AMZ Tone Control Mods

To get a better idea of what is happening with the tone control, we can examine the response at the extremes of the tone pot rotation.

When the tone knob is all the way to the treble side, the gain stage is essentially as shown here. The 20k variable resistor is essentially out of the circuit so we can drop it for clarity. Note that the 1k/0.22 RC low pass network on the input side is always in the signal path and it rolls off the highs above 720 Hz. This is part of what makes the TS so mid-range heavy. However, since the clipping stage is ahead of the RC network, it does help to take some of the fizz out of the fuzz.

If we roll the tone control back to the bass side, we get a circuit much like this example. The signal path is now heavily filtered by the 220/0.22uF pair that is now positioned in parallel with the input RC network. The low pass corner frquency is now at 360 Hz., but with a bit of a kink in the response caused by the 220 ohm resistor. This is heavy filtering for any guitar signal.

In this novel example, the parts are all the same values as in the original but by moving them around, a different tone control circuit is formed. Compare the modified circuit to the first schematic on this page.

You will see that the 0.22uF near the input is no longer directly grounded but instead is connected to the tone control pot and grounded through the wiper and its resistor.

The deep low pass filtering is no longer present as the control is opened up.

Another advantage of this tone control is that we now have independent control over the low pass (C1) and high pass (C2) filter components. In this version, the C1 capacitor is made smaller so that there are more highs passed, even when the pot is turned to the bass side. This opens up the tone control and allows more useful range.

Another way to modify the response of the tone control is to change the resistor values. This example has had all of the resistors altered to illustrate.

The values shown make a perfectly acceptable tone control.

There is more gain on this version when the treble is boosted and the opamp may be driven into distortion, which adds a bit of grit to the sound.

A further rearrangement of components gives one of my favorite versions. The resistor on the wiper of the pot has been moved to the non-innverting opamp side of the control and the wiper connects directly to ground.

The advantage here is that the bottom end of C1 can go directly to ground and give complete rolloff without a shelf in the response.

All of these tone control networks work well with overdrive pedal designs and can be a pleasant alternative to the stock TS-9 type control, especially where it is desired to removed some of the mid-range heavy character.

Lagniappe -noun An unexpected benefit or bonus (frequently used in south Louisiana)

Our lagniappe is a quirky clipping circuit derived from the modified tone control.

Give this circuit a test if you are looking for an out-of-the-ordinary clipping signal!

Update! As I played around with some variations of the tone control, I decided to toss in a 500mH (0.5H) wah inductor to see what kind of response could be had. When the inductor is placed in the potentiometer's wiper-to-ground circuit, a variable midrange filter is made.
The first image here is with the tone knob turned all the way to the bass side. A -12db notch is made in the frequency response at around 480 Hz.

With the tone knob to the treble side, a mid peak of +9db is produced at the same 480 Hz.

The center frequency (resonance) of the RLC network is calculated as follows:

F= 1000/(6.28*sqrt(L*C)), where L is in H and C is in uF (C=C1=C2)

For our example, F = 1000/(6.28*sqrt(0.5*0.22)) = 480.1 Hz

The tone control is not limited to just the wah inductor. On the Lab Notebook Tips page, I have a chart of the inductances of several small transformers available from Mouser. The primary or secondary of one of these transformers could be used as the inductor in this circuit for different resonant frequencies and responses.

Since the previous circuit lost most of the control over the high frequencies when it was modded, I decided to reintroduce that back into the new tone control by adding a single capacitor in parallel with the inductor.
Notice that the high frequency response goes up and down, but we also have the mids tracking in parallel.

With this version, we are controlling the highs and the mid-range band with a single knob!

The response will be flat when the tone is in the middle position.

A more complex control such as this one can add some unique character into an otherwise bland fuzz sound.