[comabereni]

I'd like to give my 8" passive, sealed enclosure subwoofers their own amplifier. I'm considering the 120-watt model from Parts Express.

My question:

What happens if I wire my subs in parallel to this one amplifier (i.e. splice into the wires shown on the back side)? Does this mean the amp will be operating at a 4-Ohms rather than 8-Ohms? It seems I use to see powered sub/slave sub combos--I assumed plugging the second sub in simply dropped the impedence to 4 Ohms. I believe the nominal impedence of each of my passive subs is 6 Ohms--not sure if that matters.

Amp specifications:
[size=x-small]Power: 110 watts RMS into 8 ohms @ 0.1% THD
156 watts RMS into 4 ohms @ 0.2% THD
Bass boost: 6 dB @ 30 Hz
Signal to noise ratio: 98 dB (A-weighted) [/size]

I considered using (2) 70-watt or 100-watt amps and running RCA line-in to the first, then daisy-chaining from RCA-out to RCA-in on the second amp, but I'd like to do it with one amp if I can. Not sure I like the 6 dB bass boost at 30 Hz--some of the other amps at Parts Express either do not have this or it can be varied.

 

[DrewWinters]

If you wire two 8-ohm subs in parallel then the amp will "see" 4-ohms. For most amps, this won't be a problem - but for some it will. if you wire two 6-ohm subs up in parallel then the amp will see 3-ohms. This may be a problem for a cheap amp like that.

The bass boost thing isn't a big deal unless you are trying to achieve a flat response curve. Your room response will make that impossible anyway unless you have significant acoustic treatments. If one of your room modes also happens to fall around 30hz though you may end up with a HUGE boost at 30hz that makes your bass sound "boomy" or "one-note" (i.e. BAD!). I'd rather have an amp that did not have a set bass boost (it should be at least defeatable).

If I were you I would look for a slightly larger amp. Around 200 watts - I'm afraid that this model would not have the headroom to handle dynamics very well. I've had good service in the past from this store....I recommend you check out the Adire Audio amps - but I can't promise you they are 3-ohm stable. I would ask Kyle (the owner) he's been very helpful for me.

http://www.acoustic-visions.com/~aco...rs/index.shtml

Anyway, hope this helps. Good luck.

 

[comabereni]

Thanks Drew. I agree about avoiding the bass boost, and I'll also talk to Kyle about the 3-Ohm stable situation. I confirmed my subs are 6 Ohms, so unless I daisy chain two I will need that. I may just do two subs. Thanks also for the recommendation on using an amp with more power--I'll do that.

-coma

 

[DrewWinters]

You may want to consider a two-channel amp and running your subs on opposite sides of the room. Using two smaller subs is usaully better (sound quality) than a single larger sub. Having two sources of sound helps alleviate the problems with room modes. Besides that, 6 ohms is alot easier on an amp than 3 ohms - and at 12 ohms you'll have trouble getting enough power (not to mention that alot of people think series-wiring is bad).

EDIT: Of course, this path costs more - especially if your subs end up a long way from the amp itself (preferrably an equidistance away - the length of cable affects the sound, particuarly damping factor) since you'll need rather large cables for them. Using two active subs makes wiring easier but usally costs even more! Anyway, just a thought...

 

[comabereni]

Quote:

Originally Posted by DrewWinters You may want to consider a two-channel amp and running your subs on opposite sides of the room. Using two smaller subs is usaully better (sound quality) than a single larger sub. Having two sources of sound helps alleviate the problems with room modes. Besides that, 6 ohms is alot easier on an amp than 3 ohms - and at 12 ohms you'll have trouble getting enough power (not to mention that alot of people think series-wiring is bad).

Since you seem willing to be help me out (thanks), I'll describe my project in more detail.

I am installing my two 8" subs in the ceiling near the satellites. The subs and sat's are matched and cross over well above 100 Hz, so I thought it would be advantageous to locate them close together for better imaging, to keep them off the floor to eliminate clutter, and to deal with room acoustics issues.

I can locate the amp(s) pretty much anywhere--even in the ceiling with the subs if I wanted to. In fact, I could even mount one amp next to each sub and daisy chain them using the RCA in/out jacks. Pretty much every option here is open.

The last issue I'm struggling with is that my receiver does not have a dedicated subwoofer out, it only has "A" and "B" front speaker outs. "A" goes to the family room satellites and "B" goes to the dining room. I want the subwoofers to operate at all times, whether speakers "A" or "B", or both, are playing. Thinking about this last issue a little more, I think I should probably just spend on a new receiver with subwoofer out. That should solve it with the least amount of headache.

Thanks!

 

[DrewWinters]

Is this a multi-channel system (you say "satellites")?

The last thing you want to do is run your subwoofers off of the rear speaker channels of a surround-sound setup. The rear channels have very little (if any) information below 80hz (I think the channel may be technically limited to 80hz, actually - I'm not sure). Even if you had the world's largest subs, this setup would add very little bass. If your main speakers (or only speakers if this is stereo) are not capable of less than 100 hz then it would be better for you to use your subs to extend them than it would even to add the subwoofer channel.

Putting the amp in the ceiling may be begging for heat issues. Sub amps work pretty hard and usally create a lot of heat.

But keeping them close to the speakers may be a very good idea if you need to cross them over that high - usually you don't have to worry about imaging with a sub, since low-frequencies tend to be omni-directional - but above 80 they start becoming locatable.

 

[gerG]

Comabereni, I have a couple of questions:

Subs in the ceiling, really? Do they have an enclosure, or just rely on the volume behind the drywall? The latter is not a good approach for bass response.

You mention that the subs/sats are matched and cross over above 100 hz. Is there a built in passive crossover?

I agree with a stereo amp, or at least dual sub amps, especially if you are crossing over that high.

More info on the setup would be helpful. I did a system over my bed once with ceiling sats and subs for nightstands. I would have built the subs into the pedistal base (I built the bed) but I didn't want vibrations. Fun system, and I may try another one some day.


gerG

 

[CRESCENDOPOWER]

Quote:

Subs in the ceiling, really? Do they have an enclosure, or just rely on the volume behind the drywall? The latter is not a good approach for bass response.

What he said!

Quote:

You mention that the subs/sats are matched and cross over above 100 hz. Is there a built in passive crossover?

Quote:

I agree with a stereo amp, or at least dual sub amps, especially if you are crossing over that high.

What he said! Also, if you need to cross a sub over higher than 65hz you should be looking at front speakers that go lower in frequency response.

 

[comabereni]

Thanks guys. The subs are built and in MDF enclosures already, so I'm not relying on open space in the wall--I agree that would be a bad idea. A few years ago, I pulled the rear jump seats out of a Ford Ranger and installed MDF panels with subs mounted in them in the rectangular open holes. The cubic space was just about right and I sealed all the gaps behind the panels. They sounded "OK", but the largely metal and plastic "enclosures" certainly did detract from their full potential.

I decided going to pick up one of those new Panasonic digital receivers with a separate subwoofer channel and output the bass signal that way. I will likely go with (2) ~100-watt (or more) sub amps, one per 8" subwoofer. I won't mount the amps in the ceiling due to possible heat issues (thanks for the warning).

Thanks again.

-coma