Um...silly question about integrating a HT sub

[GhettoSQ]

My dad gave me an AudioSource sw6.5 sub which I think is designed for HT use since it has no outputs for satellites. He said he used to run the speaker cables into the speaker-level inputs on the sub (from the amp), then connect the smaller speakers from those same terminals. This strikes me as a really bad idea; I think my NAD 3020 can handle 2 ohm loads, but still...am I wrong?

[Dr.EM]

Is the sub active (ie, does it plug into the mains) or passive? If its active there should be no problem at all. If its passive I just wouldn't do it, you'd need a seprate amp for it.

[GhettoSQ]

Active. I'll give it a shot then. I just figured this would be equivalent to running it parallel, which would halve the impedance...but then I got thinking about speaker selectors (which I also use for my main room) and I guess this isn't too different?

[audiobomber]

Originally posted by GhettoSQ
Active. I'll give it a shot then. I just figured this would be equivalent to running it parallel, which would halve the impedance...but then I got thinking about speaker selectors (which I also use for my main room) and I guess this isn't too different?

It's entirely different. With speaker selectors, you're running the speakers in parallel, so your impedance will be radically lowered (eg two 8-ohm speaker systems, the amp will see 4 ohms).

With an active sub having speaker level inputs and outputs for the main speakers, the load on your power amp will actually decrease. That's because the feed to the sub's internal amp needs voltage to operate, not current. Plus the sub's output that feeds the mains is high pass filtered, so your main power amp will only be driving the main speakers above the low bass range (probably 80 or 100 Hz).

Whether you like the sound of the main speakers through the sub's high pass filter is another matter. The filters aren't high quality and can lose more in sound quality than they gain by filtering the main speakers. I suggest you compare the sound of the speakers filtered through the subs, vs the sound with the mains driven from the amp and the B speaker terminals driving the subs (parallel operation). Don't worry about the load, even in parallel operation the subs use virtually no power from the NAD.

[GhettoSQ]

I actually just found a 200hz passive crossover in the closet -- not ideal but better. So now I have the main system using pre in/out to the main sub, signal comes back unfiltered (I believe), the I go to the speaker selector, then off A) to the mains and B) to the kitchen, into the crossover and from there to the sub and sats. Whew!

The NAD has onlt one set of speaker outs -- are you saying I should use different selector switches for each sub and each pair of mains? I have 4, I could try it.

[GhettoSQ]

Hey -- I just wired the main room as above, left the jumpers in the NAD and ran straight to the speaker selector, using 1 channel for mains and 1 for sub. Problem is, sub HUMS...any ideas why?

[audiobomber]

Originally posted by GhettoSQ
Hey -- I just wired the main room as above, left the jumpers in the NAD and ran straight to the speaker selector, using 1 channel for mains and 1 for sub. Problem is, sub HUMS...any ideas why?

It's likely due to different ground potentials. If your main amp and sub amp are plugged into different wall outlets, try plugging them both into the same one. If that's not possible, a cheater plug on the sub will probably cure it, but I can't advise that, as it's a safety hazard.

[GhettoSQ]

Hmmm...there's definitely a ground loop when I use the pre in/out loop from the amp, so it wouldn't surprise me, though I thought this would fix it since the signal just goes into the sub and out the driver, but nothing comes from the sub back to the amp or mains.

Maybe if I use the switched outlet on my amp for the sub's power? Right now they are on the same power strip.

[audiobomber]

Using the outlet on the rear of the amp vs using the power bar should result in the same grounding as you have now. And usually they only recommend using the outlet on the amp for source components. Have you tried gettting rid of the power bar and going straight to the wall? If the power bar has any sort of protection device in it, they're often nasty sounding things.

If that doesn't work, I don't know what to tell you. You could try searching this site, there's tons of stuff on grounding problems.

[GhettoSQ]

I tried using the wall when I had my original ground loop, to no avail. I may have to just shrug and decide this amp is not that great. I'll do some reading up on grounding though.