Simple Tone Control

Specifically designed for the 3 - 5 Watt Class-A Audio Amplifier

Traditional Bass and Treble Controls - 24V supply

Circuit diagram:

Class-A Tone Control Stage


P1______________47K  Log. Potentiometer (See Notes)
P2,P3___________47K  Linear Potentiometers

R1,R3,R5_________4K7 1/4W Resistors
R2______________22K  1/4W Resistor
R4_______________1M  1/4W Resistor
R6_______________1K8 1/4W Resistor
R7_____________560R  1/4W Resistor

C1,C4,C5,C7_____10µF  63V Electrolytic Capacitors (See Notes)
C2______________47nF  63V Polyester Capacitor
C3_______________1nF  63V Polyester Capacitor (See Notes)
C6_____________220µF  35V Electrolytic Capacitor

Q1____________BC550   45V 100mA NPN Low noise High gain NPN Transistor


A Bass and Treble frequency control to be added to the 3 - 5W Class-A Amplifier was required by some audio enthusiasts. Therefore, this circuit has been designed keeping in mind the extreme simplicity of the amplifier circuit to which it should be linked and was carried out using as few as components possible.

Q1 is the only active component forming a straightforward single-stage transistor amplifier with the tone control network in the ac feedback path. Taking this feedback from the split load of Q1 we obtain an ac stage gain of about 3: this can be useful to cope with low output voltage audio sources.

Technical data:

Input sensitivity:
160mV RMS for 0.5V RMS output
Input impedance @ 1KHz:
Volume control knob to 12 o'clock = 40K
Volume control knob to 2 o'clock = 19K
Maximum output voltage:
4.5V RMS into 10K load
Frequency response:
flat from 15Hz to 23KHz
Total harmonic distortion @ 1V RMS output:
1KHz = 0.008% 10KHz = 0.01%
Total harmonic distortion @ 4V RMS output:
1KHz = 0.06% 10KHz = 0.05%
Bass control frequency range referred to 1KHz:
+11.6dB/-8db @ 100Hz
Treble control frequency range referred to 1KHz:
+7.7dB/-7.3dB @ 10KHz