Hello, and help needed with an active Baxandall tone control variation

[EpicFail]

Hi all,

Just joined the forum, although have read posts on it for some time... Love the site.

I am trying to modify a tone control stage of a preamp for a violin pick-up (I know this is a bit off the normal beaten track for this website, but there are similarities...)

It is a version of a Baxandall Tone Control. Here is the schematic:

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/Preamp+sim+4-with-trad-buffer.jpg.html?g2_imageViewsIndex=1


My issue is that I'm trying to change the frequency response for the bass and treble controls. By having a play with it in SPICE I know that this can be done by altering the cap sizes, but I'd really like to be able to back this up with some calculations.

I have done a good web search (including http://sound.westhost.com/dwopa2.htm and http://makearadio.com/tech/tone.htm) and have found formulas for a number of Baxandall tone controls, but I can't find any calculations for the version I'm trying to alter and applying the formulas I have found to the circuit version I'm using doesn't work!

If anyone could suggest where to find calculations for this or what they are - formulas for corner frequency / turnover etc. - I'd be very grateful.

I would also be very interested to know whether people think this would be a good tone control system for this application, or whether there are others I should be considering... (pretty new to this stuff...)

Thanks in advance!

Epic

[artifus]

http://sim.okawa-denshi.jp/en/Fkeisan.htm

[Jered]

  And there is always this fun little tool.   www.duncanamps.com/tsc/tsc_setup.exe

[PRR]

> applying the formulas I have found to the circuit version I'm using doesn't work!

We can throw you a fish, or look at your fishing-technique and see why your fishing "doesn't work".

Show your math. Math skill can always come in handy.

Anyway, here's a fish:

Leave the resistors alone. Change the caps. Bigger caps (with smae resistors) work at lower frequencies.

> Here is the schematic

That does not look practical. Gain is high enough to clip e-guitar (though maybe OK for dynamic mike on violin). Should not need three-halves of a TL072 to do what it does. Boost-cut range looks odd (maybe it is meant for small changes).

Anyway... the James/Baxandall 2-knob EQs are meant for broad-brush adjustment of Full Orchestra, from basses to triangle. The violin has a narrower range and unique voice. Also being very acoustic, you may need to EQ-out narrow bands of feedback. I think you want more smaller brushes. _I'd_ go find a Boss/Ross/Berh 6-slider graphic EQ pedal.

(I once bid on a high-class modern string trio. Their sound contract called for _8_ channels of 27-band EQ.)

OTOH: Dave Baker used to play wonderful jazz on a cello through a 1969 Fender Twin Reverb. So the cannonical Fender BMT tonestack with all caps 2-4 times smaller might be a point of departure.

[EpicFail]

Hi all,

Thanks for the replies and sorry for taking a while to reply myself - been tied up with work...

artifus: that okawa site resource is great - thanks.

PRR: yeah, I knew about the fish. The formulas I was trying to apply were taken from the two sites I linked to. The whole area of filters / tone controls seem very big areas at the moment and I can assure you that I am trying to learn as I go... (currently working on understanding 2nd order / butterworth / sallen-and-key LPF and HPF).

I also take your point about using a multi-band EQ set-up, but this circuit is for a piezo-film (PVDF) pick-up for live performance and I just want to have the ability to make adjustment to bass / treble / mid. I've seen the Baxandall used in this way before for similar pick-up preamps. Perhaps finer adjustments could be done at the mixing desk end of things.

And yes, the gain in that schematic is too high... sadly found that out the hard way... The gain stage is also now an active gain control with max gain of 10, which still may be a bit too high.

I have also been thinking about ways to get the circuit into two halves of a TL072. I saw the "Bax in a Box" circuit http://www.diystompboxes.com/smfforum/index.php?topic=80191.0 recently and thought a variation on a theme of this might work, but my only concern is that the tone control in this one doesn't have any feedback...

Anyway, thanks again for all your replies.

[Bill Mountain]

http://www.ti.com/lit/ds/symlink/lm833-n.pdf

Page 14 shows the math for a Bax eq.  You can input the formulas in excel and play with cap values to see how the frequencies are effected.

[EpicFail]

That sounds great. Will definitely check that out. Thanks!