For the sake of brevity, only channel 1 components are mentioned in the
following discussion.
NOTE...
6.1 Main Audio Path
Channel 1 audio enters the Model 250 through the INPUT XLR connector to
a Servo Balanced Input Stage. Then the audio signal is routed to the Spectral
Phase Refractor (SPR) and through some electronic switching circuits on to
the Summing Stage, where it is combined with the signal from the Ax
Sidechain. This composite signal then flows to the Servo Balanced Output
Stage for routing to the OUTPUT XLR Connector via contacts controlled by
the RELAY button or a user-installed REMOTE CONTROL switch.
6.1.1 Servo Balanced Input
The Channel 1 audio signal from the INPUT XLR first passes through a
balanced fourth-order, low-pass (500 kHz) rejection filter to reduce interfer
ence from radio frequency sources. One phase of the filter is made up of
resistors R102-R103, inductors L101-L102 and capacitors C101-C102. The
other phase is made up of resistors R104-R105, inductors L103-L104 and
capacitors C l03-004.
After the rejection filter stage, the balanced audio signal is then phase
coupled to the differential input circuit through composite coupling capaci
tors, which are usually found only in high-definition audiophile circuits.
Each coupling capacitor set, C107-C108 and C105-C106, is made up of a 22 uF
non-polarized electrolytic in parallel with a small 0.1 uF mylar capacitor. At
high frequencies, this high grade mylar capacitor bypasses the inductance of
the electrolytic to improve high frequency performance and pass the fine
details of the audio.
The balanced audio signal is then transformed via RN101, VR101, and U101B
into a single-ended current that feeds differential input amplifier, U101A.
The single-ended output of U101A feeds the SPR circuit via R110. U101B
acts as a common-mode servo that modulates a correction current into
UlOlA's input nodes via RN101 (pin 5 & 6 and pin 3 &4). U101B creates the
error signal through the signals it receives from the output of U101A via
R106 and its own ouput through RN101 (pin 3 & 4). A sa result, none of
the common-mode signal appears at the inputs of either amplifier stage.
This circuit greatly improves the linearity of the entire input stage under
high common-mode conditions such as AC hum, RF interference, and DC
offsets. VR101 is a fine trim for nulling CMR and is set according to the
Input CMR test procedure discussed in Section 7.1.2.
An internal termination resistor, R101, can be placed across INPUT XLR pins
3 and 2, whenever the INPUT TERMINATION Switch, SW101, is pressed IN.
This resistor presents a 602 ohm load to the outside world at the Model 250
Channel 1 input.
6-2
Circuit Description
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