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Is it too low, 2-Ohm drivers?
Is it too low, 2-Ohm drivers?
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Old 10th May 2021, 03:54 PM   #1
presscot is offline presscot  Thailand
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Default Is it too low, 2-Ohm drivers?

I’m going to start a project of building a pair of large floor-standing speakers. I plan to use a pair of 12” woofers per cabinet.

I intended to go with 4 Ohms for the woofer section, so I could save money on inductors. I’m, thus, thinking of parallel connecting two 8-Ohm drivers together to bring the impedance/ or resistance to 4 Ohms.

I found 2 pairs of 8 Ohms drivers now. Regrettably, they are out of stock when I ask the seller and the 12” woofers + 8 Ohms are hard to find locally. Yet, I found 4-Ohm drivers instead.

So, I’m thinking to change the plan. I’d like to ask everyone that if I use 4 Ohms and parallel connect them to make it 2 Ohms totally, will it have any problems?

I’m concerning on amplifier that may be danger - because of the load on 2-Ohm woofers. But I’m not going to connect them serially since I don’t want to use too large coils - too much internal resistance and too expensive - for 8-Ohm configuration.

Actually, I saw the vintage speakers: Acoustic Research AR9, use the same configuration as my project — 2 x 12” woofers. When I search for their info, I found their woofers are parallel connected and, moreover, they are 4 Ohms. Thus, the impedance/ resistance may be 2 Ohms for woofer section. Can I conclude that it’s okay to use 2-Ohm configuration for woofer connection?
 
Old 10th May 2021, 04:36 PM   #2
Galu is offline Galu  Scotland
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The two AR9 woofers are wired in parallel to the same filter circuit, however they use a clever filter circuit.

Although each driver is nominally 4 ohms, it is much higher at its resonance frequency. With two such drivers in parallel, you would expect the impedance to be well above 4 ohms at resonance but down to 2 ohms or less at other frequencies. But this is altered by the arrangement of inductors and capacitors which increase the series impedance above and below the system bass resonance of 28 hz so that the combined impedance never goes much below 4 ohms at any frequency. This protects the amplifier and flattens the frequency response of the woofers at the same time.
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File Type: jpg AR9 XO.jpg (145.7 KB, 267 views)
 
Old 10th May 2021, 04:46 PM   #3
Inductor is offline Inductor  Portugal
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Quote:
Concerning the bass portion of the AR-9 crossover. I suggest removing the 2500 uF cap - but NOT the 470 uF cap shunt to ground. The 470 is a critical part of the 200 Hz low pass filter. The 2500 uF cap was placed there to raise the impedance at resonance - otherwise the two 12" woofers are effectively in parallel with an impedance of about 2.8 ohms. An amp killer - particularly in the world of 1979. So if you do remove the 2500 uF cap make sure you have a modern solid state amp - no tubes need apply - that can handle low impedance loads.

I have found two amps that are comfortable with that load; Odyssey Audio Khartagos and Pass XS150.5. I am relatively certain that ANY Pass amp in his XS or XA models would work well with that kind of load. There may well be other solid state amps that can handle such a load - but DO NOT try to drive the modified Nine with any "vintage" receiver - the result will be smoke, possibly flames and a dead receiver. Nor try to use an H/T receiver - those things are at best rather flimsy in terms of actual power capability.
By valkyrie (Posted November 7, 2018)
"AR 9 Crossover Rebuild - Page 2 - Acoustic Research - The Classic Speaker Pages Discussion Forums" AR 9 Crossover Rebuild - Page 2 - Acoustic Research - The Classic Speaker Pages Discussion Forums
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Old 10th May 2021, 04:53 PM   #4
Galu is offline Galu  Scotland
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Quote:
Originally Posted by presscot View Post
I’d like to ask everyone that if I use 4 Ohms and parallel connect them to make it 2 Ohms totally, will it have any problems?
Here's the quick answer!

If the impedance falls as low as 2 ohm in the bass section, you are highly likely to damage your amplifier.

Last edited by Galu; 10th May 2021 at 04:59 PM.
 
Old 10th May 2021, 10:24 PM   #5
presscot is offline presscot  Thailand
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Could anyone help to explain the principle and the calculation of this circuit, please?

Is this filter be a 3rd-order low-pass filter?

If I would go for 2nd-order with 85 Hz x/o point, could I simply calculate it by using a normal 2nd-order method and only paralleling those large capacitor and coil to the first inductor?
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File Type: jpeg AD39C5F2-BB64-4F1C-A4D8-56078D521CE2.jpeg (36.7 KB, 236 views)
 
Old 10th May 2021, 11:57 PM   #6
Galu is offline Galu  Scotland
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You ae not easily put off, are you?

Using two 4 ohm woofers in parallel is a bad idea, unless you are an accomplished loudspeaker designer.

The AR-9 bass filter not only takes into account the electrical parameters of the woofers, but also their acoustical interaction with the enclosure.

It's complicated!
 
Old 11th May 2021, 12:01 AM   #7
AllenB is offline AllenB  Australia
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Is it too low, 2-Ohm drivers?
Yes that is a third order filter but it has some added response adjustment. Don't use the extra LC normally unless you need to.
 
Old 11th May 2021, 12:35 AM   #8
Galu is offline Galu  Scotland
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Quote:
Originally Posted by AllenB View Post
Don't use the extra LC normally unless you need to.
So, Allen, are you advocating that presscot wire two 4 ohm woofers in parallel?
 
Old 11th May 2021, 12:46 AM   #9
presscot is offline presscot  Thailand
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Hi all,

Let's look at, finally, what I've found. I found a calculation of that L-C circuit!

It may be a kind of bandpass zobel circuit. The formula are already shown thoroughly. Here is a full link; Zobel network - Wikipedia

I'm not a big fan of math, so I'm needing a help on it. I'd like to try to reverse engineering that AR9's filter.

Please educate me, what is R-naught? Where can I find it? Thanks
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File Type: png bandpass.png (44.0 KB, 206 views)
 
Old 11th May 2021, 01:25 AM   #10
AllenB is offline AllenB  Australia
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Is it too low, 2-Ohm drivers?
presscot, here is an Xsim comparison. The difference is the notch. It also affects the knee of the filter but that's an incidental effect which you just need to adjust for.
Quote:
Originally Posted by Galu View Post
So, Allen, are you advocating that presscot wire two 4 ohm woofers in parallel?
I thought you were doing quite well yourself on that front but if it helps I don't disagree with you (maybe a little on the severity, but no I wouldn't go around suggesting members do this).
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File Type: png xsim.png (26.1 KB, 197 views)
 

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