Adding a sub - what will I loose?

12-02-11: Dbphd
"I've had a couple of subs that actually sounded better using the speaker connections than the RCAs."

Doesn't that imply that the main amp is superior to the amp in the sub, even within the limited frequency domain of the sub.

With a powered sub, the amplifier in the sub is being used regardless of whether the signal is sent into the sub via speaker-level or line-level inputs. If the speaker-level inputs are used, conceptually what is happening is that the voltage range that is present at the main amp's output is divided down by the sub to a line-level voltage range, and then sent into the sub's power amp. Almost no current and power is drawn from the main amp by the sub.

The main difference between the two configurations is that the sonic effects of the main amp, whatever they may be, will be seen by the sub as well as by the main speakers, if speaker-level inputs are used.

Also, if line-level inputs are used, and the preamp's output is split between main amp and sub via a y-adapter, or if the two destinations are driven from separate output jacks on the preamp that are not individually buffered (as is usually the case when two sets of output jacks are provided), adverse effects can occur under certain circumstances.

First, the preamp will see a considerably reduced load impedance, which can result in frequency response issues such as deep bass rolloff, if its own output impedance is not low enough in relation to that reduced overall load impedance. The overall load impedance seen by the preamp will be equal to the product (multiplication) of the two input impedances, divided by the sum of those numbers. For example, if the sub has an input impedance of 20K, and the main amp has an input impedance of 50K, the combined impedance is (20 x 50)/(20 + 50) = 14.3K. For best results, the preamp's output impedance should be at least a factor of 10 less than that (i.e., 1.43K) at all frequencies. If the worst case (maximum) output impedance of the preamp across the audible frequency range is not known, and just a nominal output impedance is specified, it would be safer to use a factor of 50 or so.

Second, the upper treble, as reproduced by the main speakers, may be affected by the capacitance of the cable that connects preamp to sub, as well as by the capacitance of the cable that connects preamp to main amp. The total of the two cable capacitances will interact with the output impedance of the preamp to form a low pass filter. Under extreme circumstances (high preamp output impedance at high frequencies, long cable lengths, high cable capacitance per unit length), the bandwidth of that low pass filter can be low enough to result in a small but audible upper treble rolloff, and consequent softening of transients.

There is no universally applicable answer to the speaker-level vs. line-level connection question.

Regards,
-- Al

Wow! Lots of great info here...except for SoundsReal and his $10K sub. LOL

Doak - you bring up a good point. I'll have to check with the speaker manufacturer.

My plan is to use two small 8" subs. Found a good deal on Sunfire HRS-8 that's within my budget. My listening room is 12x16 with 13' ceilings.

I'll report back.
TG

12-02-11: Tghooper
My plan is to use two small 8" subs. Found a good deal on Sunfire HRS-8 that's within my budget. My listening room is 12x16 with 13' ceilings.

I use a pair of Mirage MM8's, which are 8" drivers and passive radiators in a 9" cube. However, they really don't add much below 35 Hz. Still, they're a good match for my system at this point. And I got them for less than $280 each.

If 14" cubes aren't too big and you want strong performance down into the 20's, you might want to consider these. They're sealed and tight, and the price is right for what they do. I have a local audiobuddy who uses them with his Magnepan 1.7's.

"Doesn't that imply that the main amp is superior to the amp in the sub, even within the limited frequency domain of the sub."

No. The sub driver is always driven by the sub's amp. The speaker-level inputs use a relatively high input impedance to reduce the main amp's output to just a couple of volts, and uses the low-voltage signal to drive the sub's internal amp. More likely there's some sort of frequency response difference caused by the interaction of the amp's output stage and the sub that is fortuitous.

Lots of cool stuff.