CSS FR125 Bipole revisited.. - diyAudio
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Old 7th March 2008, 02:12 PM   #1
Bukem is offline Bukem  Belgium
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Default CSS FR125 Bipole revisited..

Dear All,

I have recently purchased as set of FR125's (initially to use these sealed with a BSC circuit and perhaps a notch filter as well).

Intrigued by the various offerings I have given this some more thought. I guess the primary reason to choose a full-range driver is it's coherency and simplicity of any filter network. Fewer components can only mean less components to affect the sound in a negative way.

Having read pretty much all the posts on the FR125 it seems that there's not a real consensus on driver loading. Bass Reflex seems to be the least popular, followed by horn/TL. Sealed and aperiodic seem to be most popular since horn/TL might emphasize CSS's impressive base response a tad too much. However there does not seem to be a "definitive" type of loading. I understand this also depends on the room..

Now personally I have a soft spot for sealed but I am drawn to the various horn designs. I might match it with a subwoofer in the future so I guess sealed is the way forward.

In order to work the benefits of full-range drivers I thing it is appropriate to use a second driver to solve the baffle step issues. Of course I could use a few passive components but If I follow this route I might as well build a 2-way speaker.

I'd like to do some more work on this type but before I move on I'd like to get a few things straight:


1) What's better for a bipole > a wider baffle or a narrow one. Somehow it seems that the width in this type isn't that important at all since the driver at the back in a push-push configuration sees the same width. Am I right or wrong here? Is it depth that is crucial?

2) To low pass or not to low pass on the rear driver..... Isn't it unnatural to run the rear driver with an inductor / low pass? What I am trying to say is that if the rear driver is rolled off early then surely there must be an unbalance below the baffle step region and the region between the baffle step and the roll off? The two drivers are partially summed aren't they - but the higher frequencies aren't hence the imbalance - right?

3) What are the advantages and disadvantages of having the two push-push drivers sharing the same physical volume. Are there advantages to be had by giving them both their own volume?

4) Is there a consensus on the ideal enclosure volume for the FR125 in a sealed config?

Many questions but hopefully interesting enough to pursue.

As a last point, one of the things that really keeps me occupied is what about using another two FR drivers (so 4 drivers in total per enclosure) but run these passively like a passive radiator. They'd need to be wired together to create a short with a rotary switch and a few resistors. This way if memory serves me right you can alter the qts of the drivers. Sonus Faber did this in the past with a KEF B139 at the back of an Extrema monitor loudspeaker. It even included a small heatsink to conduct the heat produced by the resistors. I know this effectively is a bass reflex enclosure but the possiblity to tailor the "passives" behaviour might prove interesting - both aesthetically and hopefully sonically as well.

Many thanks for thinking with me!

Bukem
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Old 7th March 2008, 04:00 PM   #2
Bukem is offline Bukem  Belgium
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I am afraid I should have posted this in the fullrange section. Apologies..

Moderator, could you please move my post?

Rgds,

Bukem
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Old 7th March 2008, 04:02 PM   #3
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Moved to full range.
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Old 7th March 2008, 05:02 PM   #4
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Hi Bukem,

During my own testing I had approx 10"W x 9"D and the bipole effect was good.

Low passing the rear took 'life' away from the sound.
The rear driver should not be directly flat facing the wall though - the reflecting HF 'pings' into the room.

I cannot see any advantage for giving each driver its own cabinet; reflection paths to the rear of each driver would be shorter, whereas some damping material or reflectors could be used between sharing drivers.

Re additional 'bass' drivers.
Without tuning or long path augmentation you will not be getting really low bass, so why not fit something larger and choke passed, front and back near the floor, then maybe the entire cabinet could be resistively vented in order to minimise LF peaking ?

Cheers ........... Graham.
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Old 7th March 2008, 05:59 PM   #5
chrisb is offline chrisb  Canada
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Our dear moderator can describe his own approach for "multi-pole" application with the CSS FR/WR drivers. i.e. see Calhoun

It's not strictly bipole, but certainly works
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Old 7th March 2008, 09:26 PM   #6
Bukem is offline Bukem  Belgium
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Quote:
Originally posted by Graham Maynard
During my own testing I had approx 10"W x 9"D and the bipole effect was good.
Hi Graham,

Thanks for thinking along with me. You mention that you've used a nearly rectangular cabinet. Is there an aesthetic reason or is there a specific calculation you made to come up with this width/depth ratio?

Quote:
Originally posted by Graham Maynard
Low passing the rear took 'life' away from the sound
Does that correspond to any measurements that you might have taken?

Quote:
Originally posted by Graham Maynard
The rear driver should not be directly flat facing the wall though - the reflecting HF 'pings' into the room.
Makes sense, especially when the rear driver's HF is not rolled off prematurely..


Quote:
Originally posted by Graham Maynard
Re additional 'bass' drivers.
Without tuning or long path augmentation you will not be getting really low bass, so why not fit something larger and choke passed, front and back near the floor, then maybe the entire cabinet could be resistively vented in order to minimise LF peaking ?
I'm not completely following you on this one. Using the FR125 as a passive radiator (albeight using resistors to alter the Q) is effectively turning it into a bass reflex system. What do you mean with choke passed, and surely you don't mean that I should mount the FR125 close to the floor...


Chrisb,

Are you refering to the horn with the side firing WR?


Any further thoughts re the "passives" idea?

Many thanks,

Bukem
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Old 7th March 2008, 09:30 PM   #7
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Quote:
Originally posted by Bukem
[B]Chrisb,

Are you refering to the horn with the side firing WR? [B]
I believe our dear friend is indeed referring to these:
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Old 7th March 2008, 09:31 PM   #8
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A very straight forward build with satisfying results.
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Old 7th March 2008, 09:51 PM   #9
Bukem is offline Bukem  Belgium
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Hi Cal,

I like your boldness - just do it !

What did surprise me is that you've mounted the 2nd driver on the side of the enclosure instead of the rear. (not ideal in this design ) I suppose you lose the push - push mechanical argument but on the other hand you reduce the distance between the two drivers.

There have been arguments that the worst thing to do is stick two FR drivers on one baffle. Isn't your design more or less approaching a "one baffle for all" scenario? Apparently there'd be severe combing issues.

I presume your design could be used with FR125's instead of the WR + tweeter? Are you currently running the side driver "fullrange"?

Regards,

Bukem

(EDIT: typo)
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Old 7th March 2008, 10:11 PM   #10
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Hi Bukem,

The side firing drivers seem to offer the most pleasing combination I tried. It allows for them to be pointed in or outward depending on the distance between the speakers and it acts like a mechanical BSC. Both drivers are running full with only a small cap and light padding on the tweet as I really hate taking a low sensitivity driver and sucking further life out of it. I couldn't even run it as a 2.5 way with both drivers on the front. It just didn't work. The Scottmoose designed enclosure goes a long way to making this a very enjoyable speaker. Combing is not a big issue at all as the side driver is so far off axis that it doesn't seem to have a detrimental effect. It's a big box but to get these drivers to really shine, you have to do something. I'm glad they finally found a home. For a while there I was beginning to wonder what all the fuss was about with them. Now I know.
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