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More Tales of Myth and Majic

[Tone Controls]
Tone Controls ???

Author: Dejan Veselinovic

Well, I'm told I sure raised a storm on the Italian list with my last editorial. It seems some caustic comments were made, but since none were told me directly, I'll assume everybody had a good time. If so, then you're in for more good time, here it comes, freshly picked, washed with morning dew.

I think tone controls are good. Are the cross and nails ready yet, should I go on?

Anyway, I really think they can be good. But they sound bad, you say, Lord knows I've read that line enough times. It's true that they can sound bad, it's even true that they mostly sound bad - but why?

To see this, we need to look at what's being offered as tone controls, what they do, how they do it and what do we gain and loose by them.

Tone controls acquired a bad name in the days when audio was becoming a popular hobby, when everybody and their dog started manufacturing audio. Those were the halcyon days of mass production, when the deal was to stick in as many features as you could for as little money as possible. Not that much has changed, but never mind. In those days, as elder readers will surely remember, manufacturers had a race on who will provide most tone control. Some split the left and right channels, some added the midrange control, others yet added selectable turnover points, and still others did all that. But all did another thing - they started the numbers race with lift/cut values.

So we ended up with ridiculously effective tone controls, because nobody needs +/- 15 dB boost/cut, with those values you could blow your speakers in no time. Remember, +15 dB is in fact 32 TIMES the output power! So, point one - they were far too effective, way above the level of reason.

Then their turnover points - for the uninitiated, the turnover point is the point taken as the place where the tone controls start to act at +/- 3 dB. In the most frequent case, with bass and treble tone controls, touch either and you had effects at around 1 kHz, i.e. well into the mid range. Obviously, this is all wrong, you don't want to fool around with the midrange, but hey, that's how they were made anyway, the sales blurb accent being on no less than +/-12 dB boost/cut. So, point two - their turnover points were poorly selected.

Now, I'm not going to tell you anything new by stating that capacitors as a rule should be avoided in all signal paths. Yet it's with capacitors that most tone controls were made, they being used to simulate inductors. Inductors are much better behaved, or at least are much more predictable, but are also costlier to manufacture, which is why so very few people ever used them. Point three - they were made the wrong way around because that was the cheaper and oh so much easier method. But there's more - capacitor simulation can actually work quite well if you just apply the teaching of one Mr Peter Baxendall, a man I feel is too little understood and far too little heeded.

Some 50 years ago or so, the said Mr Baxendall laid out the ground rules for tone controls, namely two of them: 1) make them gentle, and 2) shelve them, do not let them increase with frequency, but set a limit and make them stick to it. I only wish more people listened to him, though in all honesty, some did and do - Rotel comes to mind here, but there are others.

Lastly, tone controls, no matter how you make them, are in fact filters, and as any filter, they will by default introduce phase shifts, which we definitely don't want, no matter what. Point four - they have inherent phase shifts, which in audio is a fault.

From the above, you could conclude the following: if we must have tone controls, then we need a much smaller field of effect, no more than say +/- 3 dB or +/- 6dB at the very most, we need to have such turnover points as to have minimum interaction with the midrange in both directions, and we should use inductors rather then simulate them with capacitors, or, if using capacitors, use classic Baxendall circuits which include shelving. All this can be done, but we're still left with the fourth problem, that of phase shift, which we cannot eliminate no matter what. Because of this, we need to answer just one question - why use tone controls at all?

Well, I think they are useful, they should be there, and I readily admit they are a compromise. As any compromise, sometimes they can work wonders, other times we don't need them at all.

Consider this example. You have a smallish room, with plenty of furniture, heavy drapes and thick carpeting, and a decent number of paintings hanging on walls (instead of your mother-in-law who you'd much rather have hanging on the wall, but that's illegal). Chances are this room will be acoustically dead, and will produce a dull treble with possibly boomy bass. Other than redecorating the room, having no tone controls will leave you with some seriously blurred sound no matter what you have. If you had tone controls, you would indeed have more phase shift, which you originally never knew you never had, because everything sounded dead anyway. Now you can reduce that bass a bit, touch up the treble a bit, and with any luck, make your sound much more balanced, far more than you loose with tone control phase shift. So, are tone controls really bad by default?

Another example. You are a student, and live in a small, barren room (which will give you that desirable look of a tortured intellectual early in life, but it's good to start your career young, and anyway, you can drink your professors under the table). Your speakers, for reasons of space and budget, have to sit on shelves. Now, shelves will make that bass boomy, muddy and terrible in general, while the barren walls will make the treble prominent. You end up listening to emphasized treble and bad bass, and you don't know what all that talk of midrange is about because you can't hear any. Now, if you had tone controls, you could calm the treble down, as well as the bass, and arrive at a more even tonal picture, where the midrange would at least have a chance of doing its thing.

So, tone controls are a compromise which can actually give much more than it takes away. But they need to be well designed, with good turnover points, made of inductors rather than capacitors, with moderate action and proper shelving, and they will be expensive, because you'll need not two, but four Alps pots - actually, 6 is more like it, because good tone controls are ALWAYS separate for each channel. This is so because your left and right speakers are never working under identical conditions, and you may have to apply different settings to each. Convenience always did have a price, and this is no exception.

So, what I'd do is design for +/- 3 dB in 0.5 dB steps, using 1% metal film resistors, separate for each channel, based on inductors not capacitors, and all that in an active Baxendall-type circuit. Passive controls are hell on phase shifts, active keep it down to smallish levels, and I'd use a good op amp for them, one with small phase shifts of its own. And I'd throw in a tone defeat switch, on the off chance that only one human being with absolutely linear hearing may buy my product - the rest of us homo sapiens have to put up with congenital nonlinear hearing.

And I unfortunately do not speak Italian, so I'll be shortchanged again for the discussion on the Italian list, where most of the fire, brimstone and blood seem to be. There's only one Ferrari red.

My thanks to Mr James Bongiorno for encouraging me to think freely, and Mr Graham Slee for some technical tips.

© Copyright 2001 Dejan Veselinovic - http://www.tnt-audio.com

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