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Old 7th February 2014, 12:06 PM   #1
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Default Converting commercial speakers to bi-amplified

Hello,

I am currently planning on bi-amplifying a pair of KEF C7's. I will take out the passive crossover and power the cones with some LM3886's amplifiers I have lying around. Before them, there will be an active crossover/preamplifier.

I am not sure on the details of how to accomplish all that, so I came here. As KEF states, those are 8 Ohm speakers crossed over at 2.500Hz. So I took out the filter and drawn it, attached is the schematic of it.

I took out the cones and measured their resistance without any filter. I had to use a plain multimeter to get their impedance as I haven't any LCR meter available. I read around 4.2 Ohm on both the tweeters and the midbass', so I can round it up and suppose all of them are 6 Ohm speakers, right?

I would like to draw the filter with some simulating software to see it response curve and to check (mainly out of curiosity) if that correlates with the 2.500Hz point that the manufacturer provides.

What I don't really know (I am quite noob regarding speakers/crossovers) is why they are labeled as 8 Ohm speakers while they don't have any, isn't it strange?. Even further, they have paralleled two 6 Ohm cones that gives 3 Ohm combined. Does the passive crossover make them appear as 8 Ohm's or something?

Then, the low pass filter seems to be a second order one, I think the high pass one should be of the same 2nd order but, it really is? isn't it a 3rd order one?

Then the next question should be, what active crossover topology to mount? I have seen those very nice boards to make it, seems to me the way to go. Their filter slope is 12dB/octave (it is a second order filter, right?), as far as I know for biamping a speaker I don't need to match the original slope so in theory I could make it be any order I would like, right? I have attached the schematic provided by the seller.

Line Level Active 2 Way Crossover Filter NE5532 PCB Only DIY Audiophile | eBay

Thanks!
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Last edited by regiregi22; 7th February 2014 at 12:27 PM.
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Old 7th February 2014, 12:14 PM   #2
AndrewT is offline AndrewT  Scotland
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This is a conversion to active speaker.
Bi-amping is connecting two amplifiers to the speaker terminals of one 2way speaker.
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Old 7th February 2014, 12:30 PM   #3
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You don't have to duplicate the passive filters' transfer functions but that is probably the best way to get the result to sound decent if you aren't going to measure the response of the drivers. Stock crossover filters may or may not work well.

If you are going to measure the driver responses then you can use any filter you want and see if you do better than Kef with the drivers.
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Old 7th February 2014, 12:37 PM   #4
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Quote:
Originally Posted by BobEllis View Post
You don't have to duplicate the passive filters' transfer functions but that is probably the best way to get the result to sound decent if you aren't going to measure the response of the drivers. Stock crossover filters may or may not work well.

If you are going to measure the driver responses then you can use any filter you want and see if you do better than Kef with the drivers.
I haven't thought in measuring them, but should be a nice way to get into it now that I am getting into DIY microphones too. So I guess those kind of uncalibrated ones should be "good enough"?

DIY microphones
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Old 7th February 2014, 12:41 PM   #5
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Quote:
Originally Posted by AndrewT View Post
This is a conversion to active speaker.
Bi-amping is connecting two amplifiers to the speaker terminals of one 2way speaker.
I think you got me wrong, the amplifiers and active crossovers would be in their separated enclosures, the speaker boxes would only contain the cones and the terminals. This, or I have a problem using the correct terminology...
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Old 7th February 2014, 12:44 PM   #6
AndrewT is offline AndrewT  Scotland
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Some in the USA refer to active speakers as bi-amped.

To help with differentiation I have suggested we call the two types:
active bi-amplification and passive bi-amplification.
But that seems to have fallen on deaf ears.

In the meantime Bi-amping is as I defined.
Quote:
Bi-amping is connecting two amplifiers to the speaker terminals of one 2way speaker.
and
tri-ampling would be connecting three Power Amplifiers to the three sets of terminals on a 3way speaker.
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Old 7th February 2014, 01:08 PM   #7
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Quote:
Originally Posted by regiregi22 View Post
I haven't thought in measuring them, but should be a nice way to get into it now that I am getting into DIY microphones too. So I guess those kind of uncalibrated ones should be "good enough"?

DIY microphones
"Calibrated" using generic capsule response curves is better than not measuring. Better would be something actually calibrated.

Have you found Holm Impulse? It's free measurement program. There are all sorts of design programs out there that let you work on your filter transfer functions. I use Speaker Workshop, but most think it has a rather steep learning curve.

Andrew, I agree with your suggested terminology, and the OP has clarified that he wants to actively biamplify his speakers.
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Old 7th February 2014, 01:31 PM   #8
fpitas is offline fpitas  United States
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Quote:
Originally Posted by regiregi22 View Post
I haven't thought in measuring them, but should be a nice way to get into it now that I am getting into DIY microphones too. So I guess those kind of uncalibrated ones should be "good enough"?

DIY microphones
You're working with nice speakers, they really deserve a calibrated microphone. I got a very reasonably priced one from Cross Spectrum here in the states; but I'm sure there are places in Europe that offer them, too.
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Old 7th February 2014, 01:34 PM   #9
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Quote:
This is a conversion to active speaker.
Bi-amping is connecting two amplifiers to the speaker terminals of one 2way speaker.
Andrew is correct. You used the wrong terminology, but we understood what you meant.

Quote:
You don't have to duplicate the passive filters' transfer functions but that is probably the best way to get the result to sound decent if you aren't going to measure the response of the drivers. Stock crossover filters may or may not work well.

If you are going to measure the driver responses then you can use any filter you want and see if you do better than Kef with the drivers.
IMHO, Kef, as a company that build drivers with their own needs, probably markets speakers with a correctly designed crossover (look at the pretty unusual values of the components). That said, you probably need to replicate the passive transfer function to the active crossover. Problem is that without measuring the in-box frequency response and impedance of the drivers you're not able to find the transfer functions, as they are likely NOT a second order acoustical (as evidenced by the driver polarity). What most people here fall to understand is that a second order electrical filter merge with the driver/box characteristics and produce often a different acoustical order, and this is what you need to reproduce. Look here for an example: Zaph|Audio - ZA-SR71

Ralf
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Old 7th February 2014, 02:30 PM   #10
AndrewT is offline AndrewT  Scotland
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Add a second order (2 pole) filter of the same frequency as the inherent roll off of the driver and you get a slower roll off but of fourth order (4pole).

If you spread the filter frequency away from the inherent, or natural, roll off of the driver you end up with a pair of cascaded 2 pole roll offs with a "weird shape" and that "weird shape" is probably not what you need.

It is generally accepted that what is required for good integration of two drivers is an acoustical roll off that gets very close to your theoretical "perfect" order roll off.
But to get this you must combine the natural acoustic roll off and the active filter roll off.

Then you have to include some EQ to correct for other response anomalies.
This EQ may interact with that "theoretical" roll off and require you revisit your active filter to counter the EQ correction. This is what they mean by "test and measure" the almost finished assembly.
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