Reducing Range in active EQ

[Chugs]

I like the repsonse of the 100hz control part of this eq but the amount of boost and cut is more than I would ever use. How can I reduce the amount of Boost and cut whilst maintaining the same frequency response?

[RickL]

If I understand what you want correctly I think you could replace the 100k pot with a 50k pot with 22k or 27k resistors on each end. Effectively you would still have a 100k pot (actually 94k or 104k) that can't be turned all the way in either direction. If you want to reduce the range even more use a 20k or 10k pot and the resistors required to bring the total resistance to 100k. Too bad no one makes a pot somewhere between 50k and 100k.

[Chugs]

You understand me correctly.

I currently have it setup like you suggested with extra resistors to reduce boost/cut but maintain the same frequency response. I figure there is a way of reducing the boost/without the extra resistors if the values of the components are adjusted. Not sure how to recalculate the response though.

[ElectricDruid]

Not sure how to recalculate the response though.

I do stuff like that in LTSpice by trial and error. The only place I've seen the equations is the 1980 National Audio Radio Handbook, which you can find on the web in pdf format.

[composition4]

Bring the capacitor ratio in... i.e. for the first band, change 220n to 180n and change the 22n to 27n

[ElectricDruid]

Here you go:

http://bitsavers.trailing-edge.com/components/national/_dataBooks/1980_National_Audio_Radio_Handbook.pdf

Page 2-46 onwards.

[composition4]

Just to make my previous solution clearer, I've simulated the response you get by altering the caps. If you increase one cap in the E12 series (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2) and decrease the other cap by the same number of values, the frequency centre will stay the same. It's actually the opposite of what I previously posted (the ratio between the caps needs to increase in this circuit in order to decrease your range).

[Chugs]

Thanks, I'll give that a go.

[jatalahd]

That is an excellent solution from composition4. The Audio Handbook link is also a good resource, but does not offer exact design equations for this specific case.

With reference to the schematic (and component labels) posted in the beginning by Chugs, I calculated the general transfer function of one boost/cut circuit block and the more useful specific case of equal resistors R9 = R10 and R11 = R12. Here are the equations for Boost mode at extreme position (pot turned all the way to the left):



and here for Cut mode at extreme position (pot turned all the way to the right)



Don't know if those are useful, but they might help in design process if someone wants to dig deeper. Those equations could also be extended by evaluating the low/high cutoff frequencies and the Q-factor, but unfortunately I ran out of time.