Tube-based EQ Schematic (NOT a Baxandall) :-)

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Consul

Well-known member
Joined
Jun 3, 2004
Messages
1,653
Location
Port Huron, Michigan, USA
After reading Brad Blackwood's thread (at his own forum on PSW) to design a DIY EQ, I decided to do some research on this name Baxandall, and I found this at HeadWize:

http://www.headwize.com/projects/showproj.php?file=equal_prj.htm

If you scroll down to figure 12, you'll see a schematic for a tube-based Baxandall active EQ (at least that's what they call it). Here's a direct link to the image if you just want the schematic with no explanation:

http://headwize.com/images/eq11.gif

Any thoughts or comments? Other than needing a +/-150v power supply, it looks like an easy build.

On a side note, I didn't find much of anything else on Baxandall or his theories and designs. Any pointers anyone can point me to?
 
That is NOT a Baxandall eq circuit - it is an active one-tube gyrator type of eq.

A baxandall works on having the filter component (most often a capacitor) either in series with an inverting input, or in the feedback path of an inverting amplifier stage - controlled by a single pot:

Fc111-3.gif


From: http://www.designnotes.com/CIRCUITS/3band.htm

The first opamp is not needed, the second opamp can be substituted with a tube stage.

Jakob E.
 
And somehow I knew that it would be wrong. I'll change the topic to suit. It still looks like an interesting circuit.

This all still doesn't explain why Google doesn't return anything worthwhile for the terms "baxandall equalizer" or "baxandall equalizer design" or any other permutation. Maybe someone has the original paper that he published?
 
[quote author="Consul"]And somehow I knew that it would be wrong. I'll change the topic to suit. It still looks like an interesting circuit.

This all still doesn't explain why Google doesn't return anything worthwhile for the terms "baxandall equalizer" or "baxandall equalizer design" or any other permutation. Maybe someone has the original paper that he published?[/quote]
try using the english and not american spelling
baxandall equaliser or "baxandall tone control"
the original paper was called "Negative Feedback Tone Control - Independent Variation of Bass and Treble Without Switches"

http://cidtel.inictel.gob.pe/Publicaciones/rvargas/JBPTCN.htm - references the original paper
 
What I kept getting were a lot of results for eq plugins for DAW recording. Maybe the real pages I needed just slipped through the cracks. And I never would have thought about the British vs. American spelling thing. I know, I'm an ugly American.

One time I saw a sign for a motel in Delta (a town about 20 minutes from here) that said:

CLEAN ROOMS
REASONABLE RATES
TRUCKRES WELCOME

And the first thing that came to my mind is, "that must be the Canadian spelling." :green: :green: :green:

Thanks for the links. I think I can take it from here. Although I am still curious about that simple gyrator circuit. Think it might be worth breadboarding? I don't know if I want to build a +/-150v supply for only one experiment.
 
[quote author="gyraf"]That is NOT a Baxandall eq circuit - it is an active one-tube gyrator type of eq.

A baxandall works on having the filter component (most often a capacitor) either in series with an inverting input, or in the feedback path of an inverting amplifier stage - controlled by a single pot:

Fc111-3.gif


From: http://www.designnotes.com/CIRCUITS/3band.htm

The first opamp is not needed, the second opamp can be substituted with a tube stage.

Jakob E.[/quote]

Jakob,

I would disagree with your statement that the first opamp is not needed. Since the EQ circuit presents varying load impedance to the preceeding circuitry it should be driven with a low output impedance buffer.

I have built an all tube based Baxandall EQ. It works fine if it's driven from a cathode follower but putting all 3 three control chains in a feedback loop around just one triode gives rather poor results with controls becoming heavily interactive. This was the case even when the triode gain stage was followed by a cathode follower and fed back from it. I have split the circuit so that each freequency band is handled by it's respective triode gain stage, three such stages connected in cascade (series). Three band Baxandall type EQ can thus be built with just 2 dual triodes. Bass and Treble are switchable between peaking/shelving, the midband has switchable centerfrequencies, control range of +/-12 to 15 db is easily obtainable.

I'll gladly share the schematic if somebody can host the file.

Alex
 
I can host it with no issues. Just email them to me:

consul (at) studioconsul (dot) net

You'll need to deobfuscate that, of course. I'm looking forward to seeing the circuit.
 
I use a tube Baxandall tone control circuit in a guitar amp I am designing. I have used the bass and treble circuit many times and it works very well. The midrange circuit is a new twist for me, but preliminary tests look really good. The schematic is here: http://users.adelphia.net/~thomasholley/Guitar/Guitar Input Stage.gif

I had to seperate the bass/treble section from the midrange section because there was way too much interaction and things turned to mush. Seperated it works as it should.
 
Jakob said: "A baxandall works on having the filter component (most often a capacitor) either in series with an inverting input, or in the feedback path of an inverting amplifier stage - controlled by a single pot"

Are you sure? I thought the baxandall circuit worked like this:

t-contr.gif


The EQ doesn't have to be active, as far as I know.

That EQ sounds wonderfull, btw :wink:

www.mitedu.freeserve.co.uk/Circuits/Audio/t-ctrl.htm
 
uk said: "try using the english and not american spelling
baxandall equaliser or "baxandall tone control"
the original paper was called "Negative Feedback Tone Control - Independent Variation of Bass and Treble Without Switches"

Ok, I got it wrong. I've often seen a passive baxandall refered to, eh, baxandall.

I like the sound of the passive better :wink:
 
The passive EQ is not a Baxandall. Baxandall's innovation, in this case, was modifying that topology and placing it in a negative feedback loop.

An amplifier to be used in a Baxandall EQ should have an open-loop gain of at least 6 times (preferably 10 times) the maximum boost, which is 20dB in most versions of the circuit. So, we're talking a minimum OLG of 35 or 40dB. That rules out a simple single-triode amplifier. A pentode would be more suitable.
 
Ok, I never tried the tube one, but I did the passive one with exellent results. With demands on the opamp like that, no wonder 'tone controls' on hifi amps got such a bad rep. A 741/301 aint gonna cut it :roll:

I've never seen an active tubed baxandall, but the passive on is very common.
 
> I've never seen an active tubed baxandall

All stereo Fishers and most EICOs used a Baxandall network around a single half of a 12AX7.

Linear dual pots are much easier to get than duals with the right audio taper and well matched. (Anyway feedback was sexy.)
 
As was mentioned, Baxandall is not a passive circuit. Many passive circuits sound great, but none of them are Baxandalls.

as the schematic I posted earlier shows, I use a Baxandall around a single triode with a boost/cut of about 15dB to 20dB. I have no problem with not enough gain. They are just about my favorite tone control circuit. Pretty transparent when set to center and a nice, evenly distributed, easily controlable boost/cut frequency curve.
 
The HH Scott preamps, integrated amps, receivers such as 222, 299, etc also used the Baxandall in a feedback loop like the Fisher, Eico, etc.

Schems at http://hhscott.com

The 299C is a particularly good sounding amp

http://hhscott.com/pdf/299C.pdf
 
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