So, the P-39f is a nominal 4ohm speaker.  I forget the math, but my understanding is that a lower resistive load preceived by the amp actually makes this spekaer harder to drive.  I also see that on most (all?) amp spec sheets, the rated power in watts goes up considerably when rating a 4ohm load vs. 8ohm.  I also see, at least for my amp, that THD goes up   http://www.klipsch.com/products/discontinued/details/2005.aspx

Can anyone shed light on the principles at play here?  Obviously I like the idea of more headroom and I can clearly see that .05% THD is greater than .03% THD but I don't have any kind of feel for how audible a .02% THD diffrence is.

 The specs (if they are to be believed) for something like the Bryston 6bSST are freaking outstanding at a claimed .007%thd http://www.bryston.ca/6bsstspec.html but I'm not sure I need 500wpc!

We all know that amps run hotter when driving lower ohm speakers as the amp needs the speaker resistance to work properly. The energy is the same. When you see an amp with multiple ratings, for example, 250W at 4 ohm and 125W at 8 ohms, that is the same energy level.

Published ohm ratings on speakers are either an estimated average, or the lowest posible load (manufacturers descresion). If the P39 says 4 ohm, that almost sounds like a least value meaning that 4 ohm is the least draw, and that you probably are driving an average of 8 to 16 in reality. Maybe someone at Klipsch could comments on how that rating was determined.

In the old days, or those of us who use old amps that have multiple taps based on speaker ohms. The rule was to set your tap to the lowest ohm rating your speaker runs at. My Khorns say 8 ohm on the sticker, but 4 ohm is the least draw so I run my Khorns on the 4 ohm tap on my McIntosh amps.

Years ago, I did try my Khorns on the 8 ohm tap. The bass lacked and it did not sound as clear as the 4 ohm tap.

 Not really a black and white answer for you, but some insight to think about.

JM 

John, would you say then that if an amp shows a higher THD rating for a 4ohm load, that the amp is really "optimized" for 8ohm and some less efficient thing is going on for 4ohm?  Is it worth looking into a new amp over?

damonrpayne:

So, the P-39f is a nominal 4ohm speaker.  I forget the math, but my understanding is that a lower resistive load preceived by the amp actually makes this spekaer harder to drive.  I also see that on most (all?) amp spec sheets, the rated power in watts goes up considerably when rating a 4ohm load vs. 8ohm.  I also see, at least for my amp, that THD goes up   http://www.klipsch.com/products/discontinued/details/2005.aspx

Can anyone shed light on the principles at play here?  Obviously I like the idea of more headroom and I can clearly see that .05% THD is greater than .03% THD but I don't have any kind of feel for how audible a .02% THD diffrence is.

 The specs (if they are to be believed) for something like the Bryston 6bSST are freaking outstanding at a claimed .007%thd http://www.bryston.ca/6bsstspec.html but I'm not sure I need 500wpc!

 

If you are buying Bryston or any amp worthy of driving your speakers, forget the power THD ratings.  They will not make a difference.  I remmeber teh old Technics cheapie receivers in teh early 1980's had 0.0001 THD and the McIntosh had 0.01.  Which one did you want?

As far as the resistence, a lower resistence is harder to drive because the amp needs to be beefed up to handle the flow.  Think of it like sucking on a straw.  If you use a normal size straw you can suck on it and it will flow fine.  Now use a bigger straw (less resistence) and try to suck at the same speed for the same time.  You run out of steam faster. 

If you are interested in Bryston amps, I have a pair of 7B ST (500 wpc/800 wpc) monoblocks that I will sell for about 1/2 of what the 6Bsst will cost you.  They are mint and have 11 years fo warranty left.  Gotta love Bryston.  Drop me an email if interested.  Here is a link to a review.  They were actually preferred over the 7B SST's:

http://www.stereophile.com/solidpoweramps/1093bryston/index4.html

You have mail

 How are amplifier ratings achieved>? An amplifer is a voltage multiplier.... say a factor of 40. This means that for 1 volt in, there is 40 volts out. Power is V^2/Re.... if we have 40 volts across an 8 ohm load, then that represents 1600/8.... or 200 watts. 

The same voltage across a 4 ohm load is 1600/4 or 400 watts. Current across the speaker and in the amp outputs must rise.  As loop resistance rises with increasing temperatures, actual voltage drops some. 

How does power increase when you drop the impedance... voltage stays relatively constant ( does drop some ) but current rises. Some pro amps will drive a 2 ohm load in stereo for hours without complaints. 

Hi damonrpayne, I'm a new guy around here and have been following your P-39 threads because it would be cool as hell to own a set of speakers of this magnitude. I've been running my RF-7's off an Emotiva XPA-2 and at 8 ohms they get 250 watts/channel and run at top form, better than I ever could have hoped for. This amp is a brand new offering from Emotiva and just came out 4 weeks ago, I got one out of the first batch. At 4 ohms they are rated at 500 watts/channel and would make those Palladiums sing without breaking a sweat. Their website quotes the THD number at .007, http://emotiva.com/xpa2.html.. The price is $799 and I know that it seems ridiculous to run $30,000 speakers off such a low priced amp but I would be very curious to see what the results would be. Since they are an internet company they have a 30 day trial policy but I won't have any use for it, I'm a believer.

gilbertma:

I've been running my RF-7's off an Emotiva XPA-2 and at 8 ohms they get 250 watts/channel and run at top form, better than I could have ever hoped for. This amp is a brand new offering from Emotiva and just came out 4 weeks ago, I got one out of the first batch. At 4 ohms they are rated at 500 watts/channel and would make those Palladiums sing without breaking a sweat. Their website quotes the THD number at .007, http://emotiva.com/xpa2.html.. The price is $799 and I know that it seems ridiculous to run $30,000 speakers off such a low priced amp but I would be very curious to see what the results would be. Since they are an internet company they have a 30 day trial policy but I won't have any use for it, I'm a believer.

$799 for a 75 lbs amplifier.  That's cheap per pound! 

Very interesting gilbertma, they look cool too.  I wonder what a S/N ratio of "> 100db" means.  This is an incredibly important spec to me, my aragon amp is basicly silent.

 if you read the selling pitch about the zero autoformer, some additional info is preseented about reducing distortion relative to  increasing impedeance.

 I am glad you guys brought up this subject. It reminded me that in the owners manuell for the Bat vk 60 that I am using it clearly states it is best to try out the three different output's for the optimum ohm setting in your system. After trying all three I have to agree with JM there is improvement to be found there in the choice. I had been using the 8ohm setting since purchasing the Bat about 3 months ago,  I have been so completely delighted with the sound I just had no desire for tweaking (which I I am usually hot to do). Preliminary listening last night leads me to believe the 6ohm setting is the right spot for my ears. Isn't it a beautiful thing in our hobby how just such small tweaks can make such a difference? 

Cornman