Quote:
Originally Posted by Ted Kendall
Judging by John Crabbe's Hi Fi In The Home, there was probably a good case for flexible filtering up to the late 1960s - he describes its use for mitigating HF distortion in some detail - but the general improvement in pickups triggered by the Shure V15 caused this need to recede pretty quickly thereafter. There is nothing subtle about the general competence of an M75/6 compared with that of the M3D of ten years before. Perhaps the greater preponderance of electric music over orchestral and particularly choral material in the body of hi fi users contributed as well.
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In respect of the need for flexible low-pass filters, perhaps to some smaller extent offsetting the improvement in pickups were the general improvements in loudspeakers, which probably made some of them at least more revealing. Although one could argue that speakers in general were moving to a “revelatory” level already reached by the Quad electrostatic.
I have a vague recollection – although I can’t trace the reference - that when the Quad 44 was released in 1979, PJW was asked about the retention of the established Quad filter system; I think his answer had a hint (fractional) of ambivalence. Perhaps that pointed the way to the simplification that came with the Quad 34 of 1982, which approach was retained with the Quad 66 of 1989. Nonetheless, it does not appear to have been simply a case of inertia, as the Quad 44 incorporated a major change in tone controls with its tilt and bass lift/step controls. Evidently quite a lot of thought had gone into what “frequency bending” controls were desirable in 1979. Ambler had developed his “tone balance” control (of which the Quad tilt control was a derivative) for use in addition to conventional Baxandall bass and treble controls, but Quad abandoned the treble control and replaced the conventional bass control by a different kind. One could reasonably deduce that in the case of the filter, on balance it was considered worthwhile to retain the existing system, particularly for a control unit intended to be very flexible.
An interesting point is that Philips - wearing its device-maker hat – made provision for low- and high-pass filters in its 1979-80 range of integrated circuits developed for high-quality audio applications. Mentioned upthread was the TDA1074 electronic potentiometer that could be used as the basis for volume, balance and tone controls, with full user flexibility to set the control parameters. Accompanying the TDA1074 were the TDA1028 (two by two-pole-pole two-way) and TDA1029 (two-pole four-way) electronic switches, intended for input, tape monitor and stereo-mono switching. From the viewpoint of a signal switched through one of these, they looked like a unity gain, non-inverting buffer stage with high input impedance. Thus they could be used for RC feedback filters. An early – maybe the original – and extensive application note (60+pages) devoted a reasonable amount of space to the filter application, and a representative selection of pertinent pages is attached.
National also offered a two-pole, four-way electronic switch, the LM1037/LM1038, at about the same time, complementing its LM1035/LM1036 volume/tone/balance controls. Presumably these could have been used for RC feedback filters.
Cheers,