# Thermionic Deluxe change the frequency and "Q" of mid control



## renatofuza (Jun 9, 2020)

Hi guys!

Could someone help me with this???
I want the mid knob to operate at a higher frequency range with a little narrower bandwidth but i have ZERO knowledge about which component should I change and how to do the math for the desired results.

I found some explanations about active band pass filters but none exact like this one. Just want to know wich components does the high pass and which ones does the low pass.

Thanks


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## Bucksears (Jun 11, 2020)

Interested as well - no pressing need for it at the moment, but curious if it's possible.
One of the few things I like about the Boss MT-2 is that dual-mid control.


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## renatofuza (Jun 11, 2020)

Bucksears said:


> Interested as well - no pressing need for it at the moment, but curious if it's possible.
> One of the few things I like about the Boss MT-2 is that dual-mid control.



Exactly...the pedal sounds amazing as it is, I just want to experiment and learn.


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## Chuck D. Bones (Jun 15, 2020)

The MID control in the Friedman pedals is a strange beast.  Refer to the Thermionic Deluxe schematic.  It's a bridged-T filter, but implemented in an odd way.  As you may have noticed, it's boost only.  The freq response of the the mid control stage is flat when the MID knob is at zero.  The ratio of the caps sets the max gain.  The product of the R's & C's sets the center freq.  See below for the response curves for the stock circuit.  The red line at the bottom is with MID set at zero.  Purple curve is set to 1, orange is 2 and so on.  Light green at the top is with MID set to 10.  As you can see, the gain varies from 0dB to +10dB. and the center freq is around 650Hz.





If you want to move the set of curves left, increase the capacitors (C14 & C15) by the same percentage.  If you double both of them, the curves move down an octave.  If you halve them, the curves move up an octave.  If you want more max gain, then increase C14 and decrease C15.  If you change them both by the same percentage, then the center frequencies remain the same.  For example, changing C14 to 22nF and C15 to 1nF increases the max gain to just over 13dB.  The other thing you can do to increase the range is reduce R21.  You can take it all the way down to zero.  The downside of doing that is once you get above 4 on the MID control, the freq response starts shifting to higher frequencies.  This is with R21 replaced with a jumper:





I'd try the R21 mod first, because it's easy and has a fairly dramatic effect.


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## renatofuza (Jun 16, 2020)

Chuck D. Bones said:


> The MID control in the Friedman pedals is a strange beast.  Refer to the Thermionic Deluxe schematic.  It's a bridged-T filter, but implemented in an odd way.  As you may have noticed, it's boost only.  The freq response of the the mid control stage is flat when the MID knob is at zero.  The ratio of the caps sets the max gain.  The product of the R's & C's sets the center freq.  See below for the response curves for the stock circuit.  The red line at the bottom is with MID set at zero.  Purple curve is set to 1, orange is 2 and so on.  Light green at the top is with MID set to 10.  As you can see, the gain varies from 0dB to +10dB. and the center freq is around 650Hz.
> 
> View attachment 4969
> 
> ...



You are my hero!!!
That's what I was looking for...can't thank you enough!

I think I´m going to play with C14 and C15...but the effect of R21 jumped is quite interesting ? 

Thank you so much!


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## Chuck D. Bones (Jun 16, 2020)

You'll get a lot more out of changing the caps if you do the R21 mod first.


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