By Vivian Capel
Now in its third version, a concise selection of functional facts for these engaged on, or attracted to, sound platforms. up-to-date all through to hide the newest know-how and defense laws
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This produces a voltage drop over a load resistor, which must be very high in value if a useful signal voltage is to be developed from the small current flowing. Output impedance is therefore ultra high, around 100 ΜΩ. A connecting cable of more than a few inches would cause severe loss of high frequencies owing to cable capacitance, so a built-in pre-amp is essential. The very low mass and inertia of the diaphragm gives a flat and extended response, while output is high because of the built-in preamp.
Phantom powering via (a) centre-tapped resistors; (b) transformer. A-B powering This overcomes the disadvantage of phantom powering which requires the screen to be connected at both ends and thereby form a complete circuit whereby induced hum and noise could be applied to the input stage. With A-B powering both signal wires are used for the supply and the screen does not form part of the supply or input circuit. rQh E It Fig. 36. A-B Powering. Capacitors between windings prevent a supply s/c. 40 Acoustic characteristics Omni-directional A cone or diaphragm with a sealed back responds to pressure variations on its front.
Not being reflected to the microphone, they produce no output and result in a falling bass response. Waves reflected from the outer areas of the dish arrive at the diaphragm at an oblique angle. At wavelengths shorter than the diaphragm diameter, there is thereby a phase difference across its surface which increases with frequency. Output thus falls at high frequencies. Overall response is thus a mid-range peak with bass and treble declining rapidly on either side. Its frequency is determined by the reflector size: for a 12-inch diameter, it is about 10 kHz, for 18-inch it is around 7-8 kHz, and for 24-inch approximately 5 kHz.