connect to DSP 1124P - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 08-11-06, 08:50 AM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: connect to DSP 1124P

Quote:
ok i will try it
Good. When you're able to establish that the negative lead of the subwoofer driver is at ground potential we'll carry on. There's no use otherwise, it's a showstopper.

brucek
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Old 08-11-06, 08:56 AM Thread Starter
Shackster
neophyte

Join Date: Aug 2006
Posts: 51
Re: connect to DSP 1124P

its grounded. what is the next step?
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Old 08-11-06, 09:56 AM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: connect to DSP 1124P

Quote:
its grounded. what is the next step?
OK, as long as the negative terminal of the speaker wire on the subwoofer driver is at ground, a simple resistor attenuation modification won't short out your amplifier when connected to the BFD.

Since the power specified is 120 watts RMS into 8 ohms, then we can assume by rough approximation that the maximum RMS voltage at the sub speaker terminals is about 31volts. (e=sqrt of resistance times power).

The BFD has a maximum input level when using the -10dBV switch setting of +2dBV which calculates to about 1.26vRMS. This is the level we need to attenuate to when the speaker is at its theoretical maximum output.

So, if we want to voltage divide the subs maximum speaker voltage of 31 volts we need to drop by a factor of about 24.6 = 27.8dB.
So the speaker level needs a voltage divider (with appropriate input and output impedances) to produce a line level of 1.26vRMS. For this we can use the resistors shown in the diagram below. The 15K is close enough to the calculated 14.8Kohm when using 600 ohms as the output impedance. There's no power dropped across the resistors so 1/4 watts will do (but bigger if you find them easier to physically work with).

This has an acceptable output impedance of ~600 ohms and an appropriate 28dB attenuation. If you find the line level to the BFD too low, then you can increase the voltage output of the divider by increasing the value of the 600 ohms. I wouldn't go above 1K though.

Once you've connected the resistors to the speaker (making sure of the positive and negative speaker terminals), then connect shielded cable (with the cables pin going to the junction of the 15K and 600 ohm resistors and the shield connecting to the negative of the speaker terminal) and then terminate in an RCA connector (just cut the end off an old RCA interconnect and use that as the cable and connector).

Plug the connector into a phono to mono 1/4" jack plug adaptors as shown below and then into the 1/4" input of the BFD.

If you short your amplifier out, you can't say I didn't give ample warning.

brucek
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Old 08-11-06, 04:00 PM Thread Starter
Shackster
neophyte

Join Date: Aug 2006
Posts: 51
Re: connect to DSP 1124P

ok thanx a lot

1 thing remains: is it possible to use all 24 filters of bfd for equalizing sub? and how to connect it?
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Old 08-11-06, 05:27 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: connect to DSP 1124P

Yes you can use 24 filters. Feed one output to the other input. There should never be an occasion when you require more than 12 filters though.

See this Guide

brucek
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