Active Muff-Style Tone Control

[Bill Mountain]

So I had an idea about 10 mintues ago for an active muff style tone control.

These are two 15dB boosts blended together (one at about 100Hz and the other at about 2kHz).  I haven't built this but it looks like it should work.  As you raise the bass you'll lower the treble and vice versa.



Thoughts?

[WGTP]

I think I remember seeing a design that looked like the standard BMP tone control in the FBL of an op amp.   

[Bill Mountain]

I think I remember seeing a design that looked like the standard BMP tone control in the FBL of an op amp.  

You know what?  Right after I posted this I started thinking about ways to do it with a single opamp.  The rule of thumb I follow is that if I don't know how to do it with an active circuit, I try to find a passive circuit that works in an opposite fashion then stick it in the opamp to see if it reverses operation.  Not very technical but it hasn't failed me yet!

This design has some flaws that I need to address.  First off it has a ton of boost and will have a big mid scooped feel (which isn't always bad) and it doesn't cut any frequencies so I would still need some LPF if this was in a dirt box.

If I lowered the amount of boost it might work as a quick "Shape" or "Room Balance" knob on a preamp.

[electrosonic]

Not sure if this helps, but this reminds me of the Ambler Tone Control - I have a Wireless World pdf about it somewhere I will link to it.

http://dl.dropbox.com/u/30050282/Ambler%20Tone-balance%20Control%20DCD.pdf

(Gotta love dropbox, that took me 30 seconds to move to my public folder and copy the public download link)

In the Douglas Self Book  - Small Signal Audio Design he shows his take on this circuit.


Andrew.

[Bill Mountain]

Not sure if this helps, but this reminds me of the Ambler Tone Control - I have a Wireless World pdf about it somewhere I will link to it.

http://dl.dropbox.com/u/30050282/Ambler%20Tone-balance%20Control%20DCD.pdf

(Gotta love dropbox, that took me 30 seconds to move to my public folder and copy the public download link)

In the Douglas Self Book  - Small Signal Audio Design he shows his take on this circuit.


Andrew.

Cool link.  I also like the simple discrete opamps used in the preamp.

[ashcat_lt]

I've been working on a thing I'm calling TiltBuzzer because I recently saw somebody call something like the BM tone control a Tilt control.  Apparently it was pretty common in consumer stereos...

Anyway, mine's got the control in the feedback loop and I had trouble getting the mid-scoop as deep as I wanted it.  Also just didn't seem to have enough range between the two extremes.  Ended up putting a static version of the same stage before it.  Still not enough.  Then I added positive feedback from the second stage back to the first.  That about did it, but of course it wants to oscillate.  I kind of dig the squeal at the top end (that's how you know it's loud!), but at the bass end it's unusable.

If I use the thing you posted here can I still call it Tilt?

[Bill Mountain]

I've been working on a thing I'm calling TiltBuzzer because I recently saw somebody call something like the BM tone control a Tilt control.  Apparently it was pretty common in consumer stereos...

Anyway, mine's got the control in the feedback loop and I had trouble getting the mid-scoop as deep as I wanted it.  Also just didn't seem to have enough range between the two extremes.  Ended up putting a static version of the same stage before it.  Still not enough.  Then I added positive feedback from the second stage back to the first.  That about did it, but of course it wants to oscillate.  I kind of dig the squeal at the top end (that's how you know it's loud!), but at the bass end it's unusable.

If I use the thing you posted here can I still call it Tilt?

You can call it whatever you want but it won't function the same as the tilt control.  It just allows you to blend 2 frequency boosts.  You may run out of headroom if you try to do this in any sort of dirt box so I would suggest going for smaller boosts on each side.  I still believe this could be done with a single opamp but it probably won't look anything like what I originally posted.  Why don't you post what you're working on?  I'd love to take a crack at getting you what you want out of it.

[ashcat_lt]

Because I kinda wanna say that I sorted it out myself...  hope that doesn't sound too prickish!  That and I'd have to wind up my big computer, render and upload the schematic...  I will get it up here eventually.  It's meant to give a decent range of horrible metal sounds.  running out of headroom is very much part of the point in my case.

I'm gonna at least try your thing and see if it gives me what I'm shooting for.  Credit will definitely be given where due.  I've already designed the graphics and am looking forward to seeing it on my pedalboard, so the name has to stay.

[Bill Mountain]

Because I kinda wanna say that I sorted it out myself...  hope that doesn't sound too prickish!  That and I'd have to wind up my big computer, render and upload the schematic...  I will get it up here eventually.  It's meant to give a decent range of horrible metal sounds.  running out of headroom is very much part of the point in my case.

I'm gonna at least try your thing and see if it gives me what I'm shooting for.  Credit will definitely be given where due.  I've already designed the graphics and am looking forward to seeing it on my pedalboard, so the name has to stay.

Cool.  Well let me know if it works.  I haven't had a chance to build it yet.  Not sure if I ever will...

[ashcat_lt]

Oh I certainly will.

Now that I actually look more closely at what you've got here, though, I can see that it will take some tweaking to get what I was thinking.  The standard BMT basically just blends between an LPF and an HPF with cutoffs that are not quite the same.  The pass bands of those two filters don't overlap at all so that when you turn it all the way one way you get only the frequencies which are not passed when it's turned the other way.  What you've got here is two LPFs with different cutoffs.  The caps to ground will set some lower limit to the boost, and that should be different for each, but I expect there will be significant overlap between the two.

It's really just a matter of adjusting the cap (and maybe resistor) values to get the response I'm looking for...

Anyway, the reason I originally posted here was because you mentioned about trying the BMT in the feedback loop of a single opamp and I have done that, and it works the way you'd expect.  You have to swap the caps between the high and low sides or else you get a mid-boost in the middle rather than the characteristic mid-scoop.  Also, the boosts slope toward infinity, so it requires some bounding.  The caps to ground which are required (unless you connect to Vref instead) help set a lower limit, but it also needs some smallish cap across the whole thing to set the upper limit.  Otherwise you start to get a lot of fizz from amplifying very high harmonics, as well as all the problems of huge amounts of gain at radio frequencies.

[ashcat_lt]

Oh!  Maybe it's obvious, but with the BMT in the feedback loop the knob works backwards.  Or rather you have to wire the pot opposite if you want it to work in the same direction.

[Bill Mountain]

I really should have called it something else.  Muff style came to mind simply because I was blending two different filters.

I like the idea of a regular tilt control in the feedback loop.  It makes me wander what an FMV stack would sound like as well.

[Bill Mountain]

You've got me thinking about trying it with flatter/wider boosts that meet somewhere around 600Hz.  But it still won't cut.