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Old 3rd February 2010, 02:45 AM   #1
pjanda1 is online now pjanda1  United States
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Default OB/amp project update

The sale thread sure is taking some time to deal with! In between answering emails, I've been trying to get some work done on the OB project. As always, there is good news and bad news. I'm not as far along as I had hoped. I did build some prototype U-baffles, and I started playing with them today. I spent a good chunk of the day trying to get Room EQ Wizard (from Home Theater Shack, for FR measurements, and ultimately, to determine parametric EQ settings) to run on my PC laptop, to no avail. I know I need an actual sound card, but I can't figure out what else I'm doing wrong.

So, I charted out the OB's room response by hand. The sound less sensitive than I had hoped, but it is tough to judge playing just LF sine saves. They are responding substantially differently from my simple predictions. Basically, in room, they are not rolling off at 6 db/octave, but by considerably less. Perhaps this is all the U-baffle vs flat? In any case, I might be able to get away without any shelving. I can possibly "fudge" things a bit by using a 2nd order Butterworth with a -3 point around 100hz, and then bring in the Betsy around 150hz. I think Nelson Pass is doing something similar. I had assumed that I'd use a 150hz low pass and some shelving, but it may end up being simpler.

I received the miniDSP, and the software is simple and fun. I built a chip buffer for its low input impedance. It seems to function, but it is adding a bunch of distortion. But, it shows great promise! I even have a nice little box for it and the buffer.

I have the Akai M8's mostly stripped down (took about 4 hours to get this far, see pic), and the digikey parts arrived at the end of last week. So, I'll be starting to build those up as well. For reference, the M8 chassis will contain a 12B4 preamp which will drive an RH84 (also in the chassis) for the Betsy, and in parallel, the buffer, followed by the DSP followed by an amp (the El Cheapo or chip amp). Sure is more complicated than one amp driving a pair of fullrangers!

I hope it all works.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers
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File Type: jpg Stripped M8.jpg (67.6 KB, 389 views)
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Old 4th February 2010, 02:52 AM   #2
pjanda1 is online now pjanda1  United States
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To follow up on the miniDSP: it turns out that I was greatly overdriving the board, which is almost certainly the cause of the distortion I was hearing. My board, an early version, had an input sensitivity designed for laptops, iPods, etc, and I was trying (as a test) to drive it with a CDP that outputs twice that much rms voltage! A quick email conversation got me squared away. I believe the boards that they are shipping now are designed for 2V RMS input. I'll be fine adding a couple of pots or resistors.

BTW, I installed the software on my PC laptop, as I have more regular access to it (my wife frequently heads out of town with the Macbook). I'm fairly new to PC's, and I mainly grabbed this one to run audio simulation software (Horn Response, PSUDII, etc). The miniDSP software worked for me on the first try, and is awfully easy to use. I wish I could say that for everything I've tried to run on the crusty old machine. . .

Paul
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Old 10th February 2010, 04:04 PM   #3
pjanda1 is online now pjanda1  United States
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I've got the miniDSP working and in a box. Here you can see the buffer (unity gain OPA2132), it's power supply (Hagerman Bugle supply from the parts bin), the miniDSP supply (5V 450ma linear wall wart from the thrift store), a pair of pots (20k Alps units from an old R2R) and the miniDSP.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers
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File Type: jpg DSP 1.jpg (40.7 KB, 336 views)
File Type: jpg DSP 2.jpg (38.5 KB, 319 views)
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Old 11th February 2010, 06:08 AM   #4
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Room EQ Wizard is working! Well, kind of. My dog and wife were making too much noise to get clean measurements by the time I finally got it set up. Anybody looking for fun in-room response measurement software should check it out over at the Home Theater Shack. It is free!

Paul
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Old 10th January 2011, 12:36 PM   #5
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Paul

How are you getting along with this one? This is the sort of baffle I would like to build does it work well with the Betsy and boomer?

Cheers
Alan
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Old 10th January 2011, 01:22 PM   #6
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They sound great! I'm using the purples in them now, as I wanted to give them a good listen. With 'zilla's Boomer, you'd want to follow MJK's dimensions for the Alpha 15A. This one's folding would be too wide. The plate amp crossover certainly gives you less flexibility than the miniDSP, but it sure would be easy.

Paul
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Old 10th January 2011, 03:00 PM   #7
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Thanks Paul

I`ll look out for MJK`s dims. unless you can point me to the thread?

Cheers
Alan
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Old 10th January 2011, 10:38 PM   #8
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Basically you would build a 20" wide baffle. I like 'em close to 4' tall to get the fullranger high enough (you really need it to be a good distance from the top), but you could go shorter if you are o.k. with a lower soundstage.

Paul
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Old 11th January 2011, 08:20 AM   #9
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Paul

A 20" baffle is what I had in mind. I am going to have to find a Jensen 15" woofer. I am looking around in UK for these. Which model did you end up with?

Alan
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Old 11th January 2011, 03:12 PM   #10
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The 20" baffle and my baffle up above are entirely different things! You cannot mix and match drivers and enclosures or baffles (i.e., put the Jensen/Foster woofer into a 20" baffle with no EQ] and expect good things. If you are using the Alpha 15, a 20" baffle is for you, and it won't need any EQ. If you are going to the route I did (lower Qts speaker), you will need an effectively much wider u-baffle, and quite likely be happiest with an EQ. You'll also probably need some measurement equipment to set it all up.

Paul
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