Loudspeakers

The hi-fi universe is not short of loudspeaker brands claiming to use ‘ribbon’ tweeters although, in most instances, these are AMT or planar magnetic drivers of some description. In a true ribbon driver the diaphragm is also the electrical conductor, this thin foil tending to be both very fragile and requiring a matching transformer to raise the electrical impedance ‘seen’ by the partnering amplifier. Real world alternatives to pure foil ribbons include ‘pseudo-ribbon’ drivers like Raidho’s tweeter, which it acknowledges to be a planar magnetic type. Here a thin polymer diaphragm carries a flat ‘voice-coil’ etched from a conductive coating on one surface.

For this latest version of the tweeter, Raidho claims to have reduced the membrane’s mass to just 20mg while refining the unit’s waveguide, the rear panel, and magnets that form the surrounding field. Low mass tweeters offer benefits, but the films are not always especially rigid and their movement prone to ‘micro-resonances’ across their surface. This is distinct from the principal break-up mode of the entire membrane which Raidho puts at a high 82kHz. In practice the clamping/tensioning of the planar magnetic film, and the uniformity of the damping material behind, is critical not only in obtaining a flat response with controlled dispersion but also in ensuring adequate pair matching between different samples. Raidho is successful in this regard, its pseudo-ribbon tweeter offering a useable response beyond 50kHz while any audioband resonances are suitably quelled [see PM's Lab Report]. PM