Decca ribbon tweeter flyer en

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The New LOHClOll High Frequency Speaker

The new London High Frequency Speaker is the latest result of the continuous
development by Decca of speakers using the ribbon principle. This unit possesses
all the inherent advantages of its predecessor the DK30 while being able to handle
frequencies from 1000Hz to beyond the audio range. Now bass speakers specifically
designed for handling only the low frequencies (previously only usable with the
ribbon speaker in conjunction with a mid range unit) can be used with the London
Ribbon Speaker in a two unit system. This eliminates the added phasing problems

encountered with three-speaker systems.

The existenCe of the London Speaker is important in that it coincides with the
emergence on the Hi Fi market of pick-ups and amplifiers which can faithfully
reproduce the top end of the frequency spectrum. However, what is not generally
appreciated is that many types of conventional moving coil tweeters are unable to
reproduce the complicated transients, and often cause listening fatigue through
their attempts to do so. The quotation of a frequency response gives no indication
of a speakers transient response: standardisation of a method of transient response

quotation is still a matter of controversy.

What makes the ribbon speaker different?

The answer lies in the ribbon itself (only 1/10th the thickness of a human hair),
which, being the only moving part of the speaker, and with a very low mass has,
therefore, very low inertia. This factor allows the ribbon to reproduce accurately
the middle and top frequencies, while still preserving the transient characteristic.
An alternating current flowing through this ribbon produces its own magnetic
field which reacts with that of themagnet system. The resultant mechanical force is
applied uniformly and in phase over the whole diaphragm. This ensures a remark-
ably smooth response, free from resonance, quite unlike conventional moving coil
type tweeters where the force is applied only at the line of contact between the
voice coil and the diaphragm.

The rear face of the ribbon terminates in an acoustic resistance contained in a
sealed cavity allowing the unit to be mounted in low frequency enclosures without
any intermodulation effects whatsoever. The ribbon is coupled to the air by a
specially designed horn, which gives high coupling efficiency at all frequencies from
1K to 35KHz. The horn incorporates a fin to ensure correct dispersion of sound.

The transformer of the London Ribbon speaker is of special design and has an
insertion loss of less than 1dB and maximum distortion of less than 0.5% from the
operating frequencies band at 30 watts input. It is essential that the ribbon is pro-
tected by the use of a suitable cross-over. The ideal cross-over frequency is 1,000
Hz at which frequency cut-ofi rate should not be less than 12dB per octave.

Crossover Network CO/1000/8
Mounted on a ridged printed board, close tolerance carbon polyester capacitors
and inductors are used ensuring unvarying performance. The low pass section is a
standard, constant-impedance, 6dB per octave filter while the high section attenu-
ates at 12dB per octave. The cross-over frequency is 1,000Hz and intermodulation
distortion is virtually eliminated.