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Opinion on CP750 crossover

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Cobby View Drop Down
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    Posted: 06 August 2019 at 7:40pm
Got some custom MT121 style cabs, guy who made them is pretty good but passed away, only info I got from the previous owner is 100hz, 1.7kHz crossover points and no delay applied. 

 I dialed in those settings and some PEQ but really wasn't happy,then I opened the driver to see a beyma CP 750 + HL 26-50 horn, both of them go down to 800hz so I'm thinking of lowering the crossover to 900hz and maybe the 12 ( probably a 12g40 ) to 160hz-900hz, shoul I use a steep 48kHz LR? I'm afraid to push the drivers, those recones are expensive.

Cheers!
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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2019 at 3:31pm
General rule of thumb is you should operate a comp driver/horn combo only down to 1 octave above its "minimum" frequency - if you go lower, you will get too much distortion. The driver might well survive, but won't sound good doing it. So for your 800hz system, you should cross at 1.6kHz with 18dB slope.

The MT121 design basically does have a flaw, in that the "90 degree" dispersion from the 12" horn starts to narrow above about 800Hz and is down to ~40 degrees at 1.6kHz - so in an ideal world, you'd put a huge horn/driver above it with a 400Hz cutoff, crossed in at 800 - but then you'd have so much spare headroom in the horn driver compared to the single 12", it wouldn't make any sense; you'd be wasting the potential of a very expensive and heavy comp driver. An MT122 (2x12) with very large horn would be acoustically more sensible - but impractically large! If you build seperate 12" and horn boxes it would be OK I guess...

Essentially, most people just cross over at 1.6kHz and accept the dispersion anomaly. It is what it is. 
There are more complicated designs that don't have the issue, but the MT121 is easy, loud and versatile...depends how much effort you want to go to!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cobby Quote  Post ReplyReply Direct Link To This Post Posted: 09 August 2019 at 11:02pm
did some more reading, still trying to grasp how to choose crossover, studied some designs, recommended settings, LR 24 seems to be the most popular, better driver protection.

Lower order crossover are more natural but a 18dB slope is going to introduce a 135 degree phase shift i reckon?

The horn is 50x25 so the narrowing is in line with the dispersion at XO point. I'm more worried at the 12 acting out of its piston range and anomalies on the 12 beyond 900hz, I see a lot of modern designs with lower crossover point.

this the data on the driver and horn:

cp750:

Sensitivity (1W/1m)   110dB 1w @ 1m w/ TD-400 horn]

Frequency Response 600Hz - 20kHz

Recommended Crossover 800 Hz or higher, 12dB/octave

the horn has min 400hz frequency, cant i lower the LP on the 12, use a lower order XO and leave some gap to the driver?


Edited by Cobby - 09 August 2019 at 11:03pm
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