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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 2nd Nov 2019, 1:23 pm   #141
Craig Sawyers
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Default Re: Tone Controls

Incidentally, you can synthesise an inductor with an op-amp. So in principle you could produce the same filter curves as eg the 33, but actively. Then that would get around the problems of magnetic core non-linearity and hum pick up (that in the 33 was difficult to eliminate entirely). The (gapped) pot cores in the 44 were better in this regard.

See for instance part way down the page here https://en.wikipedia.org/wiki/Gyrator

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Old 2nd Nov 2019, 6:09 pm   #142
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I've simulated the 33 and 44 filter responses in Spice. The 22 response is identical to the 33 one, since the capacitor values are ten times smaller, so increasing the inductor values by ten gives the same result.

First the Quad 33 curves.
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Old 2nd Nov 2019, 6:10 pm   #143
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Default Re: Tone Controls

Now the Quad 44 curves
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Old 2nd Nov 2019, 8:51 pm   #144
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Quote:
Originally Posted by Radio Wrangler View Post
One glance and you could see the status of everything. One control actuation made any change you wanted...menus are a big step backwards...each and every time you do anything with it, it inconveniences you.
When I started in the BBC, I was in installation and commissioning, where all the talk was of assignable desks - one control strip to be steered to any given desk channel when you wanted to adjust it. From the time I moved into operations until I left the corporation I heard nothing more of the things, thank goodness. You wouldn't make a piano with a one-octave assignable keyboard, would you?
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Old 2nd Nov 2019, 11:33 pm   #145
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Originally Posted by Ted Kendall View Post
You wouldn't make a piano with a one-octave assignable keyboard, would you?
Perhaps with pianos the trend is in the other direction, e.g. see: https://www.stuartandsons.com/108keys.html.

Agreed that with the Quad 44, one may see all of the control settings at a glance. The Quad 66 is also very good with its visual display.

On the low-pass filter, I wonder if Quad pondered the idea of using opamp gyrators rather than inductors for the 44, given that it was largely based upon the TL072 opamp family.

Re the QCII, I had forgotten just how big the filter inductor was. Given that space was going to be tight in the 22, more compact design was probably an objective, and that may have in part driven the changes in filter circuitry.

The Quad 44 must have been about the last new design control unit with a continuously-variable-slope low-pass filter. Low-pass filters as such seem to have come into prominence in the late 1940s, for example being included in the Williamson control unit. I imagine that the improvement in amplifiers and particularly in speakers showed up the deficiencies of the then-available programme sources and equipment. One would expect this to have been the case with the Acoustical Corner Ribbon speaker of late 1949, which may have prompted PJW to develop an amplifier with a suitable low-pass filter. That appeared on the original Quad (retroactively designated as the Quad I) of late 1950. (Not 1951 as I previously said; I rechecked and found the release announcement had been made in WW 1950 November.) This is the earliest example of a variable slope filter that I have found. The rationale behind the Quad controls was covered in its advertising of the time:

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Old 3rd Nov 2019, 12:49 am   #146
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Default Re: Tone Controls

It is interesting that the continuously variable slope filter was a feature of Quad preamps from late 1950 through to the last Quad 44 in 1989, so 39 years and covering two valved, one discrete transistor and one IC preamp.

The two frequency two fixed slope HF filter lasted a little longer in the 34, which finished in 1995.

By 1989 most listening was done from CD, and sales of vinyl were plummeting. So I guess that the need for sophisticated switched frequency variable slope HF filters were no longer needed - if indeed they ever were.

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Old 3rd Nov 2019, 1:18 am   #147
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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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Old 3rd Nov 2019, 1:27 am   #148
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Default Re: Tone Controls

Note the (simulated) -0.9dB droop in flat response at 20kHz of the HF filter in the 44? That is because with the HF filter slope control set to 0, the 1k series resistor on the filter input and 2k load acts with the 6n8//10k + 33n to ground.

However that droop is within (just) Quad's spec for the 44 which is +0, -1dB from 30Hz to 20kHz.
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Old 3rd Nov 2019, 7:07 am   #149
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Default Re: Tone Controls

I'd spotted that on the schematic. It looked like a compromise to simplify switching by no longer being able to completely cancel the filter. The 1k series resistor is needed to set the source impedance for when the filter really is in use.

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Old 9th Nov 2019, 1:20 am   #150
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Quote:
Originally Posted by Ted Kendall View Post
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.
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.

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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.


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Old 9th Nov 2019, 3:52 am   #151
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Default Re: Tone Controls

Around the time of these devices, a large part of their market were car stereo radio/cassette units, and the simplification of switching, tone control and volume/balance functions that these parts gave was a great help to designers trying to cram a lot into a very small panel.

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Old 9th Nov 2019, 9:53 am   #152
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Even high end audio manufacturers, such as Audio Research (whose kit costs as much as a family car) now use analogue switching. The main problem in using recent generation switches is that they are tiny, bump mounted things that would fit under a fingernail. Intended for use in phones, tablets, GPS etc.

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