6.1.2 Spectral Phase Refractor (SPR)
The SPR circuit, U102A and U102B, is designed as two identical all-pass
filters that act as delay circuits on the overall audio signal. A greater time
delay occurs for signals above 150 Hz according to the time constant product
of C110 and R109.
Q101 and Q102 are FET switches that turn the SPR circuit on or off, depend
ing on the selection status of the SPR switch found on the front panel. When
SPR is off, the FET switches are biased on (R146 goes to ground) and Cl 10/
0 1 2 are shorted to ground. As a result, U102A and U102B essentially
become serial inverting amplifiers that maintain proper signal polarity to the
output stage.
6.1.3 Operating Level
The output signal from U102B splits into two paths. The main audio path
goes to the SUMMING STAGE consisting of U104A and U104B and their
associated electronic switches, U103A-D. The sidechain path goes through
the OPERATING LEVEL (Gain Normalizing) Switch, SW102, located on the
back panel.
The Ax Sidechain circuit needs to see an incoming signal at a -10 dB
operating level. Therefore, when the audio signal is at a +4 dBu level
(OPERATING LEVEL Switch is out), resistor R145 is grounded through one
pole of SW102 and the main audio signal is appropriately dropped to present
a -10 dB operating level for sidechain processing. When the audio signal is
at -10 dB (OPERATING LEVEL is IN), R145 is unconnected and there is no
signal drop.
The return from the Ax Sidechain circuit is routed to the Electronic Switching
stage through R118. This resistor adjusts the gain structure according the
chosen OPERATING LEVEL. For +4 dBm operation, R118 is in-line (along
with R119) to drop the returning signal. For -10 dB operation, R118 is
shorted and the returning signal is unaltered.
6.1.4 Electronic Switching for SOLO and Ax Modes
CMOS switches U103A and U103D, working in tandem, shut off the main
audio signal whenever the SOLO button is pressed to indicate on. Logic
ground is routed via SW407 (see Control Board schematic) to H101 pin 5
causing UKBD's switch to open and UKBA's switch to close to audio
ground via R143, through the use of logic invertor U301F. In addition, D104,
acting like an OR circuit, turns on the Ax Sidechain signal (described below).
When SOLO is released to indicate off, H101 pin 5 is pulled up to +15 VDC
through RN301. This causes U103D's switch to close, UKBA's switch to
open, and D104 to bias off. This dual switch action always presents a zero
potential across any switch to prevent distortion.
Circuit Description
6-3
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