Active Baxandall question

[bushidov]

Hi All,

I am playing around in LTSpice and noticed something a little odd. I built a baxandall tone stack based on Peter Vis's site:
https://www.petervis.com/record_players_and_turntables/baxandall/baxandall-tone-circuit.html

It claims it is an active tone-stack because it cuts and boosts. However, when I put it into LTSpice, it just cuts. Below are the screen shots:



In this example, I am leaving the "treble pot" at noon (50%) and doing a sweep of the bass pot (1 - 99% stepping 10% intervals)
I'm pumping a 0.123mV signal and checking 100 points per decade, starting at 10 Hz and going up to 30kHz.

When looking at the output, with both treble and bass knobs at noon (50%), I am getting a relatively flat response, but dropped to -18dB.

If this was an active baxandall, shouldn't that be at 0db instead?

[PRR]

Yes, it should be unity gain. Simplify (rip stuff out) until it at least does that.

[bushidov]

I think I figured it out. I didn't have to rip anything out. I just had to change my input voltage the under small signal AC analysis section from that of 0.123V to just 1.0V. Now it centers around 0dB.

Now for the final question, "what's the frequency Kenneth?"

But seriously, if I were to put a bass knob on a pedal with this baxandall, what would I call it? 100Hz?

[Sooner Boomer]

How about 100 Hurts ?

[PRR]

...if I were to put a bass knob on a pedal with this baxandall, what would I call it?

"Bass"?

I don't think Hertz are much help to most players, especially on a really broad-band EQ like this.

[POTL]

I have said it many times and will say it again, Baxandall / James is not the kind of equalizer we want to use for guitar or bass, it is for hi-fi, but not for musical instruments. Sure, you could say Ampeg uses it, or give examples of some boutique stompboxes that have it in their design, but does that work well for us? if the treble controls are working fine, much like any other passive tone stack or tone control, then the bass controls are below the range we want (like most passive tone stacks).
In 2021, we can use active regulators, with control of the desired frequencies. IMHO for guitar the best control is about 80Hz, maximum 100Hz. For bass guitar, probably 40-50Hz is something like E on an open thick string. I think it makes sense to use gyrators and fine control circuits, at least for low and medium frequencies, although high frequencies are better processed pointwise. 3 adjustments are enough for most musicians, maximum 5 (hello Mesa Boogie).

[bushidov]

Well, not to be contradictory to POTL, but the reason I was asking about "where do you call the frequency" is from a bass pedal company called Darkglass. One of their best selling pedals and known heavily for its tone is the B7K and all of its derivatives. It's tone stack is an active baxandall, which then goes into a two active peaking equalizer circuits, back to back. The baxandall tonestack, which comes first in their active tone-stack (although, there are some passive filters prior to it), is the one in my post. On their website, they call the low/bass "100 Hz" and their high/treble "5 kHz", because it is a wide "sweep" that a baxandall is, I am not sure where the frequency is, so much as where it isn't. As a side note, the Darkglass B7K's lo-mids and lo-highs are the two active peaking circuits.

IMHO, I think all tone stacks have their place, even in the guitar and bass realm. As of bass, I shelve stuff under 100Hz for studio, but may boost 40-50Hz in a live event in such that the front of house's system sucks. I scoop around 300-450Hz, as that's a lot of mud, boost after that till about 1.2K. If playing with a pick, 1.2K I steeply remove as that is the area where pick-clank occurs and it's like fingernail on chalkboard for me. Then I boost 2k - 5k for the sparkly highs, but also if playing slap bass, that's a good frequency to boost and then shelve everything after that. But, this is just me. Other people have their own ways that work for them.

[ElectricDruid]

I am playing around in LTSpice and noticed something a little odd.

<snip>
When looking at the output, with both treble and bass knobs at noon (50%), I am getting a relatively flat response, but dropped to -18dB.

If this was an active baxandall, shouldn't that be at 0db instead?

This is common LTspice problem. It doesn't reference the output level to the input level unless you specifically tell it to. If you leave it to its own devices, you need to recognize that what you call "input level" it calls "-18dB" (or whatever), so you've got boost above that and cut below. If you want to get it to give you a graph centred around 0dB, you either need to keep fiddling about so it finally gets it right (like you've done) or plot V(output)/V(input) instead.

[antonis]

I have said it many times and will say it again, Baxandall / James is not the kind of equalizer we want to use for guitar or bass

I'm deeply sorry but I'm obliged to inform you that some of us (including myself) aren't arsed simply by repetitive proverbs..

[iainpunk]

edit: this post is a bit of a rant on tone stacks, sorry. its also largely my opinion, not fact, you can probably skip the whole post and not miss anything of value. proceed with caution

on distortions i personally prefer the baxandall over the classic 3-band styles of tone controls, since it feels like i have better mid range control with a baxandall (or James for that matter). (despite those tone stacks not having a mid control, you can actually boost the mids, instead of only scooping.)
peaking and scooping controls with narrower bandwidth feel less natural, more polished, which is fine for some applications (modern metal/rock), but not for others (punk/blues)

less is more, especially with knobs, a 2 or 3 knob OD/DST/FZ is often way easier to get a satisfying tone out of than a 4 or more knob pedal. probably has to do with choice paralysis.
another good tone control is the BMP in the feedback loop of an opamp.
this basically makes it a boosting tone control. when its in the middle, the original mid scoop transforms in a mid boost, and turning it either boosts more bass or more trebble. it needs an extra 5k6 resistor on the former bass side (which is now the treble) to not oscillate on the brightest setting, but its really easy to use and sounds amazing. (a call this the inverted Big Muff Pi tonestack, since the frequency transfer graph is flipped upside down)

i'd also like to confess that, despite my experimentation with lots of tone controls, i generally leave out any types of tone control on my pedals, or gain control for that matter. i love the infiltered, broken but open sounds that come from un-filtered clipping!
IMHO, the best guitar sounds are from little tiny practice amps with all knobs on 10, distorting their little transistor 'power' amps to heck and back. this is always unfiltered, raw and pure.

my opinion in conclusion:
none > bax/james > inv. BMP > norm. BMP > 3-band > simple treble roll-off

cheers, Iain

[ElectricDruid]

the inverted Big Muff Pi tonestack

So the Big Muff !d tonestack??

[marcelomd]

My personal preference for bass preamps, specially that topology, is 60Hz, 250Hz, 1000Hz and 3000Hz.

Shelving 60Hz for that dub bass feeling. Add or cut for tightness.
250Hz is there the punch is.

[iainpunk]

the inverted Big Muff Pi tonestack

So the Big Muff !d tonestack??

ıԀ ɟɟnW ƃıq tone stack

cheers

[mitchelr]

Is there a schematic for the "inverted Big Muff Pi tonestack"
Cheers

[antonis]

Is there a schematic for the "inverted Big Muff Pi tonestack"

[mitchelr]

Thats clever but its not active 

[antonis]

It's followed by a CE recovery amp so it's active enough..

[iainpunk]

the filter values can and should be changed for normal tone control use, the filters are shifted apart here to get a more peaky response for WAH-ish use, (its a non-standard wah, the 'feel' is different. its bet before overdrive or fuzz, not after)
i used the Tone Stack Calculator to get the sweeps, and Photoshop to change the scales and polarity.

the input-reducer can be left out or be replaced by a volume control.



cheers, Iain

EDIT: the freq. sweep doesn't take R4 in to account. R4 makes sure that the extremely treble boosting curve is avoided, so you don't have an oscillator that also amplify's noise to heck...

[amptramp]

At what point do you say to hell with tone controls on the individual pedals and amplifier and just stuff in a graphic equalizer?

I understand that with non-linear pedals like distortions, the effect of a tone control going in is going to be different from a tone control at the output and simpler tone adjustments work for this type of case.  You can do what Jefferson Airplane pioneered in 1973 by using a Hewlett-Packard Fast Fourier Transform analysis unit to allow the sound engineer at a live performance to adjust tone in each area of the audience.

The question is, what is the problem the tone control is supposed to solve?  The answer will be different for the studio and live performances.

The three-knob Baxandall tone control could be modernized a bit to have second-order mid feedback so the boost/scoop bandwidth would be a bit more narrow.

[antonis]

The three-knob Baxandall tone control could be modernized a bit to have second-order mid feedback so the boost/scoop bandwidth would be a bit more narrow.

+1..