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Old 3rd Jun 2019, 8:07 pm   #61
Rod Watts
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Default Re: Tone Controls

Interesting discussion on tone controls. I wonder, who made the change, from using a Cut/Boost switch, to using a variable pot? One basic difference between the passive and the feedback versions, is:-

The two resistors, on either end of the bass pot, are the same value in the feedback version, but in the passive, they are different values (usually 10:1 ratio). Also the two caps on the bass pot are same value for the feedback version and once again, a 10:1 ratio in the passive version.

Is it difficult to say, who actually invented the basic passive circuit? So many were working on the same idea, around the same era. Also several different circuits were developed around the same time, such as the Pultec, and the Fender Tonestack.

The basic reason for tone controls, was to make up for deficiencies in the speakers and of course the Source material. The trend towards no controls is rather limiting and assumes that everything in the world is perfect. Possibly some purists thought that it was, so you didn't need to alter it. In fact, some amplifiers, that had tone controls, and NO loudness switch didn't sound good, with muddy bass etc.

The important thing is to get things to sound good with what you are playing so, if the source material is a bit lacking, what's wrong with improving it a bit?

Nice to see some detailed research into tone control history, very interesting.
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Old 4th Jun 2019, 5:12 am   #62
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Default Re: Tone Controls

I think it is fair to say that the passive tone control with continuously variable treble and bass had what might be called “partible paternity”. Voigt may well have been the first to publish a circuit in 1940, but Volkoff claimed to have developed in 1939 essentially the same circuit that James published in 1949. James was probably unaware of Volkoff’s work, nor the fact that Acoustical (and probably others) were already using very similar controls. James made no mention of the Voigt circuit; that which he published was essentially a different way of achieving essentially the same end, but using a pair of capacitors to standoff the bass control from having any material effect on the treble. The Volkoff/James approach is that which seems to have been the basis for many subsequent passive controls.

Villchur wrote an article on tone controls for Audio Engineering magazine, 1953 March p.24ff. This was mostly about the tone control curves themselves, but he also included a circuit with separate passive treble and bass controls separated by an amplifying stage.

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In fact the same basic circuit had previously been published by Sterling in Audio Engineering, 1949 February, and in the 1949 June, issue, Faran showed a combined version using only one valve, and looking similar to the James circuit.

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Somewhat more recently, National Semiconductor provided a worked example, with R and C calculations, of a passive tone control in its 1972 May Application Note (AN-64) for its LM381 audio preamplifier IC.

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Audio Engineering magazine (later Audio) is available at: https://www.americanradiohistory.com/Audio-Magazine.htm.


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Old 4th Jun 2019, 7:04 am   #63
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Default Re: Tone Controls

In contrast to the passive case, there is no evidence of any other origination claims to the symmetrical negative feedback type of treble and bass control, and it certainly looks as if Baxandall had “clear title”, as it were, to that one.

In Audio Engineering 1953 September, p.29ff, Barber published an American version of the Baxandall, including an interesting mathematical function presentation of its action. It was suggested that this was an improvement over Villchur’s earlier circuit.

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[QUOTE=Synchrodyne;1149750 I can find no suggestion that the LM381 or NE542 were offered for tone control use. Rather they seem to have been oriented towards pickup, tape head and microphone input stage applications. Perhaps they were not very happy when used in the nominally unity gain inverting mode?[/QUOTE]

That was a false alarm, at least in the case of the LM381 series. Here is an active tone control built around the LM387, which was the utility version of the LM381.

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This is from an article about the LM381/2/7 series of audio preamplifier ICs in Radio-Electronics 1990 February p.54ff. I’d guess that most of the (many) application circuits presented were taken direct from National Semiconductors literature.

It would seem that the LM381 family had quite a long production life, as the LM381 at least dated back to mid-1972. Interest in it might have declined once the NE5534 op-amp family became available. Still, Popular Electronics 1978 September p.61ff carried a constructional article for a disco mixer that used the NE5534N for the microphone preamplifier, the LM387AN for the disc input RIAA stages, and the TL074 for line-level functions. (More horses than courses, perhaps?)

In post #59 I mentioned the unusual RCA tone control based upon its CA3140 bimos op-amp in the non-inverting mode. Here are two more based upon the CA3140. The first is from an article in Popular Electronics (PE) 1977 September, p.85ff, on IC audio preamplifiers. It included a selection of manufacturers’ circuits on the NE542, RCA CA3140 and TI TL080. Amongst those for the CA3140 was a Baxandall tone control, fairly normal except for the use of very high impedances.

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An earlier PE article, 1977 April p.66ff, specifically covered the RCA CA3140, which was then very new. Amongst the sample circuits shown, apparently from the RCA literature, was this tone control:

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It was unusual in that the tone control R-C network was entirely within the feedback loop of a non-inverting CA3140. To a first approximation, it was if a passive tone control network had been placed in the feedback loop. In contrast, the Baxandall network is placed both in the signal arm and the feedback arm of an inverting amplifier. In this case the CA3140 would be producing a net gain with the tone controls at level, in order to provide enough “room” for whatever maximum treble and bass and attenuation was required.

It is the first example I have seen of a fully-fledged and complete “hi-fi” tone control wholly within the feedback loop of a preamplifier stage. At the radio/radiogram level, simple tone controls were sometimes placed in the power amplifier feedback loop. The Quad QCII control unit had a treble control network that was wholly within the feedback loop of a non-inverting gain stage, although the bass control network was not, being split between an output arm and the feedback loop. The Radford SC24 had treble and middle control networks that were wholly within (separate) gain stage feedback loops, but again the bass control was split.


Radio-Electronics magazine is available at: https://www.americanradiohistory.com...ster_Page.htm;

Popular Electronics magazine is available at: https://www.americanradiohistory.com...nics-Guide.htm


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Old 4th Jun 2019, 7:37 am   #64
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Default Re: Tone Controls

To a high-end audio aficionado, buying a new amplifier/cartridge/speaker with, say, a rising frequency response is an acceptable way to adjust the sound to their taste, which they consider to be definitively right. It also allows them to spend more money making their setup more impressive. It moves them along the pathway to their heaven.

To a high end audio aficionado, creating the same adjustment of frequency response using a variable tone control or equaliser is unacceptable. It exposes them to the derision of their peers, it makes them look like a cheapskate. They will break out in huge warts and they're on their way to their own hell.

To someone who is not a high-end audio aficionado, the difference is mystifying.

To someone who is a mathematically-based engineer, the characteristics of circuits and transducers can be analysed in either time or frequency domains. They are just different ways of expressing the same things. The Fourier transform and its inverse provide a mathematically provable route of converting from one to the other. Systems can be described in differential equations and their roots found to give descriptions in terms of poles and zeroes of Laplace transforms. In this world, you can create whatever responses you like, and you can see that there are many ways of doing it. It doesn't give any guidance on what your preferences ought to be, but it allows many ways of achieving it. It gives freedom.

In the olden days, amplifiers had a variety of tone controls, designed to be cheap to implement. Better circuits, as described in this thread, gave controls which didn't interact and had useful ranges of adjustment.

The seeds of the anti tone control sentiment started with it becoming anathema to be ever seen using them. The amplifier manufacturers never fitted hasps and staples to them, but it could have been a selling point. Instead they fitted a 'tone control defeat' button to take them out of circuit. No-matter how good the circuitry, the pundits reckoned the tone control circuitry, even in a neutral condition must be doing discernible damage. They will not admit to any limit to their discernment. Nothing to them can be trivial. So it became unacceptable to ever not have the defeat button in. Equipment manufacturers must have been delighted! They took out the tone control circuit with its components, pots and knobs, then they upped the price! Well, it's better, innit? and better must be worth more!

From an engineering viewpoint, tone controls can be done well if you want to do them well, though there have been some poor attempts in the past. They can save you £££££ if your religion permits their use. This thread, and Synchrodyne's research of the mainstream and side-branches of development is a delightful exposition of their history.

In terms of people's attitudes to them, and the alternatives some seem to prefer, I think it is a field offering several PhDs worth of research to anthropologists.

Me? I think it's a bit of a hoot.

My hearing IS limited. There are people and things I wish to keep out of earshot and I'm 400 miles away from London. Super! Great! Terrific!

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Old 4th Jun 2019, 8:05 am   #65
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Default Re: Tone Controls

Agreed - this is a superb thread. And Synchrodyne's research and posts ought to be put together into a document about the history and practice of tone controls.

The defeat switch has a long history. The first one I saw is in the Quad 33 preamplifier, where the defeat switch takes out any tone control settings and the filter too - introduced in 1967. Although Synchrodyne might well find an earlier product that implemented a defeat!

I think that the rationale behind a defeat is that it allows a direct comparison between what you have set the controls to, and flat, in order to hear whether you have made a positive of negative alteration.

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Old 4th Jun 2019, 9:20 am   #66
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Quote:
Originally Posted by Radio Wrangler View Post
To a high-end audio aficionado, buying a new amplifier/cartridge/speaker with, say, a rising frequency response is an acceptable way to adjust the sound to their taste, which they consider to be definitively right. It also allows them to spend more money making their setup more impressive. It moves them along the pathway to their heaven.

To a high end audio aficionado, creating the same adjustment of frequency response using a variable tone control or equaliser is unacceptable. It exposes them to the derision of their peers, it makes them look like a cheapskate. They will break out in huge warts and they're on their way to their own hell.

To someone who is not a high-end audio aficionado, the difference is mystifying......................

........................Me? I think it's a bit of a hoot.

My hearing IS limited. There are people and things I wish to keep out of earshot and I'm 400 miles away from London. Super! Great! Terrific!

David
The 'high end audio aficionado' must, at all costs, never enter the studio where his source material is recorded because in that 'holy of holies', far from an imaginary vision of a length of (oxygen-free) copper with gain, they will find all manner of infernal audio processing.

They'll find that every mike and mixdown channel will have bass and treble tone controls, normally switchable for either a shelving or peaking +- 20dB response curve. Add to those a couple of mid-frequency peaks/dips, also adjustable in frequency and amplitude and the engineer and producer have wide scope in determining the sound that they judge sounds right on their monitor speakers in their control room acoustic.

Then when a disc, a CD or MP3 file is mastered from that studio recording, the mastering engineer has a similar degree of tone control scope so that in his/her judgement, the end product sounds right. Different media tend to have subtly different characters which the mastering engineer knows well and will compensate for appropriately.

So if my home speakers and listening room acoustics are identical to those in the studio, I may well choose not to use my tone controls. Otherwise, adjustment is a matter of my personal audio taste. We can all thank Paul Voigt, Peter Baxandall et al for helping us to create the reproduced sound illusion which we enjoy.

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Old 4th Jun 2019, 4:02 pm   #67
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Originally Posted by Craig Sawyers View Post
The defeat switch has a long history. The first one I saw is in the Quad 33 preamplifier, where the defeat switch takes out any tone control settings and the filter too - introduced in 1967. Although Synchrodyne might well find an earlier product that implemented a defeat!
The oldest one I've seen is the (mono) Quad QCII control unit. It has a 'cancel' position on the filter swtich which also cuts out the tone controls. The Quad 22 does the same.

Fortunately I do not have 'golden ears'. I just want something that I find pleasant to listen to. And if that means modifying the signal in some way (e.g. by using tone controls) then I will do it.
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Old 4th Jun 2019, 7:15 pm   #68
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The 'high end audio aficionado' must, at all costs, never enter the studio where his source material is recorded because in that 'holy of holies', far from an imaginary vision of a length of (oxygen-free) copper with gain, they will find all manner of infernal audio processing.Martin
On no account should the aficionado ever have been permitted to enter the control room of a major broadcasting organisation circa 1980. The sight of a large jackfield, an enormous number of double-enders, strange and wonderful PCM encoders, valve (!) line-level amplifiers and the Lord knows what else would have caused severe palpitations.

But I once knew an aficionado who claimed that using oxygen-free mains cable to the tuner had "noticeably improved" FM reception quality.
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Old 4th Jun 2019, 8:28 pm   #69
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Lots of interesting facts to look at, it all seems to kick off before 1950. Around the same time, the famous Pultec equaliser was born.This is passive, and over the years, has been used in recording studios, to improve the sound, They are still used today. The circuitry is different to the James design. Look up Pultec EQP-1A for circuits. They have a bass lift, with ability to alter frequency and also reduce the high frequency bass, in a similar way, to the so called "Tonestack" used in guitar amps, such as the Fender and Marshall. Lots of Tonestack info available on the net. The Pultec and the Tonestack tone control, have lots of range, so most, but not all, guitar amps use Tonestacks, not James/Baxandall ones.

Having worked on tone controls systems, since I was young, I find the Hifi Audiophools interesting, It seems to be, a sort of Ignore the facts, and believe what's printed in the HiFi mags, When I worked in a Hifi shop, we met a speaker manufacturer, who said, that the Magazines give you a good review, if you let them keep the product, brilliant, not biased then.

The bit I find strange, is the audiophile's idea, that the tone controls must be set exactly in the middle, A good starting point yes, but as Hartley118 points out, when the music was recorded, lots of filters were used, and music went through lots of o/p amps and tone controls before you got it

In general, One set of tone control circuits, on its own, has not got the range, that real world things, need compensating for. It's all down to DB/octave slope. One CR network is just not enough. Two CR networks, such as Bass and treb pots and ALSO a loudness switch, is the minimum needed. Without the second CR network, the Bass when turned up, will bring up unwanted High bass frequencies, possibly one reason purists don't like tone controls. With a second RC network, low bass can be improved, without making it boomy or muddy.

Real world things like speakers, and Room acoustics, need compensating for. How an audiophile can think, that his system is absolutely perfect, and not only that, but that the music he is playing is perfect as well, amazes me. They must be deluding themselves, sort of self denial I suppose. I bet if you actually measured their amp and speaker system, they would be surprised at the results. I suppose it is a sort of Religion, do you believe or not etc.

A very good free program is the Duncan Tonestack Calculator. Even has the James in, as a model, as well as Guitar amp tonestacks see:-

http://www.duncanamps.com/tsc/
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Old 4th Jun 2019, 9:14 pm   #70
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Quote:
Originally Posted by Craig Sawyers View Post
The defeat switch has a long history. The first one I saw is in the Quad 33 preamplifier, where the defeat switch takes out any tone control settings and the filter too - introduced in 1967. Although Synchrodyne might well find an earlier product that implemented a defeat!
The oldest one I've seen is the (mono) Quad QCII control unit. It has a 'cancel' position on the filter swtich which also cuts out the tone controls. The Quad 22 does the same.
Well darn it! I have a Quad 22 lurking in the lockup (sad isn't it?). Must look at the schematic....

Craig
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Old 4th Jun 2019, 9:28 pm   #71
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Yup - and the QCII, you are quite right. Which puts the earliest introduction of a cancel function by Quad at 1953.

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Old 4th Jun 2019, 10:59 pm   #72
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Default Re: Tone Controls

Quad, or rather Acoustical as it then was, included a tone control cancel facility on its QA12/P of 1947. See post #6 for the schematic.

But before then, Voigt had included a cancel switch in his passive tone control of 1940:

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Old 4th Jun 2019, 11:35 pm   #73
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It is interesting that it had its genesis long before any audiophile fad for defeating any tone control setting.
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Old 5th Jun 2019, 8:33 am   #74
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Default Re: Tone Controls

The defeat switch is perfectly legitimate and serves a useful purpose. You can quickly compare whatever settings you have to the amplifier in a flat condition, and you can use the comparison to determine if you hear those tone control circuits doing ungodly things to your sound like they have been accused of.

Of course, the padlock has to be removed from the switch in order to do this, as well as the padlock from the mind.

Oxygen-free copper wire for the RF input to a tuner will make a large difference to the sound.... If you start with wire that is, say, 99% oxygen The audiophool world is founded on the principle that nothing is small, nothing is trivial and so they have no sense of proportion. A colleague said that 'they dismiss all numbers, except price tags'.

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Old 5th Jun 2019, 8:47 am   #75
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Writing of the usefulness of the cancel function in his book "Hi Fi For Pleasure" in the fifties, Burnett James observed that " it is remarkable how often one finds that all one's solemn twiddlings have gone into labour and produced scarcely a mouse of improvement"...
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Old 5th Jun 2019, 9:14 am   #76
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I do find it interesting that the steep-cut low-pass filter was a standard feature of top-quality pre-amps in the 1950s. 78s of course required such filtering for scratch reduction, but it was also found necessary to reduce distortion and edginess on microgroove discs. However, play those same 1950s discs today with a modern pickup and stylus profile and they frequently sound distortion-free. This suggests that at least some of those deficiencies were not in the LP discs themselves but in the limited tracking capabilities of the pickups we used back in the day.

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Old 5th Jun 2019, 10:48 am   #77
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The easing of this problem is well documented in John Crabbe's Hi Fi News reviews of top-line pickups in the mid-sixties - Ortofon SPU/SL15/SL15te/Shure V15/Empire 999VE/X and so on. As tip masses dropped and tracking improved, he remarked frequently on the need or otherwise for the Quad filter to achieve clean results. At this time, recorded velocities were generally increasing, leading to the trackability arms race.
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Old 5th Jun 2019, 4:28 pm   #78
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Why did they used to be called (no longer though) "continuously variable" tone controls. They were certainly variable, but never continuously....which suggests "infinitely"?!
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Old 6th Jun 2019, 12:23 am   #79
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Prior to the advent of the Volkoff (1939) and Voigt (1940) circuits, bass and treble controls were typically of two kinds. One was the switched type, in which for example rotary switches were used with a number of steps each for “decrease” and “increase” about a centre “flat” position. The other was the potentiometer type, with one potentiometer each for treble and bass. However, each potentiometer had to be switched between the decrease and increase function, so there was a discontinuity in the operation.

Against that, the Volkoff and Voigt circuits provided for a continuous potentiometer sweep from maximum decrease through flat to maximum increase, with no discontinuity. Thus “continuously variable” was a suitable descriptor as compared with what had gone before. It was also mathematically correct, in that the mathematical representation of each of the tone control functions were continuously variable between their lower and upper bounds. Not just in audio, but elsewhere the terms “continuously variable” and “infinitely variable” are sometimes used interchangeably. One could argue that as a continuously variable function has an infinite number of values between its lower and upper bounds, the “infinitely variable” descriptor is appropriate. As a counter-argument, given that “continuously variable” exists for the bounded case, then reserving “infinitely variable” just for unbounded functions has some merit. Take your pick…


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Old 6th Jun 2019, 12:46 am   #80
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An example of a tone control that used potentiometers but required switching between decrease (fall) and increase (rise) is found in the Williamson circuit:

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Also pertinent is an exchange between DTN Williamson and PJ Walker in WW 1949 December:

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One has the impression that back in those times, quite a bit of engineering thought was put into the development of tone controls.


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