EMI-REDD47-pre-维修说明书手册含电器原理图

ELECTRIC MUSICAL INDUSTRIES LTDRECORDS INTERNATIONAL DIVISION.Record Engineering Development Dept.Ref.REDD.M47.Line Amplifier Type REDD.47.Purpose.This unit is intended as a line voltage amplifier of small dimensions,foruse between control networks in a sound mixing and control console.To allowfor use under various conditions,a compromise has had to be made between highbridging input impedance,noise level,and frequency response.Three altern-ative values of effeotive gain are available,and the unit is capable of anoutput of o dBm under matched conditions,or +6 dBm with low output impedance;into a load of 200 ohms resistive,or it will give an output of l volt r.m.s.into open circuit.At these levels,harmonic distortion is very low.Theoutput can be increased to about +14 dBm on overload peaks before severeaudible distortion becomes apparent.Description.The amplifier consists of two valve stages,resistance capacitance coupledwith two loops of negative feedback.Input Circuit.The input transformer is designed to have the highest possible step-upratio attainable in the space available,with very generous hum screening.Increase of ratio by reducing primary turns results in too low a primaryinductance,with increase in iron distortion.If the secondary turns areincreased,leakege inductance and secondary self-capacitance both increase,and resonate too low in the frequency spectrum,leading to loading effects onthe line at high frequencies.Such resonance effect is in fact present,andis damped by the resistor Rl,the loading effect of this resistor beingremoved at low frequencies by condenser C13.First Stage.The first stage consists of an EF.86 pentode operated under conventionalconditions.The cathode is not decoupled directly to earth,but to a resist-ance across which the overall aegative feedback voltage is developed.Thisresistance R5 is kept to the low value of 120 ohms,so that the effect of humvoltages induced in the cathode from the heater is negligible.The input terminals are connected through the socket s2 and plug P2 astwo crossed pairs.Any a.c.magnetic field linking with this input pluginduces opposing voltages in the two loops,and considerably reduces hum pick..up at this point.