Is this active x-over design worth while?

Status
Not open for further replies.
Jump to Latest
klitgt

klitgt

Member
Joined 2004
#1
Hi

I have seen lots of threads about active crossovers here. They all use opamps, I want to try descrete components if possible. I found the attached diagram in an old HIFI magazine and I want to have opinions on this.

it is a 12dB/octave, why go for more if the speakers can handle it.

The Cf and Rf components have to be calculated according to the formula Fc=1/2*pi*square root 2*R*C.

Any comments??:clown:
 

Attachments

  • active x-over incl buffer.gif
    active x-over incl buffer.gif
    9.5 KB · Views: 519
#2
Line level crossover filters are invariably based on the Sallen & Key topology that you've shown. You only get nice simple equations if the amplifier within the loop has lots of gain. the circuit you've shown should have quite a bit of gain, so it's rather like an op-amp in that respect - it just doesn't have the DC stability conferred by a differential pair input stage.

A very popular configuration is the 24dB/oct Linkwitz-Riley crossover, which is made of a cascade of two 12dB/oct Butterworth filters. The reason it's so popular is that it's insensitive to the fact that the two drivers are not coincident.

Finally, all Sallen & Key filters have increased distortion within the pass band compared to that same amplifier configured for the same gain but without filtering. In other words, you want a nice low-distortion amplifier before you wrap Sallen & Key around it.
 
#3
EC8010 said:
Line level crossover filters are invariably based on the Sallen & Key topology that you've shown. You only get nice simple equations if the amplifier within the loop has lots of gain. the circuit you've shown should have quite a bit of gain, so it's rather like an op-amp in that respect - it just doesn't have the DC stability conferred by a differential pair input stage.
***
The DC offset is blocked by the caps at the output of both high pass and low pass sections
***
A very popular configuration is the 24dB/oct Linkwitz-Riley crossover, which is made of a cascade of two 12dB/oct Butterworth filters. The reason it's so popular is that it's insensitive to the fact that the two drivers are not coincident.
***
What does "not coincident" mean?
***
Finally, all Sallen & Key filters have increased distortion within the pass band compared to that same amplifier configured for the same gain but without filtering. In other words, you want a nice low-distortion amplifier before you wrap Sallen & Key around it.
***
??? I do not understand this statement???
***
Click to expand...

😀 Anyway I am happy that someone finally responded to my thread
 
#4
"it is a 12dB/octave, why go for more if the speakers can handle it."

Below the crossover frequency down to the resonance, the excursion of a hi-pass filtered loudspeaker is constant for constant input to the filter, despite the fact that electric power applied is falling.
This is a very good reason to go more than 12 dB/o.

~~~~~~ Forr

§§§
 
#5
Not coincident means the drivers don't occupy the same point in space.

Sallen & Key wrote the original paper on how to make filters with CR networks and amplifiers, rather than LC networks. The circuit you showed is a Sallen & Key filter. Unfortunately, it increases the distortion produced by the amplifier compared to that amplifier without Sallen & Key around it.
 
#6
As a professional designer of loudspeakers, I would say that at least 18dB/ Octave is preferrable. The problem that you can have with 12dB is the steepness of the slope ( or lack of it). Let's take, for example, a ribbon tweeter. These perform very well in the case of the Visaton MHT12, from around 2500 to 3KHz but, to function correctly, they need quite a steep filter slope say at least 18dB/ Octave. The 24dB Linkweitz Riley crossover is good because it gives you a nice steep slope BUT, you get the equivalent of 1st order phase response.

If it were me, I'd go for the above option.

Best of luck.

Ricky.
 
#8
My personal experience is that lower order crossovers, no matter whether active or passive, analog or digital, sound better. Which is a reason to select drivers that don't need steep slopes.

Anyway, probably a matter of taste...

klitgt, did you have a look at Nelson Pass' active crossover or the (similar) MOX? They do use opamps, but discrete ones...
 
#9
Any comments??
Click to expand...
The discrete opamps in the attachment are so primitive, that thay simply won't work correctly I think. Especially this 'open collector' operation makes little sense.
A question:
is in-built frequency compensation of opamp really neede for low-pass filter? Or does the filter compensate opamp itself?
regards
 
#10
The frequency compensation is there to achieve a stable NFB-LOOP.
In the case of a Sallen-Key filter where the op-AMP is used as a buffer full feedback (or almost full in case of gain) is applied and therefore the OP-AMP must be stable for the desired gain independant of being used for low- or high- pass.

Regards

Charles
 
#14
Re: Re: Is this active x-over design worth while?

Ricky said:
As a professional designer of loudspeakers, I would say that at least 18dB/ Octave is preferrable. The problem that you can have with 12dB is the steepness of the slope ( or lack of it). Let's take, for example, a ribbon tweeter. These perform very well in the case of the Visaton MHT12, from around 2500 to 3KHz but, to function correctly, they need quite a steep filter slope say at least 18dB/ Octave. The 24dB Linkweitz Riley crossover is good because it gives you a nice steep slope BUT, you get the equivalent of 1st order phase response.

If it were me, I'd go for the above option.

Best of luck.

Ricky.
Click to expand...

What I want to use the filter for is a bi-amp system for "bookshelf" size speakers assisted by subwoofers with a cross-over frquency of 100 Hz. The small speakers are able to go down to 60Hz, therefore I do not believe it is necessary with steper slopes than 12dB/octave.
 
Status
Not open for further replies.
Top Bottom