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PortableMicrophonePreamplifierHighheadroominputcircuitry

9VBatterypowered

Circuitdiagram:

Parts:

P1______________2K2LinearPotentiometer

R1,R2,R3______100K1/4WResistorsR4______________8K21/4WResistorR5_____________68R1/4WResistorR6______________6K81/4WResistorR7,R8___________1K1/4WResistorsR9____________150R1/4WResistor

C1______________1F63VPolyesterCapacitorC2,C3,C4______100F25VElectrolyticCapacitorsC5_____________22F25VElectrolyticCapacitor

Q1___________BC560C45V100mALownoiseHighgainPNPTransistorQ2___________BC550C45V100mALownoiseHighgainNPNTransistor

J1_____________Jacksocket(Mono3or6mm.)

Devicepurpose:

Thiscircuitismainlyintendedtoprovidecommonhomestereoamplifierswithamicrophoneinput.Thebatterysupplyisagoodcompromise:inthismannertheinputcircuitisfreefrommainslowfrequencyhumpickupandconnectiontotheamplifierismoresimple,duetotheabsenceofmainscableandpowersupply.Usingastereomicrophonethecircuitmustbedoubled.Inthiscase,twoseparatelevelcontrolsarebetterthanadualgangedstereopotentiometer.Lowcurrentdrawing(about2mA)ensuresalongbatterylife.

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Circuitoperation:

Thecircuitisbasedonalownoise,highgaintwostagePNPandNPNtransistoramplifier,usingDCnegativefeedbackthroughR6tostabilizetheworkingconditionsquiteprecisely.OutputlevelisattenuatedbyP1but,atthesametime,thestagegainisloweredduetotheincreasedvalueofR5.ThisunusualconnectionofP1,helpsinobtainingahighheadroominput,allowingtocopewithawiderangeofinputsources(0.2to200mVRMSfor1VRMSoutput).

Notes:

Harmonicdistortionisabout0.1%@1VRMSoutput(allfrequencies).Maximuminputvoltage(levelcontrolcursorsetatmaximum)=25mVRMSMaximuminputvoltage(levelcontrolcursorsetatcenterposition)=200mVRMSEnclosingthecircuitinametalcaseishighlyrecommended.SimplyconnecttheoutputofthisdevicetotheAuxinputofyouramplifierthroughscreenedcableandsuitableconnectors.