The Tone Control project was an opportunistic project, conceived out of a desire to utilise some spare parts, and to experiment with building an audio circuit on a PCB, instead of using point to point.
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.
- 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
Waiting to be done: Distortion and noise floor measurements.