# Help Getting Lows To Sub Amp



## Guest (Mar 5, 2008)

Hey all,
I recently acquired a 15" Rockford Fosgate T215D4 sub. At the same time I have a Technical Pro LX-3000 that I'll be bridging mono-4ohm to produce the 1100W RMS my sub requires. 
My problem is I don't have a clue as to getting only the lows I want for the sub to the amp. 

Should I use an active crossover or a low-pass filter or an equalizer or what?

I'm relatively used to car stereo and the equipment needed for that, but when it comes to home audio I'm completely lost.


----------



## drdoan (Aug 30, 2006)

Welcome, Bob. I would assume you have a receiver for your home theater system. If so, you can connect the LFE (or Sub out) from the receiver to the amp's line level input. Hope this helps, Dennis


----------



## bobgpsr (Apr 20, 2006)

Normally you just hook the subwoofer amplifier input to the receiver's (AVR's) line level LFE output. Most often with just a simple RCA to RCA shielded interconnect cable. Line level LFE output (aka Sub Out) have been in nearly all multichannel receivers/AVRs/Pre-Pro's for the past ten years or so.


----------



## Guest (Mar 5, 2008)

I do have a receiver, but it only has front - L/R, rear - L/R, and center outputs. (Sony TA-AV531)

It's not the greatest and I bought it on ebay so now I'm stuck trying to get by. Being in college and all I don't really have that much cash, but hopefully once I get this system hooked up I'll have the loudest 12'X13' room on campus.


----------



## bobgpsr (Apr 20, 2006)

You would need some type of line level 80 Hz (or so --- 120 Hz, 100 Hz?) low pass filter to connect between the receiver's center output and your subwoofer amplifier. Even then it would not really calibrate correctly for audio levels -- the L/R main speakers would also get the low freqs.


----------



## Guest (Mar 6, 2008)

So now I'm looking at a Nady CX22SW crossover so that I can bypass my main receiver and go directly from my CD/DVD to the - Nady - Sub Amp - Sub.

Does anyone see an immediate problem with that or should it work? I was planning on just take a y-splitter to the RCA's, but the Nady requires either a balanced XLR or an un-balanced 1/4" phone jack. Is there someway to connect the two without stripping and soldering wire?

Any advice over all?


----------



## bobgpsr (Apr 20, 2006)

You can buy 1/4" phone jack to common RCA plug -- cables or adapters at Radio Shack or sometimes your local music (guitar, etc) store. 

Also can be hand built as discussed over in the BFD section.


----------



## tonyvdb (Sep 5, 2007)

Another option is to use the receivers headphone output jack you can buy an adapter to go from the headphone jack to rca the only issue is on some receivers plugging anything into the jack disables the speaker output. The advantage is that all the signals that go to all 5 channels should go out the headphone jack.


----------



## brucek (Apr 11, 2006)

> So now I'm looking at a Nady CX22SW crossover so that I can bypass my main receiver and go directly from my CD/DVD to the - Nady - Sub Amp - Sub.
> 
> Does anyone see an immediate problem with that.....................?


Yes, how do you propose to have the volume level of the subwoofer track the receivers volume level?

Simply use a Y-splitter on the left and right mains of the receiver and feed the sub amp..

brucek


----------



## bobgpsr (Apr 20, 2006)

brucek said:


> Yes, how do you propose to have the volume level of the subwoofer track the receivers volume level?
> 
> Simpy use a Y-splitter on the left and right mains of the receiver and feed the sub amp..
> 
> brucek


He has a line level center channel output on his old receiver -- best to use that. 

Sometimes when you use a Y-splitter like you propose, you end up shorting the line level of the L/R channels together so you don't get any L/R stereo separation from your main L/R speakers. Better to put at least a 1K ohm resistor in series for each leg of the splitter before combining -- so isolation is maintained.


----------



## brucek (Apr 11, 2006)

> He has a line level center channel output on his old receiver -- best to use that.


But wouldn't the center channel only carry center channel steered bass? I would think you'd want the mains information....



> Better to put at least a 1K ohm resistor in series for each leg of the splitter before combining -- so isolation is maintained.


Well, he indicated he was feeding a stereo amplifier for the sub, so I assumed a stereo input capability (that he would be bridging), which would be fine.

I don't think the 1K ohm mixer resistors would result in an acceptable crosstalk. They would result in an input impedance of 1K ohms and an output impedance of 500 ohms. Lets assume his preamp has an output impedance of 100 ohms (reasonable assumption). The mixer divider setup would result in ~-26dB crosstalk between channels. Not really acceptable seperation.

If you scaled the resistors to 10K ohms, then the crosstalk would drop to ~46dB (assuming 100 ohm o/p Z). Quite acceptable. The resulting higher input impedance to the mixer that would normally be a problem with line driving capabilities and the unfavourable low pass distortion that results, would be fine since he's feeding a sub. You could also mount the arrangement near the sub.

But I don't think he really needs a mixer, since it sounds like he's driving stereo.... 

brucek


----------

