STM's Passive Mid-Range

[WGTP]

http://www.aronnelson.com/gallery/v/STMs-Circuit-Ideas/Simple_Mids_1.png.html

Tried this on the bread board and it works pretty well.  What with rolling off the lows for less IM distortion and the highs for less fizz, we need something to restore a more natural/less middy tone.

I'm wondering what input impedance it is expecting to produce the frequency response shown on the graph?  Thanks.   

[Electric_Death]

Cool passive WTP, you've inspired me to share my version of the Big Muff tone control which I've so properly named, The Big Merkin



Really simple and I'm sure you don't need a graph to figure out what it does to the wave form. Of course the values can be toyed with to nail your favored tone but it eliminates the need of an over sized enclosure for including a tone stack or active filtering.
The Big Merkin teamed with the passive you posted will give one he** of a scooped response                    

Sad or not but it's the heart of my overdrive, hopefully it's appreciated.

[coxter]

question, how to change the Freq of the STM?

I tried RG's EQ article, and its too confusing for me.

[stm]

Just scale both caps up or down by the same factor.
Doubling caps (68n & 6.8n) halves mid frequency. Halving caps (15n & 1.5n) doubles mid frequency.

[coxter]

Awesome! Thank you, nice to get the answers from the man direct!

One more thing if there's a formula would you mind sharing?

can Q be varied as well?

[Electric_Death]

can Q be varied as well?

coxter Q simply stands for quality and since it's a passive, I don't think think it's really an option..though I'm certain different material based caps and resistors will slightly effect the quality and sometimes dropping or jumping the value of the pot makes an improvement. I prefer a 250K L over the 100K standard used in the Big Muff tone control design.

[coxter]

  ? I thought Q is the width of the notch ?

[stm]

Hi there!

The notch frequency can be calculated as:  Fn = 1 / [ 2*PI*sqrt(R1*R2*C1*C2) ]

Where:

2*PI can be approximated as 6.3 or 6.28
R1 is the pot's total resistance
R2 is the resistor to GND
C1 and C2 are the two caps

I am not aware of a formula to calculate the notch depth or Q, however you may try this guidelines:

a. Notch depth is related to the ratio of R1/R2.  The larger that ratio, the deeper the notch, so reducing R2 will somehow affect maximum depth.  Of course changing any of these will affect notch frequency as well, but you may adjust C1 and C2 later to compensate.

b. Q may be tuned somewhat by varying the C1/C2 ratio, however you must notice since that since this is a passive circuit that Q is limited to a certain range only, probably for values well below unity.  I think that the 10:1 ratio was already chosen to narrow the notch as much as possible.  IIRC, increasing the cap ratio beyond that limit didn't narrow the notch significantly--entering into diminishing returns territory.

Cheers.

[WGTP]

Looks R2 could be a pot (25K) as well to vary the notch location.