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Old 30th August 2008, 02:01 AM   #1
grjr is offline grjr  United States
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Default designing an octave eq

Hello, I need to design an octave equalizer for a project I am working on. I would like some opinions on the matter. I am planning on using the circuit described at the bottom of this page .

I am not designing this equalizer for an audiophile system, it is simply for a school project so it doesn't need to be anything spectacular. Even if the sound is "just ok" that would be sufficient. The author runs through an example using 20dB of boost/cut and a desired Q of 1.7 . Is the 20dB of boost/cut excessive? Is the Q of 1.7 suitable for my application? Also ISO octave spacings are 31, 62, 125, 250, 500, 1k, 2k, 4k, 8k, 16k correct? Just want to make sure before I start on any actual calculations. Thanks for the help!
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Old 30th August 2008, 11:53 AM   #2
djk is offline djk
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Copy the basic McIntosh MQ104.

Make however many bands you want, whatever frequency, whatever width.

Gyrator with single transistor:

http://www.pmillett.com/file_downloa...Q104_early.pdf

Gyrator with opamp:

http://www.pmillett.com/file_downloa...MQ104_late.pdf

Instructions on how to calculate frequency and width:

http://www.roger-russell.com/room2.htm
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Old 3rd September 2008, 07:10 AM   #3
djk is offline djk
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BW=2 (5-band), Q=.67
BW=1 (10-band), Q=1.4
BW=.5 (20-band), Q=2.87
BW=.33 (30-band), Q=4.32
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Old 3rd September 2008, 07:15 AM   #4
grjr is offline grjr  United States
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Quote:
Originally posted by djk
BW=2 (5-band), Q=.67
BW=1 (10-band), Q=1.4
BW=.5 (20-band), Q=2.87
BW=.33 (30-band), Q=4.32
where do these Q values come from?
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Old 3rd September 2008, 07:52 AM   #5
djk is offline djk
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I used a bandwidth calculator at a pro sound site, I don't know the equation.
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Old 8th September 2008, 03:52 PM   #6
wg_ski is offline wg_ski  United States
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Default From the continuum, where else?

Quote:
Originally posted by grjr


where do these Q values come from?
The same place Q always comes from. Bandwidth divided by center frequency. The bandwidths are normally set so that the 3dB points of adjacent bands are the geometric means between center frequencies. Write out the center frequencies, calculate the geometric means between them. The differences FH-FL are the bandwidths. Then the ratio of BW to f0 and that gives Q. Which also tells you what the ratio of reactance to resistance must be in the RLC circuit which controls each band. For equally spaced frequencies (related by a constant multiplier) the Q will be the same for each band.

We didn't have online calculators when I was doing home-made EQ's and active crossovers back in college.
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