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Old 9th February 2013, 03:04 PM   #1
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Default MiniDSP H-Frame.

I have been building an H-frame dipole woofer that is based on the Orion's woofer.

I am using the same Peerless 830452 10" woofer and same dimensions for the H-frame as the Orion but have used a single woofer each side.


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I have been programming the MiniDSP today.
I first applied a LT biquad using the woofers TS specs as a reference and a target of 35hz, Q0.5.
Was I right to use the drivers FS as one of the poles?


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I then tried to add the 6dB/oct dipole compensation, 35hz-305hz, and soon found the MiniDSP was clipping. So I tried splitting the EQ with one low shelf and one high shelf and splitting the difference regarding the Gain. The crossover is a 100hz LR4. Again, is this a good way to do it?


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And here is the outcome measured at the opening to the front of the H-frame. Any comments or advice is welcome.



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Last edited by rich31td; 9th February 2013 at 03:24 PM.
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Old 9th February 2013, 03:09 PM   #2
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And this is what it will be attached to, the photos show my BK sub that was filling in until the H-frames were built.

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Old 9th February 2013, 11:42 PM   #3
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You don't need a LT for the Orion H-frame, just shelving filters for the dipole EQ and for the driver EQ.

You'll find some good info here:

MiniDSP as Linkwitz Orion ASP
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Old 10th February 2013, 12:16 AM   #4
OllBoll is offline OllBoll  Sweden
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If you are lazy then you could hook up REW, set your flatness target and then let it calculate all the biquads for you and then copy them to the MiniDSP as custom biquads =)
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Old 10th February 2013, 01:00 AM   #5
Davey is offline Davey  United States
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Rich,

If you're measuring at the H-baffle opening and have the 6db/octave dipole correction applied you should see a boosted response which is roughly equivalent to that correction. When you pull your microphone back and take further measurements at a more nominal distance then you would observe the measured response "flattening" to approximate your overall target.

I think I would approach this in two steps. Start by placing your microphone near the dust cap of the woofer and correct its response first to approximately achieve your 35Hz/0.5 target. Then move your microphone away from the enclosure/baffle and apply an estimated dipole correction to the miniDSP and see how the response looks. You can fiddle with the miniDSP settings to fine tune it. You don't need to use the "advanced" features of the miniDSP programming....you can easily achieve your target using standard shelving filters in the "basic" configuration.

The LT spreadsheet doesn't do you that much good in this instance since it's designed for a different application.

Cheers,

Dave.

Last edited by Davey; 10th February 2013 at 01:02 AM.
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Old 10th February 2013, 08:39 AM   #6
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That makes sense, thanks Davey, was I right in using SHP and SLP filters together to avoid clipping?

I shall try that today, I would imagine that is the same way to measure the mid range EQ?

And just for the sake of it I attempted to program the Minidsp with the figures that are on SL's ASP diagram;
Driver EQ: 20-110
Dipole EQ:20-305

and this is what I got,

Click the image to open in full size.

Last edited by rich31td; 10th February 2013 at 08:50 AM.
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Old 10th February 2013, 11:00 AM   #7
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FWIW: Gradient woofer equalization

I don't particularly care for targeting a 2nd order high pass response for a dipole woofer as it means excursion will keep increasing below the cut off point. 3rd or 4th order limits excursion below cut of, but does increase group delay. However, when you look at the GD it is apparent that the major contribution to the woofer GD above 60 Hz low pass filter between woofer and mains.Most recently I have determined that the easiest way to obtain a 4th order high pass alignment is with a single stage Q boost and a single stage Q cut. For the woofer you are using and a 35 Hz cut off, a boost with Fc = 37 Hz, Q = 0.75 and Gain = 14dB, and a cut with Fc = 325 Hz, Q = 0.45 and gain = -12.5 dB should get you close. Then couple with an LR4 low pass to the midrange. [note: the Q's specified are the analog Q's and are not exactly the same at what miniDSP uses. You will need to fine tune.]
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Old 10th February 2013, 02:59 PM   #8
Davey is offline Davey  United States
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Rich,

Yes, no problem using a combination of SHP and SLP filters together. Whatever is required to achieve your target and maintain the best gain structure and prevention of clipping in the miniDSP.

Cheers,

Dave.
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Old 11th February 2013, 03:23 AM   #9
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Hi Richard, nice speakers

Equalising H-frame (and any subs) is quite easy. What your spreadsheet is missing is the "measured" graph. I've modified that one to include it (will send you when I get home).

Basically:
- Measure at the opening plane of the H-frame
- obtain the .frd and import into the spreadsheet
- 'guess' the Fs and Qts (grey line below matching blue)
- punch in the target Fs and Qts
- copy paste in MiniDSP

After the LT is applied, then apply the dipole SLP. Normally this is simply 6db/oct from xo frequency to 20hz.


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Old 11th February 2013, 09:23 PM   #10
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Quote:
Originally Posted by john k... View Post
FWIW: Gradient woofer equalization

I don't particularly care for targeting a 2nd order high pass response for a dipole woofer as it means excursion will keep increasing below the cut off point.
Interesting. I've never actually tried a higher order highpass for the subwoofer. Always equalised with target response of 20hz, Q=0.5 which sounds great but can be SPL-limited with certain recording/movies.

So this morning I tried to have 24db/oct highpass at 20hz for the sub. To do this by following the prescription: linkwitzlab.com/filters.htm#2

- First, I equalise the woofer for F=20hz, Q=0.707 using Linkwitz Transform
- Then add another biquad for Highpass filter, F=20hz, Q=0.707



Having it for a few moments before going to work, I heard nothing wrong with this. Perhaps group delay is not that audible. I will observe further and it it's good then more SPL can be obtained !!
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