Islander Posted August 25, 2007 Share Posted August 25, 2007 Thanks, Mark. I guess I've been reading actual tests of Class D amps and spec sheets of linear amps. Incidentally, in one test of a Class D amp (the Yamaha MX-D1), it was described, due to its very low distortion harmonics, as "obviously a very linear design". No wonder people get confused. Quote Link to comment Share on other sites More sharing options...
Zen Traveler Posted August 26, 2007 Share Posted August 26, 2007 ...I wish I could say that it is seldom, but this is a prime example of a thread gone terribly awry with the oddest compendium of of personal hangups, off the wall specs and 'standards' (right...) derived from product marketing sheets, and non sequitur emotional product endorsements that we have had in a short while. One can only hope that the time between this and the seemingly inevitable next one is long. But then, hope springs eternal....despite the foreknowledge of the complete and utter futility of the venture. I usually like it when mas waxes poetic on the ignorance of others... I won't be apologetic for my sincere desire to understand this aspect of my hobby and appreciate everyone's involvement in this thread. I've read his deleted material and have added it to my notebook (along with other informative responses) for future reference . I don't wish to be part of the problem that mas identifies and hope to learn enough to be part of the solution. ;-) [C] Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted August 26, 2007 Share Posted August 26, 2007 But then, hope springs eternal....despite the foreknowledge of the complete and utter futility of the venture. Such an Optimist, Mark ......[] Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 26, 2007 Share Posted August 26, 2007 When actually based on measurement even high-quality linear amps do not usually fully double. For instance-- Bryston 14B: 690W/8 ohms and 1000W/4 ohms. Or Mark Levinson #431: 234W/ 8 ohms and 400W/4 ohms. Doubling down is not any indication of the amp's ability to dampen a difficult loudspeaker load. It is merely an indication of thermal limitations. Allow me to present a very simplistic example to help portray the concept... Let's say your amp can output a maximum 40V. If you connect an 8 ohm speaker to the output, Ohm's Law tells us that 5 Amps of current will flow through the speaker (I = V / R). We know that Power = Voltage x Current (P=VI) so we know that our amplifier is putting out 40V x 5A = 200W. Instead, let's connect a 4 ohm speaker to the output: 40V / 4 Ohms = 10A 40V x 10A = 400W Since the impedance is cut in half, we see twice the current flow and therefore twice the power output. The most important thing to realize here is that the peak voltage output does not change. The significance of this is that you can continue to reduce the speaker impedance to increase the power output from the amplifier. So for instance, let's connect a 0.01 ohm speaker to the output: 40V / 0.01 Ohms = 4,000A 40V x 4,000A = 160,000W !!! The problem with this picture is that current flowing through wires creates heat. In fact, that's how toasters work (and electric ovens and pretty much any electrical heating device). The problem with heat is two-fold. Obviously stuff melts, but more importantly distortion increases. Anyways, the point I want to make is that our devices are also current limited, which is to say that there is a peak current draw that the devices can handle. In other words, 4,000A through our transistor isn't going to happen. So let's take our same 40V max voltage amplifier and pretend that it is current limited to 8A. With an 8ohm speaker you still have 5A of current which is under the 8A limit and therefore you still have the 200W power output. But with a 4 Ohm speaker, 40V would require 10A of current to flow. Our device isn't capable of 10A so what happens? Since the amp can only output 8A, we can see from Ohm's Law (V=I x R) that the maximum voltage output at 4 Ohms is: 8A x 4 Ohms = 32V And therefore our max power into a 4 Ohm speaker is: 32V x 8A = 256W The amp is still capable of creating 40V at the output, but if it can't supply the current, the voltage is forced to drop. In other words, the amp "clips". All that said, is the amp that "doubles down" better than an amp that doesn't? No - as long as you don't run the weaker amp into clipping. In the very dramatic case I presented above (200W at 8 Ohms, 256W at 4Ohms), one could probably argue that the amplifier designer didn't use a powerful enough power supply or enough heatsinking...as such, one might wonder what other shortcuts were taken, but those kinds of things should show up in other specifications...like output impedance and the various types of distortion. Quote Link to comment Share on other sites More sharing options...
pocket.change Posted August 31, 2007 Share Posted August 31, 2007 Using 06' GDP $s and retail as a starting point, the $750 SX1250 would be around $5,400.00 in todays marketplace (no, I wouldn't buy one).There must be some reason the SX1250 has the following it has, for less than a third the cost of the Denon @ $3,200.00 (will they be around in 3 decades) you would have a very nice receiver. I've several Denon examples under roof, good product with no problems and will buy more in the future. BUT, if I was looking for a High Current Stereo Receiver and was on a budget looking for a monster that would drive most anything, a look at a SX1250 would be a positive move. And yes SX1250 value is going up and $700 or so should find you a very clean example given time. Denon value in a few decades...? Only time will tell... pc PS. I have a pair of SX1250s and enjoy them daily. Quote Link to comment Share on other sites More sharing options...
Zen Traveler Posted August 31, 2007 Share Posted August 31, 2007 Fwiw, I don't know anything about the SX1250, but the Denon AVR 4806 is a Level 6 AVR in this thread: http://www.avsforum.com/avs-vb/showthread.php?t=738511 In my personal situation, I paid $1800 for my original AVR 4802R 5 years ago and agree that the Bells and Whistles of the AVR 4806 "ci" may not be worth the extra cash to purchase new. I had what started out as a bad Customer Service experience with a Denon repair Center several months ago (lightening damage) and Denon's HQ went WAY beyond the bounds to satisfy my complaint and sent me an AVR 4806. This unit will last me through the next upgrade cycle especially for my video needs, but also for my advanced audio, which I didn't realize the benefits until I experienced them in 7.1 through my Denon-link. [Y] {edited} I still feel confident in recommending a discontinued AVR 4806 unit at close out prices because it does have a good reputation among owners and fits the criteria of "Future Proof AVR's" listed in the link above. Quote Link to comment Share on other sites More sharing options...
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