Linkwitz transform ciruit applied to midrange driver - diyAudio
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Old 7th August 2002, 08:30 PM   #1
Beggar is offline Beggar  United Kingdom
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Default Linkwitz transform ciruit applied to midrange driver

Hi everyone, i am planning to use as part of my sound system, several Linkwitz transform circuits (http://sound.westhost.com/project71.htm) to extend the low end of the midrange units i have, (some audax AP130M0's http://www.audax.com/products/details/AP130M0.shtml) so i can get them flat to about 100hz, which is an octave below my crossover point to my sub (200hz)

there is perhaps only limited point to what im going to do (a few decibels over a few hz of attenutated signal!), but i think it would be fun to try it out, has anyone ever tried this before, and if so what were the results like, or can anyone see any major problems with doing this?

btw its a tri-amped system and i'll be powering each (of 4) midrange driver with its own lm1875, the tweeters have their own lm1875 too (i have a lot of 1875 chips laying around ), and the sub is running off a couple of P3A amps again from http://sound.westhost.com/project3a.htm which is of course to use a Linkwitz transform circuit too. Xover frequencys are to be 200hz and 3600hz

thanks for any responses.
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Old 7th August 2002, 09:10 PM   #2
ergo is offline ergo  Estonia
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You seem to be decided that the cross for sub is 200Hz, then why do you need for midranges to play flat to 100Hz ???

If you have single sub I think you will run in trouble as you will be able to locate the sub very easily.....

Only downside I can see in using the transform circuit is that it increases the driver displacement and as the Xmax might not be too great the max spl can be limited by cone travel. This is of course very dependent on how loud you listen and how big is your room.

Ergo
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Old 7th August 2002, 09:20 PM   #3
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Yes I did this before in an active speaker system and even with a tweeter that works fine. No problem - just take care to place a highpass filter before the LR-circuit otherways You might run into voltage overdrive problems due to its high gain towards the low end frequencies.
I did it because I wanted to change the drivers highpass function to lower frequencies in order that it does not disturb the H-pass of the electronic filter alignment.
If You intend to implement the LR-circuit to Your Audax midrange out of the same reason, 100Hz probably is not low enough (depends of the slope) and You`ll still have some interference with Your 200Hz highpass filter.
To avoid the inference, either You have to equalize Your midrange highpass fc down to still lower frequencies or You might try to modify the Audax highpass function in a way that allows You to proper incorporate this new derived highpass (driver+LR) into a higher order alignment at 200Hz (III, IV or higher order). I would go for the latter if possible.
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Old 7th August 2002, 09:50 PM   #4
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ok, thanks dudes, good to know it works!

hmm, thanks for pointing out the locating of the single sub problem i might crossover at 150 or perhaps just lower, which will mean i will have to equalise further to perhaps 75 hz.

The reason i want to equalise an octave beyond the xover is because i thought/think there is still a fairly large amount of musical content sent to the midranges in this frequency range, even when using 24dB/octave filters, and as such i thought it might be cool for them to have a flat response in this reigon. please tell me if this isn't the case or i wont notice it, etc, because im pretty new at this kinda thing

thanks again
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Old 7th August 2002, 10:43 PM   #5
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Quote:
hmm, thanks for pointing out the locating of the single sub problem i might crossover at 150 or perhaps just lower, which will mean i will have to equalise further to perhaps 75 hz.
Naturally I assumed that You have a stereo sub because usually single subs are not crossed so high at all.
My experience is that even with 24dB octave (and not less steep than LR) You should cross the sub at least at 100Hz or better even somewhat lower (80Hz) to avoid localization! Try it out!
.
Quote:
The reason i want to equalise an octave beyond the xover is because i thought/think there is still a fairly large amount of musical content sent to the midranges in this frequency range, even when using 24dB/octave filters, and as such i thought it might be cool for them to have a flat response in this reigon. please tell me if this isn't the case or i wont notice it, etc, because im pretty new at this kinda thing
There are two ways making an "ideal" (I know there is nothing like ideal..) crossover that works acoustically like in theory: the drivers have to have flat response beyond the crossover point (one octave usually is not enough) or drivers high pass function has to become incorporated into the filter alignment.
The point is that Your Audax has fc around 100Hz (shot of the hip - depends of Your enclosure) and makes a II.-order highpass by itself, with Q somewhat around 0,7 (means Butterworth). If not maybe You can modify enclosure accordingly.
My suggestion that very likely will work fine (doing so You`ll kill to flys at once and with less effort):
Leave the Audax as it is and don`t use the LR-circuit in this case. Put a 12dB- Butterworth highpass (fc= 100Hz) in series with it and cross the sub with a 24dB Linkwitz lowpass at 100Hz and the hole thing will quite precisly make a perfect (and symetrical slope acoustically) 24dB-LR filter at 100Hz - just what You would need.
BTW: that`s what I have meant with incorporating the drivers highpass function into a filter alignment.
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Old 8th August 2002, 09:50 PM   #6
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hmm now i understand (i got there in the end lol ) thats a really cool idea and makes the whole thing a lot easier, and im pretty much going to do that now.

instead of the 12 dB highpass in series with the audax unit i might try to alter my active crossover unit so the high pass of the midrange is 12dB, i think this is possible, but not too sure, hmm i'll give it a try though.

thanks for the great idea, sorry for my slowness
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Old 8th August 2002, 11:16 PM   #7
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Quote:
instead of the 12 dB highpass in series with the audax unit i might try to alter my active crossover unit so the high pass of the midrange is 12dB, i think this is possible, but not too sure, hmm i'll give it a try though.
Yes, that`s exactly what I have meant. It`s possible and moreover it`s very easy.
From what You said earlier in another post You seem to have an electronic 24dB highpass filter with Your Audax. Right? Very likely Your 24db filter is made of two 12db OP-Amp(?) filter stages in series. Just bypass one 12dB stage and change the few resistors and capacitors of the remaining stage to make the Butterworth alignment at 100Hz accordingly to this:
http://hem.fyristorg.com/effects/f12db.jpg

When You have done this (and likely after some necessary sub level adjustment) Im pretty sure that`ll be quite a step of improvement in Your overall system performance.
Please let us know about the results!
Quote:
thanks for the great idea, sorry for my slowness
no worries at all!
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