Discrete 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, employing + and - 24V supply rails, was chosen, maintaining the transistor count to a minimum while still allowing low noise, very low distortion, and a high input overload margin. The modules forming this preamplifier could be used in different combinations and drive various power amplifiers, provided the stages presented a reasonably high input impedance (i.e. higher than 10kΩ).

Main Module:

If a Tone Control facility wasn’t required, the Preamplifier would consist solely of the Main Module. Its input would be connected to a changeover switch, allowing several audio reproduction devices, such as CD players, Tuners, Tape Recorders, iPods, MiniDiscs, and more, to be connected. The total number and type of inputs would be the choice of the home constructor. The output of the Main Module would be connected to a 22kΩ Logarithmic potentiometer (dual gang if a stereo preamp was planned). The center and ground leads of this potentiometer must be connected to the power amplifier’s input.

Circuit diagram:

Parts:

Tone Control Module:

This Module utilizes an unconventional topology, still maintaining the basic op-amp circuitry of the Main Module with several modifications to the resistor values. A key feature of this circuit is the implementation of six-way switches instead of common potentiometers: this allows for the attainment of precise “tone flat” settings or preset dB steps in bass and treble boost or cut. Tone Control switches also facilitate more precise channel matching when a stereo configuration is used, mitigating the frequent inaccuracy associated with conventional ganged potentiometers.

Six-way (two poles for stereo) rotary switches were chosen for this purpose as readily 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 observed practice that tone controls are predominantly used for frequency boosting rather than cutting. Nevertheless, +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 desiring to utilize conventional potentiometers for Tone Controls may employ the circuit enclosed within the dashed box (bottom-right of the Tone Control Module circuit diagram) to replace the switched controls. The Tone Control Module should typically be positioned after the Main Input Module, and the volume control should be inserted between the Tone Control Module output and the power amplifier input. Alternatively, the volume control can also be placed between the Main Input Module and the Tone Control Module, as desired. Furthermore, the positioning of these two modules can be interchanged.

Circuit diagram:

Parts:

Note:

If this preamplifier is used as a standalone device, 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 3.3kΩ 1/4W resistor in series to the output capacitor of the last module (i.e., the module having its output connected to the preamp’s main output socket).

Technical data: