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DESIGNPHILOSOPHY
ThecircuitryusedintheFET PREAMPLIFIER07xistheresultofanadvancedandcomplete
designprocesscombininginnovationandprovefundamentals. Thisprocessavoidsboththe
limitationsoftotaladherencetoconventionandtheflawsresultingfrominappropriate
applicationsofclevercircuitgimmicks.
ImpedanceisolationandmatchingisderivedfromFETs.Whilecarefuldesigncanyieldgood
resultsfromanydevicetype,FETsconsistentlyhavetheedgeinvoltagegain,lownoise,low
interaction,andinterfaceapplications.
FETsareinherentlytransconductancedevices,meaningthataninputvoltagecontrolsan
outputcurrent.Unlikeconventionaltransistors,FETshaveextremelyhighinputimpedance
(about10megohm-similartovacuumtubes). TheFET "senses"theaudiosignalwithout
drawingcurrentfromthesource. Thiseliminatescomplexinteractionswiththesource,allows
maximumperformancefromeachsystemelement,andgreatlyreducesthechanceofcable
characteristicsalteringthesound. TheabsenceofinputcurrentinFETsallowshighbias
currentsforlinearityandspeedwithoutsacricingDCparameters.
Noiseiskeptlowbymultipleparallelingofinputdevices,carefulselectionofcircuit
impedances,andpre-screeningofdevices. TheClass A complimentaryfollowersusedto
drivethepreamplifieroutputareofsuchspeed,linearity,andlowoutputimpedancethatno
feedbackcorrectionisrequiredorused. Theadvantageofthisisthatthecircuit'sperfect
stabilityandtransientresponsearepreservedintoawiderangeofdicultandunpredictable
loads.Variationinsound,whichcouldoccurthroughinteractionswithinterconnectcablesand
othersystemelementsarethusavoided.
Untilrecently,perfectvolumecontrols(attenuators)didnotexist. Thisisbecause
conventionalstereopotentiometershaveseriouschannelmistracking(20%typically),
becomenoisywithage,wearout,andhavepoorresolutionoflevel-particularlywhen
operatedbyamotorasrequiredforremoteoperation.Inanattempttosolvetheseproblems,
manymanufacturershavebeenusing"switchedattenuators",whicharediscrete,resistor
laddersbuiltonrotaryswitches.Butwhiletheseeliminatechannelmistracking,theyintroduce
newproblems,includinglimitedresolution,steppingtransients,andcannotbeoperated
remotely. Andtheystillwearout.
Theylimitresolutionbecausethemostcontactsavailableonrotaryswitchesarethirty-one.
ThisrequirestwoorthreedBstepsinordertogetenoughrangeoutoftheattenuator,which
isn'tafineenoughresolutionformostlisteners.Switchedattenuatorsalsointroduceswitching
transients(a"click"or"pop")eachtimetheychangepositions.
Thisnoisecomesfromtwosources. Thefirstismechanicalnoisefromtheswitchitselfasits
ball-detentmechanismmovesfromdetenttodetent.Butamoretroublesomesourceisthe
voltagedifferencecausedbythechangeinthemusicalwaveformduringthetimeittakesthe
switchtomovefromonepositiontothenext. Thegreaterthevoltagedifference,thelouder
thetransient.