Sooooo.... after a 10-year hiatus and a detour in Stax-land and some of the non-TOTL planars, I came back to my original love for the HP-3, and with the help of modern materials figured out a damping scheme that seems to work much better than what we used to do with felt or similar materials.
I've been discussing it with Kabeer privately, and it might be worth to show it here too. I skipped the last 10 years of postings, so this might have been already discussed. In which case I'm curious about your experience with similar materials.
Material #1:
synthetic fibers used to soundproof motorbike exhausts, labeled "rock wool" but it's polyamide
Material #2:
speaker cabinet polyester damping
These
absorb sound and reduce the speed of the backwave, without impeding driver movement like felt does. I can stuff cups full of dense balls of these, and I never get the typical overdamped, constrained sound that is so easy to get with felt.
In the HP-3, I use a ball of the first material against the driver, and then I shape a donut of the second lighter material to cover the vents around the cup. I also remove the paper ring and dust cover on the front of the driver and glue it in place with a ring of thin strong 3M adhesive film, very much like the old Stax Lambda phones. I also change the old cable with Mogami mini quad, but that is probably unnecessary. If pads are worn, there are excellent cheap 65mm spares on ebay, and 3M film replaces the old adhesive.
The effect is surprising: treble gets a slight boost, bass and deep bass come out, mids are the usual warm Yamaha mids. The modded HP-3 stand up very well to the lower end Audezes I have like the LCD-1 and Sine, and I actually prefer them to both. This damping scheme also requires less amplification compared to the old felt-based schemes, as the diaphragm encounters very little air resistance.
I don't have any pictures of the insides, I can open a pair if anyone is interested. This is my cheap, very good sounding balanced planar setup: