Originally Posted by hifisponge
Hey mp -
What is it that bugs you about the sound of passive x-overs? Let's see if it matches up with what I commonly hear.
It's that pain near the top end Tim, near the x-over point
for many loudspeakers. It's just hashy to my ears, and wears me out.
But no wonder, it's full of distortion, by definition.
Here we go, from another forum ...
"A while ago on this forum there was some discussion about loudspeaker crossover networks and the relative merits of the active and passive approach.* This prompted me to investigate passive crossovers further.
First, some background:
Loudspeaker drive units are designed to be driven by a very low impedance, and the published frequency response plots from the manufacturer will always be shown with the drive unit driven directly from an amplifier, without any intermediate crossover network.* If the driving impedance is not low, the response will be adversely affected, and the amplifier will lose control over the speaker cone motion.* The degree of control is termed 'damping' and is defined by a 'damping factor', which is simply the ratio of the nominal speaker impedance (usually 8 ohms) to the driving impedance.* So for example, an amplifier having an output impedance of 0.5 ohms would have a damping factor of 16.
Amplifier designers generally strive to achieve a damping factor of 50 or more, and for good ones it can be several 100s.* But what happens when you insert a passive crossover network in between the amplifier output and the speaker drive units ?
The effect of the passive crossover :
To quantify this, we need to establish the impedance of the crossover network output, since this becomes the new driving impedance for the speaker unit.* The plot below shows this impedance for a 2nd order passive Linkwitz-Riley crossover network with a crossover frequency of 2kHz.
As you can see, over most of the audio band the impedance is not very low, reaching a maximum value of 8 ohms at the 2kHz crossover frequency.* By calculating the ratio of this impedance to 8 ohms, we can establish the effective damping factor when the crossover network is in circuit.* This is shown in the plot below.
At 30Hz, the damping factor is a just about tolerable 34, but it drops sharply as the frequency is increased, dropping to less than 2 right from 500Hz to over 7kHz.* At the 2kHz crossover frequency the damping factor is just 1 !
So what does this mean ?
The results show that a typical passive crossover has an absolutely disastrous effect on speaker damping.* Over most of the audio band, the damping factor is very low, so the amplifier will have almost no control over the cone movement at all.* Were it not for the mechanical self damping in the drive unit suspension, the cone would be flapping around wildly.* As it is, the cone movement will certainly not be accurately following the applied signal voltage from the amplifier.
What about active crossovers ?
With an active speaker, the crossover network is connected at the amplifier inputs, and the amplifier outputs drive the speaker drive units directly.* In this case, the excellent damping factor of the amplifiers is maintained, and the cone motion is accurately controlled at all frequencies.* In the past, the main argument against the active speaker approach has been the cost.* But these days, power transistors are cheap, so there really is no excuse !
I must say that when you look into it as we have in considerable detail, you just keep asking yourself why anyone would even consider passive speakers, they are a horrendous bodge!
A passive crossover produces about 1% distortion and an active one 0.001%, so that's a no brainer too.
The problem probably is that non technical audiophiles are persuaded that in order to find true system synergy they must accumulate the different parts of an audio system from different companies who are best at each of these. It's arrant nonsense but it has give dealers and magazines scope for "building systems" and selling endless upgrades, something I suspect that enthusiasts will be reluctant to give up on.
I'm very glad that a non audio industry scientist has presented the facts ...
I do agree that passive speakers can sound pleasant and there is a place for them, but not for hi end if we agree that the highest possible sound quality is the goal.
Active crossovers normally use a 4th order Linkwitz-Riley response, since this gives large attenuation in the frequency ranges where the drivers can 'misbehave', but also has quite a benign transient response due to the low Q (0.5).* Another property of this type of filter is that the output from the two drivers is precisely in-phase at all frequencies, so the polar pattern (and hence imaging) is stable.
Although it is possible to implement such 4th order filters in a passive crossover, component tolerancing is a nightmare, and you also need to accurately correct for the frequency dependent impedance of the drive units.* With active filters, on the other hand, it is easy to obtain very accurate 'textbook' responses.
Yes, damping at bass frequencies is important because the cone motion is largest there.* But you can see that the damping factor is still quite poor even at low frequencies.* The biggest difference is at or near the crossover frequency, where the damping is almost non-existent so the cone/dome motion will be completely uncontrolled.
Changing the amplifier power won't really make any difference because the amplifier's output impedance will normally be very low (few milli ohms).* It will be swamped by the effect of the passive crossover network."
And finally, quoting Linkwitz ...
"Crossovers may be implemented either as passive RLC networks, as active filters with operational amplifier circuits or with DSP engines and software. The only excuse for passive crossovers is their low cost. Their behavior changes with the signal level dependent dynamics of the drivers. They block the power amplifier from taking maximum control over the voice coil motion. They are a waste of time, if accuracy of reproduction is the goal.
SL - October 2009"
I've heard most of it Tim, although I've owned only half the loudspeakers
you have, over the last 30 years. Who hasn't?
( Kidding! )
Transducer technology, cabinet claims, just never cut it for me. I don't buy it, not anymore. I think those things are stop-gap and unique selling points, and they don't solve the real issue.
Just MHO, YMMV, I respect other opinions, etc etc.