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Old 9th January 2005, 03:49 PM   #1
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Default Bi-amping

I am looking at the possibility of bi-amping without using electronic crossovers. Would it be okay if I build a passive crossover and insert it before the power amplifiers? After all, even active crossovers are a series of filters!

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Old 9th January 2005, 04:05 PM   #2
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You would be much better off using an active crossover, or putting the passive crossover components in the amp enclosure on the outputs.
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Old 9th January 2005, 06:51 PM   #3
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Default Re: Bi-amping

Quote:
Originally posted by diyman
I am looking at the possibility of bi-amping without using electronic crossovers. Would it be okay if I build a passive crossover and insert it before the power amplifiers?
What would be the point in doing it this way?
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Old 10th January 2005, 03:35 AM   #4
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If I build a passive crossover using the same specifications as per the original speakers, I suppose I do not have to tweak it as the original componets were chosen with the box design in mind eg. roll-off, Xover freq etc.
The reason I want to do it this way is to split the signal before amplification as is done by electronic crossovers. This is to allow specific power amps to amplify certain frequency range only to optimize the full benefits of bi-amping.

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Old 10th January 2005, 03:54 AM   #5
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You can use a passive line-level crossover. I believe there has been quite a bit of discussion about them here before. I would recommend using an active crossover though; they have many desirable characteristics.



Quote:
Originally posted by Bill Fitzpatrick
What would be the point in doing it this way?
I can think of two reasons: 1) Simplicity/cost. 2) Potentially lower distortion by using only linear components.



Quote:
Originally posted by diyman
If I build a passive crossover using the same specifications as per the original speakers, I suppose I do not have to tweak it as the original componets were chosen with the box design in mind eg. roll-off, Xover freq etc...
You couldn't just use the crossover as-is, as it will have been designed taking into account the variable impedance of the drivers.
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Old 10th January 2005, 03:57 AM   #6
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That and what are you going to do about the very large value inductors you will be needing?
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Old 10th January 2005, 04:13 AM   #7
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by Bill Fitzpatrick
That and what are you going to do about the very large value inductors you will be needing?
Don't ask me, I've never done one before! I know other people have though, so it's possible.
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Old 10th January 2005, 04:22 AM   #8
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Default Re: Bi-amping

Quote:
Originally posted by diyman
I am looking at the possibility of bi-amping without using electronic crossovers. Would it be okay if I build a passive crossover and insert it before the power amplifiers?
Quote:
Originally posted by diyman
If I build a passive crossover using the same specifications as per the original speakers, I suppose I do not have to tweak it as the original componets were chosen with the box design in mind eg. roll-off, Xover freq etc.
If you mean that you know the design details such as corner frequency, slope, etc., of the passive crossover for a speaker you are building, but want to build a passive line level crossover version instead to biamp, yes you can do that. But there are pitfalls you must overcome:

http://www.t-linespeakers.org/tech/f...ssiveHLxo.html
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Old 10th January 2005, 02:56 PM   #9
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Passive line-level crossovers are possible, but fairly difficult to implement in most cases. Inductor values increase dramatically, but it is still possible. The impedances involved are fairly low and a stout preamp (maybe a headphone amp) would be required.Check the Marchand XM46 crossover:

http://www.marchandelec.com/xm46.html

Passive crossovers using RC circuits (vice LC circuits) are possible, but there can be significant insertion loss, Q's are limited to 0.5 maximum, slopes are realistically limited to second-order, etc. Interconnect driving ability may also be a factor. There are many variables, but if the proper combination of preamp, power-amps, and a not too challenging topology is required then passive line-level crossovers are a viable alternative.

Active crossovers solve a multitude of problems and are preferred in most cases.

Cheers,

Davey.
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Old 10th January 2005, 08:55 PM   #10
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There is nothing wrong with using a PLLXO but you then have to involve the input impedance of the next thing in line after the PLLXO, and include the article driving the PLLXO. You can think about a PLLXO like a normal loudspeaker.

Power amplifier drives the coils, caps and resistors, and the loudpeaker drive units. The coils, caps and resistors are designed around the loudspeakers impedance curve and frequency response to achieve a desired result.

In a PLLXO the preamp drives the coils, caps and resistors, and the power amplifier input impedance. You design caps, coils and resistors in the PLLXO around the input impedance of the power amp and the drive units frequency response.

The benafit this brings is the usual coupling of the power amp directly to the loudspeakers, and xover designed around a fixed impedance that doesnt change with level. The power amp input impedance doesnt change with frequency or how many watts the amp is giving out. Loudspeaker voice coils impedance vary with frequency and increases when power is applied.

The versatility of a PLXXO is not as much as a real active xover and is harder to implement. If you want to do xover before amplifier then just throw a few opamps in the circuit too, this removes the need for large inductors (because of the power amp high input impedance) and pretty much removes the preamp having to be capable of driving a difficult load.

www.linkwitzlab.com

has some useful info on active xovers.
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