Stereo System help: Ohm and Watts

[wower]

Hello Giant Genius Brain That is Head-fi!

I am looking at downgrading - yes, downgrading - my stereo system so sound quality need not be a factor, however, I have to sleep in the same room as this thing and I rather not have it burn down my house. I need some help matching speakers to amp.

Having decided which sound I like, I am looking at the JBL Venue Stage Speaker to pair with either the Harmon Kardon 3485 or the 3385. I realize this is not the picture perfect audiophile sound (I'm sure the FR will be all over the place) but I will be using it almost exclusively with <=128kbps steaming audio and want a system that will cover its faults (whereas my headphone system ruthlessly exposes it).

Stats:
JBL Stage requires 80 Watts @ 8 Ohm. 320W peak.
HK 3385: 80W (x2) @ 8 Ohm and 100W (x2) @ 4 Ohm.
HK 3485: 120W(x2) @ 8 Ohm and 150W (x2) @ 4 Ohm.

My whole philosophy in audio has been to make sure the speakers get plent of power to grip the drivers but I don't want to damage anything. If anyone out there knows more about these numbers than I do and would like to share it would be greatly appreciated. I'm sure others have simlar questions. The different in price is negligible and I will also be sourcing a sub using some of the great advice found in this forum.

Links go to the spec pages on cnet:
http://reviews.cnet.com/separate-spe...5.html?tag=sub
http://reviews.cnet.com/av-receivers...7.html?tag=sub
http://reviews.cnet.com/av-receivers...3.html?tag=sub

 

[jcn3]

usually the specs associated with speakers are the maximum wattage that the speakers can take before blowing. so in this case, the speakers are rated to handle a maximum of 80w. they are listed as 8 ohm speakers -- most speakers are either 4 ohm or 8 ohm speakers. this is the nominal impedance -- in reality the speaker resistance variety by frequency.

amplifiers are rated in terms of watts for a given load (in ohms). so in the case of the amplifiers you listed, they provided you different output capabilities based on whether they were 4 ohm speakers or 8 ohm speakers.

from a practical point of view, you don't use anywhere near the maximum output of the amplifier in everyday listening, so either amplifier would be fine.

the real determination of whether a speaker and amp are a good match is not the wattage, but the ability to provide the current when you hit dynamic peaks in the music. there are lots of 100w amps out there that can drive speakers better than higher powered amps that don't have the ability to provide significant bursts of power.

 

[JadeEast]

You need to know the efficiency of the speaker to figure out how much of an amp you need. Jbl specs the speakers at 90db at 1 meter with 2.8 volts. Normally speakers are speced with 1 volt at one meter it makes it easier to just do the math in your head. But for some reason JBL decided to use an odd ball number.

For every 3db in volume it requires a doubling of amp power.

 

[wower]

Thank you very much. That sheads some light on the situation. This thread didn't get many responses. Theres not much love for people that are downgrading the speaker system; really, I have a great headphone rig!!

 

[tourmaline]

Quote:

Originally Posted by JadeEast /img/forum/go_quote.gif You need to know the efficiency of the speaker to figure out how much of an amp you need. Jbl specs the speakers at 90db at 1 meter with 2.8 volts. Normally speakers are speced with 1 volt at one meter it makes it easier to just do the math in your head. But for some reason JBL decided to use an odd ball number. For every 3db in volume it requires a doubling of amp power.

Exactly, efficiency of a speaker is much more important, since at 4 ohm the amp needs to deliver serious juice!
90db m3 and above is seen as efficient, under 90 db is seen as less efficient and need in general befier amps!

 

[JadeEast]

I forgot to mention that as you double the distance from the speaker you loose 6db.

 

[lini]

Quote:

Originally Posted by JadeEast /img/forum/go_quote.gif (...) Jbl specs the speakers at 90db at 1 meter with 2.8 volts. Normally speakers are speced with 1 volt at one meter it makes it easier to just do the math in your head. But for some reason JBL decided to use an odd ball number. (...)

Nothing odd there. Speaker efficiency is usually specified as sound pressure level per power for a certain distance (usually measured with a 1 kHz sine), i.e. dB/W @ 1m. 2.8 V simply is the corresponding voltage needed for 1 W on an 8 Ohm speaker, hence the JBL's 90 dB/2.8 V are just the same as 90 dB/W.

Greetings from Munich!

Manfred / lini

P.S.: wower: Gaint Genius Brian???

 

[wower]

Quote:

Originally Posted by lini /img/forum/go_quote.gif P.S.: wower: Gaint Genius Brian???

Yeash.... I need to edit that s**te or someone needs to take my computer away.

If my understanding of EE is correct, the bigger wattage amp will also provide more current no???

 

[tot]

Quote:

Originally Posted by wower /img/forum/go_quote.gif If my understanding of EE is correct, the bigger wattage amp will also provide more current no???

Yes. Simply put the relationship in voltage and current is A = V/Ohm and power W = VA.

Like the sensitivity example above, 2.8V/8Ohm = 0.35A, and 2.8V * 0.35A = 1W.

From above you can figure out that A = sqrt (W / Ohm), thus 100W amp for 8Ohm can deliver 3.5A current which using the Ohm's law gives you the effective voltage V = A * Ohm => 3.5A * 8Ohm = 28V (and again 28V * 3.5A = 100W).

 

[JadeEast]

Thanks lini you're right. I realized my mistake in crossing my volts and watts as far as the specs.