What is a high current amp??

[Islander]

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.

[Zen Traveler]

...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]

[Duke Spinner]

But then, hope springs eternal....despite the foreknowledge of the complete and utter futility of the venture.

Such an Optimist, Mark ......[]

[DrWho]

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.

[pocket.change]

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.

[Zen Traveler]

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.