How to Make Tone Controls for a Stereo Amplifier

How to Make Tone Controls for a Stereo Amplifier
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Music and audio reproducing devices like DVD, CD players and amplifiers may appear quite incomplete and “monochrome” without external enhancement facilities attached. These facilities primarily are the sound equalizing features which enable the output sound to be shaped and enhanced so that maximum desirable effects may be produced for a particular listener or perhaps an audiophile.

Basically the above feature may include bass and the treble controls. The bass control is nothing but a low pass electronic circuit which filters and allows only the specified low frequency signals to pass into the amplifier stage and blocks other irrelevant frequencies.

Switching to higher bass levels thus produces sound outputs which are “heavier” in response like drum beats, etc. The treble control produces just the opposite effects, at higher levels the treble circuit will sweep out all signals which are below the specified frequency range and suck-in only the high pitched sounds, so it’s no surprise that it’s also called the “chiller” control circuit. With this control at max, the inputs consisting of sharp sound contents like chirping of birds, guitars, violins become predominant at the amplifier output.

Usually a mix of both bass and treble effect is preferred by sensible listeners. However many audiophiles are not just satisfied with these couple of controls and go for much advanced multistage equalizers incorporations, which may have facilities of introducing pin-point accuracies with trimming or raising sound characteristics and therefore may tend to convert and generate extravagant sound effects at the speaker outputs.

In this article we won’t discuss the multi stage tone control circuits though, but the simpler two stage ones which are easier to build, yet enable quite comprehensive tailoring of the fed sound inputs. Let’s discuss them one by one:

Circuit Details

The first diagram shown below is perhaps the simplest of its kind. It uses just one transistor as the active part and a few other common passive tone control components.

T1 used is a low noise type general purpose audio transistor, along with tone optimization; T1 also provides a certain degree of pre-amplification to the applied audio input.

The circuit may be introduced in between an input audio source and the external power amplifier.

The pot at the bottom is the treble control, gives a cut of 15 dB at 100 Hz, while the upper control which the bass adjust knob, provides a boost of 15 dB at around 15 kHz.

The next tone control circuit uses an IC 741 as the main active part and provides comparatively better response than the previous circuit. The circuit configuration is basically a Baxandall type, which also includes bass and treble control facilities through the pots VR1 and VR2 respectively.

Active Tone Control Circuit Using IC, Image

The above two circuits if combined together can produce some outstanding results as far as sound enhancement is concerned. The figure below shows how it’s done by connecting the inputs and outputs of the two circuits in series. The configuration provides you with four discrete controls enabling very sharp and intense sound frequency optimizations, however with the bass at full thrust may cause considerable heating of the amplifier devices.

Active Tone Control Circuit Using IC and Transistor Stages in Series, Image

The transistor circuit operates with a single power supply; the IC circuit will however require dual supplies. The voltages may be anywhere between 6 to 12 volts DC for all the above circuits. The configurations shown above are for a single channel, two identical sets may be used for enhancing both left and right tracks with stereo amplifiers.