Tone controls are often an important feature of most audio amplifiers, and these are generally available as bass and treble controls, competent at delivering around 12dB approximately of boost or cut above their specific frequency bands.
While these are extremely straightforward circuits, numerous tone control layouts apparently reveal rather unconventional control features when you do a few mindful inspections with them! The thing is typically an absence of symmetry in the boost/cut characteristic.
This may not be truly a dangerous drawback, however it does imply that the key control configurations is not going to provide the required flat frequency response. One reason behind the issue is that several tone control circuits tend to be passive types, and they are as a result determined by having appropriate source and load impedances. Glitches in either could result in undesirable variations in the responses of the tone controls.
Passive Tone Control
Figure below exhibits the circuit diagram of a basic passive tone control which might work fairly good given that the signal supplied to it is from a low source impedance and passes into a relatively high load impedance.
Due to the passive characteristics of the circuit, it isn't actually appropriate to assess in terms of the controls delivering boost and cut. Such designs will always present losses, and if configured to offer bass or treble boost it is in reality delivering diminished losses instead of a authentic boost to the signal level. This may not be strictly academic, and the technique in general has to be built to consider the fundamental loss of around 12dB associated with such designs.
Active Tone Control
It is generally advised that instead of messing with passive circuits, it is better to go for an active tone control circuit like the one demonstrated in the following circuit diagram.
This is simply a passive tone control hooked up from the feedback circuit of a non-inverting op amp amplifier, along with an input buffer stage to ensure that the primary tone control circuit is operated through a appropriately low source impedance.
This offers a kind of inverted results, in which boost from the tone controls supplies increased feedback and decreased gain, while the cut from the controls offers reduced feedback and increased gain. If the two potentiometers are connected considering these factors, it might be able to deliver the right results through the controls (meaning clockwise movement of the pot will produce boost; and anti-clockwise rotation will deliver cut).
The indicated tone control circuit supplies slightly above 12dB of boost and cut within the opposite limits of the music range.
Simplified Tone Control Design
The next figure below exhibits the circuit diagram of a simplified Active Tone Controls using a single op amp, which is a standard set up possessing bass control VR1 and treble control VR2.
When the wiper arm of VR1 and VR2 are turned fully towards left side, it results in maximum feedback, producing full bass and treble cut.
When the wipers are moved at the opposite side of their rotation we get lowest feedback and thus highest bass and treble boost. The controls don't have any substantial impact at central audio frequencies (around 800Hz), and offer highest boost and attenuation value of approximately 12dB.
The total level of cut and boost is actually offered at the two extremes of the music frequency range, and 12dB is around the maximum that will actually be required in real life use.
1C1 is wired in the inverting mode, and its non-inverting input is as a result easily biased to 50 % of the supply voltage through R1 and R2. C2 is used for decoupling any noise that might normally be supplied to the non-inverting input through the supply rails through R1 and R2, or picked-up because of to stray coupling.
The amounts of noise and distortion generated by the circuit are minimal, even while the pot controls are adjusted for getting highest possible boost (which still allows the the circuit to have just an extremely minimal level of voltage gain).
As soon as one or both of the tone controls are adjusted for a cut, IC1 offers a closed loop gain of lower than unity. Using certain inside compensated operational amplifiers a closed loop gain of smaller than unity may trigger instability, and the internal compensation is simply for closed loop voltage gains of unity or higher.
A number of TL081 CP ICs had been experimented with in the circuit and zero complications with instability were encountered.
The circuit may additionally perform nicely using a 741C IC, and in real life use is unlikely that any kind of recognizable drop in efficiency would actually be noticeable employing IC 741. However, the noise and distortion quantities will probably be somewhat greater compared to the IC TL081CP.
The next figure below displays the estimated frequency responses from the a couple of control pots, when they are arranged for highest boost and cut. These responses include pretty excellent symmetry, and the circuit offers a characteristics that's very near to a flat response once the pots are adjusted on the center location.
Having said that, remember the potentiometer tolerance can be pretty extensive at around 20%, and that the physical center point of adjustment can be impossible to be the correct electrical center-point.
Any kind of errors caused by this situation within the theoretically flat frequency response could be rather insignificant though. Building of the tone controls provides hardly any issues.
The voltage gain is so minimal, that even if the control is adjusted for getting optimum boost, there could be absolutely no risk of instability.
How to Connect the Pots
Make sure that the pinouts of the pots controls are connected the right way round.
Referring to the two op amp tone control circuit, treble control VR1 delivers boost when its wiper is moved towards the C3 end of the rotation, or oppositely the cut is set when the control is rotated towards the C6 end.
In the same way, bass control VR2 offers boost when the pot wiper is moved towards the R3 end of the rotation, or oppositely the cut is set when it is adjusted towards the R6 end of the rotation. Observe that you will find a little amount of voltage gain of approximately 5X from the circuit at the 0dB reference level.
3 Channel Tone Control (Bass, Treble, Presence Controls)
The next concept explains a 3 channel tone control circuit, which can be used for generating bass and treble control responses and in addition to this, the circuit can be also used to produce presence control, or the mid frequency control.
The input music signal is applied through connector SKI to the 1st opamp stage configured around IC1. This is wired as a non-inverting amplifier with a gain that is fixed through the ratio of resistors R3 and R1. For this 3 channel tone control circuit the gain is fixed at unity.
The first op amp stage needs to be kept isolated from the next stage due to prevent loading effects. The ICI output is applied through 3 frequency shaping circuit configurations to IC2. The three tone controls are constructed around the pots RV1, RV2, RV3 which additionally form the part of the feedback path of IC2, which is configured like another inverting op amp stage.
The parts used around the three potentiometer are selected such that it delivers the intended frequency and tone control results.