Moduler Preamplifier Circuit Diagram


High Quality, Discrete Components Design, Input and Tone Control Modules

To complement the 60 Watt MosFet Audio Amplifier a High Quality Preamplifier design was necessary. A discrete components topology, using + and – 24V supply rails was chosen, keeping the transistor count to the minimum, but still allowing low noise, very low distortion and high input overload margin. Obviously, the modules forming this preamplifier can be used in different combinations and drive different power amplifiers, provided the following stages present a reasonably high input impedance (i.e. higher than 10KOhm).

Main Module:

If a Tone Control facility is not needed, the Preamplifier will be formed by the Main Module only. Its input will be connected to some sort of changeover switch, in order to allow several audio reproduction devices to be connected, e.g. CD player, Tuner, Tape Recorder, iPod, MiniDisc etc. The total amount and type of inputs is left to the choice of the home constructor. The output of the Main Module will be connected to a 22K Log. potentiometer (dual gang if a stereo preamp was planned). The central and ground leads of this potentiometer must be connected to the power amplifier input.

Circuit diagram:

Circuit

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:

This Module employs an unusual topology, still maintaining the basic op-amp circuitry of the Main Module with a few changes in resistor values. A special feature of this circuit is the use of six ways switches instead of the more common potentiometers: in this way, precise "tone flat" setting, or preset dB steps in bass and treble boost or cut can be obtained. Tone Control switches also allow a more precise channel matching when a stereo configuration is used, avoiding the frequent poor alignment accuracy presented by common ganged potentiometers.

Six ways (two poles for stereo) rotary switches were chosen for this purpose as easily available. This dictated the unusual "asymmetrical" configuration of three positions for boost, one for flat and two for cut. This choice was based on the fact that tone controls are used in practice more for frequency boosting than for cutting purposes. In any case, +5dB +10dB and +15dB of bass boost and -3dB and -10dB of bass cut were provided. Treble boost was also set to +5dB +10dB and +15dB and treble cut to -3.5dB and -9dB.

Those wishing to use common potentiometers in the usual way for Tone Controls may use the circuit shown enclosed in the dashed box (bottom-right of the Tone Control Module circuit diagram) to replace switched controls. The Tone Control Module should usually be placed after the Main Input Module, and the volume control inserted between the Tone Control Module output and the power amplifier input. Alternatively, the volume control can also be placed between Main Input Module and Tone Control Module, at will. Furthermore, the position of these two modules can be also interchanged.

Circuit diagram:

Circuit

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

Power supply:

The preamplifier must be feed by a dual-rail, +24 and -24V 50mA dc power supply. This is easily achieved by using a 48V 3VA center-tapped mains transformer, a 100V 1A bridge rectifier and a couple of 2200µF 50V smoothing capacitors. To these components two 24V IC regulators must be added: a 7824 (or 78L24) for the positive rail and a 7924 (or 79L24) for the negative one. The diagram of such a power supply is the same of that used in the Headphone Amplifier, but the voltages of the secondary winding of the transformer, smoothing capacitors and IC regulators must be uprated. Alternatively, the dc voltage can be directly derived from the dc supply rails of the power amplifier, provided that both 24V regulators are added.

Note:

If this preamplifier is used as a separate, stand-alone device, thus requiring a cable connection to the power amplifier, some kind of output short-circuit protection is needed, due to possible shorts caused by incorrect plugging. The simplest solution is to wire a 3K3 1/4W resistor in series to the output capacitor of the last module (i.e. the module having its output connected to the preamp main output socket).

Technical data:

  • Main Module Input sensitivity:
  • 250mV RMS for 1V RMS output
  • Tone Control Module Input sensitivity:
  • 1V RMS for 1V RMS output
  • Maximum output voltage:
  • 13.4V RMS into 100K load, 11.3V RMS into 22K load, 8.8V RMS into 10K load
  • Frequency response:
  • flat from 20Hz to 20KHz
  • Total harmonic distortion @ 1KHz:
  • 1V RMS 0.002% 5V RMS 0.003% 7V RMS 0.003%
  • Total harmonic distortion @10KHz:
  • 1V RMS 0.003% 5V RMS 0.008% 7V RMS 0.01%

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