The tone control Bass Mid Treble referred to herein based on the use of an integrated circuit OPA ( Operational Amplifier ) twice LM358 type, which can be replaced by a TL072, TL082 or an NE5532. The mounting supply is a simple type (not symmetrical), the value must be between 12 V and 18 V. The scheme presented here is a monaural type, it will be enough to build two copies for one version stereophonic.The realization requires no development, the operation must be immediate.
It is possible that you find it a little complicated pattern, and it is true that one can be easier with a fully passive circuit if one accepts a loss level.
Break this pattern into subsets to better understand how it works. The first part is the input stage focused on the first half of the LM358, which is, therefore, an operational amplifier, wired here as an inverter. The second part is the tone control itself, made up of three potentiometers settings and the second half of the LM358, the second operational amplifier, located to the right.
The input stage allows to attack the tone corrector in good conditions, it helps to have a high input impedance and has low output impedance, allowing an optimum impedance matching. This stage causes a phase inversion, the signal at the output of U1: A has undergone a polarity inversion (180-degree phase of rotation) with respect to the input signal. This poses no problem, and we will see that this signal suffered another reverse polarity in the output stage, thus giving it its original phase.
The corrector is the shelving type, mounting encountered much for two-way tone corrector or three-way (B / M / T). It consists of a filter which is placed in the feedback circuit of the amplifier centered on U1: B, and which can amplify or attenuate more or less strongly Grave frequency components (via VR1) those of center frequencies (VR2) or of high frequencies (RV3). The amplifier U1: B causes also a phase inversion, the phase of the output signal Out is, therefore, identical to that of the input signal In.
A simple power supply (not symmetrical) is required here. The two resistors R12 and R13 are used to create a point intermédiaired’alimentation, whose value is half of the supply voltage. This intermediate voltage is a virtual ground , denoted here Vbias, which is wired to the noninverting input of the two operational amplifiers. This procedure causes the appearance of a DC voltage equal to Vbias final output, this DC component is blocked by the coupling capacitor C9.
Choice of potentiometers
The proposed scheme is a monaural type, it makes sense to opt for simple potentiometers:
However, if you plan one day or another the switch to stereo version, so opt immediately for type double potentiometers, such as shown in the photo below (you do not have to take exactly the same these are two examples).
In the end, so you have three double potentiometers for application in stereo:
– One for the left Bass and right Bass channel setting (RV1 RV1 + ‘);
– One for the left Mid and right Mid channel track adjustment (RV2 RV2 + ‘);
– One for the left Treble and right Treble channel (RV3 RV3 + ‘);
It is preferable to change in the same proportion, the frequency response curve of both left and right channels simultaneously. In summary, do not take simple potentiometers that if you intend to make a monaural correction, or if you absolutely have independent settings for both channels of a stereo version of the tone control.
A simple power of + 12V to + 18V is required.