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josefr 23rd April 2003 10:26 AM

HQ tone regulator needed

Can somebody recomend me more than descent tone regulator (low,mid,hi or just low,hi). Schematics of course :nod:

ThorstenL 23rd April 2003 07:52 PM

Re: HQ tone regulator needed


Originally posted by josefr

Can somebody recomend me more than descent tone regulator (low,mid,hi or just low,hi). Schematics of course :nod:

Conventional "Tone" controls are pretty awful and are way too blunt instroments to get anything. I would recommend the "Pultec" EQ Circuit as being very musical and an ideal mastering/remastering EQ, unless you go for a digital remastering EQ.


lohk 23rd April 2003 11:35 PM

2 Attachment(s)
This is another Pulteq goodie from the Gyraf pages You will find more infos there.

Only the passive EQ section of the EQP-1R is shown. Although the orginal devivce uses tubes for amplification, you can also use decent SS circuits.
I suggest to put a buffer before the EQ section or an input transformer and an amplifier with 20-24db gain following the EQ.
I use old NAIM 42 boards which include unity gain buffers and amps, the EQ section comes in instead of the balance and volume pots.


josefr 25th April 2003 08:43 AM

nice but.....
Well - these circuits look nice but what is there a high quality tone regulation which uses potentiometers instead of multiposition switchs and inductors? Maybe some newer designs (and it would be nice if there would be not opamps).

lohk 25th April 2003 04:43 PM


are you sure to have read the schematics ?
The actual control in this circuits is done by potentiometers, only the center frequencies are changed with switches.

A passive tone control (tone correction) with inductors and capacitors like the Pulteq is BY FAR the best sounding possibilty - specially for hifi purposes or in the mastering studio.
But if you prefer opamps you will find something in the Gyraf pages too, and 1001 others in the net. You did not do a search yet, did you ?


josefr 28th April 2003 06:21 AM

I did a search and yes - I found more than 1k hits about tone regulation what was too much :cannotbe: - too much pages worth nothing :dead: - that's why I post here. And I DON'T like opamps ;)

Maybe I try this LC solution in the future....

THX anyway

ThorstenL 28th April 2003 07:24 PM



Originally posted by josefr
I did a search and yes - I found more than 1k hits about tone regulation what was too much :cannotbe: - too much pages worth nothing :dead: - that's why I post here. And I DON'T like opamps ;)

Maybe I try this LC solution in the future....

I would like to re-enforce a few points.

First, you need to be clear what you want to use the "tone" control for. The Tonecontrols found no most "stereo" gear are worse than useless. They have not much usability when processing single instruments through a single channel on a Band Mixer, for dealing with a whole "mix" they are completely off.

These "Bass, Mid, Treble" (or "Bass, Mid1, Mid2, Treble") controls can be realised in any funloving form, active, passive, partially parametric, or where things start working well fully parametric.

The best "HiFi" Tonecontrol I know is the Cello Palette.

This is more or less a 6-Way Equaliser, similar to graphic EQ's, but each band as we move towards the midrange has a different Q and a reduced range of control. While at the extremes the range was (IIRC) around 20db, in the actual midrange it was only 6db.

Circuitwise the Palette has loads of discrete Op-Amp's and is highly complex, prices are not just out of africa or china, they are not even out of the USoA, they are strictly off planet.

I find reasonably affordable digital Pro-Audio EQ's nearly as good and a lot more flexible (see my review of one such in the attached link).

Now back to the Pultec EQ recommended by lohk and myself. This is a circuit that goes back to the days before multitrack recording, before even stereo originally and is designed (by ear) to offer a "musically meaningfull" correction of the music. It is for very good reasons legendary among mastering engineers and has been "re-produced" in various slightly to very different versions over the Years.

Manufactures of some repute who have Pultec variations in their program include Manley, Ear Pro (Tim de Paravichinis pro Outfit) and several others.

What is being done as tonecontrol in the Pultec is purely passive, absolutely no active components or feedback for the actual tonecontrol. There are downsides.

The Pultec EQ produces a variable, rather low impedance load on the source. Original it was specified for use in 600 Ohm bridging operation, expecting thusly 600 Ohm source impedance. Secondly, in order to allow a given frequency range to be boosted we first must attenuate the signal, in the case of the Pultec by over 20db. This gain must be re-attained from somewhere.

So ahead of the Pultec circuit you need some form of impedance matching, unless you can be certain that you will always have a source with Z-Out < 600 Ohm and you will need around 20db+ gain after the circuit, with sufficient headroom and low output impedance.

For DIY with Solid State you could use a complementary J-Fet (2SK170/2SJ74) push-pull source follower buffer, or the most excellent Burr Brown BUF634 (loop) feedback free buffer. As gainstage a 2SK170 J-Fet cascoded with a BJT may offer itself, followed by J-Fet/pnp-BJT compund emitter follower to get low output impedance. Alternatively a Burr Brown OPA-627 (I know you don't like Op-Amp's - but the best of these sound as good or better as all but the most extremely refined discrete circuits).

For a Valve based circuit I would likely use a 10k:600 (4:1) stepdown input transformer and then a 6072A SRPP (2k Cathode Resistors) followed by a 5687WB SRPP (500 Ohm Cathode Resistors) which in turn drives a 10k:600 (4:1) stepdown output Transformer via parafeed.

Either version of buffering and output amplifier will have a unique sound signature, it may be worth when working on a studio version to build in all three options of input buffering and all three options of output circuitry, to have more "sounds" to play with.

Me? I'll just get the new Behringer 24Bit/96KHz digital EQ as soon as released, mod the analogue stages and be done. For home anyway. But that's just me.... :Pirate:


rafbraw 19th April 2015 04:34 PM

2 Attachment(s)
Hi all
I think this one is worth of publication, quite interesting and good sounding.. coming from Revox B-251.

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