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Basic question about impedance and watts

I just bought a pair of Thiel cs3.5's and am now looking for an amp. If the impedance is 4Ohms (i think this is the correct value, i didn't get the owners manual) and the recommended power is 50-150 watts (once again, a guess), how much power should the amp have? Do i need an amp that is 50-150 watts at 8ohms, or do i need an amp that is 25-75 watts at 8Ohms (assuming the amps watts double as the impedance halves)?
I guess the reason i had no luck searching for this is because its so basic.
if anyone knows the specs for these speakers, could you let me know? the owners manuals are available on Thiel's website for most, if not all, models except the 3.5s.
ketchup
unsound
7,329 posts
 
3B ST!
ketchup OP
698 posts
 
do you like the 3B st?
bombaywalla's avatar
bombaywalla
1,958 posts
 
Ketchup,

IMHO, Elvick is the only one who seems to have gone thru a bit of number crunching to give you some idea of what kind of power you need for your Thiels. Suffice it to say that you need a *brute* of an amp that "doubles down"!

These days there is a large influx of Class-D power amps on the market. There was a thread called "Green Mountain owners please read" just a few days ago so it should be on the 1st page of the Amps/Preamp or Speaker forum. Therein they were discussing the new Panasonic $600 power amp (I think that I'm remembering this correctly) that many user's considered to be one heck of an amp. Do read that thread as this amp *could* work for you wattage-wise & budget-wise.

To give you an estimate of power required:-
89dB @ 1m using 1W input
implies 80dB SPL at 9.9 feet from speaker.
implies 90dB SPL at 9.9 feet needs 10W (10dB increase implies 10X the power)
implies 100dB SPL at 9.9 feet needs 100W
implies 103dB SPL at 9.9 feet needs 200W
implies 106dB SPL at 9.9 feet needs 400W
implies 109dB SPL at 9.9 feet needs 800W
This is using just 1 speaker. Using 2 speakers, just add 3dB to each of the above numbers.

If you are listening to a Chopin piece (for example), there are very quiet passages then suddenly the piano takes off easily creating transients that are 109dB SPL. Your amp would need to provide 400W/ch momentarily IFFFF you want to get a perception of reality/live-event.
At the min. you should have a high current 100W amp with a 3dB dynamic headroom (this is going to cost you!). However, it appears from the discussion from above mentioned thread that this Panasonic class-D amp is a killer amp.
ketchup OP
698 posts
 
thanks, Bombaywalla.
what do you mean by a 3dB dynamic headroom? are you referring to a particular amps ability to double its wattage going from 8 to 4 ohms?
unsound
7,329 posts
 
Ketchup, while its not my first choice, I do like the Bryston 3B ST. As I said before, while the Thiels appreciate as much power as they can get, depending on your room and desired listening levels, I think a really top quality solid state amp rated at 100 watts into 8 Ohms that can double down is the place to start. The Thiel 3.5's are not very forgiving of abberations in the upper registers and can be a touch soft in the upper bass/lower mid range and a bit on the forward side.
They require the proverbial iron fist in a velvet glove.
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