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Which principle should I prefer?
Which principle should I prefer?
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Old 20th June 2004, 11:42 AM   #1
ChocoHolic is offline ChocoHolic  Germany
diyAudio Member
Join Date: Dec 2003
Location: Munich
Default Which principle should I prefer?

Hi All!

I am considering different solutions for my active sub.

Clear points:
- I will go for a sealed design because plan to run the system down to 12Hz. A vented design with a tuning frequency of 12Hz would be large and you would have to add a lot of mass to the driver.
Also the ratio of length and diameter of the tube would be extreme...
-The natural resonance frequency of that sealed box will be somewhere around 50Hz.

In fact I already bad experiences 10 years back with a sub that had a resonance frequency of 60Hz and was booming like mad in combination with the room resonance modes.

My last design was small URPS design with a natural resonance frequency of 150Hz, LT and negative output impedance of the amp for contolling the damping. It was great. Simply great, but now I want to have something more powerfull. Also at small size.
My current plan is to set up two small Sadhara units (about 20l volume for each box). This will allow about 11dB higher output levels than my current design.

Open Question is the way of electronic low frequency correction:
A: Should I go for a LT?
B: Or better excursion/acceleration regulation, ?
C: ACE electronic?

My current opinion on each point:
A: Is fine but does not fight the resonance at 50Hz.
B: I have doubts that I can handle the phase jump at the resonance frequency.
C: Looks promising. But the weak point seems to be that the ACE electronic must incorporate an accurate compensation of the speakers DC-resistance by negative output impedance only if this is achieved, then theory works and I could control the speakers behaviour and adjust any resonance frequency & damping by a further complex output impedance of the amp.
Unfortunately the DC-resistance of the speakers coil is depending on the temperature. At 125C it will have 38% more DC resistance than at normal room temperature.....
Further on I am not sure how the phase shift of the output filter
of my planned class D amp will behave together with the feedback of the ACE electronic....

Uhps...., seems like that this project is getting more complicated than I thought in the beginning....
In fact this project offers three areas, which will require some
efforts ....
- SMPS with PFC
- Class D amp
- Electronic low frequency correction

Good thing: Not boring! And I will gain further knowledge..

Comments, opinions and experiences are highly welcome!

Cheers Markus
Attached Images
File Type: jpg sadhara1.jpg (86.1 KB, 68 views)
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Old 20th June 2004, 04:11 PM   #2
BillFitzmaurice is offline BillFitzmaurice  United States
Join Date: May 2004
Location: New Hampshire
Running to 12 Hz in a small box is not realistic, but frankly it doesn't matter as I doubt you'll be able to find anything that has prgram material that low. A bottom end of 20Hz is quite adequate.

If you want a box Fb at 50 Hz you'll need not only electronic correction to reach below that but also a lot of power. With the 12dB/octave roll off below Fb you'd need 24dB additional to reach 12 Hz than that at 50 Hz; that means that to equal 50 watts input at 50 Hz you'd need about 12,000 watts at 12Hz. For that matter you'd need 800 watts at 24hz- and a driver that can handle it. Possible, yes. Practical, no.

A more reasonable approach would be to go for a larger box with an Fb of perhaps 30 Hz. The you'd be able to reach 20 Hz without a kilowatt of power, and if you actually find some 12Hz program you might be able to reproduce it.
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Old 20th June 2004, 09:17 PM   #3
ChocoHolic is offline ChocoHolic  Germany
diyAudio Member
Join Date: Dec 2003
Location: Munich
Hi BFm!
Yes, I will need a lot of power.
Power is not an issue as long as the speakers do
not burn. But size of the enclosure is a major concern for me.
My current old subwo has a max. linear volume displacement of +/- 0.3l.
With LT it goes down to 15Hz. The netto volume is only 12l.
Already my old amp could deliver more than 1.5kw...
Please refer to one of my first postings last winter.

You are right 12Hz are not really required.
In fact also the volume displacement of two Sadharas is not
a definite must in my living room...
The entire project is just a mad overkill for a crazy DIY-guy.

I simply love small subwoofers with excessive properties....

And I am curious how far I can bring such a concept!
In first step I plan to make the signal conditioning
of the LT (or ACE electronic or whatever...) and the crossover with traditional OP amp circuits. Just, because I know how this works
and there are enough other points which will give me a struggle.

But in fact this application would also be the perfect thing for
a modern DSP.
The signal conditioning and the class D amp, both
are somehow screaming for a DSP .... may be in future...
Currently I have no idea about DSP and I cannot learn everthing right now..... so first I will stick to traditional OP amps....

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Old 20th June 2004, 09:46 PM   #4
454Casull is offline 454Casull  Canada
diyAudio Member
Join Date: Jan 2004
Location: Toronto, ON, Canada
What driver are you planning to use?
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Old 20th June 2004, 11:28 PM   #5
ChocoHolic is offline ChocoHolic  Germany
diyAudio Member
Join Date: Dec 2003
Location: Munich
Hi Casull !

I am using a Sadhara. Please refer to the attached pic of
my first posting.
... well, two SADHARAs and two enclosures...
each enclosure about 24l , 24l minus the volume of the magnet
and the cone makes around 20l air volume.
The stiffness of that air will shift resonance frequency from
26Hz (Sadhara free air) to about 50Hz.
I plan to use this sub mainly below that.
Up to now I had best results with such Under Resonance Principle Subs.

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Old 20th June 2004, 11:30 PM   #6
ChocoHolic is offline ChocoHolic  Germany
diyAudio Member
Join Date: Dec 2003
Location: Munich
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Old 20th June 2004, 11:56 PM   #7
ThomasW is offline ThomasW
diyAudio Member
Join Date: Oct 2001
Location: 5280'
I hope you already have the drivers in hand.

Dan Wiggins posted a while back that no more Sadhara drivers were going to be sold to DIYers.............
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