Interactive tone controls

Phil_aka_Pip

Was given Dave Hunter's "The Guitar Amp Handbook" for christmas. One of the things he goes on about is how the usual tone controls are "interactive". I realised I'd never thought about it much, and lets face it if you want less bass you turn the bass down or if you want more treble you turn the treble up ... so what's with this "interactive"? I suppose it means that the behaviour of the two other controls depends on how you set the third one - eg how the middle & treble behave depends on how you set the bass?

Thing is he doesn't explain HOW they are interactive, and I'm not smart enough to read a circuit schematic and tell you what the behaviour is going to be. So what should you expect from the bass middle & treble controls on say, a Plexi?

ThePrettyDamned

Mostly, just that - bass, mid and treble.

But you can 'boost' the bass by turning the treble down. Not sure if that's what he means. Or boost mids by turning the bass and treble down?

Interesting question.

guitarfishbay

A classic case of very interactive tone controls is the Mesa Rectifier.

The treble control basically voices the whole EQ. The higher you run the treble control the less signal gets passed on to the mid and bass, meaning that high treble values mean a less thick and less bassy tone. If you run the treble control very low the amp will get thicker, bassier, and if you turn the treble off the amp actually gets a bit louder too. You have to balance the brightness/darkness of the tone with the presence pot.

This makes it very versatile but also throws anyone unaware of how the interactive EQ works - hence (IMO) the reputation of being hard to EQ.

The Mesa Roadster manual has explanations of the interactive EQ in it across the different preamp modes. I can't figure how to link you to it from my phone, I keep accidentally starting a Pdf download. But if you google it you should find it easily.

Maxi

In some cases changing the value of one pot has more than expected impact on other parts of the tone stack in other cases its the opposite  . The pots are just variable resistors and the signal chooses the path of least resistance so opening or closing one filter shunts or reduces signal to the other parts of the tone stack . I didnt fully understand this impact (and still dont) until I bread boarded a dumble preamp clone with tone stack . Even the gain control had some impact as it shared a filter path with the treble control , basically cranking it up boosted low end but over all those individual controls dont seem to do much when you turn them one at a time they only really work when you balance them against each other and then the changes leap out , im not really a fan of that as it forces you to twiddle everything even for small tweeks . I dont really see an advantage to it at the moment , possibly could give you more head room where you might normally max out one of your tone controls ?!....

 

grungebob

My latest amp buy has interactive tone Controls and active. Interactive to me seems the normal i.e. As you increase treble less bass or middle comes through and visa versa, the active tone Controls on my amp only changes the parameter your changing and doesn't effect the rest (the active also makes the amp a damn lot louder too!!)

octatonic

Before ICBM comes in with a much more comprehensive answer I understand it to mean the following.

When you boost the bass in an amp you aren't boosting one frequency- you are boosting a range of frequencies with the central one being boosted more and the closest ones slightly less and the furthest ones quite a bit less- for example:

100hz might be boosted by 6db (I'm exaggerating here of course)

110hz and 90 hz might be boosted by 5db

120hz and 80hz by 4db

130hz and 60hz by 3db

140hz and 50hz by 2db

Amplifiers have a wide Q to their EQ circuits- if you had a fully parametric EQ in an amp then you could control how wide a range that particular control would affect across the frequency band.

http://www.bobgolds.com/WideQ_NarrowQ.gif

As you keep going up the frequency range of the amplifier the upper range will eventually creep into the midrange.

Boosting or cutting the bass will have an impact on the structure of the midrange, but obviously to a lesser degree than it will to the fixed frequency point of the bass.

Also- as you remove bass you are able to hear more of the mids and vice versa (as grungebob says just above).

Richardj

I have to be honest and say I usually struggle initially to find a sweet spot on an amp with a full tone stack, compounded by running either a Strat or LP which really need a different setup to get the best out of them. Are we getting lost in the need to minutely tweek?

Whilst I love being able to fine tune my tone I am still reveling, after more than a year of ownership, in the simplicity of my Pro Junior. Volume and tone controls, one makes it louder and ruder, one makes it brighter or darker. Simples!

ICBM

Maxi said:

In some cases changing the value of one pot has more than expected impact on other parts of the tone stack in other cases its the opposite  . The pots are just variable resistors and the signal chooses the path of least resistance so opening or closing one filter shunts or reduces signal to the other parts of the tone stack .

That's as good a way of thinking about it as any really!

In a standard guitar amp tone stack, all three controls are connected to each other, and all are passive 'cut' controls - there's no boosting anywhere. The frequencies each controls are determined by capacitors between the previous stage and the controls. A simple analogy would be three taps all connected to the same water supply - opening one tap reduces the flow to the other two. In some cases the mid control is so large a value that opening it up a long way not only reduces the effectiveness of the other two, it increases the overall 'flow' as well, ie reducing the total cut of the tone stack and appearing to boost gain. The same can be done by fitting the treble control with a larger cap, which allows more signal through in the upper mids as well.

grungebob said:

My latest amp buy has interactive tone Controls and active. Interactive to me seems the normal i.e. As you increase treble less bass or middle comes through and visa versa, the active tone Controls on my amp only changes the parameter your changing and doesn't effect the rest (the active also makes the amp a damn lot louder too!!)

Yes. Active controls can boost as well as cut. Usually the frequencies are divided into separate bands and then amplified independently, so the controls aren't interactive.

For some reason, the crude interactive passive tone stack just sounds 'right' for guitar amps and the active type doesn't, usually - it's not just that it's more powerful and needs setting more carefully.

Richardj

Just realised my post didn't add in any way to this discussion so have given myself a mild slap. I am however re-visiting my Koch TTII to mess around with it more and use some of the knowledge here to better understand why I cannot get as good a tone out of it as I should be able to. Thanks for the wisdom on show here.

ecc83

http://www.duncanamps.com/tsc/

If you can make sense of response plots the above gives you a good idea of how tone stacks work. If you can find the CR values for your particular amp you can "plug them in" and check it out....Bit nerdyacademic tho!

Of course ALL CR networks just produce a loss, "active" tone circuits use filters to release negative feedback to provide boost. 

Naturally the hi-fi bods do not want interactive controls so the Baxendall circuit was developed and this uses the virtual earth principle to keep the tone filter sections electrically isolated well, to a degree. You can in fact use a third, MID section in a Bdall control unit but after 3 sections interaction becomes too bad....But that is another story! As has been said, we don't like 'em for rock and roll!

Dave.

ICBM

ecc83 said:

http://www.duncanamps.com/tsc/

If you can make sense of response plots the above gives you a good idea of how tone stacks work. If you can find the CR values for your particular amp you can "plug them in" and check it out...

It's important to remember that the frequency graphs this produces are *for the tone stack alone*.

A lot of people read into it that it's for the whole guitar amp - and think that in order to get a 'flat response' you turn the mids up full and the bass and treble down low.

The actual response of a guitar amp can be quite different in the other stages, and even more importantly guitar *speakers* have nowhere near a flat response, especially not in open-back cabinets… they're almost all midrange, and have no real top-end at all. The reason the standard guitar amp tone stacks sound 'right' is because they have a huge mid cut when all the knobs are set halfway, precisely to compensate for the speakers. You can't ever get a truly flat response from a guitar amp - in fact why you would even want to is due to not understanding the problem!

Most guitar amps which use the Baxandall tone stack are really a simpler form of it called the 'James' circuit. (I'm sure you know that Dave!) One of the reasons this, and the standard TMB tone stack, are good for guitar amps (but bad for hi-fi) is that they introduce 'phase smear' which is a form of harmonic distortion - basically different frequencies are phase shifted by slightly different amounts, which produces a richer and more complex sound… bad for pure audio reproduction, good for tone generation.

Maxi

ICBM said:

For some reason, the crude interactive passive tone stack just sounds 'right' for guitar amps and the active type doesn't, usually - it's not just that it's more powerful and needs setting more carefully.

I was wondering about that ..Ive often used a graphic on the input and they sound great there but easy to get lost with your gain settings when you tweek them like you would a tone stack . Then you start changing the gain and the tone changes so you start messing with the tone stack on the amp and so on and so forth .

 

ICBM said:

Maxi said:

In some cases changing the value of one pot has more than expected impact on other parts of the tone stack in other cases its the opposite  . The pots are just variable resistors and the signal chooses the path of least resistance so opening or closing one filter shunts or reduces signal to the other parts of the tone stack .

That's as good a way of thinking about it as any really!

In a standard guitar amp tone stack, all three controls are connected to each other, and all are passive 'cut' controls - there's no boosting anywhere. The frequencies each controls are determined by capacitors between the previous stage and the controls. A simple analogy would be three taps all connected to the same water supply - opening one tap reduces the flow to the other two. In some cases the mid control is so large a value that opening it up a long way not only reduces the effectiveness of the other two, it increases the overall 'flow' as well, ie reducing the total cut of the tone stack and appearing to boost gain. The same can be done by fitting the treble control with a larger cap, which allows more signal through in the upper mids as well.

grungebob said:

My latest amp buy has interactive tone Controls and active. Interactive to me seems the normal i.e. As you increase treble less bass or middle comes through and visa versa, the active tone Controls on my amp only changes the parameter your changing and doesn't effect the rest (the active also makes the amp a damn lot louder too!!)

Yes. Active controls can boost as well as cut. Usually the frequencies are divided into separate bands and then amplified independently, so the controls aren't interactive.

For some reason, the crude interactive passive tone stack just sounds 'right' for guitar amps and the active type doesn't, usually - it's not just that it's more powerful and needs setting more carefully.

ecc83

ICBM said:

ecc83 said:

http://www.duncanamps.com/tsc/

If you can make sense of response plots the above gives you a good idea of how tone stacks work. If you can find the CR values for your particular amp you can "plug them in" and check it out...

It's important to remember that the frequency graphs this produces are *for the tone stack alone*.

A lot of people read into it that it's for the whole guitar amp - and think that in order to get a 'flat response' you turn the mids up full and the bass and treble down low.

The actual response of a guitar amp can be quite different in the other stages, and even more importantly guitar *speakers* have nowhere near a flat response, especially not in open-back cabinets… they're almost all midrange, and have no real top-end at all. The reason the standard guitar amp tone stacks sound 'right' is because they have a huge mid cut when all the knobs are set halfway, precisely to compensate for the speakers. You can't ever get a truly flat response from a guitar amp - in fact why you would even want to is due to not understanding the problem!

Most guitar amps which use the Baxandall tone stack are really a simpler form of it called the 'James' circuit. (I'm sure you know that Dave!) One of the reasons this, and the standard TMB tone stack, are good for guitar amps (but bad for hi-fi) is that they introduce 'phase smear' which is a form of harmonic distortion - basically different frequencies are phase shifted by slightly different amounts, which produces a richer and more complex sound… bad for pure audio reproduction, good for tone generation.

Yes indeed! Only take the DATSC as modifying the WHOLE amp response. Never heard of "phase smear" and I am not going to get into THAT discussion! Caps cause phase shift. Period.

BTW. If  you wanted a Baxendall tone control but interactive, build it with 4 sections and reduce the isolating resistors.

Dave.

ICBM

ecc83 said:

Never heard of "phase smear" and I am not going to get into THAT discussion! Caps cause phase shift. Period.

I will have to apologise for slipping into audiophool terminology .

Caps definitely cause phase shift. The 'problem' (from a hi-fi point of view) with the standard passive tone stack is that it's not consistent across the frequency range - I think it may make a genuinely audible difference too, since I pretty much always prefer amps with that sort of stack over ones without.

koneguitarist

I like amps with a single tone control.

Funnily enough though, when I had my first boogie, I struggled like hell with settings at gigs as they did so much more than fender. It was only after setting the treble and bass the way I wanted and learning the mid control did I get the hang of it. The Mid control on a boogie is a helluva tool.

midiglitch

Does that mean, that on paper at least, all tone controls at 10 is 'neutral' on something like my jtm45?

What about 'presence' is that a tone control in the same sense?  What does it do? (Electrically i mean- i can hear what it does to the sound.)

I run the bass *really* low on the jtm45 ( like 0 sometimes).  What would be the knock on effect of that on the rest of the signal path?

ThePrettyDamned

midiglitch;116805" said:

Does that mean, that on paper at least, all tone controls at 10 is 'neutral' on something like my jtm45?

What about 'presence' is that a tone control in the same sense?  What does it do? (Electrically i mean- i can hear what it does to the sound.)

I run the bass *really* low on the jtm45 ( like 0 sometimes).  What would be the knock on effect of that on the rest of the signal path?

I think that's the wrong way of looking at it.

If you like the bass on zero, and it sounds flat, treat it as such. The eq is a small component in what makes the sound.

Phil_aka_Pip

"Presence" is usually a control in the feedback loop, that mucks about with high frequencies. I (naievely perhaps) set my JTM & Bluesbreaker tone controls to 468 (BMT) when playing a Lester. What would other people do?

ThePrettyDamned

Phil_aka_Pip;116826" said:

"Presence" is usually a control in the feedback loop, that mucks about with high frequencies. I (naievely perhaps) set my JTM & Bluesbreaker tone controls to 468 (BMT) when playing a Lester. What would other people do?

On my amp, clean channel is (bmt) 8, 0.5, 6, bright switch off, with a compressor (with very little compression, it just makes cleans sound better... Like a klon maybe).

Dirty - gain 6 and a bit, bass 7 ish, mid 5, treble between 2 and 4 (newer strings are at 4, older ones get nasty overtones so I take the mids down a bit and the treble down to 2).

My mate came over and complained I had no treble, till he plugged in. It's just about finding what sounds good at the volume you're at, and if that happens to be bass 0, mids 0, treble 10, so be it.

ICBM

There is no such thing as a neutral response from a guitar amp. This is the most important thing of all to remember... there's no point in thinking about it or trying to achieve it, because it doesn't exist, even if it did it wouldn't sound good, and you wouldn't want it anyway!

A guitar amp is a tone producer, not a sound reproducer. The *only* thing that matters about its frequency response is whether it sounds *good* to you.

Presence is usually a negative feedback control affecting the amount of high frequency damping in the power amp, not a 'tone' control as such.

There are no 'rules' - experiment with them all, over their full ranges and in all combinations, and don't think that full or zero settings are wrong. For what it's worth I almost always set the bass full up in a passive tone stack - not just because I like "bassy" sounds but because it seems to loosen and soften the sound of the amp.

koneguitarist

I have different settings for different amps, for example with my tele and old JCM800 I always had it about 480 BMT, but amp cranked 6and 6 on gain and volume. All other changes was via guitar pots. Fender twin was more 688.