AB amps and power consumption

schotter · « **on:** 29 Jul 2014, 05:57 am »

Probably a dumb question for most of you but here goes: How much power does a class AB design consume when idle?
Reason I am asking is that while I love my Shigaraki amp (2x20W chip amp), I find it quite hot and I suspect 'always on' means 'always the full 40Watts'. Am I right?

G Georgopoulos · « **Reply #1 on:** 29 Jul 2014, 06:09 am »

Maybe it's got a small heatsink,if it's a chip amp may have a small heatsink!!

schotter · « **Reply #2 on:** 29 Jul 2014, 09:13 am »

Possibly. I guess everything works as intended by the designer (still gorgeous to me), but, while not INSANELY hot, it still is on the hot side given it sits mostly in a closed sideboard.

I just wonder how much power it consumes as I rarely do dedicated listening these days.

srb · « **Reply #3 on:** 29 Jul 2014, 11:42 am »

Consuming the full rated power consumption at idle is more indicative of Class A amplifiers, Class AB will consume some fraction of full rated power at idle depending on the bias setting (how many watts of Class A operation before switching to Class AB operation).

Most manufacturers don't list power consumption at idle specs, but for example here are the specs on a couple of Bryston Class AB power amplifier models:

Bryston 3BSST²
Power Output: 300W total (150W/ch X 2)
Power Consumption at full power: 668W (45% efficiency)
Power Consumption at idle: 127W

Bryston 4BSST²
Power Output: 600W total (300W/ch X 2)
Power Consumption at full power: 1280W (47% efficiency)
Power Consumption at idle: 170W

In contrast the Musical Fidelity AMS35i Class A integrated amplifier has a power output of 70W total (35W/ch X 2) and consumes 330W continuously at idle or full power! (21% efficiency)

A handy device to test the power consumption of each component as well as the whole system would be one of the affordable P3 International Kill A Watt Power Monitors such as the P4400, P4455 or P4460.

Steve

schotter · « **Reply #4 on:** 29 Jul 2014, 12:39 pm »

Thanks Steve,
helpful answer.
I will check in with Sakurasystems /47Labs and see what they say. And good idea about those kill-a-watt devices, I saw similar ones here in Europe.

srb · « **Reply #5 on:** 29 Jul 2014, 02:58 pm »

I couldn't find any specs on the 4717 Shigaraki other than the 2 X 20W output.

Amplifiers that have extruded multi-finned aluminum heat sinks distribute the heat generated from their output devices across a relatively larger number of square inches of thicker material that is inherently efficient for convection cooling, so when you feel for heat on the heat sink or chassis it doesn't feel as hot in any area.

It could simply be that because your amplifier's chip module is sinked directly to the enclosure itself, it feels much hotter near the output device even though the dissipation may be adequate enough to prevent destruction.

Still, it would be interesting to measure the power consumption with a metering device. Here in the US, the Kill A Watt meters sell for as low as ~ $20 US.

Steve