simple active tone control

[antonis]

The 820k feedback resistor used or biasing probably screws up the behaviour of the Baxandall tone control a bit. 

I'd say more than a bit..

A rough estimation of its apparent value [ 1 / (Q4/Q5 gain + 1), due to Miller effect ] tells me that it should be more wise for Q4 to be biased via a +15/0 V voltage divider..
Although there are designs with even lower biasing resistor value (like R12 below)..


https://keith-snook.info/wireless-world-articles/Wireless-World-1974/Baxandall%20tone%20control%20revisited%20DCD.pdf

[Lino22]

So the C11 cap raises the apparent value of TR2 collector resistor, by which it helps to raise the input Z of the TR2 stage for the bax signal no to bleed?

[antonis]

No.. 

C11 raises R15 Value (not the total Collector resistance) for higher open-loop gain, hence more predictable closed-loop one..
(see what happens for an inverting configuration op-amp, disregarding the fact that Tr2 Base could hardly be considered virtual ground..)

Tr2 Zin raising indeed exists but should be considered negligible..

[PRR]

I'd say more than a bit..

Compare it to the ~~50k+10k of the audio path feedback network. 820k ~~ 10X 60k. Someone has cleverly held the nominal error down to about 1dB.

There is another error b/c the open-loop gain of one transistor is not infinite. Typically another part-dB at "flat" and several dB at "+20dB boost".

If you really care you will throw the 23 cents for TL072.