Crown VFX2 Instruction Manual page 18

The HI VERNIER potentiometer (R14, R16, R18) and
LO VERNIER potentiometer (R23, R25, R27) consist
of matched sets of three 15K-ohm resistive elements
used to determine the location of the desired corner
frequency of the filter. Refer to paragraph 2.5.1 for
the proper method of using the VERNIER/ RANGE
controls to select the corner frequency for the filters.
The signal from IC1B is applied through selected RC
filter components to dual operational amplifier IC2A,
which, along with IC2B and IC3A, forms a 3-pole,
high-pass, 18 dB/octave Butterworth filter. This
produces the non-inverted HI PASS OUTPUT.
Operational amplifier IC3B creates the HI PASS
INVERTED OUTPUT.
The HI PASS OUTPUT signal from IC3A is also
applied through the mode switch to the selected RC
filter components to dual operational amplifier |C4A,
which, with IC4B and IC5A, forms a3-pole, low-pass,
18 dB/octave Butterworth filter. This filter produces
the non-inverted LO PASS OUTPUT. Operational
amplifier IC5B creates the LO PASS INVERTED
OUTPUT.
c. CROSSOVER (Low Pass), FILTER (Bandpass)
Switches
The rear panel mode switches select either a LOW
PASS or BANDPASS output at the LOW PASS
QUTPUT jacks. With the mode switch inCROSSOVER
(LOW PASS), the input, from IC1B, is tied directly to
the LOW PASS filter sectian; in FILTER (BANDPASS),
the output of the H! PASS filter signal, from iC3A, is
tied directly to the LOW PASS filter section.
3.2.2 Power Supply Circuits
AC power is applied to the power transformer, which
may be wired for either 120 VAC or 240 VAC
operation. The AC input line is fused with a 3AG 1/8
A fuse for 120 VAC or an AGC 1/16 A fuse for 240
VAC. The bridge rectifier, D1-D4, together with C1,
R1, Q1 and Zener D5 provides a regulated + 15VDC;
the bridge rectifier and C2, R2, Q2 and Zener D6
provide a regulated -15 VDC. The+15 VDC is applied
to pin 8 and the -15 VDC is applied to pin 4 of all dual
operational amplifier IC's.
18
3.3 TEST PROCEDURES
This paragraph lists precautions essential to obtain
accurate test measurements when dealing with high-
purity filtering equipment such as the VFX2.
1. Use the proper line voltage (120 VAC or 240 VAC)
for which the power supply has been wired for normal
operation. The line voltage should be measured with
a peak reading AC voltmeter and adjusted to the rms
equivalent voltage.
2. The input LEVEL controls (attenuators) should be
set to maximum for all distortion tests to assure
repeatability of all test measurements. Filter settings
may greatly change distortion measurements
(usually reducing these measurements and many
times making them invalid). Most distortion
measurements should be made with the RANGE
switches set to OFF, for the same reason.
3. When measuring hum and noise, all inputs should
be disconnected from the VFX2 and the LEVEL
(attenuator) controls set to minimum (or to
maximum for certain tests).
4. Always monitor the test oscillator when
measuring frequency response. Use a wideband AC
voltmeter or use the same meter for both input and
output level measurements, if the meter's frequency
response is known not to be dependent on attenuator
settings.
5. Residual distortion and noise levels should be
fully Known for all test equipment in order to
accurately evaluate the performance of the VFX2.
6. Never measure hum and noise when in the
presence of strong magnetic fields. The VFX2 should
be at least 4 inches from any large metallic objects or
shield plates for a reading to be*meaningful.
7. Noise measurement should be taken with the
output bandpass filtered for 20Hz - 20KHz. For audio
purposes the measurement of noise above 20KHz is
meaningless.