Question about ohms

Peter Andrew Fraser New member Username: Pete81m Post Number: 1 Registered: Apr-08	Posted on Sunday, April 20, 2008 - 12:47 GMT Hi everyone. Im new to the forum. I have a question about an amp and 2 speakers I just purchased. The amp puts out 2x100W rms at 8 ohms and it also puts out 2x170W rms at 4 ohms. The problem is that the speakers put out 180W rms at 8 ohms. How can I get the full 2x170W amp power transmitted to the speakers? Im assuming that the speakers will only receive 100W rms each because of their ohms rating.
P Galbraith Gold Member Username: Petergalbraith Canada Post Number: 2160 Registered: Feb-04	Posted on Sunday, April 20, 2008 - 14:49 GMT Hi Peter, You are misunderstanding this. The amplifier can put out as much as 2x100W under an 8 ohm load, or as much as 2x170W under a more difficult 4 ohm load. Your speakers represent an average load of 8 ohms (but this varies tremendously by frequency) and will reach their maximum excursion at around 180W. There's nothing you can do to further match the ohm ratings of both to maximize the power output, but nor would it change much of anything. Acoustic power is not perceived linearly as loudness, but rather exponentially. To hear an extra 10 decibels (dB), the power has to be multiplied by 10. A further increase of 10 dB means another 10-fold increase in power. The plus side of this for you are two-fold: First, there is hardly any perceptible difference in loudness from your speakers receiving 100W or 170W, so it's not something you should worrry about optimizing. Secondly, unless you are cranking up the volume to the max close to the point of distorsion, it's likely that you are using less than 10W per channel, sometimes much less. I play jazz at -30 to -40 dB on my volume dial; At -30 dB, it's already as low as 1/1000 of my maximum rating, so less than 0.1W, and only a tenth of that at -40dB (< 0.01W).
Jan Vigne Platinum Member Username: Jan_b_vigne Dallas, TX Post Number: 12477 Registered: May-04	Posted on Sunday, April 20, 2008 - 15:44 GMT . The bottom line is you do not require all of the wattage available from your amplifier to be present at all times. You must have sufficient power available to reach a desired loudness level (SPL) without distorting (clipping) the amplifier's output. That means there must be sufficient power in reserve which would allow loud peaks to remain clean while also allowing the average (lower in level) program material to be reproduced. Your average level will probably be well beneath your peak requirements. If your desired peak loudness levels cannot be reached, you should investigate buying speakers with a higher electrical sensitivity. Power ratings for speakers are all but meaningless, your speakers do not put out 180 watts at any point. Speakers use watts provided by the amplifier, but do not effectively put out any watts. The number you need to look at on your speakers is the sensitivity spec. The higher that number is, the more volume you can achieve with your present amplifier. The difference between 100 and 180 watts is all but insignificant. If you need more volume, the way to get it is with more sensitive speakers. Your present amplifier would then play loud enough no matter how much power it is rated for. http://diyaudiocorner.tripod.com/def.htm http://www.enjoythemusic.com/audioterminology.htm#A .
P Galbraith Gold Member Username: Petergalbraith Canada Post Number: 2161 Registered: Feb-04	Posted on Monday, April 21, 2008 - 03:13 GMT Speakers use watts provided by the amplifier, but do not effectively put out any watts Technically, they do. Just mostly as heat since they are very inefficient, and a little bit as acoustical watts. Thus the sensitivity measurement. The difference between 100 and 180 watts is all but insignificant. Agreed.
Peter Andrew Fraser New member Username: Pete81m Post Number: 2 Registered: Apr-08	Posted on Monday, April 21, 2008 - 11:20 GMT Thanks everyone. I wasn't expecting so much help. Much appreciated One more question... I was thinking of getting another set of the same speakers and doubling up the speaker wire on the amp channels. That way I would have 4 speakers running. How would 4 X 8 ohm speakers go with the 2 channel amp? Would that provide bigger sound or am I just wasting my time?
Jim Bay Silver Member Username: Jrbay Livonia Detroit area, Michigan USA Post Number: 134 Registered: Feb-08	Posted on Monday, April 21, 2008 - 11:53 GMT If you wire a second set of speakers in parallel (with two wires to each receiver binding post as you suggest by saying "doubling up the speaker wire") the load, in ohms, decreases: Zt = (Za x Zb) / (Za + Zb) So then if your speakers are 8 ohm you have: Zt = (8 x 8) / (8 + 8) = (64 / 16) = 4 Then you can set your receiver's output to 4 ohms and it should blow you away!!! Keep in mind though that technically the facts mentioned above are all still true, you are increasing the SLP more by additional sound sources than by the wattage increase. OK, enough of all of this, your question has been answered, now I have one for you! What are you trying to do, drive your neighbors insane? If you truly want such a big sound at these deafening SLP levels you need a huge subwoofer!
P Galbraith Gold Member Username: Petergalbraith Canada Post Number: 2162 Registered: Feb-04	Posted on Monday, April 21, 2008 - 12:12 GMT Jim is correct that pairing up speakers on one amplifier output will drop the ohm rating in half, making the package harder to drive. But there are major caveats to consider: (1) your speakers are not simple 8-ohm loads and likely have swings below 8 at certain frequencies such that halving that could be too difficult to drive. (2) Doubling the sources doesn't lead to a better source image so it's best avoided anyway. So... what are you trying to do? If playing louder, then get speakers that have a higher sensitivity rating. If playing better, you are going about this the wrong way.
Jan Vigne Platinum Member Username: Jan_b_vigne Dallas, TX Post Number: 12485 Registered: May-04	Posted on Monday, April 21, 2008 - 14:58 GMT . "OK, enough of all of this, your question has been answered ... " Take it easy, JB, someone is trying to learn something. I admit he's not going about it in the best fashion, but we all started somewhere and had to learn. "Then you can set your receiver's output to 4 ohms and it should blow you away!!!" I'm not so sure that's true. If the receiver has an 8/4 switch, the four Ohm position will simply induce current limiting which is not what you want when you drop the overall impedance load. It may get louder from having more speakers in the room but it's unlikely it will get better - or stay as good - unless SPL is the only quality hat matters. In this case I think PG's last remark says it best. What are you trying to do, Peter? Because, whatever it is, you are probably not going about it very well. .
Jim Bay Silver Member Username: Jrbay Livonia Detroit area, Michigan USA Post Number: 136 Registered: Feb-08	Posted on Monday, April 21, 2008 - 15:32 GMT Sorry if I sounded rude, it would appear as though I did not put my smiley in soon enough. Perhaps I should have put it right after, "now I have one for you!" I well remember, though not too fondly, the days of QUANTITY rather than QUALITY and am trying to find out which direction Peter is heading. In other words, is SPL all we want here or not?
Peter Andrew Fraser New member Username: Pete81m Post Number: 3 Registered: Apr-08	Posted on Tuesday, April 22, 2008 - 11:20 GMT To answer your questions... great quality sound is what I am after. Basically I just want to get the most out of my current setup without a loss in quality. So I guess doubling up the amp channels with another set of speakers would not be the go. Thanks for all the help people. This is no doubt a very good forum.
Nuck Platinum Member Username: Nuck Post Number: 10087 Registered: Dec-04	Posted on Tuesday, April 22, 2008 - 13:48 GMT It can be better, Peter. The more info you provide, the easier it is for all of us to get some idea of what you want to know.

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