Impedance.

[just_grass]

I have a 100w 4ohm sub from a sharp 5.1 system.

and a cambridge audio a500 RC unit coupled to 2x Wharfedale 6R's (100w 8ohm) 65 watts per channel @ 8 ohm and 85 @ 6 ohm....

Is there anychance of using these two together? as i'm going to uni, and as much as i'd like to, its completely impractical to take the 5.1 setup with me...

If its any use, the amp has 2sets of speaker terminals per channel to enable bi-wiring, a pre-out and rec-out... *shrug*

over to you? Would be great if you could shed some light on this, i'm used to the kick i get from having a sub next to me, and the 5.1.. and would obviously quite like to take a fair setup with me... (it was a cheap pickup a yr ago...) and i#d like to add the sub to it somehow?

Cheers!!
Alex.

 

[Greenlizard0]

It will depend on what your amp can churn out. Mine can churn out only 4 and 8 Ohm's, therefore I can only use speakers that match.

Not a lot of speakers (correct me if I'm wrong) are made with such a low resistance these days, most go for 6-8+.

 

[just_grass]

well the sub is from a sharp SD-AT1000 5.1 system. all the speakers are 4ohm... afaik.

i was just hoping that it could be put to use with this amp too...

i bit the bullet and jus connected it up. to the 2nd set of terminals to one of the channels. it seemed very low volume wise.

is this due to the fact the amp is rated for 6-8ohm? and the sub being 4? or another reason??

Thanks for your reply,
Alex

 

[Greenlizard0]

well the sub is from a sharp SD-AT1000 5.1 system. all the speakers are 4ohm... afaik.

i was just hoping that it could be put to use with this amp too...

i bit the bullet and jus connected it up. to the 2nd set of terminals to one of the channels. it seemed very low volume wise.

is this due to the fact the amp is rated for 6-8ohm? and the sub being 4? or another reason??

Thanks for your reply,
Alex

No that's pretty much it.

 

[james.miller]

doing that isnt such a great idea lol. that amp only has 1 set of stereo outputs - Left and Right. the second set of terminals are for another set of speakers which you can turn on and off. if you look in the small print, or on the back of the amp, it'll tell you that if you use both sets of terminals the impedance of each set of speakers must be higher than if you used one set only...normally double the impedance and there's a good reason for that.


with both sets hooked up, your are connecting two speakers in parallel to each channel. assume the amp is rated to drive a 4 ohm load. it's fine to use one set of 4 ohm speakers. however if you use two sets of 4 ohm speakers, the overall impedance is actually halved because they are in parallel. the amplifer no longer sees a 4 ohm load, it sees a 2 ohm load.

its a simple formula : Rt = (R1 * R2) / (R1 + R2)
where Rt is the impedance the amp will see, R1 is the impedance of speaker 1 and R2 of speaker to respectively.

connecting that sub woofer to one channel youve basically lowered the load to a rough 2.6ohm. that might not do anything,. it might melt your amplifier down to a lump of solid metal. its not recommended

as for the volume, thats easier to explain. the subwoofer your using wont be very efficiant. ie: per watt, it'll be a lot quieter than the speakers you are using. subwoofers can get away with a much higher power input so thats not really a problems when they have their own amplifiers - you can just match the signal levels and job done. as youve found out though, driven from the same amp youve got no chance because there's no real way to match the outputs of the sub and main speaker when driven by the same channel