A usable tone control

[Michael Bean]

Tone controls are the bane of true audiophiles, or so they say. And if truth be told, there's some real justification for that sentiment. Most traditional tone controls are based on the Baxandall circuit, which in my opinion leaves much to be desired as far as sound quality goes. I've never heard an implementation of that type of tone control that didn't seem to muck up the sound in some way that made things worse. And considering the way the phase and frequency response are treated, and with the inter-channel variation caused by crappy-cheap potentiometers performing thier special kind of "magic", it's no wonder most prefere no tone controls at all.

But inspite of all that, I've always felt the need to have some way of correcting the response aberrations that are almost inevitable given the fact that the recording artist and/or engineer has no way of knowing what the environment that the recording is being played back in will be like. So I've put together a circuit that I believe is functional and good sounding while avoiding the pifalls and shortcomings of the usual designs.

First, the range of adjustment is limited to +/- 7.5dB using 1.5dB switched steps, I can't think of any reason why anyone would need the 15 to 20 dB boost or cut that most controls provide. Using rotary switches instead of pots also avoids the inter-channel variation thing, and allows using a center position to ground to completely disable the tone control action when it's not needed.
Second, instead of the usual 1kHz center point, I've put this one around the mid-600Hz range, which is actually the middle of the audible range of the Human ear.
Third, the slopes are shallower, which makes the adjustments much more subtle but still effective, and also causes less phase shift. There's also a provision for adjusting the turnover frequencies if desired. The final result is a tone control that is usable and won't cause more problems than it solves.

Below are the frequency response plots, schematic and bill of materials.

Tone Control Amplitude Curves.jpg
Tone Control.JPG
Tone Control BOM.txt

Mike

[PeteMcK]

I like the adjustable turnover freq capability. I usually change the turnover freq of the bass control on any amps I have to 250Hz, that seems to solve the muddiness that the 1Khz turnover point gives rise to.

[Michael Bean]

Yeah, that's a big problem with the tone controls in many systems. In addition to the built in trimmer adjustment, it's also easy to replace just one cap in the filters to change the range of adjustment if desired.
Woops, I just noticed that I put R1 on the wrong side of C1. Oh well, can't be perfect all the time.

Mike

[jerryo]

A pcb for this would seem to be the next step............nice!

[Michael Bean]

Hi Jerryo,

A PCB is certainly more professional looking, but there isn't any reason building it on perfboard would be a problem if one were so inclined. The circuit uses very modest gain levels and is inherently stable, so as long as good layout and construction techniques are used, it's all good. Probably the hardest part of building this is soldering the resistors to the rotary switches, everything else is pretty straight forward and relativly easy. Also, the 1k resistors shown on the non-inverting inputs to ground on the input and output summing amps are there for current limiting bi-polar input OpAmps like the LM4562 to minimize turn-on thumps, they aren't really necessary for FET OpAmps like the OPA2132 shown.

Mike

[nigelwright7557]

I have found instability problems with tone controls especially the treble starting to feedback and oscillate.

With all those steps it might be worth looking at a digital pot with 128 or 256 steps.
A push button for up and down could change the pot value.

I am currently working on a USB controlled mixer that drives digital pots. The next step is a tone control using a digital pot.

[Michael Bean]

No instability with this one. With the minimal gain of this design, oscillation is unlikely. Also, my goal was to build something simple and functional that would sound just as good using it as not. I achieved my goal.

Mike

[stratus46]

Originally Posted by Michael Bean

No instability with this one. With the minimal gain of this design, oscillation is unlikely. Also, my goal was to build something simple and functional that would sound just as good using it as not. I achieved my goal.

Mike

Looks pretty nice but is the step size a little coarse?

G²

[bcmbob]

Michael, It is my understanding the LDR attenuators also eliminate many of the traditional problems of volume controls. Is it possible/practical to incorporate a gain component into a LDR V control?

[Michael Bean]

Stratus46, the main reason I went with 1.5 dB steps was because I found that it seemed like the best compromise with the shallow slope filters. No reason one couldn't use more steps if desired.
Bcmbob, LDRs would certainly work, but you would lose the center position defeat feature, I suppose you could add a defeat swich if you wanted to go that way. But one of my goals was simplicity and ease of construction. Rotary switches work well here and are simple to use.

Mike