One may wonder if the Baxandall’s association with Quad went back to the time when the Quad II was in development.
The original Q.U.A.D. of 1951 (retroactively the Quad I) had a passive tone control system similar to that used in the QA12/P of 1947:
The QCII, released I think in 1953 September, had an active tone control system. I have endeavoured to extract the essence of this circuit (from the complex-looking actual circuit) in this hand-drawn diagram:
The tone controls were incorporated in the control unit output stage, which was built around a triode pair (ECC83) and so was non-inverting. The feedback was of the series type, fed into the cathode of the first triode. This stage provided quite a reasonable amount of voltage gain, taking an input of 100 mV to a final output of 1.4 V, thus 23 dB. As there was some loss in the output network, the actual output gain would have been higher than 23 dB. Here are the published curves:
Quad described the QCII tone control thus:
“The continuously variable bass and treble controls are so arranged that the central position of their travel will always give a level response with virtually no error. The bass rise operates by varying an impedance in the feedback circuit of the two final stages and bass fall is obtained by varying an impedance in the top arm of the feed to the filters. Those two impedances are of like value so that a linear bass control is used. This is advantageous because a linear control does not appreciably change its scaling and any change in overall value mill be symmetrical. Further. the two fixed capacitors are small and of like value, thus easy to match and extremely unlikely to drift.
“In the maximum rise and fall positions. the maximum slope is 5db/octave and in intermediate settings the slope is gradually reduced to zero and at the same time the turnover point reduces in frequency. This is preferred to the step obtained by variation of amplitude or the constant slope obtained by variation of capacity alone.
“The treble control operates both rise and fall in the feedback circuit. The slope is always asymptotic to zero so that a boost in musical brilliance in the treble musical register is not accompanied by boosted distortion at very high frequencies, nor is a reduction in the treble musical register accompanied by a complete loss of harmonics. This feature of musical balance control is. of course, only made practicable because of the independent filtering controls provided.”
The bass control looks to me as if it is approximately where one would arrive if the objective was to adapt the Baxandall control for use in the series feedback loop of a non-inverting amplifier. In the Baxandall case, the tone control potentiometers are partly in the input arm and partly in the feedback arm, where they effectively act as frequency-selective active gain controls. In the series feedback case, the input arm and the feedback arms do not connect (in a signal sense), but the output arm and the feedback arm do, so the tone control potentiometers could be place there.
So perhaps the QCII bass control at least was an adaptation of the Baxandall circuit. The latter was published in 1952 October, although Baxandall is on later record as saying he developed it in 1950. So the timing would be right for the QCII tone control to have been a derivative. The QCII/II probably had a longish gestation period. For example, Quad had asked G. Horn to develop an FM tuner to match the QCII/II (as well as an AM tuner), and the prototype of that was first shown in mid-1952. (The final design work for the production version – released in 1955 - was done by J. Collinson of Quad.)
I imagine that Quad’s use of a treble control entirely in the feedback loop was done as the best way of achieving its desired curves – with some eventual flattening. A pointer to that may be found in an unusual place, namely Stephen Spicer’s excellent book on the history of Leak. On page 133 the Baxandall tone control (adopted by Leak in 1954) was discussed, and there was a side panel contributed by Australian audio enthusiast Peter Stinson, from which I quote:
“An important characteristic of the Baxandall circuit is the sliding turnover point, which differentiates it from the fixed hinge point of conventional systems. However, Peter Walker would say, and I would agree with him, that while the Baxandall circuit is almost ideal in the bass, it is not so well suited to treble, particularly treble boost, where it can result in excessive boost at the extreme top when only a little mid-top lift is wanted”
Actually, with the Baxandall tone control, some flattening of the treble curves could be obtained by omitting the treble potentiometer centre-tap of the 1952 circuit. This was a later refinement, probably not available when the QCII was designed. However, the Quad 22 of 1959, whilst having essentially the same bass control as the QCII, had a different form treble control, one that was partly in the output arm and partly in the feedback arm, thus making it analogous to the bass control, and so perhaps the series-feedback analogue of the Baxandall variant without the centre-tap. Here is the hand-drawn simplified circuit:
And the resultant curve-set:
The Quad description was as follows:
“Bass and treble control is effected in two frequency discriminating networks of identical impedance, one in the signal chain and one in a feedback chain. Boosts and cuts are therefore symmetrical. The bass control varies both slope and turnover. The treble operates midway between variable step and variable slope in order to facilitate adjustment of musical brilliance while maintaining natural harmonic balance.”
That description of the treble control is, I think, consistent with the curves provided by the non-centre tap Baxandall.
Cheers,