Framus Mid Control - What can I do with a DPDT?

[j-stylez]

Hey,

I am building a small top unit that will only have 3 controls, a gain, tone, and master volume knob. I've got a 250k push-pull pot with a DPDT switch that I want to incorporate as the tone control. Now I am looking for something interesting to do with the DPDT switch.

I have used the Framus mid control, as described here: http://chasingtone.com/yourguitaramp...tone-controls/ before, with adjusted values (1M pot, 100k resistor, 1n caps) to not load the signal too much:



Now I would use it with 5n caps, 25k resistor, and 250k pot. If that loads down my signal too much, I can install an IRF-820 MOSFET cathode follower before the tone stack.

My first idea, which I would really like, is to use the DPDT switch to change the whole tone stack to a Big Muff Pi control (you can also see it on the site linked above):




What I really like about the Framus mid control is that you can dial in that fender tone on pretty much any guitar, but also get some broader and rawer sounds with the mids not attenuated so much. I figured the Big Muff Pi control would give me something else, with more emphasis on the T/B ratio. Maybe someone got an idea how else the Framus mid control can be mixed up with a DPDT switch if my plan does not work? Maybe some kind of control for mostly the bass, with a fixed high/mid range?

TL,DR:
Is this going to effectively switch between the BMP and Framus mid control on a single 250k pot? I put in some Red color to See what happens, when the Framus circuit is activated.


Any feedback and ideas welcome, and happy building to you all! Cheers, Julius

[j-stylez]

I See that with the proposed way of switching, I will still have effectively a 10n capacitance across the outside Lugs of the 250k pot when the BMP tone stack is activated. Can someone help me understand what that is going to do?

[Chuck H]

I'm not really sure what your question is.?. But I modeled your circuits on spice. Each shown with the switch elements removed for clarity. Traces are with pot at full, pot at zero and pot at half resistance (since I don't know your pot taper). Honestly, I don't see where the "Fender" tone is for these circuits. Neither looks to be very dynamic with one acting primarily as a weak Big Muff control and the other as a very weak mid control. But they do seem to be relatively low loss as you intended.

[j-stylez]

Thanks for These models! I want to use the dpdt switch to change what my 250k pot does. On one setting, it's supposed to be the big muff pi tone circuit, on the other setting it is supposed to be the framus mid control. I have used the framus mid control before, with 1M pot, 1n caps and a 100k resistor. So I would like to switch between a mid scoop and the T/B thing the big muff pi circuit supposedly does.
My question therefore is, with my switching setup, whats the effect the ground lifted framus circuit still has on the signal... I think your Simulation Shows that I am doing something wrong.

Or, maybe I should ask a different question then: I must work out how to increase the responsiveness of these Circuits given that I can only use the 250k pot and dpdt switching.

[echuta13]

Not sure if it's helpful to you but there is a Framus mid control tab @ the online tone-stack-calculator: https://www.guitarscience.net/tsc/framus.htm

[j-stylez]

Thanks to both of you! I went ahead and learned how to use ltspice to do the simulations. That is a really nice tool. I compared my result to the online thing, and it checks out:



So now I will play around with my larger circuit above and see if I can make it work...

It is still my goal to be able to turn the framus mid control into something very different with the DPDT switch. But the thing is the ground-lift doesnt seem to detach it from my circuit enough.

[j-stylez]

I love this software. Thanks again for pointing me there!

So here is what I am getting when I simulate...it looks a bit different from your output:



I made 2 simulations, one with the switch lifting the ground of the Framus circuit and disconnecting that part from the input, the other one is lifting the ground on the Big Muff Pi circuit and disconnecting that respective input.

I think I just need to find a way to avoid the strong treble attenuation in the BMP circuit. Will try out some more values now...

[j-stylez]

Ok, I am getting closer!



What do you think in terms of responsiveness in this example? I have a bit less maximum scoop and the range is wider on the Framus style mid control. I tried to maximize the effect of both controls without changing the shape too much.

Do you think I get a fairly decent range and combinations on the BMP side of things? I would set the midrange focus a bit higher but it doesnt work out because then the Framus circuit scoop gets really wide and moves to the wrong range.

I think I will try this out on the workbench...any words of advice in terms of usability?

I figured that since I never maxed out the Framus mid control I have anyway, a little less attenuation compared to the original means I can use more range of the pot for precise settings.

[Chuck H]

That's looking pretty good. More range, less loss. You win. I might fudge around and try to get the Framus to where there's less bass cut and still keep the adjustment range you have now. It's a tall order, but there's nothing like a cad program for working things out.

I think it's interesting that your initial run got results different from mine. I assumed a typical 12ax7 gain stage and plugged in a 38k resistance for my voltage source. You have a different spice program from mine but I think you plugged in 12k? Anyway... My sims show a pronounced HF loss compared to yours.

[j-stylez]

Ahhh...then I think I was not properly taking into account the source impedance!

For the simulation of the signal, I just used a 1 V AC source. Would I have to add the value as the series resistance of that voltage source? Because when I add 36k to that, I get a pronounced attenuation of treble frequencies as well. It's under "parasitic properties" of the voltage source settings in my program:



BUT - I've still got my IRF-820 at hand, and if I plug in a really low series resistance value there, like 1k, my simulation is still close to the previous result:


I've tried a lot of combinations, but I think this is the best I can do. If I get the mid scoop more narrow in the correct frequency range, it screws up the BMP part. I will now try out this compromise and see how it sounds.

[Chuck H]

Or, if this is a build as yet to happen, you could just feed the tone stack from a cathode follower. Output impedance about 600R according to Duncan TSC. I'm not a silicootie hater or anything. But cathode followers "look" more eloquent in a design and don't require any additional voltage source considerations.

[j-stylez]

I have actually already built it, all parts except some chassis work and the tone circuit. But you are right on with the cathode follower!

I tested it - first straight off the plate of my 12ax7 first gain stage - and unsurprisingly, it was too dull. Just a treble cut on both controls. Then I installed the cathode follower (using an IRF-820, since this is a small and tight build in a tiny chassis, with just 4 tubes: 12ax7, 6au6, EL-84 x 2). And man, it works out nicely. Two different options, all on one knob. Pull out the knob, and you got from full flat signal to a nice mid scoop. Push it in and you have a choice of bass/treb seasaw kind of thing going. All in one pot, nothing to fiddle too much with. Although I'm yet to test it on high volumes, since its already past midnight here in Germany, it seems to work really well. I think I am actually able to use the whole range of the knob on both settings. I will report how it goes tomorrow when I test it on higher volumes.

Just in case anyone is interested, this is the final circuit I went with:



If you do build it, it is essential you do it after a cathode follower - it wont work as intended otherwise. I also have a thread going with the full circuit where I was contemplating the use of the IRF-820, I will upload the full schematic there once this is done.

[walkman]

I’ve used the framus mid control on several amps now and with 1M and 1n and 100k. When the control is at zero rotation there is a slight bass and mid boost. It would be great to get some bass cut at zero rotation. Any ideas?

[j-stylez]

Walkman, I played around with some values but I cant think of a way how you could achieve your goal. It's easy to add some attenuation for the lower frequencies in general, I also did that to my circuit. But only on zero rotation? I have no idea.

I tweaked the circuit a little more after testing it on louder volumes. There was not enough control for my taste, so I added a couple of components to cut out some bass and increase the attenuation. I increased the amount of attenuation in both variants. I'm getting closer to what I was having in mind.

[walkman]

I like the look of that lower plot... though I’m not really making sense of your circuit.

Can you put it in context in a full schematic


I had been thinking of a stacked pot with a bass cut wired in reverse ... but that may be too lossy