Impedance matching

[Tom Swenson]

My Yamaha A700amp @100 wpc spec'ed only for 8ohms has no line out or sub out. I want to add a powered Polk xt12 (8ohm) sub via amp speaker outputs that also feed Polk rti8 tower speakers (8ohm). My plan is to leave sub's speaker level outputs disconnected, and rely on existing amp wiring to both tower speakers. I believe this still results in 4 ohms to amp. What I need is to net 8 amps???

 

[voodooless]

If you use the high-level inputs of the subwoofer, it will not strain the amp additionally. These inputs are high-impedance and are used when there is no subwoofer output available on your amp. So your amp will still drive the 8Ohm impedance from your speakers.

 

[Tom Swenson]

Won't the amp see four ohms with the combo of two stated 8 ohm speakers per channel? Thank you

 

[voodooless]

Won't the amp see four ohms with the combo of two stated 8 ohm speakers per channel? Thank you

No! The subwoofer is driven by itself own amp, regardless of how it’s connected. Like I said before: that’s high level input is high impedance, so is almost no additional load to your Yamaha amp.

I also doubt the sub would use an 8 Ohm woofer simply because a 4 ohm woofer will let the amp output more power for the same voltage. Meaning amp can be cheaper and more powerful. In any case, this is totally irrelevant.

 

[RandomEar]

To rephrase what @voodooless already said: The sub speaker level inputs are not 8 Ohms, they are much higher - maybe 10000 Ohms or so. There is essentially no power flowing from your speaker amp into the subwoofer. The sub gets its power from the internal PSU.

I also couldn't find anything about the impedance of the xt12 driver. In any case, that would not be a useful metric, as the sub driver is being powered by the internal amp and only an external line level audio signal is required to transmit the music. The speaker level inputs on the xt12 are directly connected to the speaker level outputs and to an internal level shifter which generates a line level signal to feed the internal amplifier.

 

[Tom Swenson]

Thank you @voodooles and RandomEar. I learned a lot. One more question please; will sub's high level input along with the sub's high impedance draw only a little main amp power?
If you go to Polk's web site you can see xt12 sub specs.
Thanks again

 

[solderdude]

The load impedance is not specified, nor for RCA in nor for high level.
Nor does any reviewer ever measure it nor of other Polk SW or any other high-level (nor low level) input of any sub I could find.

It most likely is > 1000 ohm so will NOT 'load' the amp it is connected to.

Safe to connect them in parallel, the SW will not 'load' the amplifier.

 

[voodooless]

Asking the same question three times will not change it’s answer