
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Analog Line Level Preamplifiers , Passive Preamps, Crossovers, etc. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
30th August 2013, 08:56 AM  #1 
diyAudio Member

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
__________________
Speculation, conjecture and discussion. If a fact pops out of my speech, its because I did some math. I KISS because I am. Last edited by mondogenerator; 30th August 2013 at 09:05 AM. 
30th August 2013, 10:37 AM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

How did you work out the combined Q of the cascaded stages?
__________________
regards Andrew T. 
30th August 2013, 11:33 AM  #3 
diyAudio Member

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 )
__________________
Speculation, conjecture and discussion. If a fact pops out of my speech, its because I did some math. I KISS because I am. Last edited by mondogenerator; 30th August 2013 at 11:40 AM. 
30th August 2013, 11:41 AM  #4 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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 !
__________________
regards Andrew T. 
30th August 2013, 12:27 PM  #5 
diyAudio Member
Join Date: May 2007

I thought Q only has meaning for a secondorder filter. A firstorder filter has no such parameter; higher orders presumably have a set of them.

30th August 2013, 12:29 PM  #6 
diyAudio Member

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.
__________________
Speculation, conjecture and discussion. If a fact pops out of my speech, its because I did some math. I KISS because I am. Last edited by mondogenerator; 30th August 2013 at 12:33 PM. 
30th August 2013, 12:38 PM  #7 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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, LR, 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.
__________________
regards Andrew T. 
30th August 2013, 12:57 PM  #8 
diyAudio Member

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.
__________________
Speculation, conjecture and discussion. If a fact pops out of my speech, its because I did some math. I KISS because I am. Last edited by mondogenerator; 30th August 2013 at 01:05 PM. 
30th August 2013, 01:07 PM  #9 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Have a read of the Linkwitz site.
His different presentation might help with understanding.
__________________
regards Andrew T. 
30th August 2013, 01:26 PM  #10 
diyAudio Member

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
__________________
Speculation, conjecture and discussion. If a fact pops out of my speech, its because I did some math. I KISS because I am. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
lattice filter question  eccdbb  MultiWay  14  20th May 2014 05:28 PM 
Filter sequence in a Bandpass filter question  BachAudioDK  MultiWay  3  28th January 2012 10:20 PM 
Question about input filter?  Rafael L  Solid State  4  2nd February 2010 11:30 AM 
Active filter question  youyoung21147  Parts  3  21st May 2006 05:27 PM 
Pi filter question  Eccu  Solid State  1  6th May 2002 03:45 PM 
New To Site?  Need Help? 