Maestro Brass Master Notch Filter Questions

[Joe Viau]

Hello:

I have a few quick questions (yeah, right, is there such a thing as a "quick question") about RG’s layout for the Maestro Bass Brass Master (a.k.a. Brass-O-Blaster).  I was curious about the notch filter that is near the upper right-hand section of RG’s slightly redrawn schematic.  I’ve searched the archive and couldn’t find anything definitive.  Also, please bear in mind that while I’ve built stuff in the past, I’m deficient in the theory area, so please bear with me.  The stuff below is the result of a couple of hours worth of work with my nose happily buried in books and articles.  How else are you gonna learn this stuff?

Looking at the original schematic, the Brassmaster switched between two sets of resistors.  I looked up the twin T filter in “The Art of Electronics” by Horowitz and Hill (pp. 279-281), and found that the notch frequency was calculated by using Fc = 1 / 2 * Pi * R * C.  Keying this into a spreadsheet yielded frequencies of 677.25 using the 47K set of resistors and 4681.03 using the 6K8 resistors, with C = .005 uF.  However, I did have to multiply the result by a million to get a result that makes sense.  I did Google and couldn’t really find anything as to why had to be done, C was expressed as uF in all of the Googled examples that I could find.  Can someone set me straight, is my math right, and are these useful notch frequencies?

I would also like to ask a question about the twin T filter used in this application.  Usually, the “tail” end of the T goes to ground, at the junction of the R divided by 2 and C times two connection.  Here, though, the tail is grounded via a 2K2 resistor.  What does this do?

Using RG’s layout, he uses a 3PST switch rather than a 3PDT, with the 47K resistor always in circuit.  Using Ohm’s Law--
R = (R1 * R2) / (R1 + R2)--I messed around and found that an 8060 ohm resistor--with 8.06 being a standard resistor value--would work, supplying a resistance of about 6K8 (okay, 6880 ohms) when the 3PST switch is closed.  Is that consistent with everyone else’s experience with this layout? The Art of Electronics, henceforth known as TAOE says that the values used in the filter need to be matched very closely for best results.  It’s also neat that such a simple circuit can do what it does.

There’s also a variable notch filter in TAOE, along with some good twin T stuff on RG’s site.  I was thinking of breadboarding a version of TAOE variable notch filter tuned to some arbitrary frequency range.  If anyone’s interested, I’ll do the math and come up with values for acceptable low / high frequencies for the variable notch circuit.  I can also post an image of the schematic soon if anyone’s interested.

Thanks for your time in reading this,
Joe

[Joe Viau]

<shameless bump>

[moosapotamus]

Looking at the original schematic, the Brassmaster switched between two sets of resistors.

It actually switched between three. Anyway...

I don't now... I have to go by ear, not by calculator, to find what sounds good to me. But, I definately like the idea of tinkering with that filter section. My personal inclination would be to try something parametric... to be able to independently vary the frequency, Q, and boost/cut.

~ Charlie

[DiyFreaque]

Looking at the original schematic, the Brassmaster switched between two sets of resistors.  I looked up the twin T filter in “The Art of Electronics” by Horowitz and Hill (pp. 279-281), and found that the notch frequency was calculated by using Fc = 1 / 2 * Pi * R * C.  Keying this into a spreadsheet yielded frequencies of 677.25 using the 47K set of resistors and 4681.03 using the 6K8 resistors, with C = .005 uF.  However, I did have to multiply the result by a million to get a result that makes sense.  I did Google and couldn’t really find anything as to why had to be done, C was expressed as uF in all of the Googled examples that I could find.  Can someone set me straight, is my math right, and are these useful notch frequencies?

Hi Joe,

I am not by any stretch of the imagination a math whiz, but I think you may be confusing notation with units.  IE, the reason you have to multiply your answer by a million is that C is expressed in Farads,  not uF, just as R is expressed in Ohms.  .005 uF is .000000005 Farads (or .005 exp-6) just as 6k8 is 6800 Ohms.   

Cheerio,
Scott

edit: prolly should be .005 exp-6 rather than .005^-6