Changing a passive crossover to active

[sreten]

Hi,

the esteemed Rod Elliot is no expert on domestic hi-fi loudspeakers.

/sreten.

[Eva]

Originally posted by sreten



Active 2-ways are a waste of exspense driving the tweeters
unless you have suitably lower power amplifiers for them.

/sreten.

This is unless you want the system to play loud without lots of distortion and smoked tweeters.

When playing loud, passive crossovers are the #1 cause of tweeter death, particularly 6dB/oct and 12dB/oct ones and the ones that achieve the most ideal acoustic response, because they are also the ones allowing huge amounts of low frequency energy to reach the tweeter. Also, when coils and voice coils get hot their Rdc can increase by 30% or more thus detuning the system.

Only active crossovers allow you to use all-pass filters (and delays) to compensate for driver/enclosure phase shifts and get phase matching and optimum summing without requiring ideal acoustic responses.

Given the same sensitivity, peak power requirements for tweeters are actually equal or higher than for woofers, particularly with recordings involving real instruments and little compression. It's the average HF power what is much less than average LF power, but the crest factor is also much lower for lows, so a smaller amplifier is a huge mistake unless the tweeter has a much greater sensitivity. This is something that you easily learn when setting up the limiters in active filters, because it gives you the chance to analyse actual signal levels after frequency division.

[AndrewT]

Originally posted by Eva
Given the same sensitivity, peak power requirements for tweeters are actually equal or higher than for woofers, particularly with recordings involving real instruments and little compression.

agreed, but no one else wants to believe us.

[sreten]

Originally posted by Eva


Given the same sensitivity, peak power requirements for tweeters are actually equal or higher than for woofers, particularly with recordings involving real instruments and little compression. It's the average HF power what is much less than average LF power, but the crest factor is also much lower for lows, so a smaller amplifier is a huge mistake unless the tweeter has a much greater sensitivity. This is something that you easily learn when setting up the limiters in active filters, because it gives you the chance to analyse actual signal levels after frequency division.

Originally posted by AndrewT
agreed, but no one else wants to believe us.

Hi,

This one has been done to death.

The simple fact is for active bi-amping a good c/o point for 2
identical amplifiers is around 350Hz, i.e. a 3-way with active
bass/mid and passive mid/ treble.
Move this up to 3.5kHz and you are simply dead wrong.

The argument implies active amplification is a complete waste
of time, i.e. a 200W amplifier will always be better than 2x100W.
However the active 2x100W can swing the equivalent of 400W ....

/sreten.

[AndrewT]

Originally posted by sreten

This one has been done to death...................................
Move this up to 3.5kHz and you are simply dead wrong........

as I said, few if any agree.

[Robert GS]

why is a 200Watt amplifier always better than 2x100W.

Then a 20000Watt ampilfier will be unbeatable unless someone comes up with a 20001Watt

[sreten]

Originally posted by Robert GS
why is a 200Watt amplifier always better than 2x100W.

Hi,

If voltage swing at one frequency was all that mattered then the
200W would always be "better" than frequency split 2 x 100W.

/sreten.

[AndrewT]

Originally posted by Robert GS
why is a 200Watt amplifier always better than 2x100W.

Then a 20000Watt ampilfier will be unbeatable unless someone comes up with a 20001Watt

it depends very much on the spread of frequencies in the music signal.

A single tone played through a 200W amp to a speaker is capable of playing louder than the same single tone played through a 100W amplifier to the same speaker. The other 100W amp is sitting idle waiting for it's frequency range to be utilised.

Now apply a two tone signal. If each tone is in the passband of the respective 100W amps and speakers then you can get xSPL from the pair.
Play the same two tone signal through the 200W amp and the maximum SPL is similar to the 100W+100W amps.

But when the frequencies are widely distributed and playing through multiple amp/speaker combinations, the multiple speaker arrangement is capable of playing louder than the single amp/speaker with the same TOTAL power.
Pink and/or white noise would be the logical extension to this SPL contest. 5 active channels each with 100W available into 90dB sensitivity speakers, will play louder than a single channel 500W amplifier playing into a 90dB speaker operating in wideband mode.

Most commentators forget to tell you the conditions for which their statement applies.

[Robert GS]

I doubt that this thread started out to be a quest for higher SPL, but a gain of better sound quality. In both cases I would simply say active rules, but there is a financial aspect - which often prevents the obvious choice

[Eva]

Hi sreten

The 350Hz "equal power distribution" point is only valid when average spectral power content is considered. However, when peak power requirements are considered (the true criteria for amplifier size selection), the middle point may be around 1Khz. Have you ever seen the waveform from a trumpet on oscilloscope? It has big killer >5Khz peaks repeating at the fundamental frequency. This requires big tweeter amplifiers for proper playback.

Active systems can produce 6dB higher output than initially expected in the crossover overlap region. Bessel filters (with global EQ to lift the wide 1dB hole that results) may be employed to make that region intentionally wider (like one octave for LR24) without resorting to lower crossover slopes. 250Hz is a great place for this (lots of kick without clipping).

Not to mention that several amplifiers clipping each one its own frequency range sound much better than a single one clipping all frequencies together.