(2) In an electronic crossover device, it is practical to vary the crossover frequency over a
wide range to accommodate almost countless woofer-tweeter combinations and acoustic en-
vironments. Changing speakers in your hi-fi system will not obsolete the XO-1. Also,
there are no complex impedance matching problems in assembling your own speaker cross-
over system.
(3) It is possible to achieve anear ideal crossover characteristic by means of electronic cir-
cuitry, without significant peaks or valleys in the overall response. Also, the response and
crossover characteristics are unaffected by differences in speakers and enclosures.
(4) Amplifier damping factor is not altered by the crossover device.
(5) The possibility of intermodulation distortion (modulation of the high frequencies by high-
amplitude low frequencies) in the amplifiers is reduced, particularly at high power levels.
(6) No reactive-load conditions, sometimes caused by conventional crossover units, are im-
posed on the power amplifier. Such load conditions can result in instability (oscillation)
in amplifiers with heavy feedback.
It should be noted that while the above discussion is confined to two-way speaker systems, the
XO-1 may be used to good advantage in three or four- way systems. Anumber of suggested
arrangements are given in the section entitled ’’Installation and Operation”.
CIRCUIT DESCRIPTION
The circuitry of the XO-1 is quite simple and straightforward. Refer to the Schematic. It con-
sists basically of ahigh-frequency channel and alow-frequency channel. A12AX7 dual triode is
used in each channel. A6X4 rectifier tube is used in aconventional full wave power supply with
atwo-section RC filter.
The high-frequency channel is actually ahigh- pass filter; it passes only those frequencies above
the cutoff frequency, as shown in the curves of Figure 1. It will be noted that the cutoff fre-
quency may be adjusted to 100, 200, 400, 700, 1200, 2000 or 3500 cycles by means of arotary
switch. Similarly, the low-frequency channel is alow-pass filter which will pass only frequen-
cies below the cutoff frequency, as shown in Figure 2. The cutoff frequency of this channel is
adjustable over the same range by means of another switch. The input signal is applied through
the high and low-frequency LEVEL controls, through suitable isolating resistors to the input of
both channels. The 12AX7 dual triode in each channel is utilized so that one section functions
as again stage and the other as an output cathode follower. Each channel employs two RC net-
works in order to achieve acutoff slope of 12 db per octave since each network contributes 6db
per octave. In each channel the 12AX7 gain stage serves to separate the two RC networks. Ap-
proximately 14 db negative feedback is applied around each channel from the cathode-follower
output back to the grid of the gain stage. The function of this negative feedback is three-fold:
(1) It modifies the shape of the response curves in the region near the cutoff frequency, so that
amuch sharper ’’knee” is imparted to the curve than if feedback were not used. This makes it
possible to maintain essentially flat response up to and including the cutoff frequency, with as
much as 12 db attenuation at twice and one-half this frequency for the low and high-frequency
channel, respectively. (2) Any harmonic distortion developed is reduced by feedback, at fre-
quencies within the passband of either channel. (3) The amount of feedback used reduces the
overall gain to unity since gain is not required in the unit.
The cutoff frequency is varied by switching different values of capacity in both RC networks of
each channel. Although this could be accomplished by switching resistors instead, the level in
the ’’flat” region would vary appreciably with the cutoff frequency due to the finite values of resis-
tance already in the circuit. Aunique method of switching capacitors has been devised whereby
each capacitor (with exceptions) serves for two successive cutoff frequencies. This greatly re-
duces the number of components required and is made possible by ’’staggering” the turnover
frequencies of the two RC networks in each channel.
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