Did anyone try making custom pads for the SR-007? I'm glad to share the experience.
I own the SR-007 SZ1 with heavily used brown pads. At first I wanted to buy modern (black) STAX earpads per Birgir's advice, however they are pricey, especially if you take into account the shipping cost from Japan to Russia. So I decided to ask the local guy (nickname snorry on the Russian forum) to make custom earpads for my SR-007. This guy is experienced in modding planar magnetic headphones (he even built some nearly from scratch) and he also has the necessary measurement equipment.
First of all, the grills were removed from the earpads.
The grills turned out to have strong windage and very poor permeability. Try to blow through it keeping your hand behind it and you won't feel any air coming through. On one hand, it isolates the membrane from reflections, on the other hand it supposedly degrades microdynamics and conceals everything above 10 kHz. We decided to change the grills with more transparent ones, that according to snorry already proofed itself to be useful in his planar magnetic projects.
Different materials and forms were tried. Here's what I've decided to stay with.
The form remains roughly the same, however new earpads are more rigid, they don't flatten on head unlike the original ones. Because of this they keep the driver slightly further away from the ear while sitting tight and comfy. This slightly improves the bass. The notch is oval, this form was chosen experimentally for its most balanced FR. Velour strips were added to the internal side of the earpads to reduce reflections of high frequencies.
Here is the measurements.
Old earpads:
Note the channel imbalance, it's caused solely by the earpads. No doubt, not every single pair of brown MKI pads out there are as terrible as mine, however that's what I got.
New earpads:
The channel imbalance is gone, the bass roll-off is more gentle and there is no midbass emphasis anymore. HF now go well beyond 10 kHz. The second-order harmonics level is slightly lower than before.
There was also a more SR-009-like variant with a little peak near 1 kHz:
This forwards the vocals a bit, however I prefer mids to be flat.
I want to thank Birgir who strongly advised changing the earpads. That really helped to make already great headphones ever better.