DIY Audio Projects Forum

Simple Tone Control
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Author:  Johnny2Bad [ 09 Feb 2017, 07:01 ]
Post subject:  Simple Tone Control

Hello All.

Although my topic title is "Simple Tone Control" what I am seeking is not a tone control as we normally might think of one ... that is a potentiometer that offers some variable amount of bass and treble boost / cut ... maybe +/- 10 dB @ 200 and 8 KHz, for example.

What I want is something much more subtle, and if possible, much more transparent; something that does not advertise it's in the circuit versus no tone adjustment at all.

If anyone is familar with the AMCL QUAD preamps, like the 33, or various Luxman preamplifiers ... they used this a lot, actually, but for argument's sake, say the 5C50 Lab Reference Series (solid state), or CL32 (vacuum state) preamps. Both examples had the feature I'm thinking of with a very good overall Sound Quality (SQ) when engaged or disengaged.

The control consisted of a knob controlling a switch with five positions; I'll refer to these as P1, P2, P3, etc.

The middle position (P3) offered no adjustment; in other words the "flat" control output, possibly this could be a "Tone Out" position as well, with the adjustment circuit out of the signal path. If the control was transparent enough, I'd be OK with either.

The four other positions offered a subtle degree of tonal adjustment. Using the leftmost position of the switch (let's call it P1) we might have +1 dB @ 100 Hz, Centred at 1 KHz (0 dB), and -1 dB @ 10 KHz. A "Down Tilt" position, if you will.

The second (P2) having a similar slope but adjusting the two extremes by 0.5 dB, and the two right-most positions (P4, P5) making the same level of adjustment, but the other way, two "Up Tilt" positions, one - 0.5 dB @ 100 Hz and + 0.5 dB @ 10 KHz, and the other (P5) making the same adjustment points but with -1 dB @ 100 Hz and + 1 dB @ 10 KHz.

Some might thing this too subtle to be useful, but having had experience with both QUAD and Lux gear in this regard, I can assure you it is the perfect way to make subtle tonal adjustments for, say, loudspeaker or phono cartridge characteristics, or even room acoustics. The maximum adjustment is 2 dB overall, and it is quite audible.

Virtually all the preamp circuits I've seen, and I have looked, utilize the usual Bandaxal circuits or some other rather aggressive level of adjustment, and in many cases skewing the frequency response with what amounts to three bands of tonal adjustment, notably the middle (second ) band being some arbitrary width and consisting of 0 dB change, regardless of what the extremes were set to provide.

I would like something more linear; if you drew it up on the typical log frequency scale, it would look like a straight line with a centre of (perhaps) 1 KHz, rather than the U-shaped curve you get with conventional tone controls.

My gut instinct is I should be looking at perhaps the type of circuit used in an RIAA equalization curve, or ?

Any ideas?

EDIT: Perhaps the switch positions should be reversed from what I described above? So that if your tone control switch was a bar type (like on the QUAD 44) instead of a knob, the bar would visually represent the frequency response curve. A minor detail at this stage, but I like it, and there's no point in doing it wrong right off the bat.

So P1 becomes (-1 dB @ 100 Hz / + 1 dB @ 10 KHz). If you visualize a bar control "knob" with the control markings on the left side of the arc of the switch's movement, and the "Flat" position at P3 being straight across, then P1 would be tilted low on the left and high on the right, just as the frequency curve would look.

[ / ] P1 Uptilt -/+ 1 dB
[ - ] P3 Flat
[ ] P5 DownTilt +/- 1 dB

EDIT Again: For some reason, the backslash character doesn't show up in my post. It's supposed to go between the brackets at P5 in my edit above.

Author:  Peter W. [ 09 Feb 2017, 09:56 ]
Post subject:  Re: Simple Tone Control

How about an equalizer? That is the snarky answer.

Now, you have defined the problem, and with it, the solution - with a little bit of cleverness and thought.

We can agree that said tone-controls add or remove resistance around a center point to achieve what they do.
We can agree that the amount achieved is directly related to the resistance added or deleted.
We can agree that 1% resistors come in many, many values.
We can agree that 'blank' stepper controls are out there where the user may install resistors of his/her choice.

By experimentation, add the correct resistors as needed to each control. Center-out may either be no-resistor, straight wire, or unity resistor for null gain/loss.

Given the variations in steppers out there, you may be able to combine both functions on a single control. -3,-2,-1,0,+1,+2,+3 being as you describe. Then -4 to -8 and +4 to +8 being more conventional and aggressive settings.


Author:  gofar99 [ 09 Feb 2017, 14:59 ]
Post subject:  Re: Simple Tone Control

Hi, I agree on the range of adjustements as being the most useful. In my main system I use none, and none are needed. In my shop system (Marantz gear and compact speakers) I use only the first click on the controls for fine adjustment as the speaker setup is not optimal. There is a free program that may help you determine the values you want. It is for guitar amps...but will will work for hi-fi. It has several different types of controls and allows you to enter the values to see what happens. It also plots the changes on a display. It is called Tonestack. If you can't find it let me know and I look up the source.

Good listening

Author:  Johnny2Bad [ 09 Feb 2017, 17:55 ]
Post subject:  Re: Simple Tone Control

Thanks Bruce I will look into ToneStack.

As an FYI, there is the one I believe you are referring to, known as "the Tone Stack Calculator" found at Duncan's Amp Pages, and there is a commercial modelling application for iOS called ToneStack ($10) by Yonac (that's it, just "Yonac"). It actually looks pretty cool, but first things first.

The System Requirements for TSC are " Windows 95, Windows 98 or Windows NT. "

I will install Duncan's app in my Virtual Machine for WinXP Audio and see what happens next. ... 69510?mt=8

Author:  gofar99 [ 09 Feb 2017, 20:46 ]
Post subject:  Re: Simple Tone Control

Hi, the Duncan one was the one I had in mind.

Good listening

Author:  Johnny2Bad [ 16 Feb 2017, 14:31 ]
Post subject:  Re: Simple Tone Control

Peter W. wrote:
... (snip) ...
We can agree that 'blank' stepper controls are out there where the user may install resistors of his/her choice.

Provide an example of a stepper control ... whatever that is ... that "is out there".

Author:  Johnny2Bad [ 15 Mar 2017, 07:47 ]
Post subject:  Re: Simple Tone Control

Upon further reading, it seems a circuit whereby the tone control is part of a feedback loop is how this might be implemented. Any ideas someone would like to offer, say, a sample circuit? Vacuum State would be a bonus.

Author:  Peter W. [ 17 Mar 2017, 06:17 ]
Post subject:  Re: Simple Tone Control

Johnny2Bad wrote:
Peter W. wrote:
... (snip) ...
We can agree that 'blank' stepper controls are out there where the user may install resistors of his/her choice.

Provide an example of a stepper control ... whatever that is ... that "is out there". ... ttenuator/

Author:  Johnny2Bad [ 04 Apr 2017, 22:35 ]
Post subject:  Re: Simple Tone Control

Peter: Thanks for the link; I had assumed you meant something other than an attenuator.

RE: my Original Post:

Just to update, I've found some more information online since first posting this topic. It's always worth repeating that the right search terms do actually produce results. I had previously come upon dead ends, but persistence pays off.

Some may find the following useful, I certainly did.

A good discussion of the tilt control can be found at the following links: ... zer-filter

And a PCB/Parts kit for $US 40 to implement the above passive tilt control:

1.5 dB steps at the cost of a -3dB gain reduction:

1.0 dB steps at the cost of a -2dB insertion loss:

Now that we have some more concrete examples to work with, maybe a few comments / ideas / criticism can commence. I haven't spent much time digging into the problem, just read through quickly, but it's enough to have me believe this is doable and possibly practical to implement in a vacuum state preamp project.

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