Active BMP Mid Control

[D_Ex_Patria]

So a few months ago, I was looking over some old Matamp schematics, and after staring hard at one of the inductors running from a tube cathode (the presence control, if I remember rightly) and started to wonder if an equivalent circuit could work in the silicon world.

I have very limited electrical knowledge, so I started a process of guessing and testing with different values of common inductor, and eventually I came up with an active mid boost that can be tacked onto a common emitter stage. The 'special' part is a 100mH inductor, which I got in the shielded variety. Not a totally off-the-wall value, and available from DigiKey and Mouser.

Here it is combined with a Russian Muff tone control:



It can go from totally flat, to about 15dB of really sharp boost, without 'rolling in' from either the low or high end, like I've seen most mid controls do. The flavor of tone stack I chose puts a scoop above the center of this boost, but I imagine you could make an arrangement where the boost aligns more precisely with the Muff mid scoop. I imagine that would give you a more Hi-Fi kind of equalization. I like the Russian BMP, though, so I didn't bother overengineering.

This is built and living inside my Russian Big Muff clone, and I've got to say, I'm really pleased with the results. I know this is more parts heavy than the usual mid controls on Muffs, which are usually a single pot and a capacitor at most, but I think it's a worthwhile alternative.

[idy]

Thanks for this. I just ordered a few 100mh inductors. Sounds worth a try especially for BMP. I have not really liked the other "mid" controls I've tried. Love using the existing recovery stage!

[KarenColumbo]

Whoa! I still have a "custom Muff" in my long list of pedals-yet-to-build. I also wanted an active tone control, but with parametric mids. I just made three bands of R. G.s parametric design (http://www.geofex.com/article_folders/eqs/paramet.htm), bass and treble had fixed values. Sadly the layout of the (trapezoid) PCB (7 pots) left no space for this construction - I'm still terrible at layouting.
Thank you for this - I have to try it.

[Elektrojänis]

Hmmm... Could that inductor be replaced with a gyrator circuit? To me it looks like it could, but it's way beyond my skill level. That way you would not need an inductor at all.

[KarenColumbo]

Hmmm... Could that inductor be replaced with a gyrator circuit? To me it looks like it could, but it's way beyond my skill level. That way you would not need an inductor at all.

If you cast a glance at the URL I posted before there's a quick, but informative explanation of gyrators.
And of course there's always Rod Elliott: https://sound-au.com/articles/gyrator-filters.htm

[iainpunk]

Hmmm... Could that inductor be replaced with a gyrator circuit? To me it looks like it could, but it's way beyond my skill level. That way you would not need an inductor at all.

a gyrator bandpass filter is the same thing that came to mind when i saw this.
its probably cheaper as well.
i think a double opamp is way easier to implement, i like using eh HM-2 tone stack and leave out the higher feq. gyrator and the bass control from it, so just boost that 850-ish hz range with a huge amount. having it not on the same center frequency as the BMP scoop makes it more musical IMHO

cheers

[D_Ex_Patria]

I did think of translating this to a gyrator; there's no technical reason that final gain stage couldn't be replaced with a dual op-amp providing boost and the gyrator equivalent of this circuit.

I went with passive components in the end mostly as an artistic choice (and I think I heard on a separate thread that a passive inductor almost always has less noise than a gyrator... but I'm justifying after the fact, there).

Also, BJT saturation, which I'm pretty sure IS happening at max gain in this circuit, seems to blend nicely with all the clipping that's already happening. Not sure if that would hold true with an op-amp. There's probably solutions to that problem as well.

What I haven't tried is using a way cheaper non-shielded inductor. If it works in all those classic Wah circuits, why not here? I'd probably have to pay closer attention to layout if I went that route.

[D_Ex_Patria]

Whoa! I still have a "custom Muff" in my long list of pedals-yet-to-build. I also wanted an active tone control, but with parametric mids. I just made three bands of R. G.s parametric design (http://www.geofex.com/article_folders/eqs/paramet.htm), bass and treble had fixed values. Sadly the layout of the (trapezoid) PCB (7 pots) left no space for this construction - I'm still terrible at layouting.
Thank you for this - I have to try it.

Thanks for this. I just ordered a few 100mh inductors. Sounds worth a try especially for BMP. I have not really liked the other "mid" controls I've tried. Love using the existing recovery stage!

Let me know how your own experiments go! Mine went really smoothly, but I'd be interested what other builders experience.

[PRR]

> active mid boost that can be tacked onto a common emitter stage

Echos the Ampeg mid boost/cut network:

[antonis]

Yeappp... 

[D_Ex_Patria]

> active mid boost that can be tacked onto a common emitter stage

Echos the Ampeg mid boost/cut network:

Yeappp... 

Yeappp indeed. Not surprised to see it popping up elsewhere. It was adapted from an old Matamp schematic, so I know I'm not inventing anything. Haven't seen it in a Muff though, and I think it's a nice fit.

[idy]

Just tried this on a muff and it works. Pretty broad but absolutely allows you to dial in a VERY present and aggressive mid boosted sound. And at minimum you have the stock sounds.

Nice way to get some tone shaping out of the recovery stages instead of adding more stuff on to the lossy tonestack.

[D_Ex_Patria]

Just tried this on a muff and it works. Pretty broad but absolutely allows you to dial in a VERY present and aggressive mid boosted sound. And at minimum you have the stock sounds.

Nice way to get some tone shaping out of the recovery stages instead of adding more stuff on to the lossy tonestack.

Which inductor did you use? I went with a shielded one in mine, but I was wondering if one could get away with something smaller and cheaper.

[idy]

I just did E-bay.


US Stock 10pcs 100mH 104 Inductor Choke Radial Lead Power Inductor 8mm x 10mm

https://www.ebay.com/itm/391736067127