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THEORY OF OPERATION
1.4 Power Supply Electronics
The PS1 is powered by a total of four distinct switch-mode power supplies. All of these are fed
approximately 340VDC from the single input filter PC Board which contains numerous EMI
filtering components, in addition to a bridge rectifier and storage capacitors C607 and C608
[FCC Filter PCB sheet1, C4]. NTC thermistors TH601 & TH602 [C2] provide added resistance
at start-up to lower the inrush current.
Three identical board assemblies provide a separate ±27VDC to each of the power amplifier
channels. These are regulated with a transformer-coupled, two-FET forward converter topology.
Isolation is provided by the main transformer T503 [27 Volt Power Supply PCB sheet 1, C4], and
by the gate-drive transformer T502 [A3]. In addition to the internal over-current protection pro-
vided in the controller IC, there is an external input which shuts down an individual channel's
power supply if a fault is sensed by the respective amplifier channel.
The fourth SMPS PC Board assembly, known as the Auxiliary Power Supply, provides all of the
other low-voltage, low-power DC voltages required by the remainder of the PS1's electronics.
These are ±15V, +24V, +5V and +3.3V. They are regulated with a direct-coupled, single-FET
flyback converter topology. Isolation is provided by the main flyback transformer T601 [Auxiliary
Power Supply PCB sheet 1, C2], and by two opto-couplers IC602 and IC606 [B3]. Feedback for
regulation is derived from the 3.3V digital and +15V analog outputs. Regulation of the other
outputs depends on close coupling of transformer T601's secondaries.
The +24V output is used to power the cooling fans [sheet 2, D3], which are in turn driven in
proportion to the output signal level. +24V is also used to provide phantom power for the micro-
phone inputs. +5 and +3.3V outputs are used primarily to power the DSP, A/D & D/A converters,
microcontrollers, remote control communications, etc. The ±15V outputs power the numerous
OpAmps and other analog circuitry.
1.5 Audio Signal Path
1.5.1 General Notes - All Analog Inputs and Outputs
All user-accessible inputs and outputs are protected from RF interference (EMI) and static
discharge (ESD) by rail-rail diode clamps, spark gaps, and RC filtering. Extensive filtering is also
applied to speaker, power and Remote connections to prevent transmitted or received interfer-
ence. All Analog inputs and outputs are configured as non-inverting, i.e. all ins and outs are in
phase with each other. Standard polarity conventions apply; XLR connectors are wired pin 2
pos, pin 3 neg.
The remote control does not carry any analog signals, rather it transmits MIDI code bi-
directionally over an 8-conductor MIDI cable.
1.5.2 Preamplifier
The INA163 low-noise instrumentation amplifier forms the core of the PreAmp section. Gain of
both the Mic and Line inputs is determined by the impedance between the RG pins 12 and 3,
which consists of Trim control VR101/102 [input panel PCB sheet 1, A3 and C3] and R133/
R168. The balanced microphone input is protected from EMI, ESD and DC voltages (including
phantom power) by a rail-rail diode clamp, spark gaps, and numerous R's and C's. The un-
balanced Line input is buffered by FET-input OpAmp U101A [D2] / U103A [B2] to maintain the
very high input impedance which is often required by instruments with magnetic pickups. R105
and 107 attenuate the Line input to an appropriate level as it is fed into the INA163.
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