# Please explain: RCA active filter / crossover on sub



## david.faik (Apr 14, 2018)

I’m looking for a sub that has left and right RCA in and out, and further that acts as an active crossover (thus not sending the bass that the subwoofer is handling to the main speaker amps).

The reason for this is that I have a great stereo pre-amp with only one pair left and right RCA outputs. I would like to buy small but high quality monitor type speakers and leave them free from the struggle of producing lots of bass (as the rest of the range IMHO suffers).

I am not clear on the terms used to describe the above. For example I see many KEF PSW 2010’s for sale 2nd hand. They have a pair of RCA in and a pair out. The pair out is labelled “high pass output”. Does that mean it is only passing out the frequency above what the subwoofer is handling?

My system is a Meridian Director DAC ->Prime / PSU pre-amp -> four Musical Fidelity X-A50 monoblocks (bi-amping)

I’d like it to be Meridian Director DAC ->Prime / PSU pre-amp -> **Sub Woofer with Active Crossover** -> four Musical Fidelity X-A50 monoblocks (bi-amping)

Appreciate most subs in home theater will use an active crossover in the AV amp (that doesn’t exist in my arrangement).

Any help from you smart folks greatly appreciated



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## ajinfla (May 10, 2009)

david.faik said:


> I’m looking for a sub that has left and right RCA in and out, and further that acts as an active crossover (thus not sending the bass that the subwoofer is handling to the main speaker amps).
> 
> I am not clear on the terms used to describe the above. For example I see many KEF PSW 2010’s for sale 2nd hand. They have a pair of RCA in and a pair out. The pair out is labelled “*high pass output*”. Does that mean it is only passing out the frequency above what the subwoofer is handling?


Yes, that is exactly what "Hi pass" means, so this or other similar subs with high pass out is what you desire.
So preamp > sub input, then "hipass out" > amp driving main speakers. One possible drawback is the KEFs highpass seems fixed, say 100hz etc.
That might either be too high or low. So the options are to find a sub with a variable high pass frequency (rare), or choose the high pass yourself, with an inline filter such as these https://www.parts-express.com/cat/in-line-crossovers/405?N=20477+4294967118+4294965810+4294965805&Ne=10166&Nrs=collection%28%29%2Frecord%5Bendeca%3Amatches%28.%2C%22P_PortalID%22%2C%221%22%29+and+endeca%3Amatches%28.%2C%22P_Searchable%22%2C%221%22%29%5D&PortalID=1
So if your mains are comfortable down to 50hz, select a 50hz filter. Or 70. Or 100, etc.
The lower the main can go (without distortion on heavy bass passages), the easier it will be to blend with the sub.


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## david.faik (Apr 14, 2018)

Brilliant, so “hi pass” out is what I’m after.

These KEF subs have dials to control the “cut off” frequency and two “slope” options, so was wondering where you saw that the hi pass was fixed at 100hz?

Having found a manual it describes the outputs as “actively filtered @ 12db/octave @ 130Hz” ... so I am guessing this is the fixed part and the cutoff isn’t linked?

As speakers give their range to +/- 3db and not flat, I am guessing if the main speakers state down to 50 or 60Hz that should be about right?

Getting there I think LOL  




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## RLouis (Jan 20, 2010)

Instead of looking for a subwoofer with a line level high pass built in you'd be better off looking for an actual "stand-alone" active crossover then just run the low pass out to the sub. Then you can run ANY sub you'd like or even switch subs easily. And, a stand alone active crossover will give you many more tuning functions and more flexibility than a simple built-in. Example:Behringer Super-X Pro


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## RTS100x5 (Sep 12, 2009)

RLouis said:


> Instead of looking for a subwoofer with a line level high pass built in you'd be better off looking for an actual "stand-alone" active crossover then just run the low pass out to the sub. Then you can run ANY sub you'd like or even switch subs easily. And, a stand alone active crossover will give you many more tuning functions and more flexibility than a simple built-in. Example:Behringer Super-X Pro


https://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4-hd


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