Author Topic: Tube-based EQ Schematic (NOT a Baxandall) :-)  (Read 18701 times)

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« on: July 28, 2004, 12:05:53 PM »
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?
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.


gyraf

  • Global Moderator
  • Member
  • ****
  • Posts: 7130
  • Aarhus, Denmark
    • http://www.gyraf.dk
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #1 on: July 28, 2004, 12:15:54 PM »
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:



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.
..note to self: don't let Harman run your company..

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #2 on: July 28, 2004, 12:18:29 PM »
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?
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.

gyraf

  • Global Moderator
  • Member
  • ****
  • Posts: 7130
  • Aarhus, Denmark
    • http://www.gyraf.dk
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #3 on: July 28, 2004, 12:22:58 PM »
Both "baxAndall tone" and "baxEndall tone" comes up with results?
..note to self: don't let Harman run your company..

  • Guest
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #4 on: July 28, 2004, 12:26:24 PM »
Quote from: "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?

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

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #5 on: July 28, 2004, 12:40:27 PM »
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.
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #6 on: July 28, 2004, 12:57:18 PM »
Cool, I found a Rod Elliot article explaining the Baxandall tone circuit, so all is good now, I think.
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.

VacuumVoodoo

  • Member
  • Posts: 216
  • Gothenburg, Sweden
    • http://www.tubewonder.com
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #7 on: July 28, 2004, 01:47:10 PM »
Quote from: "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:



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.


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
Alex Niemand
_____________________________________
Life's a party but you get invited only once...
Tubewonder amps
"L

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #8 on: July 28, 2004, 01:50:29 PM »
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.
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.

  • Guest
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #9 on: July 28, 2004, 02:54:22 PM »
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%20Input%20Stage.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.


sismofyt

  • Member
  • Posts: 1002
  • Denmark, Aros
    • makeb4break
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #10 on: July 28, 2004, 03:54:56 PM »
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:



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

sismofyt

  • Member
  • Posts: 1002
  • Denmark, Aros
    • makeb4break
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #11 on: July 28, 2004, 03:57:47 PM »
Here is a tubed one:


sismofyt

  • Member
  • Posts: 1002
  • Denmark, Aros
    • makeb4break
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #12 on: July 28, 2004, 04:01:05 PM »
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:

NewYorkDave

  • Member
  • Posts: 4402
  • New York (Hudson Valley)
    • Phantom Playland
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #13 on: July 28, 2004, 04:07:52 PM »
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.

sismofyt

  • Member
  • Posts: 1002
  • Denmark, Aros
    • makeb4break
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #14 on: July 28, 2004, 04:14:41 PM »
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.

PRR

  • Member
  • Posts: 7441
  • Maine USA
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #15 on: July 28, 2004, 04:37:28 PM »
> 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.)

sismofyt

  • Member
  • Posts: 1002
  • Denmark, Aros
    • makeb4break
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #16 on: July 28, 2004, 04:40:41 PM »
Ok, thanx for the info. That stuff ain't regular in euroland, allthough I know it's common in the US.

  • Guest
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #17 on: July 29, 2004, 12:30:33 AM »
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.

Scott S.

  • Member
  • Posts: 12
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #18 on: July 29, 2004, 11:21:08 AM »
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

dmlandrum

  • Member
  • Posts: 1649
  • Port Huron, Michigan, USA
  • Cosmic Muffin
    • The Eternal Prototyper
Tube-based EQ Schematic (NOT a Baxandall) :-)
« Reply #19 on: July 29, 2004, 11:31:37 AM »
Here's VacuumVoodoo's tube EQ schematic:

http://studioconsul.net/techtalk/niemand/TubeBaxandall.pdf

It looks pretty cool, and not too hard to build.
Darren Landrum

Be comforted that in the face of all aridity and disillusionment
And despite the changing fortunes of time,
There is always a big future in computer maintenance.


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
918 Views
Last post March 26, 2008, 04:34:50 AM
by zebra50
2 Replies
1488 Views
Last post April 16, 2008, 02:28:12 AM
by PRR
12 Replies
1628 Views
Last post October 12, 2011, 04:53:36 AM
by Junction
8 Replies
775 Views
Last post July 18, 2013, 05:08:26 PM
by ruffrecords
7 Replies
620 Views
Last post October 30, 2013, 07:16:44 PM
by abbey road d enfer