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Old 30th August 2013, 08:56 AM   #1
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Default filter Q Question

Hello.
I have started to model an active filter design using Boxsim to sim the acoustic output and i have arrived at a 5 pole lowpass and 4pole highpass for my loudspeakers.

I have used each 2order stage Q to account for driver response vagaries, and in on case i have Q=1.5.

This filter has similar output to what i am using passively at the moment, in the active design im working on i was just a little concerned about ringing in the underdamped stages, and with that im seeking advice as im just starting out in active filter and a noob

Can someone here help me? Im concerned that the transient effect of this will be audible. Or am I concerned over nothing?

Thanks for reading
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Old 30th August 2013, 10:37 AM   #2
AndrewT is online now AndrewT  Scotland
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How did you work out the combined Q of the cascaded stages?
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Old 30th August 2013, 11:33 AM   #3
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I havent worked out the combined Q Andrew.

In boxsim i can add a single pole, then add a 2pole 2nd stage in series and so on to reach 5 poles cascaded.

Each 2 pole stage has Fc and Q, with the single pole stage fixed at 0.7 as you would expect.
This is a LP btw.

1st stage 1st order at 400hz,
2nd stage 2nd order Q = 1.2 at 3500hz,
3rd stage 2nd order at 3300hz Q=1

IF memory serves. Ill check my numbers and post them a little later (I shouldve started there really lol )
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Old 30th August 2013, 11:41 AM   #4
AndrewT is online now AndrewT  Scotland
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Q=0.7 for the passive single pole, cascaded with Q=1.2 for the active 2pole, cascaded with Q=1 for another active 2pole, results in a combined Q of: 0.7 * 1.2 * 1 = 0.84
Where did you get Q=1.5 from?
That was my question !
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Old 30th August 2013, 12:27 PM   #5
DF96 is offline DF96  England
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I thought Q only has meaning for a second-order filter. A first-order filter has no such parameter; higher orders presumably have a set of them.
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Old 30th August 2013, 12:29 PM   #6
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Default thank you!

thanks Andrew. You filled in a gap in my uni (read basic) filter theory. I didnt realise you multiplied Qs for the total filter Q. They stopped at 2nd order at uni...

One of the 2 pole stages has a Q of between 1.2 and 1.5 for a good acoustic response, and i was just concerned that it may not be a great idea. But then im a noob and this is my 1st serious attempt at making a sallen key filter of practical use for my hifi.
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Old 30th August 2013, 12:38 PM   #7
AndrewT is online now AndrewT  Scotland
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The standard RC is a Butterworth roll off.
That Butterworth roll off implies a Q = sqrt(2) (~0.7)

When you go active, you use component values to arrange the Q you want, Butterworth, L-R, Bessel, or any of the many rippled and anywhere in the infinite series that is available.

When you create a 3pole, using a passive RC, plus an active 2pole, to create a Butterworth, you assume Q=0.7 for the passive and set the active Q=1 for a combined Q = 0.7 for the 3pole. It's the same for all the other odd order Butterworths.

The same assumption would apply if your desired Q for the cascaded filters were not Butterworth.

If anybody sees this as wrong, please correct me with references to where I can learn the correct way.

I have been in previous Threads where some Members simply stated I was wrong without any attempt to help me to learn. I am open to learning and to correcting my mistakes.
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Old 30th August 2013, 12:57 PM   #8
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Default Thanks again.

Im familiar with how to structure the filter, but the individual stage Q just foxed me a bit. I could also do the RC with opamp to match impedance or gain. A system Q of ~0.8 isnt too bad then. I had visions of making an oscillator rather than a filter .

Im not designing to cheby 3db dips or a bessel curve per se, just adjusting Q and Fc in boxsims active editor to smooth the response and align phase as best as i can. In doing that i required a stage Q higher than 1 and didnt know if it was a problem. After further reading i found the LR filter Q of
.55 and 1.3 ish so now im not worried.
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Old 30th August 2013, 01:07 PM   #9
AndrewT is online now AndrewT  Scotland
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Have a read of the Linkwitz site.
His different presentation might help with understanding.
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Old 30th August 2013, 01:26 PM   #10
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thanks again. I am doing that in conjunction with Active Filter Cookbook to make the rather lumpy (electrical) filter curve. Acoustically what i have yields around a 4th order roll off and phase alignment within 20'. Andrew i think you have been more help than you realise
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