Standalone Tone Control

The tone control was a scrapyard project… A few leftover parts from previous projects were sitting in the workshop so the idea was conceived of building something useful to add to the big KT88 amplifier (featured in a previous entry). It was also my first attempt at an audio circuit on a PCB.

Update Aug 2023

The Standalone tone control on this page was a one-off project, it is currently not in production.

Since it has been consistently the most popular project on this site, in Sep 2023 we will be starting on a re-design of the tone control as a separate unit for individual purchase – watch this space!

There is still a practical application for a tone control, particularly with analog sources such as vinyl. In this case, the tone control was designed to sit between the RIAA phono preamp and the linelevel integrated implifier.

The circuit was taken from this site and then modified. The values in the treble arm were changed to make the lift and cut amounts symmetrical on treble and bass.
Running this circuit through LTSpice shows a symmetrical lift and cut, at the various setting positions of the two tone adjusters
Measurements using the function generator set to a sine sweep showed similar results, though the amount of boost and cut was less than LTSpice predicted, at ±10dB.
The project was constructed in an AliExpress aluminium chassis with the top panel removed and replaced with a clear acrylic / perspex cover, to allow viewing of the internals.
Also a bypass relay was added to each channel to allow a complete tone defeat if needed.

Designing the tone control presented the opportunity of designing a tube circuit on a printed circuit board.

By way of explanation: Traditional tube circuits tend to use so-called “point-to-point” wiring, rather than using a circuit board. However, circuit boards allow an item to be designed once and then built multiple times. So there are definite advantages.

In this case, an existing published circuit schematic was used, with some changes to make it more balanced. A power supply was designed – again using existing parts to hand, and a bypass capability was added into the design of the board.

The idea of placing all these electronics within view led to the replacement of the aluminium case top with clear acrylic, to allow easy viewing of the insides.

  • This circuit works extremely well; listening tests reveal a completely neutral sonic signature, and that is using the cheap Shuguang 12AX7 tubes (all I had to hand)
  • The circuit is “quiet as the grave” – hum and hiss are inaudible even with the amplifier on maximum volume and ears pressed right up to speakers

7 thoughts on “Standalone Tone Control

    1. Good question. The switch just connected the input and output together and disengaged the circuit, by way of a couple of signal relays. If power was off, the relays would close and it would automatically enter “bypass” mode.

  1. i purchased 3 of these banxandall units with the 12ax7 on ebay to add to my KT88 SE amp right after the volume potentiometer. the thing is, i have built the KT88 with a 6N1P in the preamp stage.
    the queston is: is this banxandall also a preamp? or are the tubes just for the tone? i hope you understand my queston correctly. could i theoretically go from this tonecontroller direct into the KT88? (pin 5) or is the signal comming out of the banxandall not enough to drive the power section?

    1. The tone control as shown on this page is unity gain. So if you put a 1v p-p signal into it, it will output the same level.
      It was not designed to swing the kinds of voltages you would need to drive a KT88.
      I would run your input signal straight into this circuit (load the input with 47K or so) and then run the output signal into the top of the volume potentiometer. 50K is fine. From there, take the signal into the rest of the amplifier.
      You could experiment with driving the KT88 straight from it, assuming your gain stage was before it, but you may need to use something better to drive the circuit with than a 12AX7 cathode follower if so – the K voltage is only about 18 volts. I’d look at a 6N1 or 12AU7 and aim for the K voltage to be around half the RMS voltage you need to run your KT88, plus about 20 volts.

  2. I have one Question about the parts you used.
    Are the potentiometer in this circuit linear or logarithmic ones?

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