Power amps, heat generation, class A, A/B, & power output? (1 Viewer)

[Kevin C Brown]

I am not understanding the following:

Both amps are class A/B. Both amps are rated at ~125W/channel. Both amps are monoblock per channel designs.

Amp X has a 225 VA transformer for each channel.

Amp Y has a 300 VA transformer for each channel.

But I have seen comparable real world power test results, that Amp X is closer to 150W/ch, but Amp Y is more like 135W/ch. 1.0% THD.

But, Amp Y seems to run a lot hotter than Amp X. Now, I haven't measured heat sink area, and it is obviously possible to design a more efficient amp with less heat sink area that will run hotter but... Let's just say that Amp Y runs so much hotter than Amp X that I can't believe it's just due to less heat sink area. (The heat sinks get too hot to be touched even at just idle.)

Would I be correct in assuming that Amp Y is running deeper into the power band with class A operation?

And that, yes, the next time I leave Amp Y on long enough for it to reach equilibrium, that I should compare heat sink temperature at idle to after stressing it, in that I remember that class A amps actually are hotter at idle and run cooler when they're playing something?

Thanks!

 

[Michael R Price]

Amp Y probably has more bias current, but it's hard to make a judgement of the sound from that alone. It will, however, get slightly warmer when you use it, since even relatively high amounts of Class AB bias correspond to low output power. A "pure" Class A amp (which draws a constant current) will indeed get cooler when used.

 

[Yogi]

Yup the harder you drive it the cooler it runs.

The VA rating of power trannies is not directly proportional to how much real world power the amp will deliver. The PS capacitance, The number and rating of the output devices, the design etc all have an influence on the output power of an amp.

 

[Kevin C Brown]

Cool. Actually, I didn't think it was a simple as I was hoping it'd be. (It never is, right?)