Passive Midrange revisited

[Natman]

Hey all,

I posted this thread years ago:

http://music-electronics-forum.com/t20252/

I made one mid control just as listed, works fine.

Recently I wanted to try another one, minus the "boost" feature but came across this:

Turn Your Tone Control into a Passive Midrange Cut | GuitarPlayer

Here, it sounds like they do not use the cap & resistor as Rothstein does; the inductor goes in series with the tone cap and that's it (there is a youtube video demonstrating this mod on a Dean).

So my question is: What role do the cap & resistor play in this circuit? What would a higher/lower value do to the sound?

Thanks for your help!

[Natman]

Ok, minor update:

I just wired up one of these for cut only using a 0.047 uF cap and 220k resistor on a 500k log pot. It is definitely rolling treble off, something I didn't notice on my last one. Sounds pretty good but I feel it could be better.

This time, instead of using the choke's primary only, I joined it with the secondary to increase inductance.

Does anybody know what function the cap and resistor play? I realize it is "tuning" the frequency somehow, but what's not clear to me is what/how each does their thing. On a low pass filter, a larger cap gives less treble. But when a resistor is in parallel, all my theory goes out the window!

Thanks for your help!

[Dave H]

Does anybody know what function the cap and resistor play? I realize it is "tuning" the frequency somehow, but what's not clear to me is what/how each does their thing. On a low pass filter, a larger cap gives less treble. But when a resistor is in parallel, all my theory goes out the window!

The 0.039u cap is the original 'Tone' cap which forms a series resonant circuit with the 1.5H inductor giving a mid range notch at a frequency of f = 1/[2 x pi x sqrt(LC)]. The Tone pot sets the 'Q' of the notch or degree of mid cut. I'm guessing the resistor across the 0.039u cap limits the Q when the pot is short circuit so the mid notch isn't too deep and narrow..

[Natman]

huh -that's interesting. So here I was thinking that I was smart by increasing the inductance, but really all I did was shift the resonant frequency down. If I got this right, I need to reduce the cap to get it a little higher. Rothstein says his is at 800 Hz.

Stay tuned (pun intended) I'll try it and see what happens. I wish I could generate frequency plots to see graphically what's going on. Thanks Dave!

[Dave H]

So here I was thinking that I was smart by increasing the inductance, but really all I did was shift the resonant frequency down. If I got this right, I need to reduce the cap to get it a little higher. Rothstein says his is at 800 Hz.

Reducing the cap, inductor or both will shift the resonant frequency up.

[Dave H]

I wish I could generate frequency plots to see graphically what's going on.

Here's a quick simulation with the tone pot set to 10k.

[Steve A.]

I've used the Dan Torres version of the Craig Anderton mid boost/cut control many times. In a typical SSS strat I mainly use the mid boost while in a Les Paul the mid cut works really well in getting a really nice clean tone with reduced mud. Remove the .001uF cap and 1M1 resistor for a mid cut control.





Steve Ahola

[Natman]

Here's an interesting set of calculations. They are pretty approximate but give the right idea.

The first one is using the "standard" Torres/Rothstein values.
Formula gives 658 Hz, apparently right at 800 Hz in practice according to Rothstein website. Since this is a theoretical exercise, we'll keep the calculated frequencies as it's only for comparison.

I joined the primary & secondary of my inductor (42TL021) for about 2.7 H and I did not have a 0.039 uF cap on hand, so I used a 0.047. You can see that by doing these 2 things, I shifted the frequency down a bunch to ~447 Hz! So I can change the cap (I have a 0.022) and I would get 653 Hz, which is awfully close to the original (indiscernible most likely given tolerance of these components).

Lastly I wanted to see if I removed the secondary leaving the current cap,I'd get 599 Hz, which is not as close.

F / L/ C
658.0 / 1.5/ 0.039

446.8 / 2.7/ 0.047

653.0 / 2.7/ 0.022

599.4 / 1.5/ 0.047

So barring any interactions/parameters that have not been included (if any?) I get right back up to the "proper" frequency by using a 0.022 uF cap. So I soldered it in this morning before heading to work! I have to wait til I get home to try it Believe me when I say I'm itching to get home...

[Natman]

Yep -this is wildly useful! Having the toggle switch to chose regular tone or mid cut gives tons of versatility. Something to note is that using the "boost" portion of the original design limits the travel of your cut to half the roation of your knob. I get more range and subtle shades than my other axe (with "boost"). I REALLY like it now, at least enough not to change anything for a while.

This is a highly recommended mod for passive guitars. You can add a mid cut on the fly without having to touch your amp or pedals. This is handy for changing the character of a dirty tone in particular because the mids are where distortion seems to live most. The only thing I CAN'T do on this guitar is cut bass, so I get oodles of options at my fingertips! Thanks everyone for your input!

[Natman]

Yep -this is wildly useful! Having the toggle switch to chose regular tone or mid cut gives tons of versatility. Something to note is that using the "boost" portion of the original design limits the travel of your cut to half the roation of your knob. I get more range and subtle shades than my other axe (with "boost"). I REALLY like it now, at least enough not to change anything for a while.

This is a highly recommended mod for passive guitars. You can add a mid cut on the fly without having to touch your amp or pedals. This is handy for changing the character of a dirty tone in particular because the mids are where distortion seems to live most. The only thing I CAN'T do on this guitar is cut bass, so I get oodles of options at my fingertips! Thanks everyone for your input!

And a final update for posterity:

I went back to the original specs once I received the correct 0.039 uF caps in the mail. I used a new inductor with only the primary, so it's wired exactly as shown without the "boost" portion. It definitely sounds better than the version I made with 0.022 uF cap and joined inductor windings (which also sounded quite good). The effect is just like it feels better tuned for lack of a better description. I'm all for experimentation, but I'm convinced the guys who designed this circuit did plenty of that to arrive at the values for each component.

Right now I have a HH guitar with PAF style pickups and it is SO versatile having the mid cut and regular tone control in a switch. Just turning the mid cut ON, without even rolling the knob down sounds cool. There is a limit though, below halfway I find it gets less interesting and you're losing signal because so much is being filtered out. But regular tone controls do that too near their lower range.