Altec 288 permanent magnet
Field coil Conversion
Altec 288 (B,C,D) permanent magnet drivers conversion.
Similar to the older Altec 287 drivers.
For the ultimate in sound reproduction a field coil driver is unsurpassed !. Very musical, smooth and warm !
each: 24 Lbs.
High quality silver coated copper with Teflon insulation cables are used for the audio and power connections.
Shipped in one double box with an approximated weight of 52 Lbs. measuring 12 x 12 x 12 inches approximately
Shown in the graphic the response curves of a CH 288287 15V dc Field Coil converted 288 driver with 5, 10,15 and 20 Volts applied to the field coil. Measurements taken at the throat of the driver very closed to the phase plug. Final frequency response will vary according to the load and frequency characteristics of the horn used.
Measurements taken with color identifying curves and the following field coil applied potentials:
The magnetic field strength is directly proportional to the power applied to the field coil up to saturation at which an increase of power to the field coil won't increase the strength of the magnetic field and all that extra energy will be wasted as heat.
In this configuration the best compromise is at 15 Volts dc, keeping the heat generation at a minimum with a very flat and high efficiency output throughout the audio response of the driver, extending very well up to he higher frequencies.
This frequency response is rarely accomplished with the permanent magnet 288 driver.
The frequency response as shown in the graphic is very flat and smooth even using a 1/24 octave spectra instead of 1/3 as is usually measured for a smoother more forgiving curve. A very high sound pressure level of over 110db with only 1W
The frequency response clearly deteriorating as the power to the coil is reduced specially at the top and bottom ends of the frequency spectrum as shown in the gray curve, the response is very similar with 15 and 20 Volts applied, only showing a relatively small increase of output at the frequency extremes at higher Voltages.
Measurements are taken at the throat of a converted driver with no horn, 1/24 octave spectra with an 16 Ohms diaphragm at 1 Watt (2.83 Volts RMS) sweep sinusoidal signal from 100HZ to 22,000 Hz., yielding a very high output.
No dividing network crossover was used.
The following graphic compares a unit before and after conversion with the recommended 15 V dc applied to the field coil.
The red curve shows the response with the Field coil, the black curve with the Alnico magnet.