Stereo tone controller with transistors

Build an active Baxandall tone control circuit. Only two transistors required for stereo controller. PCB provided.

The tone controller is a circuit inserted before an audio power amplifier. Its purpose is to allow the user to adjust the gain of specific frequencies of the audio spectrum. Tone controllers range from simple bass boost circuits to complex equalizers. The circuit presented here is a simple one, with a potentiometer which alters the lower third of the audio spectrum (for bass adjustment) and another one which alters the upper third of the audio spectrum (for treble adjustment).

The schematic does not contain a volume adjustment potentiometer, but that can be added before the controller input. The signal input level should not exceed 1 Vp-p. When both potentiometers are turned up to the middle (flat), the relative gain of the circuit is -1 to -2 dB. Therefore, with an input signal of 1 Vp-p, you get no less than 0.8 Vp-p at the output.

Built prototype of the stereo tone controller

Things get different when you turn the bass potentiometer to the right (boost). The maximum gain is about 18 dB at 40 Hz. That means an output of 8 Vp-p. In real world, most audio amplifiers will distort such high level signal. So, at maximum volume, remember not to turn the bass potentiometer to the maximum right. Same for treble. The maximum gain appears at the end of the audio spectrum, close to 20 kHz. Anyway, these are extreme frequencies, rarely heard in what you'll usually feed to an audio system. That is how Baxandall tone controller performs (the greatest gain modification appears at the extremities at the spectrum).

The circuit is very simple and consists of a passive tone control network (Baxandall type) wired into the feedback path of a single transistor amplifier. The schematic of a single channel is shown below.

Schematic of the tone controller (only left channel shown)

Right channel is identical, only part references are different. There is a parts list below. Note that the input of the controller is capacitor coupled and can be connected directly to an audio source or through a potentiometer. The output holds a voltage of 6.5 V (when the circuit is powered from 12 V) and must be connected to a capacitor coupled input of the amplifier.

Wiring to a volume potentiometer and amplifier (single channel shown)

The capacitor at the input of the power amplifier is mandatory. All schematics I have seen contained such a capacitor. But if it isn't there, you must add it. Anything from 2.2 up to 22 uF will do.

PCB of the tone controller (stereo)

The PCB of the stereo tone controller is 108 by 40 mm wide, with the potentiometers 75 mm apart. The A points, B points and C points must be connected with wires.

Here are all the parts you need.

Reference (L ch.)	Reference (R ch.)	Value, Info
C1	C2	47 uF, electrolytic
C5	C6	39...47 nF, film capacitor
C3, C7	C4, C8	2.2 nF, film capacitor
C9, C11	C10, C12	10 uF, electrolytic
C13	C14	47 uF, electrolytic
R1, R7, R13	R2, R8, R15	4.7 k
R3	R5	39 k
R4	R6	5.6 k
R9	R11	150...180 k
R10	R12	33 k
R14	R16	1.2 k
Q1	Q2	2N3904, general purpose NPN
RV1A, RV2A	RV1B, RV2B	100 k stereo potentiometer
INPUT, OUT	the same	2.54 mm pinheader or similar connector
+12V	the same	5.08 mm PCB screw terminal or similar

The above circuit may be added to an audio amplifier project. If built on a separate PCB, you must use separate leads from supply to the controller and the amplifier. The output of the amplifier must be kept well away from the tone controller and audio input. Each module should have only one connection to ground and all ground connections must converge in a single point (including loudspeakers if the case). The power supply voltage for the tone controller may be regulated with a linear 78xx IC (7809, 7812, 7815, even 7805 with zener diode to ground).

Resources

KiCad Project, with schematic and PCB
LTspice simulation file

References

Ray Marston, Bipolar Transistor Cookbook - Part 4, in Nuts & Volts Magazine, October 2003. Available from https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_4 (accessed March 9, 2019)
Austereo Control Amplifier (circuit no. 44) in Elektor 5 - Summer Circuits, July-August 1975. Available from https://www.americanradiohistory.com/UK/Elektor/70s/Elektor-1975-07-08.pdf (accessed March 9, 2019)
M.V. Thomas, Baxandall tone control revisited in Wireless World, September 1974, pg. 341. Available from https://www.harbeth.co.uk/usergroup/filedata/fetch?id=62450 (accessed March 9, 2019)
Cătălin Lăzăroiu, Corector de ton in Almanah Tehnium, 1989, pg. 65. Available from https://pro-electronica.net/tehnium-almanah1989/ (accessed March 9, 2019)