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Old 2nd November 2013, 07:07 PM   #1
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Default Adding passive highpass

Hello!

I currently own a pair of Monitor Audio BX2, with a Denon Ceol Piccolo amp.

Am buying an additional subwoofer for some more 30-60Hz "ooph", but I am concerned about excursion on the BX2s when pushed hard.

SO, my plan is to make a 6dB / 12dB passive highpass to protect it from VLF.

Would a 6dB/oct LR at 80Hz be sufficient? Would give -6dB at 40Hz, and -12dB at 20Hz . And how would I calculate the capacitor value?

Thanks in advance,
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Old 2nd November 2013, 08:46 PM   #2
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Usually this can be done at line level, by changing input capacitor at the amplifier. So it's not a basic level and involves some knowledge..I'd say 2nd or 3 rd grade of DIYing You'd end up with a little cap ranging from 0.1 to 0.05 uF depending on amplifier's input impedance ( usually 22 to 100 K Ω ) .
Doing it after the amplifier involves working with higher currents and reactances given by the crossover/ driver / box system. Of course it's doable and it wouldn't imply that the signal has to go in two paths before getting amplified.
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Old 2nd November 2013, 09:01 PM   #3
tvrgeek is offline tvrgeek  United States
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A passive crossover that low is possible, we used to do that in the 70's It will cost hundreds of dollars in massive coils and generate copious harmonic distortion. PLEASE use an active crossover. A dead-bug with u741's has lower distortion than the massive coils you would need let alone the OHMS you will be adding in all that wire. Ohms, not miliohms. I might suggest the Marchand crossover if you did not want to DIY one. You have very nice speakers, don't screw it up!

Don't believe me? Go price a 35mH coil and a 220 uf worth of film caps for a second order passive.

With a nice LR4 active, you can cross at 60. This will prevent virtually any localization of the subs and lower the point where sub to main phase cancellation becomes a problem. Your Monitor's can handle anything you can stand at 60 with a nice crossover like that.
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Old 2nd November 2013, 09:05 PM   #4
tvrgeek is offline tvrgeek  United States
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Pico,
Changing the input cap is even worse than the passive crossover. Do the SPICE sim on a typical LTP bipolar input you will find it can raise the amps distortion by orders of magnitude. Zero has high fidelity speakers and amp. Don't bring him down to K-Mart.
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Old 2nd November 2013, 09:25 PM   #5
tinitus is offline tinitus  Europe
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you could try a big 'motor-run' cap in series with speaker

but doesn't most or many sub plate amps have filtered high pass option for satelite speaker ?
and even with mono sub, I think the sat out still is in stereo

if you have no line out from your amp, your sub will need option for driving with speaker cable signal
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Old 2nd November 2013, 09:31 PM   #6
tvrgeek is offline tvrgeek  United States
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Most plate amps are pass-through on the mains. Bummer. They assume we all have AVR's and are decoding DD 5.1 or something, not listening to music on a real stereo.
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Old 2nd November 2013, 09:52 PM   #7
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Thanks for all the responses!

I had a feeling that the inductors may be a problem, I naively assumed that the size & cost would scale down from PA applications (where most of my experience lies).

I should at this point explain better what I'm doing with the system. The Ceol is used for both the DAC (SPDIF) and via wifi. I planned to use the SUB out from the device to drive a REL Q-200E. So, unfortunately there is no way to provide the highpass corrections upstream of the speaker terminals. I fully intend to upgrade the system with seperate amps, airport express, MiniDSP etc as funds permit. Until then...

So, with 2nd order out the door - 1st order is the only "easy" solution to the situation in my eyes? If this were placed at 80Hz, it would provide a nice bit of LF protection (4x less excursion @ 40Hz, 8x less excursion @ 20Hz).

From my testing, the cone seems to become unloaded at ~45Hz - so a shallow filter may suffice?

Thanks!
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Old 2nd November 2013, 10:43 PM   #8
DavidL is offline DavidL  United States
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Just placing a cap in series with a speaker can cause all sorts of problems. The cap will resonate with the driver/box resonance and can actually boost the low frequencies going to the driver. Why not just try the subwoofer as planned and see if the system sounds okay first? You aren't over driving the current speakers are you? Adding the subwoofer will only make it sound better and you may find yourself turning down the bass going to the smaller speakers. I suggest crossing over the sub from 80 to 100 Hz depending upon the extension of your current speakers.
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Old 2nd November 2013, 10:47 PM   #9
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Quote:
Originally Posted by tvrgeek View Post
Zero has high fidelity speakers and amp.
Quote:
Originally Posted by zerokelvin99 View Post
s. I fully intend to upgrade the system with seperate amps, airport express, MiniDSP
That's going to worsen it !
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Old 3rd November 2013, 12:07 AM   #10
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Quote:
Originally Posted by DavidL View Post
Just placing a cap in series with a speaker can cause all sorts of problems. The cap will resonate with the driver/box resonance and can actually boost the low frequencies going to the driver.
Oh, I wasn't aware of this - can you link to some further reading?

Quote:
Originally Posted by DavidL View Post
Why not just try the subwoofer as planned and see if the system sounds okay first? You aren't over driving the current speakers are you? Adding the subwoofer will only make it sound better and you may find yourself turning down the bass going to the smaller speakers. I suggest crossing over the sub from 80 to 100 Hz depending upon the extension of your current speakers.
Yeah, I guess I'll try that first. It's just that running without a HPF scares me a little, especially with HT content! I don't plan to run the Ceol into clip (45WRMS), so they won't be overdriven

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Originally Posted by picowallspeaker View Post
That's going to worsen it !
Hah, explain?
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