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Multi-Way Conventional loudspeakers with crossovers

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Old 18th August 2010, 03:37 AM   #7221
Pano is offline Pano  United States
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Originally Posted by Lynn Olson View Post
I always start with the most-accurate inductance compensation for every driver, extending it well beyond the audio band.
OK, point taken. But I have to ask why? So let's talk crossovers here, passive crossovers.

A flat impedance curve makes calculating a passive filter easier, sure. But as long as you hit your target - either electrical or acoustic - does it matter? Throw an inductor or 2 in front of that woofer and the impedance seen by the amp will be anything but flat. It will rise sharply above the crossover point. Combine that with the tweeter section where the impedance is rising quickly under the crossover point, and things flatten out.

Let me cite and example.
Using a 3.3mH+30uF low pass in front of a flat 8 ohm load yields an electrical low pass of about 450Hz, 2nd order. But place those values in front of an Altec 415, 515 or woofer with a similar impedance curve and you get an electrical 3rd order low pass at about 775Hz. The rising impedance changes the filter function. A little value tweaking and the acoustical curve falls right into 3rd order 775, too. The same could be done with a Zobel and 3 element low pass, but why? 2 passive components vs 5 to achieve the same response.

The amp will still see basically the same impedance thru the crossover.
With the flat impedance curve that many modern woofers have, why the need to flatten it further? It's flat enough thru crossover area already, isn't it? Does the Zobel help to combine impedances with the high pass section? (Legit question, not rhetorical).

I do see how impedance compensation can help in some crossovers. If you are well into the rise when you want to cross, or right at a resonance, it can be very difficult to achieve the target filter curve without flattening the impedance. But if you don't have to do it, why do it? What is the benefit?
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Old 18th August 2010, 04:42 AM   #7222
Sheldon is offline Sheldon  United States
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Originally Posted by john k... View Post
And suffer the loss of power dissipated in the series R?
Why not? Maximum Z for a single ended, no feedback, amp might be around 4 Ohms. That will typically be also the lowest powered amp. Minimum speaker impedance might be around 4 ohms, so maximum loss would be about half. On the other hand, the lowest impedance amps will be SS designs, where power is more readily available. In between would be PP tube designs with some feedback. And we are talking high efficiency designs in this thread.

However, this is DIY, so there's no excuse for not designing the crossover to fit the entire system.

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Old 18th August 2010, 05:23 AM   #7223
gedlee is offline gedlee  United States
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Originally Posted by panomaniac View Post
But as long as you hit your target - either electrical or acoustic - does it matter?
Quite correct, except ... only the resultant acoustical matters, whatever the electrical needs to be to do this is correct. It would be nice if it didn't have to depend on a particular amps output impedance. Zero is a good choice - very common in fact.
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Old 18th August 2010, 11:03 AM   #7224
mige0 is offline mige0  Austria
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Originally Posted by gedlee View Post
Quite correct, except ... only the resultant acoustical matters,

The point Lynn actually made is that not only "the acoustical result" matters - meant that there are impacts from far out - and I definitely agree on the "importance" of subtle electronics design decisions (passive XO that is in this case).

This is possibly not within your scope until now as its *slightly* into the "golden ear" direction...

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Old 18th August 2010, 11:22 AM   #7225
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Originally Posted by panomaniac View Post
A flat impedance curve makes calculating a passive filter easier, sure. But as long as you hit your target - either electrical or acoustic - does it matter?
Yes and no. There is another factor that enters the picture beyond steady state frequency response. That is the roll of electromotive damping due to the driver's back EMF. The driver, acting as a generator, see the impedance of the crossover connected to the amplifier, looking back toward the amplifier, as its load. When this load impedance is very small the full electromotive damping is present to help control the driver motion. When this load impedance is larger the electromotive damping is decreased. Under steady state conditions, as in matching the steady state acoustic output of a driver to a specific target, this isn't of concern because what ever the damping is, it is always in balance (through the design of the crossover) with the driving force supplied through the crossover so that the output matches the target. But when subjected to transients, such as are present in music, how the driver response will be, in part, dependent on the damping. So different crossover topologies which achieve the same steady state frequency response when connected to a given driver can sound different when playing music. Now, the electromotive damping decreases in magnitude by 6dB/octave on each side of the driver's resonant peak. So the magnitude of these differences is further complicated by how the load impedance varies with frequency.
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Old 18th August 2010, 11:37 AM   #7226
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Originally Posted by soongsc View Post
Have you looked at the distortion figures under the different loading conditions? Regardless how much the amp acts like a particular type of source model, it cannot match exactly. How far from the ideal model just makes that much difference. I think every little bit of different just accumulates to those audible differences between different products.
I think we all recognize that all amplifiers will have different distortion characteristics, to some extent , depending on the load. But the difference between higher output Z amps that tend toward current sources and amps with low output Z that tend to voltage sources is the speakers, in generally, designed to be driven by voltage sources with result that the frequency response of the speaker will vary from the desing target when driver by a high output Z amp. Of course, if designed for a specific amplifier then the amp's output Z can be taken into consideration. Call it a system approach to speaker design.
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Old 18th August 2010, 12:49 PM   #7227
soongsc is offline soongsc  Taiwan
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I think that if we design a speaker with relatively flat impedance, the SPL would be pretty much the same regardless whether driven by a voltage source or current source. My current experence with tweaking the impedance characteristics makes me more aware of impedance matching.
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Old 18th August 2010, 02:47 PM   #7228
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Originally Posted by soongsc View Post
I think that if we design a speaker with relatively flat impedance, the SPL would be pretty much the same regardless whether driven by a voltage source or current source. My current experence with tweaking the impedance characteristics makes me more aware of impedance matching.
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Old 18th August 2010, 03:06 PM   #7229
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Hmm, output impedance may not be flat.
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Old 18th August 2010, 03:24 PM   #7230
Pano is offline Pano  United States
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Thanks JohnK, that makes good sense.

Actually after posting I spent some time simulating and measuring different approaches. Even when the same electrical result (and thus acoustic) was achieved, impedance thru the crossover region could be quite different. That should certainly effect damping.

The impedance peaks that I saw in the crossover region were not huge, a few ohms or double the baseline impedance, but they were certainly there. No idea (yet) as to whether they are audible or not.
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