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hahfran 4th February 2008 08:48 PM

Passive crossover into active crossover
 
I designed a bookshelf loudspeaker with Visaton AL130 and Scan Speak D2904/9800. The preliminary crossover was designed with
Boxsim, a freeware which can be tricked into accepting other speakers than Visaton brand, such as the ScanSpeak tweeter.
The final design was made with LAUD measurements and listening tests.
As I am going to make 4 or even more of these I tried to replace the passive crossover with an active. The reason for this is simple:
a second power amp costs me virtually nothing as all parts are on stock and insofar are written off, but the high quality air conductors and special capacitors of the passive crossover are
really expensive. I took the frequency and phase response of the low pass and the high pass passive crossover with LAUD and designed an active filter which has very close the same characteristics. Although the acoustical response of the active and the passive speakers match, the listening tests clearly favored the passive version especially in complex chorus peaces of music with female voices dominating. Somehow the acoustical resolution of the passive is better.
I don't have a plausible explanation.

Dieter F.

Magnetar 5th February 2008 12:06 AM

The active crossover has to be designed differently. If you modeled the passive reactive crossover and used it's reacted responses in the active design there lies the problem

Thunau 5th February 2008 12:36 AM

Re: Passive crossover into active crossover
 
Quote:

Originally posted by hahfran
I designed a bookshelf loudspeaker with Visaton AL130 and Scan Speak D2904/9800. The preliminary crossover was designed with
Boxsim, a freeware which can be tricked into accepting other speakers than Visaton brand, such as the ScanSpeak tweeter.
The final design was made with LAUD measurements and listening tests.
As I am going to make 4 or even more of these I tried to replace the passive crossover with an active. The reason for this is simple:
a second power amp costs me virtually nothing as all parts are on stock and insofar are written off, but the high quality air conductors and special capacitors of the passive crossover are
really expensive. I took the frequency and phase response of the low pass and the high pass passive crossover with LAUD and designed an active filter which has very close the same characteristics. Although the acoustical response of the active and the passive speakers match, the listening tests clearly favored the passive version especially in complex chorus peaces of music with female voices dominating. Somehow the acoustical resolution of the passive is better.
I don't have a plausible explanation.

Dieter F.


As Magnetar said, you have to derive the electrical transfer function of your passive crossover while taking the reactive nature (impedance) of the drivers in account. If you simulate the transfer function of your passive crossover into a fixed impedance it will not have much in common with the transfer function of it into a varying impedance of a real world loudspeaker.
So, you have to export the real transfer function of your passive from your sim, then construct and optimize an active circuit to that transfer function- down to at least 0.5dB. It might take shelving or parametric filters in addition to the standard LP and HP filters to arrive at a close match.
I use lspCAD which makes this fairly easy.

ttan98 5th February 2008 12:37 AM

Re: Passive crossover into active crossover
 
Quote:

Originally posted by hahfran
I designed a bookshelf loudspeaker with Visaton AL130 and Scan Speak D2904/9800. The preliminary crossover was designed with
Boxsim, a freeware which can be tricked into accepting other speakers than Visaton brand, such as the ScanSpeak tweeter.
The final design was made with LAUD measurements and listening tests.
As I am going to make 4 or even more of these I tried to replace the passive crossover with an active. The reason for this is simple:
a second power amp costs me virtually nothing as all parts are on stock and insofar are written off, but the high quality air conductors and special capacitors of the passive crossover are
really expensive. I took the frequency and phase response of the low pass and the high pass passive crossover with LAUD and designed an active filter which has very close the same characteristics. Although the acoustical response of the active and the passive speakers match, the listening tests clearly favored the passive version especially in complex chorus peaces of music with female voices dominating. Somehow the acoustical resolution of the passive is better.
I don't have a plausible explanation.

Dieter F.


Excellent thread, I wrote a similar thread in another forum, the response was very poor indeed.

I have limited experience with passive x-over design however I am now using active x-over, digital DCX2496, which one are you using, analog or digital?

There is even a difference between pure analog and digital x-over. I used an analog Behringer before and now I am using a digital x-over, ie DCX2496.

The adv. of DCX is that signal from the source is digitised all the way till the analog stage prior reaching the amplifier. DCX also accepts analog input from the source , digitises it and converts it back to analog. There is Real difference between the 2 options, digital is much better.

I like my DCX because it is very flexible and I can design many different types of speakers and don't have to spend lots of money on passive components. If I like the sound speakers I may change the X-over to passive. That is the option I have.

I am sure there is a difference between active and passive x-over. I hope there will be more contributors here than I experienced before. I am sure there are many here who have experience in both technologies.

cheers.

dublin78 5th February 2008 09:02 AM

I too am very interested in some of the background to this subject, as I am contemplating going digital active with a dcx2496 sometime soon.

hahfran: What active crossover are you using? Does it simply split the signal or is it capable of using slopes/filters such as Butterworth, Bessel and Linkwitz-Riley?
Maybe this is the problem, and you are not achieving the ideal integration of your drivers?

I have been led to believe that an active crossover should be able to achieve better results as it does not have passive components in the signal path. In theory the drivers are performing unhindered and free, which must have similar advantages as fullrange drivers.

bzfcocon 5th February 2008 10:01 AM

Re: Re: Passive crossover into active crossover
 
Quote:

Originally posted by ttan98


If I like the sound speakers I may change the X-over to passive. That is the option I have.

Well, that might not be a trivial task to do, if you want to achieve an exact match.

There are pros and cons for both active and passive, what really matters is the end result. Passive crossovers are tightly coupled to the drivers, while actives are separated: this can be an advantage or a drawback, it depends on the application.

I compared once an analog active, OPAMP-based crossover with a passive on the same drivers. While I wasnt been able to achieve the exact same response, I ended up with the passive version, as being both simpler and better sounding.

Depending on the driver combination, sometimes you can achieve a simple and elegant solution using passive components.

There is however one area where I definitely think active is much better: very low (100-200 Hz) crossover points. I wouldn't bother using can-sized coils there, where active can do it a lot better.


There are also hybrid approaches, as JohnK describes on his site: use of passive for the actual filter and active equalization to flatten the response curve.

ttan98 5th February 2008 11:02 AM

Re: Re: Re: Passive crossover into active crossover
 
Quote:

Originally posted by bzfcocon


There is however one area where I definitely think active is much better: very low (100-200 Hz) crossover points. I wouldn't bother using can-sized coils there, where active can do it a lot better.


There are also hybrid approaches, as JohnK describes on his site: use of passive for the actual filter and active equalization to flatten the response curve.

When I say if I like the sound I will change to passive I mean x-over between the mid and tweeter. I will still maintain active x-over between the woofer and mid. That we are in agreement.

hahfran 5th February 2008 08:09 PM

Thanks for the responses. So when I dig out my sparse knowledge of physics , the acoustical side of a loudspeaker is the
radiation resistance which is frequency dependent and complex.
It depends also on the acoustical properties of the listening room which is never an echo-free half-sphere.
This radiation resistance must be transformed into an electrical circuit consisting of L,C,R which is the electrical load of the crossover.
The current which a voltage source ( the ideal amp ) drives depends then on the paralleled crossover and the electrical equivalent of the speaker's radiation resistance.
So far so good or bad.
Insofar the active crossover has to simulate the passive plus a correction which is due to the interaction of the passive loaded with the electrical equivalent of the radiation resistance.
That should be solvable at least for the simulated loudspeaker.

Davey 6th February 2008 04:04 AM

The best way to obtain a transfer function (electrical response) of an existing passive crossover is to actually measure it. Perform a differential measurement from the crossover input (reference point) to each drivers terminals (measurement point.) This will yield the electrical response of what you would need to duplicate with an active crossover to obtain the same acoustic result. Most of the soundcard-based speaker/analyzer design programs will do this. ARTA, Spectraplus, SW, etc, etc.

A really good simulation would also get you there, but since you already have the speakers it's easier and better to take an actual measurement. The result can be saved as a text file and used for other purposes as well. Imported into Jan's excellent program....."target" file for some other speaker design program......whatever.

However, I think if you're going to use an active crossover setup you can most likely improve upon your existing design quite a bit. :)

Cheers,

Dave.

Speedskater 6th February 2008 03:19 PM

Quote:

Originally posted by Davey
The best way to obtain a transfer function (electrical response) of an existing passive crossover is to actually measure it. Perform a differential measurement from the crossover input (reference point) to each drivers terminals (measurement point.) This will yield the electrical response of what you would need to duplicate with an active crossover to obtain the same acoustic result. Most of the soundcard-based speaker/analyzer design programs will do this. ARTA, Spectraplus, SW, etc, etc. (snip)
Dave.

Dave, is there much information on doing this measurement?


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