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Old 15th November 2011, 09:04 AM   #1
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Question Active crossover for old Wharfedale Diamonds

I've always wanted to play around with active speakers and the chance happening of a pair of Quad 303s coming my way makes it seem within reach. For my first effort I intend to convert a pair of old Wharfedale Diamond III speakers. I believe these are quite different to the current speakers of the same name but still seem a pretty good budget box to play with.

Inside, I find that the LF crossover part only has a 450uF capacitor in it so I think that is being driven full range with a 1st order high pass filter at about 45Hz to prevent bass boom. The HF unit has a conventional 3rd order high pass filter (5KHz is stated on the back of the box and the component values look correct for that). There is also a small 65pF capacitor and what is probably (it is corroded so I can't see any markings) a 2.2R resistor. I'm guessing the resistor is some kind of gain matching between the two drivers, not sure what the capacitor does. Would it be true to say that the 90 degree phase shift of the 1st order filter and the 270 degree shift of the 3rd order filter would be counteracted by wiring one of the units out of phase? It isn't possible to tell from the drivers as they are completely unmarked.

I don't have any circuit drawing software but I hope this helps: <http://home.btconnect.com/www.cosmo.com/Xover.pdf>

So, should I just completely mimic the same filters in my active configuration? There are plenty of examples for 1st & 3rd order active filters that would do the job. If the resistor is there for gain matching should I also do that actively with an adjustable gain stage in the active filter? Or, should I keep the original 5KHz cross-over frequency and use something different for the filters that doesn't have any phase shift problems?
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Old 15th November 2011, 12:25 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

The bass capacitor interacts with the bass unit in way that is more
complex than a first order high pass at 45Hz, due to the driver and
port impedance peaks.

Making the speakers bi-wirable and simply bi-amping is the easiest.

Best option is a 3 Way with active bass/mid c/o and passive mid/treble.

rgds, sreten.
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Old 16th November 2011, 06:43 AM   #3
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Quote:
Originally Posted by sreten View Post
The bass capacitor interacts with the bass unit in way that is more complex than a first order high pass at 45Hz, due to the driver and port impedance peaks.
So, I have no choice but to include some passive components? If I remove the requirement for passive filtering then any other passive components for cabinet compensation should be minimal. Would there be much interaction with the HF driver and the port impedance?

Quote:
Making the speakers bi-wirable and simply bi-amping is the easiest.
Sure, that would be trivial, but that isn't my goal here.

Quote:
Best option is a 3 Way with active bass/mid c/o and passive mid/treble.
I'm starting with a small 2 way bookshelf speaker, and want to end up with the same small 2 way bookshelf speaker but actively driven.
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Old 16th November 2011, 07:25 AM   #4
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The 65pF?? is not a capacitor, maybe a Polyfuse?
I would start by measuring the drivers, one at a time by disconnecting from the binding posts, with PC software like Holmimpulse

That woofer really needs a 2nd order lowpass to stop cone breakup
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Old 16th November 2011, 11:06 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by 68Spider View Post

Sure, that would be trivial, but that isn't my goal here.
Hi,

I know that, but IMO anything else is a waste of time for that speaker model.

The 303 in pairs is very good for passive bi-amping*, and for most
2-ways it's the best and simplest option, YMMV, so try it and see.

rgds, sreten.

*Current limits of the shared regulated supply are relaxed with
only one channel of the two providing the bass/mid levels needed.
Its fair to say the treble end of two ways doesn't need much current.
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Old 17th November 2011, 07:52 AM   #6
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Quote:
Originally Posted by sreten View Post
I know that, but IMO anything else is a waste of time for that speaker model.

The 303 in pairs is very good for passive bi-amping*, and for most 2-ways it's the best and simplest option, YMMV, so try it and see.
Yes, I realise the humble Diamond will not be transformed into a Super Speaker and passive bi-amping is definitely on my incremental development list.

But this is just a learning exercise before I attack my Sonus Fabers ;-).
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Old 17th November 2011, 12:20 PM   #7
Colin is offline Colin  United Kingdom
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Have a look at the MiniDSP or investigate an eBay Behringer external x/over as a starting point. That way you can modify values a you learn. You can always build a fixed crossover later when you have established the x/over point and slope you wish to use. They'll also come in handy when you get ready to do the same on the Sonus Fabers.

My inclination would be to forget the passive crossover - placement and values are likely to be restricted by manufacturing budget. Go for a proper 24dB octave crossover on both drivers, centred at 5kHz as a starting point.

I think the LF roll-off cap can be removed for now too - fi the system starts to show undesirable booking, try blocking the port or damping it with foam.

Passive crossovers are a compromise so going active gives you the chance to approach something more ideal.
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Old 17th November 2011, 02:41 PM   #8
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I did look on eBay at ready built kit like the Behringer CX2310 (or even something cheap and nasty for in-car systems) but simply twiddling a couple of knobs to alter the frequency takes a good deal of the fun out the exercise. It also looks to be a fixed 24dB slope so no adjustment there.

I'll have a look at the miniDSP thing but that seems way beyond my simple analogue tinkering.
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Old 18th November 2011, 06:33 PM   #9
juz400 is offline juz400  United Kingdom
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I would recomend the miniDSP!

It is VERY versitile, plug it in, load the software on your computer and your away
you can tinker quickly and in REAL time!
once you have the FR sorted you can then plan something analogue easier.

I have a pair of 2x4`s with 3 poweramps running a set of standmounts and a pair of
subwoofers. I love it!

Thier customer service is EXCELLENT and at the price you cant go wrong!
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Old 22nd November 2011, 11:24 AM   #10
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My first search for miniDSP pointed me at the low level DSP chips (a bit intimidating) but now I've found the miniDSP site with all the kit projects. It does look just the business for a lot of tweakery - a bit like the type of car ECU where you can fine tune the fuel maps with a laptop.

I've now scored a cheap 2 way + sub active crossover from eBay to play with. That should keep me amused until Christmas.
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