Active sub via headphone out?

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A simple one.

Can I drive an active sub via the headphone jack, on any other connection, when using an amp without a proper preamp out? And is this wise? If yes, would I have to construct some kind of custom cable?

Why you ask? I may have to make due with a cheapy EBay amp as my nice old Sansui integrated checked out, but I would REALLY like to keep rolling on my DIY sub project.

It should work fine. You will have to sum the two headphone channels together using resistors, but this may not be necessary because most headphone outputs just use resistors off the speaker outputs.
I assume you won't be limiting low frequencies to your main speakers, just enhancing them with the sub.
Quality won't be as good as using the preamp output but hey, it's a subwoofer.
For a cable, you should be able to buy something at Rat Shack. When you build your sub amp, use two phono jacks as inputs and tie them to the actual amplifier input using the resistors I referred to above. Value of the resistors is not critical; 1K or so would be okay I'm sure. If you plan to use this later with a proper preamp output, maybe use 10K resistors.
Thanks Paul

Hate to be so ignorant (hey won't be the last time) but here I go....

First off I'll be using a plain ole 250W Parts Express plate amp that "features gold plated high and low level inputs" which well they look like, regular binding posts? Hummm, thought there should be a RCA jack for connection to a HT LFE channel?

So how do I wire this beast to the headphone jacks and how do I tell if I need to use resistors? I guess for that matter couldn't I just use the high level speaker outs with a resistor?

Many thanks for help with these mundane questions.

Wait a minute... you're using a plate amp with high-level inputs? Then forget about the headphone outs... You should be able to connect inline with the speaker outs. There should be a high-level in and a high-level out for both left and right channels. This will include a crossover cutting the bass frequencies to your speakers.

As to low-level, yes there should be RCA connections somewhere on the amp as well. I just took a look at the PE site, and not sure if I have the right amp, but check the top right corner, (with the heat sinks to the left) there's some RCA inputs there...
Thanks Schaef

I suppose could use the high-level inputs but two issues come to mind:

1.) I always thought using a preamp out signal was the preferred and higher quality way to connect/integrate a sub

2.) My main speakers are fairly large full range towers and I was hoping to have the sub augment their sound - letting them roll off naturally

Any thoughts?
Just trying to get something straight here... do you have an LFE out, or a left/right mains out on your amp? If you do, you can use these with your plate amp. I really wouldn't suggest using those high pass binding posts unless you really have to, they're really designed for use with small satellite systems.

Ah.... the full story comes out.

The amp I am trying to connect is a vintage El cheapo I bought on EBay as a stop gap until I can afford something good in a HT system. It has L & R headphone jacks and RCA jacks for tape in and out. Definitely no LFE.

This amp is literally in the mail and my sub is still on the drawing board, so the options are still open, but the basic concept is to use this amp for basic stereo and upgrade later to six channel.

You could use the tape output as a preamp output (although you couldn't play tapes through the sub, I guess). This may be the best solution.
Using the headphone output to the sub amp low level inputs is almost certainly exactly the same as connecting the speaker outputs to the sub amp high level inputs, because the headphone output is very likely just connected via resistors to the speaker outputs. Either will work fine, I'm sure. If you don't want to cut the bass to your main speakers just don't connect them through the sub amp.
By the way, plugging something into the headphone jack may cut out the speaker outputs. Depends on the receiver.

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