Adding passive highpass

[zerokelvin99]

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,

 

[picowallspeaker]

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.

 

[tvrgeek]

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.

 

[tvrgeek]

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.

 

[tinitus]

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

 

[tvrgeek]

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.

 

[zerokelvin99]

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!

 

[DavidL]

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.

 

[picowallspeaker]

Zero has high fidelity speakers and amp.

s. I fully intend to upgrade the system with seperate amps, airport express, MiniDSP

That's going to worsen it !

 

[zerokelvin99]

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?

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

That's going to worsen it !

Hah, explain?

 

[adason]

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,

just use bipolar capacitor, in parallel with small MP cap, bipolar comes in high values and are cheap

you can calculate required value here:
2-Way Crossover Designer / Calculator

 

[sreten]

Hi,

A series capacitor on the speakers is generally a disaster area. It won't work.

Sod all information in the manual about the sub out or any options in the
menus to apply any sort of high pass to the speakers when a sub is used.

A proper service manual is needed to do it but there isn't one.

rgds, sreten.

 

[tvrgeek]

It won't block the low frequency power, but if you just plug the port you will have a second order rolloff and sealed systems ar a bit more tolerant of sub-resonance frequencies than ported speakers. This will not degrade the fidelity above the rolloff. Most subs have a second order crossover, so just match the crossover frequency. Most subs can tap the speaker level output, so no line level is needed.

 

[tinitus]

... sealed systems are a bit more tolerant of sub-resonance frequencies than ported speakers. This will not degrade the fidelity above the rolloff ...

best post so far

 

[AllenB]

Oh, I wasn't aware of this - can you link to some further reading?

I have an example of a capacitor increasing response. It isn't always this bad. Some components do more than just divide the signal between them and the driver impedance.. and they react with each other. It will work properly if you compensate the impedance peak (RLC), though the components need to be larger values.

Another way to reduce this effect is a second order filter (cap and coil) as these are less sensitive to the driver impedance.

First order might be a good choice. I'd try to find a cap somewhere earlier in the chain which the signal is passing through, and make it smaller as mentioned above.

 

[AndrewT]

............Changing the input cap is even worse than the passive crossover. .................

What is wrong with passive filtering of the input signal to the amplifier?
That is the best way to ensure the amplifier can handle the remaining signal that gets past the input filters.

A single pole roll off at the amp input and a further single pole capacitor only filter at the speaker would give a "gentle" roll off aiming for 40dB/decade (2pole 12dB/oct)

 

[tinitus]

That is the best way to ensure the amplifier can handle the remaining signal that gets past the input filters.

I hope you are not telling him to take the lid off his amp, and poke around inside it, without having any clew about how it works

 

[Dissi]

A series capacitor on the speakers is generally a disaster area. It won't work.

That's undoubtedly true. A voltage divider consisting of a series capacitor and the double impedance peak of a vented loudspeaker does not yield the desired attenuation at all.

 

[tinitus]

ahh, right, yes, ofcourse

now I remember why a very big series 'motor-run' cap was used
it was back in the times of vinyl and turntables
and a quite different problem to deal with
very low frequency 'noise'

it is ofcourse far from being very good
but a possible option in case of desperation when looking at a small woofer moving like it was going to fly

sorry I suggested that
please just forget about it

I should know better
it will have to be crossed much higher to have any significant effect with ordinary music signal

 

[tvrgeek]

What is wrong with passive filtering of the input signal to the amplifier?
That is the best way to ensure the amplifier can handle the remaining signal that gets past the input filters.

A single pole roll off at the amp input and a further single pole capacitor only filter at the speaker would give a "gentle" roll off aiming for 40dB/decade (2pole 12dB/oct)

It has to do with the impedance feeding the input filter and LTP. I don't pretend to understand the finer points. I leave that to Jan, John, Nelson and the rest. Seems a bit abstract until you model it in SPICE. It really surprised me.