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A Modular Audio Preamplifier

2015-01-03 13:47  
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This article describes a modular audio preamplifier. The amplifier can be used in different combinations and drive different power amplifiers provides the following stages provide a reasonably high input  impedance  (ie, greater than 10 kohm). The main modules of the circuit input will be connected to some form of switch, to allow the connection of several audio reproducing apparatus, for example. CD players, tuners, tape recorders, iPod, compact disks, etc. total number and type of inputs are left home constructor choice. The output of the main module will be connected to a 22 k log. Potentiometer (dual gang if a stereo preamplifier is planned). Central and ground lead to the potentiometer must be connected to the power amplifier input. And the amplifier's tone control module uses an unusual topology, still maintaining the basic op amp circuit main module has a little bit of change in the resistor values. The circuit is using six kinds of mode switch, precise "tone flat" setting, or preset dB steps in the bass and treble to increase or decrease available. Tone control switch also allows more accurate channel matching when a stereo configuration is used, to avoid frequent poor alignment accuracy proposed common connection potentiometer. Six (two stereo) rotary switch was chosen so useful for this purpose. This determines the unusual "asymmetrical" configuration to enhance the three positions, one for the plane and two in order to reduce. This selection is based on the fact that in practice the frequency of the use of tones to control more than the increased cutting purposes. In any case, 5 db 10 db and 15 db 3 db bass boost and bass cut and -10 dB up. Threefold increase is also set to 5 db 10 db and 15 db and triple reduced to -3.5 db and 9 db. Tone control module normally be placed on the primary input module, and the volume control of the tone control module inserted between the output and the input of the power amplifier. In addition, the volume control module can be placed between the primary input and the tone control module, when the. In addition, the position of these two modules can be interchanged. The power amplifier is a dual track, 24 and -24 volts 50 ma DC power supply. This is easily achieved by using 48 v 3 va center tap of the power transformer, a 100 v 1 a bridge rectifier and a few 2200 do? F 50 v smoothing capacitor. These components are two 24 v IC regulator must be added: 7824 (or 78 l24) railways and 7924 was positive (or 79 l24) a negative one. Such a power supply in the figure is the same for, but the voltage of the secondary winding of the transformer, a smoothing capacitor and IC regulators must increase the power. In addition, the DC voltage can be directly derived from the DC power supply rails of the power amplifier provides two 24 v regulator is added.

 

Main Module circuit diagram:

Modular Audio Preamplifier Main Module

Parts:

R1_______________1K5  1/4W Resistor
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R7,R10__________10K   1/4W Resistors
R8______________33K   1/4W Resistor
R9_____________150R   1/4W Resistor
R11_____________ 6K8  1/4W Resistor
R12,R13________100R   1/4W Resistors
R14____________100K   1/4W Resistor

C1_____________220nF   63V Polyester Capacitor
C2_____________220pF   63V Polystyrene or ceramic Capacitor
C3_______________1nF   63V Polyester or ceramic Capacitor
C4,C7___________47µF   50V Electrolytic Capacitors
C5,C6__________100µF   50V Electrolytic Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor

Tone Control Module circuit diagram:

Modular Audio Preamplifier Tone Control Module

Parts:

R1,R7___________47K   1/4W Resistors
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R8_____________150R   1/4W Resistor
R9______________10K   1/4W Resistor
R10,R16__________6K8  1/4W Resistors
R11,R12________100R   1/4W Resistors
R13____________100K   1/4W Resistor
R14______________1K5  1/4W Resistor
R15,R21,R22______4K7  1/4W Resistors
R17,R24,R26______8K2  1/4W Resistors
R18______________3K3  1/4W Resistor
R19______________1K   1/4W Resistor
R20____________470R   1/4W Resistor
R23,R25_________12K   1/4W Resistors
R27,R28__________4K7  1/4W Resistors

C1_____________220nF   63V Polyester Capacitor
C2_______________1nF   63V Polyester or ceramic Capacitor
C3,C6___________47µF   50V Electrolytic Capacitors
C4,C5__________100µF   50V Electrolytic Capacitors
C7______________10nF   63V Polyester Capacitor
C8,C9__________100nF   63V Polyester Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor
SW1,SW2_______2 poles 6 ways Rotary Switches

Simpler, alternative Tone Control parts:

P1______________22K   Linear Potentiometer
P2______________47K   Linear Potentiometer

R29,R30________470R   1/4W Resistors
R31,R32__________4K7  1/4W Resistors

C10_____________10nF   63V Polyester Capacitor
C11,C12________100nF   63V Polyester Capacitors
 

Technical data:

Main Module Input sensitivity:250mV RMS for 1V RMS outputTone Control Module Input sensitivity:1V RMS for 1V RMS outputMaximum output voltage:13.4V RMSinto 100K load,11.3V RMSinto 22K load,8.8V RMSinto 10K loadFrequency response:flat from 20Hz to 20KHzTotal harmonic distortion @ 1KHz:1V RMS0.002%5V RMS0.003%7V RMS0.003%Total harmonic distortion @10KHz:1V RMS0.003%5V RMS0.008%7V RMS0.01%
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