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previous: Ed Rembold R.G. The frequencies on th... -- 10/10/1999 9:32 PM

Re: EQ at GEO

10/11/1999 2:01 AM
R.G.	Re: EQ at GEO No, they're not exactly on for guitar. They're more generic, like you'll find for a hifi graphic EQ - which is what the circuit was. To recompute the frequencies: (1) pick equal values for R1 and R2. (2) Set the desired Q = 1/(2* SQRT(c2/C1)) [ holds true if R1=R2] (3) f = 1/(2piSQRT(R1R2C1C2)) to set the final frequency. Note that the circuits are NOT Twin T filters, but are a variation of the gyrator based inductor. The math for the Twin T shouldn't work here. You have to pick Q as well as f because for a graphic EQ each band needs to have it's half power frequency coincide with the half power frequency of the adjacent bands. If they don't coincide, you get "holes" where neither filter affects the response, or overlaps where there are extraneous peaks and notches. In this case, you'll notice that in all seven cases, C1 is 1/10 C2. That makes the Q be 0.5SQRT (10), or a bit more than 1.5 (just doing the rough square root in my head). I think you just pick the ratio of C1 and C2 to make the Q right for the band, and then pick the product of C1 and C2 to make the frequency come out right. You're getting ahead of me (although that's not all that tough to do * - I was going to dig through the design and reassign frequencies and Q's for better EQ for guitar. I suspect that a full-bore, gonzo, all out guitar based EQ will have the EQ bands bunched up in the lower frequencies and stretched out in the treble; this presents the specter of having a graphic EQ with non-equal Q's per band. That's where I was headed anyway. I'm looking forward to seeing what you come up with.

10/11/1999 2:01 AM

R.G.

Re: EQ at GEO

No, they're not exactly on for guitar. They're more generic, like you'll find for a hifi graphic EQ - which is what the circuit was.

To recompute the frequencies:
(1) pick equal values for R1 and R2.
(2) Set the desired Q = 1/(2* SQRT(c2/C1)) [ holds true if R1=R2]
(3) f = 1/(2*pi*SQRT(R1*R2*C1*C2)) to set the final frequency.

Note that the circuits are NOT Twin T filters, but are a variation of the gyrator based inductor. The math for the Twin T shouldn't work here.

You have to pick Q as well as f because for a graphic EQ each band needs to have it's half power frequency coincide with the half power frequency of the adjacent bands. If they don't coincide, you get "holes" where neither filter affects the response, or overlaps where there are extraneous peaks and notches.

In this case, you'll notice that in all seven cases, C1 is 1/10 C2. That makes the Q be 0.5*SQRT (10), or a bit more than 1.5 (just doing the rough square root in my head). I think you just pick the ratio of C1 and C2 to make the Q right for the band, and then pick the product of C1 and C2 to make the frequency come out right.

You're getting ahead of me (although that's not all that tough to do * 8-)

- I was going to dig through the design and reassign frequencies and Q's for better EQ for guitar.

I suspect that a full-bore, gonzo, all out guitar based EQ will have the EQ bands bunched up in the lower frequencies and stretched out in the treble; this presents the specter of having a graphic EQ with non-equal Q's per band. That's where I was headed anyway. I'm looking forward to seeing what you come up with.

Replies:

Scott Scholz R & D made a guitar EQ with ... -- 10/11/1999 1:44 PM