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 ?
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.