Active tone stack?

[ricach]

I'm probably posting this in the wrong section but....

I'm researching my first Vintage build and want it to be simple, but would like gain, volume along with 3-band eq. I read how much adding tone controls rob power (and possibly character) from the amp and I'm wondering if there is a good simple 'active' tone stack design out there. Rather than re-amplify to make up for the loss, I'm thinking a tone stack that doesn't suck power from the main circuit would provide a wider range of control for the sound.

For that matter, is there already a simple tube amp that incorporates this already? I've seen that the Epi Galaxie 10 uses 3-band eq but from user reviews of this amp, it does appear the 3-band is very limited in its usefullness. And from looking at the schematic it does appear to suck power.

Thanks for any suggestions.

[Satamax]

Do you know how active filters work? Look at sound city, they're somewhat like an active, but not real active. Some baxandall coupled with active circuitry could be intresting. IIRC tube active filters use two tubes feeding the output of the second one back to the first one, cuting the frequencies that the filter let pass back to the first stage.

http://www.linkwitzlab.com/filters.htm

[ricach]

Baxandall

Yes and no. I envisioned a preamp filtering circuit driven by a preamp valve specific to that. Looking at the Baxandall-style tone control, it seems exactly that. Thanks for the info.

[Satamax]

Well, i'll try to explain a bit better he way an active filter works. Basicaly on a tube input and output are out of phase there a local negative feedback between output and input, which includes a filter, and that filter lets some frequencies through back to the input, thoses frequencies being 180 out of phase they cancel each other, nearly completely cuting out thoses frequencies. And by regulating the amount of local negative feedback you can control the volume of thoses cut frequencies compared to the ones not cut out. There's the other type, active regenerative feedback, where there's two valves the filter can be between the two valves or after the second, the out of the second valve is in phase with the input of the first one (360°) then when you feed back the filtered output of the second valve back to the input of the first one that raises the volume of the frequencies that the filter lets through, in turn increasing the volume output at thoses frequencies at the output of the second valve; this can quickly get out of hand. Oscilating like crazy. But imho, the only real active filter is the combination of the two, local negative (degenerative) feedback on the first valve, then regenerative (or positibve) feedback fed back from the output of second valve to the input of the first valve. This system can provide extreme boost or cut at the given frequencies. If you want to implement this over bass mids and trebles, it starts to get complicater. For example the "active" tone stack of a sound city is a simple ladder type filter having big cuts like -24db followed by amplification stages. Thoses amplification stages also ahave another -3db filter on their input, thoses two type of filters making high and low cuting points of a bandpass filter, there's no feedback involved. But their big problem is the high power of cutting or amplifying the passed signal, and some frequencies are overlaping making spikes or dips in the sound spectrum. Realy, the most natural sounding imho is a mid control with variable center frequency variable width and variable boost or cut, plus a bass control with top variable frequency and boost or cut, and treble with bottom variable frequency and again boost or cut. That's takes 3 12AX7 and minimum 7 knobs (that means boost or cut are controled by a single dual ganged pot for each chanel! otherwise it takes two pots!) Remember, this is all theory, i've never aplied it. And i've only seen it with solid state and not valves!

Hth

Max.

[Enzo]

The passive EQs in most amps are cut only, yes, but I think you sound overly concerned about it. YOu used the term sucking power several times.

Active EQ means you can actually boost a freq band, not just have it less cut than the others. There are lots of examples out there. Look at some recent loud PV amps.

Actives also tend to be a lot harder to dial in a tome than teh passives. YOu can get used to it and do real well, but many experience a steep learning curve.

To implement an active EQ, there needs to be a gain stage within it. Well your passive often uses a recovery gain stage too, so either way you need one.

The amoint of power your amp puts out depands on the power stage and how much signal it gets. Doesn't matter how it gets there.

ALl a cut only EQ means is that the overall signal level is lower coming out than it is going in. But so what? Unless the tone stack is feeding the power tubes directly, there will be more opportunities to bring the level up.

[ricach]

Not overly concerned

While researching what amp i wanted to duplicate, I read the user reviews on different amps and find an almost consistant problem with having too many tone dials on simple tube amps. it appears they degrade the character, and in some instances have minimal effect on the sound. I'm not so concerned with loss of power because I figured you can boost it back up as needed, but I felt it was inherit in the design that has so few parts, that changing the resistance anywhere in the circuit will have noticeable (but not desirable) changes in the overall characteristic. Therefore - moving the tone controls 'offline' so to speak, it would isolate the rest of the circuit from the resistance changes will 'dialing' in your tone.

I realize not a very scientific explanation, but more a logical one.

[Satamax]

Ricah, what are you trying to do? Metal at low volume? Fender cleans? American blues? English rock?

[ricach]

Learn. But not necessarily the hard way. lol. I'm not going after a particular sound, I just want the ability to dial in the widest possible sounds a Class A valve can produce. From that maybe I'll determine just what I really want - tweak or mod it as necessary - or build something completely different. All for the fun of it.

[Satamax]

Ricah, try to get your hands for a few hours on a mark4 sound city (120/200/50 pluses) not the PA ones. And see if you like dem! Otherwise, i think you could try a bax, it's a different and nice circuit. Or try the 15.43a

http://satamax.free.fr/P663.jpg

[hasserl]

It's cool and fun to think about such things, but for a first build I would strongly recommend against undertaking such an excersize. I always recommend simple circuits for first build projects, there is enough to figure out with a simple circuit. Save the experiments for future projects.

Like Enzo mentioned, amps with active EQ controls can be hard to dial in a good tone, small changes at the controls result in large changes in sound. It's real easy to dial in bad tones. Unless you enjoy sitting and tweaking an amps controls instead of playing, they can be frustrating. I am a big beliver in the KISS prioncipal, the best sounding amps, with the best touch sensitivity, IMO have very simple circuits. Adding gain stages and active filters increases the distance from the player to the sound coming out of the speaker. Keeping that distance short and uncluttered seems to be the best approach to me.

[Steve Conner]

My Ninja Toaster has all active EQ.

http://www.scopeboy.com/toaster.html

It actually has two "tone stacks", a 2-band Baxandall that uses a tube for gain, and a 2-band parametric that uses a tube and a bunch of op-amps to help out. The parametric is similar to the SVT midrange control, but the op-amp circuit simulates an "inductor" that can be continuously varied with a pot, to sweep the frequency.

I still run out of mids and wish I'd fitted an ordinary presence control too, though.

[ricach]

**snip** Unless you enjoy sitting and tweaking an amps controls instead of playing, they can be frustrating. I am a big beliver in the KISS prioncipal, **snip**

Thing is, I feel the same way, just didn't think to apply it to this project. I was looking to build the swiss army knife of Class A amps, without realizing the endless tweaking of controls needed to dial in the sounds I want. Only to have it change completely when i switch guitars/rooms/speakers/etc.

And Steve, looks like I was heading down the same path you did 8 years ago. Thanks to your efforts, and your conclusions, I think I'll change directions.

Maybe I'll build in an effects loop so later down the line if I so desire, I'll slide an active tone stack in there.

[Steve Conner]

Well, it's not like my active EQ didn't work, but in retrospect it just didn't seem worth the hassle. Sure, it gives extra tones that the usual combination of bass, mid, treble and presence don't, but the question is whether those extra tones are worth the extra hassle.

I mostly used to use it to dial in mid scoop and bass boost for making aggressive metal noises, but I grew out of that, when I discovered that most metal guitar tones are actually quite mid-heavy and all the thump actually comes from your bass player doubling the riff.

Recently it ended up in the backline at a local venue along with a Plexi reissue, and someone (maybe me) had left the mid control in the cut position, where it was last used in the studio recording dirty basslines for a punk album. It sounded very flat and dull compared to the plexi.

Now I mostly leave the bass EQ flat or even turned down, and use the mid EQ for a presence boost. Except if I use it as a bass amp, in which case the bass boost is nice. It has been used in backlines and on records as a bass amp, and works fine.

But again these results could maybe have been achieved with a standard tone circuit, plus presence (and maybe even resonance) controls on the power amp. The resonance control is cool, it boosts the bass partly by allowing your speaker cabinet to flab around at its natural resonant frequencies. So it's creating a bass boost by working with the speaker, instead of trying to fight it.

The Toaster will stay in my amp collection for the foreseeable future, and I have no plans to get rid of the active tone stacks, but I'm not sure whether I'd build another like it. (10 years older and wiser, I'd probably just add an effects loop and stick a Boss graphic EQ pedal in it.)