What does the ECO mode actually do, in terms of the technical implementation? Its effect is remarkable! When listening to music or videos on a 5.0 system at medium-low volume, which I guess is no more than 2 watts/channel average, or when playing nothing, it reduces the AVR's power draw in half! From 60-65 W off the wall socket down to 31-33. How?? I don't hear any difference in the sound.
The most curious thing about it is that when switching ECO on/off a relay clicks inside the unit, but since the sound doesn't change the only thing it can be switching is something in the power supply circuit, right?
The ECO mode is implemented by switching between two AC output voltages from the power transformer. This switch is accomplished by using a relay to switch between two different transformer windings, one with a high AC voltage and the other a lower AC voltage. The switching of this relay is the sound you hear.
The selected AC voltage is converted to +/- DC voltages by a rectifier circuit. These DC voltages are the rail voltages of the power amplifier channels. In a D/M AVR the higher rail voltage will be about +/- 62V and the lower voltage around +/- 25V with no load, depending on the AVR. These voltages of course sag significantly as load is put on the power supply by loading the power amplifier channels.
The resultant power outputs with two channels driven, based on some of Amir's measurements and other measurements, will be
approximately as follows for an X6700H. Other D/M AVR's will show similar relative results.
High Rail Voltage
o 8 ohm - 120 watt
o 4 ohm - 180 watt
Low Rail Voltage
o 8 ohm - 19 watt
o 4 ohm - 38 watt
In return for a reduction in the operating temperature of the AVR and electric power usage, the amplifier power output level has been cut substantially.
One way of looking at it this fact is that you're the proud owner of an AVR with an output of about 20 watts into 8 ohm. An alternative view is that if the AVR operates satisfactorily at the lower output level then all is well.
Setting the ECO mode to Auto seems the best choice if use of the ECO mode is desired. When using the Auto setting in a D/M AVR, the ECO mode is used at low output. The AVR switches out of the ECO mode when load on the AVR increases past a preset level.
Note that using D/M AVR's in the all preamp mode will engage the ECO mode. This feature saves power and operating cost that can somewhat offset the power usage of the added external power amplifiers.