail, but smoother. Upper-mids and lower-treble seem to be better integrated and less recessed. Tone and smoothness reminds me more of the PM-1, but without the constricting soundstage and noticeably dark treble. I'm enjoying it.
Is it perfect now? Well, no, but IMO a big step up in balance and refinement over either the PM-1 or 2. The best part is I no longer listen to these and just feel like something isn't right or that something is missing. I think these dri
vers have a lot of potential but are held back by the enclosure design and stock damping. Someone that has more patience and audio knowledge than me could certainly get something great out of these.
Attached measurement 1: This is the stock PM-2 with stock pads after I stretched it out for a wider, less clampy fit. Do note that stretching them out for less clamp did affect measurements, and I do believe I've posted these results before.
Attached measurement 2: Modded PM-2 with velour ear pads. The distortion characteristics in the low-bass might be a bit weirder and worse, but I'm not too concerned about that. Didn't save it, but the impulse response on this looked great. CSDs still good as always, but didn't save that either. You can find my later PM-1 measurements here for some level of comparison (also velour pads):
http://www.changstar.com/index.php/topic,1590.msg44503.html#msg44503Anyway, I must say I am happy with the measured results, especially since I did this all by ear.
The first mod screenshot shows the cups mods with some of the exterior grill portion blocked with foam squares (this is the left cup, so the foam is concentrated towards the rear of the head). You'll also notice a bunch of drawer liner. The second screenshot shows my foam damping sandwich. Then a picture of it inside the cup (thin foam on top). Lastly is a shot of the driver's naked backside. Please don't attempt dangus insertion.
