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Old 10th May 2016, 02:49 PM   #1
Mrcloc is offline Mrcloc  South Africa
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Default Passive crossovers in an active bi-amp setup

Hi. I have read a number of threads here and found some useful information (noob here).

I want to find out more about bi-amplification. I have read some threads here on the subject, but I would like to be sure of something. I have designed and built many amplifiers, my latest being an amplifier intended for use as a bridged amplifier. I have an 18-0-18 transformer with enough VA for a bridged design to provide 100W stereo into 8 ohms.

But I have been thinking that I should rather use the amplifiers to bi-amp my speakers (Celestion F20). I definitely want to put in an active crossover (it's a very minor modification on the board), and it will only be 1st order. I want to do this to benefit from the separation of high and low spectra to compress my power a bit, and get more headroom.

What's baking my noodle is that I want to leave the passive crossovers in, but I don't want them to interfere with the potential quality of the signal reaching the drivers. The preamplifier filters are a LPF at 2700Hz and an HPF at 2400Hz, and the speakers have a crossover frequency of 2500Hz. I want to leave the passive crossovers in because each amplifier is identical, so I need the speakers to attenuate for the highs.

I know this will work, but is there truly a measurable benefit to taking the passive crossovers out?

This is a DIY project which I'm keen to make a post about (the amplifier, that is).
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Old 10th May 2016, 03:20 PM   #2
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Quote:
Originally Posted by Mrcloc View Post
I want to leave the passive crossovers in, but I don't want them to interfere with the potential quality of the signal reaching the drivers
Hi, I guess that you know that a filter introduces some phase shift.
In passive crossovers ( at power level) that issue is earable at crossover frequency, when one driver starts to emit at full power and the other is decreasing in level; so particular attention must be given to that.
That happens also with electronic crossovers; you don't want to mix & match the phase issues of the two kinds of filters.
You should put the speaker's terminals out of the box and make your experiments. Usually a capacitor before the tweeter is in line for DC protection, and it could be used as an extra 6 dB attenuation below Fc- better an octave or two below Fc, so about 150 uF for 2500 Hz.

Sooo, at the end ( and in the beginning ) you should focus on the acoustical side of the thing - filters introduce phase shift, speakers also- and what you hear is the processment made before ( and after, 'cos the environment etc etc)
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Old 10th May 2016, 03:57 PM   #3
Mrcloc is offline Mrcloc  South Africa
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Hi, thank you for the reply. My active filters are 1st order Butterworth, so there will be no phase shift there. The inherent phase shifts in the passive crossovers will be the same as using 1 amplifier per speaker, but does the inherent phase shift justify the effort required to remove the passive crossovers?
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Old 10th May 2016, 04:44 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

TBH I think you'll be better off bridging for 100W than bi-amping with 25W.
Bridging theoretically has the same headroom as active bi-amping.

You can't use simple 1st order active x/o's instead of the passive x/o's.

rgds, sreten.
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Old 10th May 2016, 04:49 PM   #5
TMM is offline TMM  Australia
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There are two ways you can go about bi-amping and adding active crossovers to the system while retaining the passive crossovers.

Method 1:
Apply the active crossovers at exactly the same frequency as the passive crossover. This comes with the requirement that the passive crossover in the speaker needs to be reasonably well designed to achieve symmetric crossover slopes. If the speaker runs some kind of asymmetric acoustic slope then adding additional active crossovers will mess things up.
The active filters need to be even-order Linkwitz Riley filters. 1st order butterworth won't work because it will introduce a 90degree phase shift between the woofer and tweeter at the crossover frequency.
If you use 2nd order filters then you'll need to reverse the polarity of the tweeter connection just as you would when implementing a 2nd order passive filter.

Method 2:
This method is the safer bet however does not come with the maximum benefit of reducing the signal level going to each amp. What you want to do is set your woofer active lowpass sufficiently higher than the passive crossover point and the tweeter active highpass sufficiently lower. The effect is that the active filters are sufficiently into the stop band of the passive crossover filters that they have no significant acoustic effect. 1st order filters are ok! Here's the catch: the filters need to be at frequencies approximately 10 times higher/lower than the passive crossover point to avoid significant phase shift effects misaligning the drivers at the crossover frequency. So your woofer lowpass needs to be at 25kHz (so basically just don't bother doing an active filter for the woofer) and the tweeter highpass at 250Hz.

Last edited by TMM; 10th May 2016 at 04:54 PM.
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Old 10th May 2016, 06:04 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by TMM View Post
Method 1:
Apply the active crossovers at exactly the same frequency as the passive crossover. This comes with the requirement that the passive crossover in the speaker needs to be reasonably well designed to achieve symmetric crossover slopes. If the speaker runs some kind of asymmetric acoustic slope then adding additional active crossovers will mess things up.
The active filters need to be even-order Linkwitz Riley filters. 1st order butterworth won't work because it will introduce a 90degree phase shift between the woofer and tweeter at the crossover frequency.
If you use 2nd order filters then you'll need to reverse the polarity of the tweeter connection just as you would when implementing a 2nd order passive filter.
Hi,

Hmmm.... Lets say the speaker has perfectly implemented LR2
acoustic at its x/o point. Your saying if you add active LR2
at the same x/o point (and reverse a driver) it will fine.

That is simply not true, it will be wrong. LR4 is two cascaded
Butterworth2's not two cascaded LR2's. You can't add LR2
active in the manner you describe, it simply doesn't work.

rgds, sreten.
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Old 10th May 2016, 09:00 PM   #7
AllenB is offline AllenB  Australia
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Your new filters will create phase variations as well. The differences will be subtle at best unless you don't maintain the response shape.
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Old 11th May 2016, 12:44 AM   #8
Mrcloc is offline Mrcloc  South Africa
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Thank you all for the useful info! In summary, I take away that I should only really worry about overall phase response at the crossover point. That shouldn't be an issue. I'll probably post some results here when I'm done - I'm just waiting for my transistors to arrive.

PS: The amplifier will be 100W bridged, but 33W/channel without bridging. The supply ripple effect on clipping also doubles when bridging - part of the reason I'm choosing to bi-amp.
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Old 11th May 2016, 02:06 AM   #9
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Maybe you should try think about the answer to the question:
"Why should I bi-amp my speakers"
to help you figure out what to do.

From my understanding the primary benefit of bi-amping is to prevent high frequency signals that are generated when an amplifier clips from reaching the tweeter. The idea is that the demands of the woofer will primarily cause the amplifier to clip. When using a single amplifier, the new high frequencies that are generated by the clipping are routed to the tweeter by the passive crossover network. The relatively delicate voice coil of the tweeter cannot absorb the extra energy without heating up and this can have several drawbacks including frequency response changes and then the death of the tweeter. In a bi-amp setup, clipping of the woofer's amplifier can only affect the woofer because a completely separate amplifier is connected to the tweeter. The relatively beefier voice coil of the woofer can easily adsorb the energy of the clipping. The tweeter is unaffected.

Is this really a concern in this day and age? I say no. Just use a higher power amplifier that isn't constantly being driven into clipping! High power amplifiers are very inexpensive (whether you buy or build)! It's not 1970 anymore.

So what other so called "benefits" does bi-amping bring? Not many, if any, really. You are still using the passive crossover networks so that aspect isn't improved. There really isn't any magic happening.

Going back a few decades this approach was what a raging audiophile "name dropped" when they wanted to impress their friends. Now it's mostly marketing hype, and a non-problem. If you are really concerned about keeping drivers from interacting in bad ways, build or buy a completely active loudspeaker.
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Old 11th May 2016, 03:19 AM   #10
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Well maybe just to impress my friends, but I've been bi-amped since 1968.

Hard for any owner of a Behringer DCX2496 (digital processor with MANY tricks as well as a crossover with precise settings and bass mixer too) to imagine life before. I wonder if there is any piece of kit that has more happy owners. A good mic and REW software and you're on the fast road to great sound.

Seems unfeasible to me to shape driver input using a pound of copper. There's no way to get sharp slopes, keep crossover points outside sensitive freq ranges, and make the two sides match.

Bi-amping power is a statistical thing, not a numerical one. Very easy for womens choruses (Brahms Lullaby??) to have transients off the end of the scale. Keeping the bands separated will reduce the probability that tsunami excessive peaks occur.

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Last edited by bentoronto; 11th May 2016 at 03:24 AM.
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