Need technical explanation

I can answer your first question. All discrete, means no integrated
circuits in the design. At least in the signal path. Thus the design uses individual transistors,fet's, etc in the signal path.
This is how the best is designed. Most audio designers consider
integrated devices to be inferior.

If you were to distill down an amplifier circuit to a basic "triangle" block diagram, there are two inputs, one inverting and one non-inverting . . . and one output. The amplifier derives its output from the difference between the two inputs. A perfect, ideal amplifier would have infinite gain at all frequencies, thus the actual circuit gain is set by the feedback network between the output and the input(s).

Most audio amplifier circuits (power amps, preamp stages, phono stages, etc.) are "voltage feedback", that is, the amplifier output is based on the voltage difference between the inputs. Since most amplifiers (that are modeled in this manner) have a very high gain, you can think of it in the sense that the amplifier circuit is always working as hard as it can to bring the voltage between the inputs to zero.

A "current feedback" amplifier is one that derives its output from the current flowing between the two inputs, rather than the voltage . . . that is, it's always trying to bring the input "error current" to zero, rather than the input "error voltage". In the simplified world of ideal, perfect amplifiers, there's really very little difference between the two. This is because when you add resistors to form a feedback network, the voltage and current differnce tend to be the same thing due to Ohm's law.

The differnce between the two topologies is only apparent in implementation, because in the real world, amplifier circuits don't have infinite gain and bandwidth. The main difference to the engineer is that in the voltage-feedback design, gain and bandwith are always linked together, and one must frequently choose between them. The current-feedback design has bandwidth linked to output/feedback loop current . . . and because of this, the gain and bandwidth can be much more independent of each other.

Just as everything in life, and moreso in audio, neither is "better", they're just different weapons, if you will. Voltage-feedback designs typically have better DC performance, and lower noise. They're also much better suited to designing active filters. Current-feedback designs allow the engineer to acheive higher slew rate and bandwidth for a given gain, which usually means a little lower distortion. The output load current must be held to a smaller range, however.

Wow. Both excellent replies. Thanks very much. I thought discreet must have meant separate power amps but wasn't sure. So a 7.1 A-V receiver (for example) then has 7 separate and distinct amplifiers.

Kirkus, you must be a graduate electrical engineer or audio engineer. Fantastic explanation. To me, distortion is of the utmost importance since I listen at at an average of 90 dB's and it's distortion that damages hearing more than background noise (my best guess on that). So "current feedback amps" would be the better choice but probably a very subtle difference.

Can I ask you another question? Do the internal amps inside these A-V receviers get physically larger the more watts RMS they produce and output? For example, a 110 x 7 watt into 8 ohms receiver would have physically larger amps inside the unit than say a 90. Is that correct? Many thanks again to you both.

Well actually, the "number of separate amplifiers" is simply the same as the number of channels . . . in the same sense that all "stereo" power amplifiers have two channels. Most of the time these days I'd expect a "7.1" receiver to have seven channels of amplification, one for each of the surround-sound speaker outputs. Usually when there's a "speaker B" or "area B" outputs, this doesn't include extra amplifiers to run them . . . they just tack on a switching relay and a couple more binding posts so you can easily hook up more speakers to the same amplifier. Usually also there isn't an amplifier for the ".1" output (subwoofer or LFE channel), because most people hook this up to a powered sub.

When they say "discrete", they're referring to the individual parts that make up each amplifier. Most semiconductor manufacturers offer a smattering of "amps on a chip", that are the audio-engineering equivalent of a cake mix - most of the parts are ready to go in a pre-done package. By saying that it's an "all-discrete" amplifier, Marantz is basically bragging that they made all seven cakes (amplifiers) from scratch, rather than using a mix.

And all else being equal, yes, the amplifier parts for a higher-output receiver are physically larger. However, all else is rarely equal . . . and humans have an infinate capacity for cutting corners.

And again, no one can say whether a current-feedback topology is better or worse in your case. An automotive analogy would be a V-6 engine versus a straight-six . . . much ink has been spilled on the subject, but what it really comes down to is who built each engine, and how good of a job they did on thousands of little details. But if Marantz is proud of their current-feedback architecture, and they feel that it differentiates them from their competition, it's a perfectly legitimate thing to put in their advertising literature.