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Old 13th May 2019, 1:56 am   #21
Synchrodyne
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Default Re: Tone Controls

Quote:
Originally Posted by Craig Sawyers View Post
Peter Baxandall was indeed very involved in the 303. There is a Linear Audio book called "Baxandall and Self on Audio Power". That is a collection of papers by both designers. And then a long set of notes by Baxandall, hand typed with sketches in his own hand, just before his fairly early death aged 74. Well worth a read, certainly at Euro 9.50 https://linearaudio.net/books/2219
I have now received my copy of that book, and it is definitely well worth a read. Thanks for the recommendation. I’ll resist further digression in this thread and thus I’ll degress, as it were, back to tone controls.

As mentioned above, the Quad 33 had a near-standard Baxandall tone control of the variety without the treble potentiometer centre tap to earth. Whilst I suggested that this may have stemmed from Baxandall’s work with Quad at the time, which probably encompassed the 33 as well as the 303, it was also logical given the progression through the QCII and 22.

Unlike the case with the 22, the tone control and output amplifier part of the 33 was inverting, and so could be the basis for a Baxandall tone control. I have drawn out a simplified version of the circuit:

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This provided a net gain of 5 times (14 dB) from input to output, nominally from 100 mV to 500 mV. The actual gain was a bit higher to cover some loss in the output network. The gain stage was a single BC109 with its output load bootstrapped by a BC109 emitter follower. This was essentially the later well-known Quilter circuit. Quilter had modified the 1966 Bailey single transistor circuit, and one has the impression from his WW article that he was unaware of the prior Quad circuit when he did his work. H.P. Walker also used the same configuration for his stereo mixer. He published in WW from 1971 May onwards, whereas Quilter had published in WW 1971 April. So it is likely that Walker arrived at this circuit independently of Quilter. Such are the vagaries as to who gets the credit for an innovation. Given that Baxandall was working with Quad at the time, it should not be too surprising if he had a hand in the 33 tone control circuit. His original circuit had unity gain, and he had noted that for best performance, a high slope pentode was required. Thus one expect might that he was rather careful when it came to extracting some gain from a tone control stage without compromising overall performance.

A slight departure from the standard Baxandall R-C network was that the 10k treble potentiometer was shunted by a 10k resistor. I haven’t worked out just what effect this would have had. With the potentiometer wiper at one of other extreme, it would have been as if its value was 5k not 10k. I wonder if at intermediate settings it had some flattening effect on the treble curves, which as noted above was evidently something that Quad preferred. Here is the curve set:

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In summary, Quad started with a James-type passive tone control before James “invented” it (QA12/P and Quad I), then moved to what might have been a Baxandall-inspired active bass control reconfigured to work in a series feedback loop, initially coupled with a fully-feedback treble control (QCII) but later with a matching treble control (22), then to a slightly modified Baxandall control using the Quilter circuit before Quilter “invented” it (33). From there it took what might be called a sideways step to the Quad 44 tilt control, in part based upon the work by Ambler.


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Old 13th May 2019, 2:48 am   #22
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Default Re: Tone Controls

Some idea of the development history of treble-and-bass form of tone control may be gleaned from the attached exchange between P.J. Walker and D.T.N. Williamson in the letters pages of WW 1949 December:

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The Williamson circuit to which the comments referred was this one:

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One has the impression that PJW had put some work into what presumably was the tone control for the QA12/P in an effort to get the right curve shapes and not just single-knob control for each of the treble and bass. An interesting comment he made was: ‘Tone control should be applied with the greatest discretion, but a small amount of “compensation” correctly shaped can on occasions prove beneficial”.’ I’d say that the tilt control on the Quad 44 et al is a model of discretion.


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Old 13th May 2019, 4:11 am   #23
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Default Re: Tone Controls

A possibly unusual implementation of the Baxandall tone control was in the Armstrong FC38 radio/radiogram chassis, as shown here:

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The tone control circuitry itself looks quite normal, and was built around one half of a 6L19 double-triode valve. The unusual part was that the tone control is completely within the main feedback loop, which also included an active volume control. That loop ran from the 3 ohm output transformer tap back to one side of the volume control, the other side of which took the output from the demodulator or the gram input. Thus all of the AF stages were within the main loop, that is 6LD20 (triode) amplifier, 6L19 (half) tone control, 6L19 (half) phase splitter an 6P25 output pair. One may derive a block schematic showing the Baxandall loop within the main loop as follows:

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I am not sure quite what the effect would have been from including the tone controls within the main feedback loop, but perhaps the initial slope of the curves would have been reduced, with marked steepening at the extremities.

The FC38 was released in 1953 June as the direct replacement for the EXP73 at the bottom end of Armstrong’s then chassis range. One of its features was that it had separate treble and bass tone controls, whereas its predecessor had had a domestic radio type single tone control, probably in the amplifier main feedback loop. I haven’t seen the EXP73 circuit details, but the EXP83/3 before it had an AF section comprising 6LD20 (triode) amplifier, 6LD20 (triode) phase splitter, 6P25 output pair. So it followed typical radio set practice in not having a defined preamplifier section. It looks as if Armstrong stayed with that layout for the FC38, simply inserting a Baxandall tone control stage ahead of the phase-splitter, and using a 6L19 in place of the second 6LD20 to avoid an increase in valve envelope count.

Whatever its reasons for the FC38 loop-within-a-loop circuit, Armstrong reverted to more conventional AF amplifier practice for the FC48, which superseded the FC38 in 1954 September. In this case, the control and power amplification functions were separated. The control section comprised an ECC83, the first section as 1st AF amplifier and the 2nd as the Baxandall tone control virtual earth stage. The power amplifier included an ECC81 and a 6P25 pair, with the main feedback loop returned to the 1st half ECC81 cathode. The FC48 was initially advertised as having “heavy negative feedback”, stated as 20 dB in later advertising). Presumably it had quite a bit more than was the case for the FC38, whose rather long loop might have been limiting, stability-wise.

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Quite possibly the FC38-style tone control was also used for the RF41 model, which was available from 1953 September or thereabouts and superseded the RF104, the main apparent difference being that the RF41 had separate bass and treble tone controls, whereas the RF104 would have had a single tone control.

Another apparent tone control oddity from Armstrong was that in the later iteration of the BS125 chassis. I think that it was also used in the EXP125/A and EXP125C models.

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At first glance it looks like a regular Baxandall circuit, following an EBC41 (triode section) 1st AF amplifier and using a 6F13 high-slope pentode as the virtual earth stage. But the catch was that the 6F13 cathode was the return point for the main feedback loop. The power amplifier section, on a separate chassis, used a 6LD20 (triode section) phase splitter and a 6P25 foursome in parallel push-pull. Thus the Baxandall loop and the main feedback loop overlapped, as may be shown in block schematic form.

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This circuitry probably post-dated that for the FC38 by a couple of months or so, and had what might be described as a shorter main loop. The BS125/4 circuit dated from 1954 May. The EXP125/A was available from around 1953 September, replacing the EXP125/3. The EXP125/2 and /3, and the original BS125 had separate bass and treble controls of the switched kind. Whether they were passive or active is unknown, but they were between two 6LD20 triode sections. As far as I know, the EXP125C (from 1954 September) and revised BS125 then survived as such until around 1959 March, when they were replaced by the RF/BS125R/T group, which might have had passive tone controls.

Armstrong’s use of a 6F13 valve in this case followed Baxandall’s original recommendation that a high-slope pentode be used. He used an SP61 in his published circuit, and the 6F13 could be considered as the successor to it.


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Old 13th May 2019, 7:13 am   #24
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Default Re: Tone Controls

In simple terms please, what does the "tilt" control do?
Is it an ultrasonic filter or something?
On a friends amp that I was checking over and replacing the Rifa smoke generator, I found that to my ears it made hardly any difference at all.

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Old 13th May 2019, 7:16 am   #25
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Default Re: Tone Controls

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Originally Posted by Synchrodyne View Post
I am not sure quite what the effect would have been from including the tone controls within the main feedback loop, but perhaps the initial slope of the curves would have been reduced, with marked steepening at the extremities.
Good grief!

There are several ways of looking at it

The overall feedback would act to reduce the slopes created by the tone controls, but the reduction factor would depend on the amount of gain reduction the feedback loop was giving... (open loop gain)/(closed loop gain) which depends on the setting of the volume control.

With the volume control at the loud end there will be a scale model of the Baxandall curves available through use of the tone controls. As the volume control is reduced, so the Baxandall curves will be progressively scaled-down further.

This is weird, because the human ear becomes progressively less flat at lower sound pressure levels - the Fletcher-Munson curves - and the listener is most likely to want to add some bass and treble boost at low levels.

It looks like someone at Armstrong had invented the unloudness control

1953 is about 14 years too early to ask what on earth were they smoking. It looks like someone who didn't really understand what they were doing had tried to amalgamate two circuits they'd come across.

Another way of looking at the thing is that the operation of the tone controls will affect the stability margin of the outer loop. Treble boost looking particularly risky at first sight. But treble boost in the tone control produces a phase lead as it produces a rising response. These two effects tend to compensate each other's effects on the outer loop's stability margin.

With the number of active stages enclosed in the outer loop, and no doubt an economy model output transformer, you might expect them not to be using agressive amounts of feedback, but the volume control needs plenty at the low end if it is to have much range.

'Orrible! No wonder they dropped it.

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Old 13th May 2019, 8:07 am   #26
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Default Re: Tone Controls

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In simple terms please, what does the "tilt" control do?
Is it an ultrasonic filter or something?
It is what it implies - it tilts the overall frequency response up or down by a maximum of 3dB, in 1dB increments. Its effect is intended to compensate for imbalances in recording quality, and is rather subtle in its effect.

In other words it is not anything like as aggressive as the +/- 10dB to 15dB of a traditional Baxandall.

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Old 13th May 2019, 8:23 am   #27
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Default Re: Tone Controls

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In simple terms please, what does the "tilt" control do?
Is it an ultrasonic filter or something?
Back in post 14 of this thread, Craig posted a link to a Ken Rockwell review, and if you scroll a long way down that review there is a plot of the measured response of all the settings of the Quad tilt control.

It is indeed rather subtle but it is very effective for mildly tweaking the tonal balance of recordings. It's something that grows on you.

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Old 13th May 2019, 2:57 pm   #28
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Default Re: Tone Controls

Turns out that incorporating the Baxandall circuit into a power amp feedback path turned into fairly common use. There is even a short section of Doug Self's Audio Power Amplifier Design book, page 101 of 6th ed. "Power Amplifiers combined with Tone Controls". He says it was/is fairly common in Japanese amplifiers, and picks the Yamaha AX500 from 1987 as just one example of "many many more".

He says that audio power amplifier design is challenging enough without putting gain variation in the feedback loop of 20dB or so, that he finds the whole business puzzling.

So although the Armstrong might seem uniquely quirky, the Japanese took the idea up with a gusto.

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Old 13th May 2019, 6:25 pm   #29
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Default Re: Tone Controls

The incorporation of the baxandall circuit into a power amps's feedback network was just that, the tone controls varied the feedback transfer function to create the wanted closed-loop response.

The Armstrong circuit above looks similar but is quite different in operation. The Baxandall circuit in this case is entirely in the forwards path of the amplifier and is completely enclosed by the feedback loop.

With a Baxandall tone control AS the feedback system, the gain of the amplifier is controlled by the tone control.

With the Baxandall tone control connected as part of the amplifier's forwards path, the amplifier's feedback acts to control (override) the tone control.

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Old 14th May 2019, 3:19 am   #30
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Default Re: Tone Controls

I think that the inclusion of tone control networks of varying complexity in the main feedback loop of power amplifiers was reasonably commonplace for radio receivers and radiograms in the valve era, but less so for higher quality audio equipment where separation of the control and power amplification functions was considered to be desirable and normal.

By was of an example, Armstrong did this with its RF103/3 radio/radiogram chassis, which had a simple, single tone control:

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I suspect that something similar would have been used in the Armstrong EXP83/2, EXP83/3, EXP73 and RF104 models.

Here is another more complex example, although still with a single control.

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I don’t know of any actual examples, but presumably it would have been possible to include one of the established passive treble-and-bass tone control networks, such as the Voigt or the James (Volkoff) in the feedback loop.

The inclusion of a true Baxandall tone control in a feedback loop would be a more complex exercise, although perhaps doable. The defining feature of the Baxandall control is not its specific RC network (which is similar to that of the James passive control), but the fact that it is built around a virtual earth amplifying stage. Absent the latter, it is not really a Baxandall control. So if included in a feedback loop, then that loop would need to include a suitable active device such as a valve, transistor or IC to act as the virtual earth amplifier. That would seem to be a complication in general and also specifically because it would invert phase. But anyway, it should be possible to arrive at reasonable curve shapes by appropriate choice of RC network in the feedback loop, without the need to go to a true Baxandall system within the loop.


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Old 14th May 2019, 6:24 am   #31
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I suppose that another approach might be to use the whole power amplifier as the Baxandall virtual earth stage. Then the power amplifier would need to be inverting overall. The Baxandall network would be at the input, partly in the forward path and partly in the feedback path. But when all said and done, I think it would be hard to argue persuasively against strict demarcation between tone control and power amplification.

As seen, Armstrong had used rather ersatz implementations of the Baxandall tone control in some of its radio/radiogram chassis in 1953, followed by a “proper” implementation in its FC48 radio/radiogram chassis of 1954. Between those events it had stepped into the hi-fi market proper with its A-10 amplifier and control unit, released circa 1954 June. For that it used a passive tone control circuit that is described as being of the Voigt type, but which was more like the James type.

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Insofar as the Baxandall tone control was thought to produce a better set of curves than the passive type, the choice of the latter seems to have been a retrograde step. As typically deployed, the Baxandall required an extra triode or pentode as compared with the passive type, so the latter was favoured in situations where cost constraints favoured minimizing the number of valves used.

It could have been that Armstrong nonetheless simply preferred the curve set produced by its passive tone control. The A-10 was in any event a little unusual. The control unit employed a single ECC83 double triode, which provided initial gain, equalization where required, and the low-pass filter. This was followed by the passive tone and then the volume control, from which the AF signal was fed to the power amplifier. The core of the latter was fairly conventional, comprising an EF86 gain stage, half of an ECC82 as a phase splitter and an ultra-linear EL37 pair output. But that was preceded half an ECC82 gain stage that was outside of the main feedback loop – a pre-stage, as it were. I imagine that that made up for the loss in the passive tone control network (say 14 dB, perhaps more), and avoided the need for higher gain in the power amplifier. Normally one would have expected that extra gain stage to have been provided in the control unit, not the power amplifier. But the spare triode, as it were, happened to be in the power amplifier, which required only one half of an ECC82 for its phase splitter. So that might have been the reason for placing the gain stage there. And it just might have been the reason for the choice of a passive tone control. A Baxandall control would have required another triode (or pentode) in the control unit, and given that the A-10 was intended to be keenly priced for a 10-watt hi-fi amplifier, the budget may not have allowed for that.

Armstrong then used a similar passive tone control in its later radio/radiogram chassis, certainly in the Stereo Twelve/Jubilee/Stereo 44/AF208 generation, but I think before that in the PB409. Why Armstrong referred to it as being of the Voigt-type is unknown. Perhaps it saw that as the original of its kind, with others, such as the James, simply being a variation, in which capacitors rather than resistors were used to avoid the bass potentiometer from affecting the treble.


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Old 14th May 2019, 6:30 am   #32
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Default Re: Tone Controls

Those two circuits show the tone control as part of the feedback path which is OK. It varies the amount of feedback to impose control of gain versus frequency.

The FC38 diagram in post 23 is fundamentally different. The tone control elements (a full active Baxandall circuit in this case) is entirely in the forwards path, and there is flat feedback around the whole thing. If the tone control is used, the feedback system sees it as the same as unflatness in the power amp, and sets about improving (flattening) it. Essentially the feedback fights the tone control.

This is the difference between the effects of the forward path transfer function and the effects of the reverse path transfer function. Any text book should have the classic diagram and equation for a feedback system.

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Old 14th May 2019, 9:27 am   #33
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The FC38 diagram in post 23 is fundamentally different. The tone control elements (a full active Baxandall circuit in this case) is entirely in the forwards path, and there is flat feedback around the whole thing. If the tone control is used, the feedback system sees it as the same as unflatness in the power amp, and sets about improving (flattening) it. Essentially the feedback fights the tone control.
That's what I thought when I first looked closely at the FC38 circuit. It didn't make sense to me. Initially I thought that I must be missing something, but I guess that the real reason is that it actually doesn't make sense. To a lesser extent I think that the BS125/4 circuit, with overlapping feedback loops, might have a similar problem.


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Old 14th May 2019, 11:21 am   #34
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Default Re: Tone Controls

Certainly the case with the FC38 circuit. It looks like they put rather a lot of parts into it and yet the system was fundamentally flawed. There's evidence that no-one with much of a clue could have looked at that circuit before it hit production.

It happens.

Just because someone makes and sells something doesn't guarantee it's any good. The human race has a track record of producing plenty of goof-ups and the occasional stinker. This one is probably just very disappointing to the end user, and potentially embarrassing to the manufacturer. They really should have known better

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Old 16th May 2019, 3:26 am   #35
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Perhaps what Armstrong did with the FC38 tone controls was an earlier form of audiophoolery, albeit probably inadvertent rather than deliberate. A much later form of probably inadvertent audiophoolery in respect of tone controls is the so-called “passive Baxandall” control, for which one may find web references.

Firstly, typically as drawn, the so-called passive Baxandall circuit is, or is very close to the James circuit, which preceded the Baxandall and which was acknowledged by Baxandall in his WW 1952 October article (a). So “James tone control” would be more accurate terminology in the historical context, although even there, and as mentioned upthread, Volkoff had a prior claim to the circuit.

Secondly, the defining feature of the Baxandall control was that it was an active type. This was not an optional feature. This was made very clear in the opening to WW 1952 October article, as follows:

“The circuit to be described is the outcome of a prolonged investigation of tone-control circuits of the continuously adjustable type, and provides independent control of bass and treble response by means of two potentiometers, without the need for switches to change over from "lift" to "cut." Unusual features are the wide range of control available and the fact that a level response is obtained with both potentiometers at mid-setting. The treble-response curves are of almost constant shape, being shifted along the frequency axis when the control is operated, and there is practically no tendency for the curves to "flatten off" towards the upper limit of the audio range. The shape of the bass-response curves, though not constant, varies less than with most continuously adjustable circuits.

“The "Virtual-Earth" Concept. - The performance outlined above has been achieved by the use of a negative-feedback circuit instead of the more usual passive type of network1,2 and it is desirable that the reader should become familiar with the "virtual-earth" concept3 as applied to feedback amplifiers, before the operation of the tone-control circuit is considered in detail.”

References 1 and 2 were to the Voigt 1940 and James 1949 passive tone controls respectively.

Thirdly, thereafter, Baxandall never offered a passive tone control circuit of his own, and so one which could have carried his name. In Amos (b), he reviewed the Voigt passive tone control and presented a set of curves for it, before moving on to his own active design, and then to the Ambler. In Talbot-Smith (c), he reviewed his own tone control, his derivative of the Ambler, then moved on to parametric and graphic equalizers.

I suspect that the “passive Baxandall” tone control name was coined by someone who was unaware of the James circuit, and who developed or saw a passive circuit whose RC network looked somewhat that used in the Baxandall control. So “passive Baxandall” might have seemed to be an appropriate name. The Baxandall RC network existed in two slightly different basic forms. An important characteristic of both forms was that they had two inputs (forward arm and feedback arm) and one output (to the inverting input of the amplifying stage). In a passive tone control, the RC network had one input and one output. This I think is a significant difference, and meant that the two-input Baxandall network could not simply be used unchanged in a passive control. This point also seems to have been missed by whomever coined the “passive Baxandall” name.

Possibly that name was first used at about the time that passive RIAA equalization circuits garnered some measure of interest. That seems to have been driven by the strange notion that feedback was inherently bad – the work of the devil as it were and so beyond rational analysis. Of course, the usual RIAA RC network was of the one input, one output type, and so was as easily placed in the forward arm as in the feedback arm. Although tone controls were by then often frowned upon, perhaps it was also thought where they were to be used, then the passive type was preferable to the feedback type. So with the Voigt, Volkoff/James and other passive circuits by then forgotten about, they were effectively reinvented. And given that the Baxandall tone control was the dominant type of treble and bass control, with its original raison d’être forgotten, it was also thought that what had been reinvented was logically a passive Baxandall control, despite the fact that the nature of the RC network (but not its superficial appearance) had been changed significantly.

One may also posit that Baxandall would have had no reason to develop a passive tone control. His active type had been developed to improve upon what was possible with passive controls. At no stage did he join the anti-feedback brigade. Rather his approach to audio circuit design was one of utmost rigour and objectivity. That was apparent in the sometimes rather heated exchanges in the letters pages of WW during the 1970s. (In one of those exchanges both he and Reg Williamson were the subject of a not very nice ad hominem remark from Stan Curtis. Baxandall brushed it off light-heartedly and continued with his logical arguments, leaving the reader to deduce – without too much difficulty - who was the mental giant and who was the mental midget in that exchange.)


(a) Wireless World 1952 October, p.402ff

(b) S.W. Amos (Editor); Radio, TV and Audio Technical Reference Book; Newnes-Butterworth, 1977; ISBN 0 408 00259 X. See p.14-24ff.

(c ) Michael Talbot-Smith (Editor); Audio Engineer’s Reference Book, Second Edition; Focal Press, 1999; ISBN 0 240 51528 5. See p.2-143ff.



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Old Today, 7:04 am   #36
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Default Re: Tone Controls

In his 1977 writings in Amos, as attached above, Baxandall suggested that as transistors and integrated circuits were by then “so cheap”, there was much to be said for employing separate circuits for bass and treble, and examples of such were shown in the article.

I don’t recall seeing a control unit circuit in which such separation was done. But a rather complex tone control circuit, with some separation, was used in the Radford SC24 control unit of 1971. In this case it certainly looks as if device count was not constrained, but left to lie where it fell.

I have endeavoured to extract a simplified block schematic of the SC24 tone control section for one channel. I was working from a schematic that was in the first place not easy to follow and which was in “small print”, so I am not completely sure that I have gotten it right. Corrections are welcome.

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The bass control used a one-input, two output network that was partly in the feedback arm and partly in the output arm of a two-stage amplifier (emitter follower Tr4 and common emitter gain stage Tr5). The treble control used a one-input, one-output network in the feedback arm of the same two-stage amplifier, but there was an emitter follower buffer (Tr6) between the output of the bass network and the treble network.

The output from the bass control network fed into a three-stage amplifier (Tr7 common emitter gain stage followed by Tr8 common emitter gain stage DC-coupled to Tr9 emitter follower). The feedback loop for that amplifier went via the middle tone control network (which was of the one input, one output type) to an emitter follower buffer (Tr10) to the emitter of the Tr7 gain stage.

Notable is that each of the two tone control feedback loops included an emitter follower buffer stage entirely within the feedback path. The basic form of the bass control, with a network fed from a gain stage and dividing into an output arm and a feedback arm was similar to that used in the Quad QCII and 22. Inclusion of the treble control network wholly within that same loop also followed the QCII precedent.


Cheers,
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