ts24-user操作说明书手册-guide-6-8

3.MX900 FADER CIRCUIT DESCRIPTION3.1 Logarithmic amplifier.The circuitry around the transistor array CA3046 and half the LM348comprises a logarithmic amplifier.This amplifier takes the voltagetapped off the wiper of the fader track (which has the +15 voltsupply across it in "auto")and coverts it into a current using R55.This current is then compared with a reference current passingthrough R32 and VR3 taken from the same voltage (+15V supply).The difference between the two currents appears as an offset at theoutput of the first section of the LM348.However,for each side ofthis bridge (made up of R55 and its transistor on one side andR32/VR3 and its transistor on the other)to be balanced,all thecurrent entering by the collectors must leave by the common emitters.Also,the left hand side transistor has its base grourded.When thetwo currents entering the inverting inputs are made equal,the outputof the left hand ampifier becomes zero.This is the condition whichcorresponds to fader full up.Since there are minor differences inthe geometry of the two transistor chips,the right hand resistorR32,is arranged to be variable (VR3)to compensate for theinequality and give the balanced condition referred to above.As the fader is moved down the scale,the left hand input currentreduces but the right hand current,being fixed,must still be drawnaway through the common emitter connection.Since the emitters arecommoned,and the left hand transistor is only having to pass the nowsmaller input current,the right hand transistor must be kept in thesame conductivity as in the original case when both transistors wereconducting the same currents.Thus the base of the right hand device must rise above the level ofthe left hand device.Since the current through a transistor isproportional to the exponent of the base-emitter voltage,the baseemitter voltage of the transistor must rise in an anti-exponential orlogarithmic manner.If the voltage required by the base is attenuatedfrom a larger source,then this source can be made to be logarithmicover a scale determined purely by the attenuator value.The output of the left hand amplifier is the logarithm of its inputcurrent with a value of zero when the current is maximum and risingto the limit of the amplifiers swing for reducing currents.If anaudio taper fader (whose output is in fact an exponent,not a log),is applied to the input of the logarithmic amplifier,the voltage atthe output will become a more or less linear function of the faderposition.(More or less'because the exponential taper is furtherbent to give finer control over the upper half of the scale andensure good shutoff at the bottom of the travel).The only other point to note is that the log of zero isindeterminate,so when the fader is at the bottom'of its scale,asmall amount of current is stolen from the input by R36 to make surePH900REL:850320Page 5Mastermix Service Manual MX900 MX190 in the Soundcraft TS24 Console