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Multi-Way Conventional loudspeakers with crossovers

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Old 8th November 2012, 09:02 AM   #691
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Amplifier IM will be reduced in a bi-amp system.
Driver IM may be reduced if steeper crossover filters are used.
If the passive crossover acts as a significant source impedance, it may act more like current drive and get better driver IM than active, see Distortion example: Vifa - Current-Drive - The Natural Way of Loudspeaker Operation
Current drive vs voltage drive is one way that "traditional" active systems may sound harsh
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Old 8th November 2012, 01:04 PM   #692
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I'm dubious of that webpage.

10dB = x10 power.

So the distortion at 1900Hz is <0.1% (first graph), not the 2.2% they claim.

It is entirely possible, of course, that I've missed something.


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Old 8th November 2012, 02:47 PM   #693
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Originally Posted by Pano View Post
Not sure about "purely objective" but certainly from a "purely engineering" standpoint it's superior. That does not necessarily mean it sounds better. Unfortunately.
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Originally Posted by CLS View Post
Superior engineering doesn't bring better results, hmmm... what a pity.

Or, that engineeing is still not fully optimized.

To name a few, in an active system, normally there's no passive component between amps and drivers, so their individual flaws or any mis-match between them would be more obvious - more clearly revealed.

For example, some combinations might still need zobel on the driver. Damping characters of amp and speaker alignment need to be optimized for each other. And it's to be remembered some physical issues of drivers can't be cured by filters... etc.

In some cases I've seen, people tend to be over-confident on active xover in the first place, believing it would be better anyway -- because the pros say so. But then, they don't pay enough attention on optimizing everything, like they do to the passive ones. Initial excitement turns to dissappointment and is given up early.

Just my 2c. (and I haven't followed this fast-growing thread very closely, so sorry if those have been brought up before.)
Great post, CLS!!

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Originally Posted by davidsrsb View Post
Amplifier IM will be reduced in a bi-amp system.
Driver IM may be reduced if steeper crossover filters are used.
Exactly, that's how i see it too.

Regarding current vs voltage drive, in any case using a fully active setup you could precisely choose how to drive each driver. Note that there is also "mixed drive" where the transducer is voltage-driven below a certain freq, and current-driven above certain freq.

On one of the amplifier-design books there was an explanation of how most distortions diminished when the amplifier (think of your typical class AB transistor amp) was asked to drive a much higher impedance load.

So the benefits may not be exactly due to "current drive" but simply because of presenting a higher impedance load to the amplifier.
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Old 8th November 2012, 04:00 PM   #694
AndrewT is offline AndrewT  Scotland
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Originally Posted by flavio81 View Post
..................asked to drive a much higher impedance load.

So the benefits may not be exactly due to "current drive" but simply because of presenting a higher impedance load to the amplifier.
That's one of the benefits of Bi-amping (not the active version).
The amplifiers handling the full passband see an average load impedance that is higher than that presented by the "whole passive crossover".
Each amp sees only a high pass filter and treble driver, or sees a low pass filter and a bass/mid driver.
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Old 8th November 2012, 04:18 PM   #695
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I don't understand what you mean, Andrew. My brain seems to be working slowly today.
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Old 8th November 2012, 04:27 PM   #696
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Passive components between amp and driver help in covering up some flaws.
that could have been nice in some cases(many)
but can't say I have had that luck, quite the opposite

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That's one of the benefits of Bi-amping (not the active version).
why not the active ?
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Old 8th November 2012, 04:45 PM   #697
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There are certainly a lot of issue to take into consideration with passive crossovers, but aren't there just as many with active? Is it really a panacea?
Hopefully, nobody's seriously claiming it is a panacea. (Except down low - given the size and cost of passive parts required, the greatly increased power demands, and the utility of easily being able to vary levels to fit a room make active a no-brainer for the modal region and below.)

In a system where large delays aren't needed, with gain structures optimized for active and the same transfer function for both an active and passive crossover, I highly doubt there would be any difference whatsoever between the two, let alone any real superiority to either approach..

Academically, I think the NAO Note design, which has a hybrid approach (active bass - midbass, passive midbass-midrange-treble) is the optimal from a cost-benefit standpoint, which is why that's what I'm doing in my next system. Sonically I just don't see the penalty, and there are fewer electronics boxes in the room than with an all-active approach. Someone with the time and comfort-level to build amps, or someone using drivers widely differing in sensitivity, may see a different balance as optimal.

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106dB or better at 2.83 volts means you need very quiet amps. I tend to use 16 ohm drivers just for that little extra attenuation.

Keep in mind it's not just the amps. Amps with perfect S/N ratios will still amplify the noise passed though to them. So optimizing the gain structure all the way through to minimize noise is important.
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Old 8th November 2012, 04:46 PM   #698
AndrewT is offline AndrewT  Scotland
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Split a traditional two way speaker so that it has two sets of speaker terminals.
Parallel the terminals and you have traditional passive crossover feeding two equal impedance drivers.
For simplicity I am going to assume perfect driver behaviour in their own pass bands and that each has a single pole filter.

Back to the split version.
One pair of terminals feeds a capacitor in series with the treble driver.
The impedance seen by the amplifier starts at around 8ohms in the lower treble and rises slightly as the VC inductive reactance increases with increasing frequency. As frequency falls the series cap becomes dominant and the capacitance reactance increases with falling frequency.
A wideband amplifier driving this treble + high pass filter sees an average impedance that is significantly higher than the nominal impedance of the treble driver. I'll pluck at a guess and suggest the average impedance is 20ohms for an 8ohms treble driver with a 6.8uF seres capacitor.
Considering the other amplifier connected to the other pair of terminals. It is driving a Bass Mid 8ohms driver with a series connected inductor acting as the low pass filter.
As with the above amplifier it too sees an average impedance that is higher than the nominal driver impedance of 8ohms. Again I'll guess and suggest 12ohms.

The two amplifiers, in Bi-amplifying mode, each handle the full bandwidth but each sees an average impedance that is higher than the nominal 8ohms impedance of the speaker. The amplifiers have an easier job driving the load and as with all SS amplifiers they perform better when presented with a higher load impedance.
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Old 8th November 2012, 04:50 PM   #699
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........why not the active ?
I had to add the (not active) because the stateside of the Atlantic have adopted a quite different definition for Bi-amping. We on the east side of the pond seem to universally see Bi-amping as using two amplifiers to drive the passive crossover (high and low pass filters) that is inside the passive speaker.
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Old 8th November 2012, 04:58 PM   #700
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OK, got it. Thanks.

I don't know if average impedance matters much (maybe it does) but rather the impedance at a particular frequency. An amp driving a passive low-pass filter will see a very high impedance above the crossover point. Vice Versa the high pass filter. Is that better or worse for the amp? Maybe it depends on the amp. Certainly there will be little current flow outside the pass-band, but will the amp deal well with the very high impedance?

A fullrange passive might have a lumpy impedance, but at least it's somewhat even over the audio range. I really don't know how much of a difference that makes - does anyone here know?
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