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NooBee Question [message #44676] Thu, 27 May 2004 15:01 Go to next message
Dean Kukral is currently offline  Dean Kukral
Messages: 177
Registered: May 2009
Master
I am planning on building a Pi-seven using a JBL woofer and BMS 4592 compression driver and horn.

So if I buy (or design, but I would prefer to buy) an 800Hz crossover, then I assume the two should work together, but how does one balance the two drivers sound-level wise? (I assume that this is what is meant by using compensation.) Is it simply a matter of experimentation by adding resistance in series with the 4592? If so, do I just order some resistors from Parts Express and then try them? If so, what wattage should they be, and what variation of ohmage? I have quite a few large ceramic resistors left over from my slot-car days; I suspect that I could experiment with these.

I know that this has already been done with the kits that Wayne sells, but how does this get done from scratch?

(My original plans were different or I would just buy a kit.)

Re: NooBee Question [message #44677 is a reply to message #44676] Thu, 27 May 2004 15:45 Go to previous messageGo to next message
GarMan is currently offline  GarMan
Messages: 960
Registered: May 2009
Illuminati (2nd Degree)
This is where a tool like SPICE becomes handy as it allows you to design a attenuation/compensation network specifically for your driver. Wayne's network attenuates the HF driver to match the LF, but the level of attenuation decreases in the upper frequencies, to compensate for the rolloff of the driver after 16KHz.

I'm not familiar with the the driver you're planning to use, but if it doesn't have the rolloff at the top end, a straightforward L-Pad configuration might do. Below is a link to help you calculate the values.

Gar.


Re: NooBee Question [message #44678 is a reply to message #44677] Thu, 27 May 2004 16:17 Go to previous messageGo to next message
Dean Kukral is currently offline  Dean Kukral
Messages: 177
Registered: May 2009
Master
Thanks.

I got to thinking that a resistor in series and one in parallel would attenuate the level of the drivers without changing the resistance in the crossover design. I did not know what the design of an "l-pad," was, but the reference you gave makes that obvious!

I wonder what the mathematical model is that they use to make the calculation to figure the drop in acoustic power. They probably ignore frequency.

Re: NooBee Question [message #44679 is a reply to message #44678] Thu, 27 May 2004 17:32 Go to previous messageGo to next message
GarMan is currently offline  GarMan
Messages: 960
Registered: May 2009
Illuminati (2nd Degree)
In a straightforward L-pad with just two resistors, the attenuation is frequency independent. However, Wayne's compensation network incorporates a capacitor in parallel with the series resistor, and this in turn adjusts the level of attenuation by frequency.

Since you asked for mathematical models, everytime you reduce the voltage to a driver by half, you attenuate by 6dB. Level of attenuation is calulated by:

dB = 20*log(Vo/Vi)
where Vo/Vi is the percentage of the original voltage.

I think the easiest thing to do is to just use one of Wayne's crossover networks that he developed for his PI 18" speakers.

Gar.

Re: NooBee Question [message #44680 is a reply to message #44676] Thu, 27 May 2004 20:28 Go to previous messageGo to next message
steve f is currently offline  steve f
Messages: 236
Registered: May 2009
Master
I believe Bill Martinelli sells BMS drivers for his wood horns. I'd ask him about your problem. www.woodhorn.com

Steve

Re: NooBee Question [message #44681 is a reply to message #44676] Fri, 28 May 2004 04:26 Go to previous messageGo to next message
ToFo is currently offline  ToFo
Messages: 219
Registered: May 2009
Master
I'm with GarMan on the idea of using a Pi 800 Crossover. You may have to fiddle with the attenuation a bit, but that is good fun. You will learn stuff about your speakers that you might never imagine a couple of dB's had anything to do with. What horn are you using? There may be someone here who has run the BMS in a similar setup.

Thomas

Re: NooBee Question [message #44682 is a reply to message #44680] Fri, 28 May 2004 10:04 Go to previous messageGo to next message
Dean Kukral is currently offline  Dean Kukral
Messages: 177
Registered: May 2009
Master
Thanks for the suggestion. I did ask Bill and he gave me some great advice. The guys on this forum are really great about helping out!!

Re: NooBee Question [message #44683 is a reply to message #44681] Fri, 28 May 2004 10:20 Go to previous messageGo to next message
Dean Kukral is currently offline  Dean Kukral
Messages: 177
Registered: May 2009
Master
Thanks for the advice from all. I had intended to do a seven - Pi 18", but just as I made up my mind,

WAYNE POSTED NEW KITS ON HIS WEB SITE

for the pro series using his new midrange. I have fallen to the siren song of the new midrange, but, unfortunately, that has upped the cost of the project for a temporary setback. However, I consider it my patriotic duty to help President Bush get the economy going again, so I will proceed undaunted.

My inclinations at this time are to put a 2226 into a Klipsch-style folded corner horn. (BTW, Wayne has great prices posted for this driver on his web site.) Then use Wayne's midrange design with a 2012. Tnen use my BMS 4592's for the higher end.

The only issue that I have to resolve [other than money :( ] is the crossover points and crossover. I would like to crossover at about 400 or 500 Hz and, maybe, 1600 Hz.

I think that this would be a nice system, and if I don't like it, the parts are good parts and I can build something else, eh?

Re: NooBee Question [message #44684 is a reply to message #44676] Fri, 28 May 2004 11:05 Go to previous message
GrantMarshall is currently offline  GrantMarshall
Messages: 77
Registered: May 2009
Viscount

Hi Dean.

You're right the resistors are the compensation and they are cheap to buy.

You may want to check 1) if the horn you are using is meant to go to 800 hz and if it does is it 2) a smoother response curve if you crossover higher and 3) if it's kinder to the compression driver to cross at 1600. Many of Waynes designs use the JBL's to 1600 hz. I've been around this forum long enough to know he usually does things intentionally.

Enjoy.

Grant.


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