Cyrus 8 DAC - TECHNICAL DESCRIPTION; Volume control; Power amplifier stage; DC offset at the output

TECHNICALDESCRIPTION
©CyrusAudioLtdJun2014 9 Cyrus6DAC,Cyrus8DACservicemanualIssue1
Volumecontrol
Theoutputsignalfromtheinputselectorbuffer (IC103)iscoupledtotheinputofthe digitally
controlled volume IC (IC100). The output of the volume control IC is then fed to the
preamplifieroutputandthepoweramplifiers.
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.
DCoffsetattheoutputof
theamplifiermaybeset underconditionsofnosignalbyadjusting
RV203.Onceset,DCworkingconditionsarestabilisedwithinthepoweramplifierbyaDCservo
(IC200),measuringtheDCoffsetattheamplifieroutputandadjustingthebiasvoltageapplied
toT202Btocompensate.
Power amplifier standby control
 is achieved via the CCS point. With this line, the
microcontroller can switch the power amplifiers on or off via the level shifting and buffer
circuitonthe‘ChargePump’schematicsheet.
Headphones
Thedrive fortheheadphonescomesfromadedicatedpoweramplifierIC901.Theheadphone
amplifier is muted by the microcontroller when the power amplifiers are on and activated
when the  power amplifiers are off. The PHONES_IN signal enables the microcontroller to
detectwhena3.5mmjackplugisinsertedintheheadphonesocket.
Powersupply
Thefrontpanelcontrol  circuit(withtheexceptionofthedisplaybacklight)ispoweredfroma
smallpowertransformermountedonthemainPCB.Whenexitingstandby,thecontrolsystem
switchesrelayRL401,applyingmainspowertothe  main power transformer.Thisenablesthe
controlsystem, including microcontroller,to consume
verylittlepower,particularlywhenset
toStandby.
Power for the display backlight and the digital input PCB is provided separately from a low
voltagewindingofthemainpowertransformer.
The power supplies for the main amplifier (VCC) are 42V (Cyrus 8 DAC) or 39V (Cyrus 6
DAC).
TheseareprotectedbyfastblowfusesFS401andFS402onthemainPCB.Inadditiona
regulated42Visderivedfroma charge pumpcircuitandsuppliesthelowcurrentsectionof
thepoweramplifiers.Aregulatedpowersupplyof15Visderivedfromthepowersupplyfor
themainamplifiervia regulators VR402 andVR403.The15Vsuppliesareusedfortheinput
selector,buffersandpartsoftheprotectioncircuits.5Visderivedfromthe15Vsuppliesfor
thevolumecontrolIC100.