quick question on the SWTC 2 tone control

[mordechai]

I want to make sure I understand the interaction of parts on the SWTC 2 (on the Muzique website):



On this circuit...will increasing the value of R1 increase the corner frequence where the tone shift happens?  So, for example, if R1 is 8.2K and I want the tone control to he a bit brighter when it is at 12:00, I could do that by increasing it to, say, 10K or 12K...do I have that right?

[Mark Hammer]

Hi Mark,
When the pot wiper is rotated to ground, R/C1 form a one-pole lowpass filter.  So, if R1=8k2 and C1=.01uf, it will roll off treble starting around 1940hz.
Conversely, when the wiper is rotated fully the other way, such that C1 is fully in parallel with R2, C1/pot form a highpass filter.  If C1 is still .01uf, and the pot is 100k as shown, that rolls off lows starting around 160hz.  R2, meanwhile, forms a full-bandwidth attenuator in conjunction with the pot's value.

The trick seems to be selecting a value of R1 and C1 that yields a useful-but-not-overwhelming treble cut and complementary bass cut.  If we leave the pot at 100k, and make C1=.0039uf, then the max bass rolloff begins at 408hz.  If C1 = .0039uf, and we make R1=22k, then max treble cut begins around 1850hz.  Neither of the two rolloffs are especially life-altering, but they should provide some reasonable tone-tailoring to either remove harsh fizz or annoying woof.

[mordechai]

Hi Mark...my current setup has R1 at 8.2K and R2 at 47K, with C1 at 15n.  It seems to work nicely, though I've tuned it by ear rather than by the frequency calculation you noted.  Compared to the values you noted in your response, it seems that my setup will have a more dramatic effect because of the value of the cap relative to the value of the resistors.  Does my initial query make sense, though, that by upping the value of one of the resistors, I would consequently up the corner frequency affected by the rotation of the pot?

[Mark Hammer]

With R1=8k2 and C1=15nf, the max treble cut begins at just under 1300hz.  If R1 is increased to, say, 10k, max treble cut begins around 1060hz.
Note as well that the combination of R2=47k and P1=100k will always reduce max output amplitude by 1/3.