I am quoting my own impressions so that people can get an idea about these headphones since the information about these is nearly non existent on web.
Here is a rather unique & rare planar headphone : Rotel RH-930 back-electret headphones.
This is an OEM version of Toshiba ?HR-910.?
The wikiphonia entry suggests 910 but the pics on this japanese site are suggest hat it is OEM version of HR-810.
The transformer is built into the TRS plug & so it can be used with usual dynamic amplifiers.
The driver is 65 mm. No backwave damping here.
Most electrets are considered infereior to the 'electrostats' but back-electrets are a bit different. Check this wikiphonia link for electret driver principle:
From an engineering standpoint, the ideal diaphragm material is a polyester called boPET, biaxially-oriented polyethylene terephthalate film, aka Mylar or Hostaphan or Melinex. Problem is, boPET doesn't make a very good electret. So an engineer must either accept a compromise in the sound, or he can re-engineer the typical electrostatic headphone layout by metallizing a boPET diaphragm and applying the audio signal to it and relegating the electret material to the stators where its inferior mechanical properties won't matter. This inside-out or back-electret design was used by only two manufacturers: Toshiba (most notably the HR 910; Rotel also marketed an OEM model) and Audio-Technica (the ATH-8, aka the Signet TK33).
About the Sound :
The first thing that struck me was how smooth they were in mids & treble. I can't seem to pick any major peak/dip in there frequency response.I can play them as loud as I can from my amp & they don't get shouty.
The Treble is very extended & without a hint of that plasticky sound that seems to bug most dynamic cans.
In the stock form they have significant bass roll-off starting from 175 Hz & lower meaning that one would miss the mid-bass & that the male vocals sounded a bit thin.
I improved the backwave seal of the cans & that has cured the vocals. The bass still rolls off but most of it is now in sub-bass region & doesn't hurt the the timbre of most instruments.
Pressing a bit on the earcups improves the sub-bass too which suggest that experiments with better sealing earpads are in the order.
Head-fi thread about back-electrets
Price : Rs. 7k + shipping
This should be of special interest to people who have been wanting to try out electrostatic headphones but have been put off either by price or by the need for a separate electrostatic amplifier/adapter.
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