Design this - L/R channel combiner, but with isolation on the inputs

[jbeckva]

Here's something I'm thinking about. Basically I'd like to fire up the Polk monitor 70's I have (until now) been using for shelf space.

But obviously they can't quite do it on the low end. I have an old Frankensteined JBL 12" subwoofer tho, and thinking I would use one of my NAD 2400PE's in bridge mode to power it.

The trick - I don't have a preamp with a sub output. I don't so much need it to split off the LF (the sub has a LP filter already), but more making sure I have the sum of left and right coming into the NAD.

So, the challenge is this - combine the left and right signals from the preamp and feed it to the NAD for the sub, but in such a way as to NOT interfere with the original signals enroute to the main amp. I can visualize the design I think... start with a couple of buffers, then combine their outputs into a final buffer amp.... something like that?

Any of ya EE's want to take a stab?

 

[mlucitt]

It may be cheaper to buy than to build, a question that often confronts project managers. You need a summing amplifier, there will always be some effect on the original signal but if the summing/combining amp is designed well and implemented correctly, the overall effect will be to only lose a few db of signal strength.

If your amp has more than one set of speaker outputs (A and B) then you could use the second output channel in a dedicated summing amp, just be aware that basic summing amps will output an inverted signal to the sub and that can make the bass sound mushy because it is out of phase with the rest of the program material.

Here is one solution, though you may need an adapter on the output:

http://www.directproaudio.com/product.cfm?directid=59128

Mark

 

[speakerman1]

Doesn't a sub run off a mono signal? Is it a powered sub? Been so long since I messed with a sub I forget.

 

[mlucitt]

If it is a Dolby source, then the mono signal is encoded in the musical content for a powered or non-powered subwoofer. If non-Dolby, like mp3 or wav, then combining (summing) the L and R signals at the preamp or speakers is an accepted method of deriving a mono signal. Just don't use a 2x RCA "Y" to 1x RCA cable to combine the L and R signals because it presents a low impedance signal back to the source and deletes the stereo mixing in the entire signal path.

 

[jbeckva]

It may be cheaper to buy than to build, a question that often confronts project managers. You need a summing amplifier, there will always be some effect on the original signal but if the summing/combining amp is designed well and implemented correctly, the overall effect will be to only lose a few db of signal strength.

If your amp has more than one set of speaker outputs (A and B) then you could use the second output channel in a dedicated summing amp, just be aware that basic summing amps will output an inverted signal to the sub and that can make the bass sound mushy because it is out of phase with the rest of the program material.

Here is one solution, though you may need an adapter on the output:

http://www.directproaudio.com/product.cfm?directid=59128

Mark

Seems like the buy option's definitely the way to go. Thanks for the link Mark.

Yeah Larry sub's one channel, but what I need to do here is make sure that music signal from both L and R is downmixed into a single channel for feeding the sub - think of a tune where you hear the drummer belt out a series of bass kicks, but they alternate between L and R.... the LF component would only be "heard" every other beat if I just fed a single channel into the sub amp. The directional cues would still be there since they'd be coming out of the main speakers (higher freq), but the bottom would drop out every time a beat is heard from the channel I didn't feed to the sub.

 

[orange]

Here's something I'm thinking about. Basically I'd like to fire up the Polk monitor 70's I have (until now) been using for shelf space.

But obviously they can't quite do it on the low end. I have an old Frankensteined JBL 12" subwoofer tho, and thinking I would use one of my NAD 2400PE's in bridge mode to power it.

The trick - I don't have a preamp with a sub output. I don't so much need it to split off the LF (the sub has a LP filter already), but more making sure I have the sum of left and right coming into the NAD.

So, the challenge is this - combine the left and right signals from the preamp and feed it to the NAD for the sub, but in such a way as to NOT interfere with the original signals enroute to the main amp. I can visualize the design I think... start with a couple of buffers, then combine their outputs into a final buffer amp.... something like that?

Any of ya EE's want to take a stab?

My SX-6000 has a center out...filter that, bingo.

 

[speakerman1]

Find a schematic of the internals of a amp with an output. That should tell you what to do.

 

[rtp_burnsville]

Does your sub have a filter in the input to block the high frequency input?

If so, a summing circuit should be all that you would need. A simple resistive mixer circuit would give you gain control as well. No active parts required.

If not, then the above summing circuit plus a filter to block high frequencies would be the solution.


Robert

 

[rtp_burnsville]

Here is a line-level example:

 

[laatsch55]

Love your sourcve Robert...,

 

[jbeckva]

Here is a line-level example:

How does that work without crosstalk between L and R outputs? I should know the answer to that, but ... esplain it if ya could.

 

[laatsch55]

Impedence Jer, Impedence......