Audio Preamplifier with Tone Control

The Audio Preamplifier Circuit sensitivity and overload margin were designed to cope with most modern music programme sources like CD players, Tape recorders, iPods, Computer audio outputs, Tuners etc. The source selecting switches and input connectors are not shown and their number and arrangement are left to the constructor's choice.







To obtain a very high input overload margin of the audio preamplifier, the volume control was placed at the preamplifier input. After a unity gain, impedance converter stage (Q1) a negative-feedback Baxandall-type Bass and Treble tone control stage was added. As this stage must provide some gain (about 5.6 times) a very low noise, "bootstrapped" two-transistors circuitry with FET-input was implemented. This stage features also excellent THD figures up to 4V RMS output and a low output impedance, necessary to drive properly the Mini-MosFet Power Amplifier, but can also be used for other purposes.

Audio Preamplifier Parts List:

R1=220K 1/4W Resistor
R2=100K 1/4W Resistor
R3=2K7 1/4W Resistor
R4,R5=8K2 1/4W Resistors
R6=4K7 1/4W Resistor
R7,R8,R13=2K2 1/4W Resistors
R9=2M2 1/4W Resistor
R10,R11=47K 1/4W Resistor
R12=33K 1/4W Resistor
R14=470R 1/4W Resistor
R15=10K 1/4W Resistor
R16=3K3 1/4W Resistor (See Notes)

C1,C2,C9=470nF 63V Polyester Capacitors
C3,C4=47nF 63V Polyester Capacitors
C5,C6=6n8 63V Polyester Capacitors
C7=10µF 63V Electrolytic Capacitor
C8,C10=22µF 25V Electrolytic Capacitors
C11=470µF 25V Electrolytic Capacitor (See Notes)

Q1,Q3=BC550C 45V 100mA Low noise High gain NPN Transistors
Q2=2N3819 General-purpose N-Channel FET