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Old 8th December 2012, 01:11 AM   #1
jcandy is offline jcandy  United States
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Default Advantages of flat LF speaker impedance for amps

In designing a 3-way speaker its not uncommon to use a parallel LRC notch filter to flatten the woofer impedance. If this is not done, the crossover will interact with the woofer impedance to produce a strong woofer response anomaly. The value of R in the filter is roughly the desired impedance -- say 6 Ohms -- at resonance. Without the LRC filter, the impedance may rise to 40 Ohms or more at resonance. My question is, in terms of the amplifier current draw and power dissipation, are there any *disadvantages* to such a notch filter? The resistor in the LRC filter will in general have to dissipate alot of power, and that in itself is worrisome. So, since I am normally focused on the acoustic side, I thought I'd pose this question to those more in the know about the amplifier.
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Last edited by jcandy; 8th December 2012 at 01:41 AM.
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Old 1st July 2013, 03:06 AM   #2
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Any help to him??

And how to flattening that "nominal" woofer/tweeter of 4-8 ohms to all the frequencies??

Click the image to open in full size.
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Old 1st July 2013, 04:27 AM   #3
jcandy is offline jcandy  United States
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Quote:
Originally Posted by Karl vd Berg View Post
Any help to him??

And how to flattening that "nominal" woofer/tweeter of 4-8 ohms to all the frequencies??

Click the image to open in full size.
Hi Karl,

Its too bad the original question fell flat on its face. In a general sense, there are two aspects the question: (1) can the amplifier supply the required current at low voltages, (2) is the output impedance low enough so that a huge impedance swing does not alter the applied voltage. I am mostly concerned with (2).

To answer your question, one adds LRC notch filters to flatten out each impedance peak. However, this can be very costly in terms of parts so its not normally done. I guess the essence of the question is "is it worth it" to flatten the peaks.

Groeten,

Jeff
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Old 1st July 2013, 09:32 AM   #4
AndrewT is offline AndrewT  Scotland
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If you add extra components to the speaker to reduce the narrow bandwidth impedance peaks you are effectively pulling a lot more current from the amplifier.
This will make the amplifier run hotter and you will gain NO advantage in speaker performance.

If you have a passive crossover then that should already be designed to take account of the normal impedance variations that occur in drivers.
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Old 1st July 2013, 11:43 AM   #5
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Ok, let me see if I got it.

In the image below (the blue line), a given speaker (Peerless SLS830667 woofer, $66.00) has a "bump" of over 45 ohms around the 40Hz area. Then, around 80Hz it starts to be at the 10-8 ohms of "nominal" impedance up to nearly 1000Hz and then it rises again, right?

Click the image to open in full size.

Now, what happen if trying to reproduce frequencies below the 80Hz?

I remember a DIYer here, way before placing the crossover in his project, putting first a resistor in parallel to "flatten" the speaker's impedance across the bandwidth. Can't find that thread/post anymore...

But would be VERY interesting to know how to do this, if one is looking for a good 8" woofer capable to deliver low ends without pushing the amp in those low frequencies...
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Old 1st July 2013, 03:34 PM   #6
Zero D is offline Zero D  United Kingdom
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@ jcandy

A parallel LRC notch filter can be helpful in flattening the impedance, of not just the Woofer, but Mids & Tweeters too. If all drivers are compensated in such a way, it's as below. But there's no free lunch, as the impedance/s are reduced downwards towards the DC resistance/s of each driver. But this makes it/them more sensitive by 3dB.

@ Karl vd Berg

The screenie of the Peerless SLS830667 is tested Unmounted in Free Air. In ANY box design the impedance peak will change, & in many designs there will be more than one peak.

KEF pioneered smoothing out the impedance curve, across the Whole frequency response, in the 1980's. This is called Conjugate Load Matching

Quote:
a crossover optimisation technique that presents a constant (albeit low) ohmic load to the amplifier https://en.wikipedia.org/wiki/KEF
http://www.kef.com/uploads/files/en/...el_104_2_r.pdf
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Old 1st July 2013, 04:49 PM   #7
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Thanks, Zero D!

I'm looking for a decent 8" woofer with good extenion in the low end. The idea is to have a "3-way bookshelf".

I was also considering this unit, but it's a bit expensive.

Otherwise the use of a coaxial driver + a midrange in a two-way fitting...
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Old 2nd July 2013, 09:24 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Zero D View Post
.............................. But there's no free lunch, as the impedance/s are reduced downwards towards the DC resistance/s of each driver. But this makes it/them more sensitive by 3dB...................
Your partially right, there is no free lunch.

If the amplifier were able to deliver the same voltage into the parallel pair of the driver and the extra components , then the driver will reproduce exactly the same output. There is no extra 3dB.
BUT !!!!
the amplifier is now trying to drive a lower impedance and it must deliver more current and this is the "no free lunch". More heat and more stress.
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Old 2nd July 2013, 10:15 AM   #9
Zero D is offline Zero D  United Kingdom
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@ Karl vd Berg

Yeah the RCF L8S800 is a nice driver, but with an fs of 60Hz, unless you tune the box lower you won't get down to 40Hz as you "appear" to want, & it won't be efficient down there either ! Have a look @ the 8" on here for more ideas US SPEAKER Home Page Menu - Speaker Cabinets, Guitar & amp; Bass Amps, upgrades and custom design. The world’s widest choice of speaker parts. “If you have listened to live music, you have almost certainly listened to Eminence” Anyway this is getting OT now, so it'd be better if you started a new thread in Multiway

@ AndrewT

I recommend you take it up with KEF, as the info in the screenie is from their PDF i linked to earlier !

Here ya go - Contact KEF KEF
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Old 2nd July 2013, 10:32 AM   #10
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I suspect that the marketing dept, rather than the engineering dept wrote that

If you took a nominally 8 ohm driver and put an 8 ohm resistor in parallel with it The amplifier would see a nominally 4 ohm load. However the sound coming out of the speaker is not going to be magically doubled.

I think what you will find with the above copy is that the important piece of information is "and yet demand no more peak current from an amplifier"

This to me implies that they are actually increasing the impedance at points where it may dip below 4 ohms (when using nominally 4 ohm drivers). As when using 4 ohm drivers you may have some nasty dips to 2 ohms or lower which will cause stress to the amplifier.

So basically what I think that they are saying is that they have devised a way to use 4 ohm drivers without any nasty impedance dips.. In other words, it's marketing speak

Tony.
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