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CYRUS 6 & 8 Circuit Description
NOTE: - Reference numbers in the text are for the left channel.
Input selection and Tape Monitor
The input and tape monitor selection is via an electronic audio selector switch. The switch is a
6way 2pole switch followed by a 2way 2pole for tape monitor (IC101). The signal from the input
selection is then sent to the TAPE OUT sockets via 330R resistors.
Volume control
The output signal from the input selector buffer is AC coupled to the input of the digitally
controlled volume IC: IC104. The output is then buffered by the FET input op-amp IC105.
Preamp Output
The Preamp output is taken from the volume control buffer IC105.
Power amplifier stage
The power amplifier is a discrete voltage feedback design. RV201 allows adjustment of the
output transistor bias current. T208 and T210 are the driver transistors. T206, T208 and T210
are bolted together to ensure good thermal coupling. T209 and T211 are the output transistors.
Headphones
The drive for the headphones comes from a dedicated power amplifier IC401. The headphone
amplifier is muted when the power amplifiers are on and activated when the power amplifiers are
off, when a 6.3mm jack plug is inserted in the headphone socket.
Power supply
The power supply for the main amplifier is 37VDC (Cyrus 6) or 43VDC (Cyrus 8), protected
by fast blow fuses. In addition a regulated 34VDC (Cyrus 6) or 42VDC (Cyrus 8) is derived
from a charge pump circuit and supplies the low current section of the power amplifiers.
Regulated 15V is derived from the power supply for the main amplifier via regulators VR402
and VR403. The 15V supplies are used for volume control, buffers and parts of the protection
circuits. 6VDC is derived from the 15VDC supply. When a PSX-R is connected (Cyrus 8
only), changes to the internal power supply connections route the 35V regulated power from
the PSX-R to the 15V Regulators.
The protection circuits
Over-current protection is achieved by detecting the voltage across the emitter resistors
R218/219. When a high level of current is detected T212 will turn on, switching T225 on etc.
The output signal from this detector is then sent to the control microprocessor.
Power supply failure is detected by IC402 arranged as a window comparator. The window range
is set between 19V and 60V, as the amplifier will work between these voltages. If the supply
voltage is outside that range a power supply error signal is sent to the control microprocessor.
Mains disconnection is detected by the control microprocessor, to prevent switch off noise.
The control system
IC501 is the control microprocessor, located on the front panel. A memory EPROM IC501 is
fitted in a socket to allow the control software to be upgraded if necessary. The microprocessor
controls all the amplifier functions including storage of settings in an internal memory. IC508 is a
3
Jan 2006
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