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Old 27th July 2007, 01:30 PM   #1
jnb is offline jnb  Australia
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Default Filtering dipole peak

I am trying to EQ this and a peak is interfering. How would I go about fixing it?

The driver is an Eminence Delta 15 on a 60cm x 85cm baffle. Shown is the response at 2m, and an attempt to EQ it. The peak is the one between 80-130Hz. If I reduce the frequency of the filter, the lower bass flattens further but the peak remains.

I would be open to active or passive solutions. Here's the plot:
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Old 2nd August 2007, 09:29 PM   #2
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You could probably just add a notch filter to your lowpass.
http://www.linkwitzlab.com/filters.htm#7
I use LTSPice/SwitcherCAD III to model the response of active filters. It's free and available here: http://www.linear.com/company/software.jsp

If this is not enough information, let me know and I'll try to be more helpful.

Regards,
David
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Old 2nd August 2007, 10:03 PM   #3
jnb is offline jnb  Australia
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Thanks gtforme00 for responding, I didn't think anyone would at this stage.

I was looking at the Linkwitz site last week and found a filter with a "heel" at resonance. I think it is called a bridged 'T' and consists of 3 resistors and 2 capacitors. This worked very well.

I found I could do the same thing passively by using a large series inductor and putting a series RC across the inductor. Then simulating the speakers true impedance I found I could remove the capacitor and just have the resistor across the inductor though the values have to be just right.

This loses me 10dB or so though and doesn't sound as good as I think it could so I am moving to go active.
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Old 6th August 2007, 02:17 PM   #4
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That is some clever and resourceful work you've done! 10 db loss though, ouch. Once you get into active, I think you will have alot of fun. Definitely play around with some modeling software like SwitcherCAD though, it makes predicting circuit response much easier.

Even if you don't use Speaker Workshop, I recommend looking at Jay Butterman's Speaker Workshop Manual, Section 6 page 203, on using active filter simulation in modeling programs. He includes a spreadsheet that will convert the data output of SwitcherCAD to the frd format, and he includes a sample lowpass filter that you can quickly open and modify to any other type of filter you please.

Once you get a circuit topology you like, you can easily wire up most active circuits on perfboard or the like (I've even used breadboard and not noticed any detrimental effects.) It's alot less expensive to swap out 1/8W resistors and cheap poly capacitors than it is to buy big inductors and big mylar capacitors!

Have fun, and feel free to pm me or post to this thread (I've subscribed to it) if you would like to discuss further issues with this design. I've got a couple of active dipoles running and don't mind sharing my experience.

Regards,
David
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Old 6th August 2007, 11:10 PM   #5
jnb is offline jnb  Australia
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I've been trying different values and as usual many don't sound too promising. I did, however hear some smooth and strong clear bass with one config and it improved with a tweeter (eventually I want to cross this woofer over at 400Hz).

I've D/L the SW manual to read this evening, thanks.

The bass has a punchy feel (when the source goes that way) and sounds on the level, balance wise. I am finding it a little quiet and I suspect this is because I am used to room resonances. I seem to like a 3dB/octave slope added to flat. Do you do this too or have you heard of others doing it?
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