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-   -   Crossover Help Needed - bass rolloff wanted (http://www.diyaudio.com/forums/multi-way/185207-crossover-help-needed-bass-rolloff-wanted.html)

BobM 16th March 2011 02:42 PM

Crossover Help Needed - bass rolloff wanted
 
I recently picked up a pair of Apogee Calipers (ribbon drivers) and the left one has the dreaded Apogee buzz. The good news is it is really only audible at about 30Hz, using a test tone disk.

So I would like to modify the crossover to rolloff below 40Hz. Here's the schematic, can someone please suggest how I can mod this to implement a rolloff starting at 40Hz so that I can get a reasonably low output down at 30Hz where the problem seems to mainly exist?

Thanks,
Bob

http://www.thomas-schick.com/apogee/Caliperweiche.gif

tinitus 16th March 2011 03:21 PM

Your speakers are purely passive
the buzz comes from amp side, signal, etc

if you still want a low cut, you need to place it in the line signal, before power amp

even so, it might be a good idea to find where the buzz is coming from

BobM 16th March 2011 03:41 PM

The buzz is purely mechanical on the speaker. It's not the amp. The ribbon is actually flapping against the magnets. Either the tensioin on the ribbon is insufficient, or the (inaccessable) foam damping is drying out - probably both.

But the good news is there is no audible buzzing above 30Hz, so I think rolling things off will work for me.

Any suggestions on how to modify the crossover to accomplish this would be appreciated.

Thanks

BobM 16th March 2011 08:37 PM

I think I would need a cap the size of a house to implement this in the crossover, so maybe it's better to try somethign simple at line level on the interconnect to the amp.

What do you suggest?

BobM 17th March 2011 01:39 AM

Here's the details.

My amp has a 10K input impediance

So a second order line level crossover is probably out.

But if I want to start the cutoff at 40 - 50Hz then the math doesn't seem to work for me, as far as I can tell (I am no math wiz) no matter what size cap I throw into the equation.

Is the input impediance of my amp too low to do this successfully?

Help, I'm stuck and not really sure what values to try.

BobM 17th March 2011 12:26 PM

After looking at frequency charts, starting the cutoff at 40-50Hz is probably not going to do it, even with 12db/octave. I'll probably have to go as high as 70Hz to accomplish anything meaningful in the 30's.

Luckily there is an inexpensive in-line 70Hz high pass filter available from FMOD that I can try before I go for something else. The real question is how transparent will it be with these in the line? I understand they work differently depending on the input impedance of the amp (mine is 10K).

FMOD Crossover Pair 70 Hz High Pass

tinitus 17th March 2011 02:43 PM

TLS.org | Passive Line-Level Crossover

hey Bob
I moved your thread to multiway speaker
maybe it helps

btw, maybe you should try the above line filter, and see what happens, instead of looking at numbers

there may actually already be one or several caps in your signal chain, to block DC
depends on your amps

chrisb 17th March 2011 04:12 PM

What do the folks at Apogee technical department or the user groups/forums have to say about this?


If your buzz is from deterioration or misalignment due to shipping damage, etc., will HP filtering truly solve the problem?

If this was a moving coil driver with any of a number of potential defects in the motor/suspension systems, it would be dubious that filtering could mitigate a mechanical problem for all but the lowest output levels.

BobM 17th March 2011 04:56 PM

I agree, the proper course of action is to replace the driver/ribbon itself or have it refurbished. However this can only be done by a certified Apogee installer and will run to the thousands of dollars. And per my initial experiments, the mechanical noise is only in the lowest bass notes.

I'm looking for an inexpensive experiment to see if I can bypass the issue by rolling off the lowest bass notes and adjusting my subwoofer accordingly. It may or may not work, but surely it is worth the cost of the experiment to find out before I commit (or not) to a full driver replacement.

I picked up a pair of those FMOD in line high pass filters, so we'll see how this all plays out.

Thanks

sts9fan 17th March 2011 05:23 PM

miniDSP?


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