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Old 1st April 2008, 09:09 PM   #1
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Default How to use a subwoofer with a 2-channel amp

I have a 2-channel tube amplifier. It does not have a subwoofer out. There's a sub I want that has both line-level and speaker-level inputs. Can I split the speaker outputs on the amplifier, so that both the speakers and the subwoofer are hooked up at the same time to the same speaker terminals? Or is that going to degrade sound quality or somehow hurt my amplifier?

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Charles.
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Old 1st April 2008, 09:12 PM   #2
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It should be fine, just check that the output transformers are fully floating, i.e. one side is not grounded. Else when you common one side to ground at the sub and you get the wrong one you will have melted wires and hot transformers.

The only thing I would say coming from experience of this kind of setup before, is that the quality of bass you will get will experience from the sub will be a very poor representation of what is possible, because the early bass roll off of the output transformers and poor speaker damping spoils the LF signal.
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Old 1st April 2008, 09:25 PM   #3
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That's exactly the kind of information I need!

My tube amp is an integrated amp. I could open the amp and split the signal right after the master volume control, but before the first preamp stage, and wire that to a dedicated set of outputs for the subwoofer.

Would this be a better approach, or would I end up with the same problems?

Charles.
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Old 1st April 2008, 09:27 PM   #4
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If you are happy doing that, the results would be a far better quality of bass. The only thing to watch is that driving possible long cables to the sub from the volume control, might alter the treble sound. Ideally the sub output would be buffered.
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Old 2nd April 2008, 04:57 AM   #5
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Buffered? You mean, like a preamp stage? Or something else?

Charles.
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Old 2nd April 2008, 03:16 PM   #6
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Yes it's kind of like a pre-amp. Another name for it is a line driver.
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Old 2nd April 2008, 10:52 PM   #7
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A buffer is an op-amp set up with unity gain to amplify the current of a signal, while the voltage stays the same.

This is to get the output signal from your preamp without loading it down. The buffer takes a small bit of current + voltage from your preamps output, and makes an exact replica of the signal, but with higher current and better drive - to send a good strong signal to your subwoofer amp.
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Old 3rd April 2008, 12:47 AM   #8
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Something like this? http://cgi.ebay.com/ws/eBayISAPI.dll...m=190209928110

Ideally, I'd like to hook up two coffee-table sized powered subwoofers on either side of my couch (and replace my coffee tables).

Eventually I might get a surround receiver, but for now music is my primary concern.

Charles.
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Old 3rd April 2008, 12:36 PM   #9
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If the preamp stage has a dual supply of the appropriate voltage I can build you a small buffer circuit (no charge).

I will build it and post a pic and you can tell me if you want it or not.
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Old 3rd April 2008, 04:20 PM   #10
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OK, done. I was going to just P2P on a small proto board but my OCD got the best of me. I tested it on my system and bass is clean and plentiful.

OPA2134 buffer
Input impedance: 100k
Output impedance: 100R
DC offset Out_1: 1mv
DC offset Out_2: -1mv

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