Amplifier Power Stage Differences

[dohanc]

I'm looking at a few different Receiver, mainly the Denon and Yamaha and I noticed that (especially with the Yamaha) they like to spec different models of their amplifiers with 10 Watt differences. Now 10W around 100W is negilable however it brings up my question.

How do they have designs that have 10W output differences. The size of MOSFETs an amplifier uses usually dictates how much current and therefore power the amplifier is capable of. Obviously there needs to be a just as capable power supply. I can't imagine they are using 3 different MOSFETs for 3 different receiver models that each give a 10W difference. I'm guessing that they use the same FETs for all or most of their models and either intentionally limit the maximum current at different values for different models or use really use a smaller power supply to save cost and just spec the amp a little lower even though the power stage is just as capable of larger models even though the power stage is identical.

I do realize some models really do use larger power supplies and FETs, however I am mainly referring to models that are very similiar to each other. For example I think the Yamaha Rx-V550, 650, 750.

Any thoughts?

 

[Leprkon]

An amp jump of 10 watts will actually require 70 additional watts from the power supply.... 10 watts times 7 channels... they are not just adding 10 watts...

 

[dohanc]

But this does not mean they are using a power supply that can supply the extra current needed for 70 watts. This is due to the sneaky way they rate their amps, which I think only drives one channel or at least a very limitied amount of channels at a time. This allows them to not limit their power output by the power supply but by the power stage.

 

[crashguy]

For the Yamaha 2500 and 1500, the specs state that the 1500 and 2500 have the same power supply (at least the same weight listed in the brochure), but the 2500 had 15000 uF caps, and the 1500 has 10000 uF caps. I don't think this accounts for the rating difference, but it is another way they "upgrade" the amp in the higher priced model.

 

[mtrycrafts]

I'm looking at a few different Receiver, mainly the Denon and Yamaha and I noticed that (especially with the Yamaha) they like to spec different models of their amplifiers with 10 Watt differences. Now 10W around 100W is negilable however it brings up my question.

How do they have designs that have 10W output differences. The size of MOSFETs an amplifier uses usually dictates how much current and therefore power the amplifier is capable of. Obviously there needs to be a just as capable power supply. I can't imagine they are using 3 different MOSFETs for 3 different receiver models that each give a 10W difference. I'm guessing that they use the same FETs for all or most of their models and either intentionally limit the maximum current at different values for different models or use really use a smaller power supply to save cost and just spec the amp a little lower even though the power stage is just as capable of larger models even though the power stage is identical.

I do realize some models really do use larger power supplies and FETs, however I am mainly referring to models that are very similiar to each other. For example I think the Yamaha Rx-V550, 650, 750.

Any thoughts?

You would have a different output if different THD limit is used to measure. 10 watts is doable by this method.

 

[annunaki]

It is also doable by increasing the rail voltage slightly.

 

[plhart]

See my article Product Managing Receiver Power Ratings for the answers.

http://www.audioholics.com/techtips/audioprinciples/amplifiers/receiverpower.php

 

[dohanc]

Thank you plhart. Your article was very informative. It seems that the MOSFETs are most likely the same for most models and that they only increase the cap or the voltage rails slightly.

Two other questions: I would be interested in calculating the power that these caps are actually storing. The can be done by:

U = 1/2*C*V^2 where U is energy in joules. Joules is Power times time, or Watt seconds. Therefore by dividing out by some discharge time we will know the AVERAGE "power" the capacitor is supplying. In fact this curve is a decaying exponential (based on a constant impedence load). Patrick Hart notes in his article that manufactures may "cheat" an RMS value by instead calling their RMS value 500mS of dynamic power. Therefore we get

Pavg = (1/2*C*V^2)/0.5

This of course is power supply power, not at the output after any inefficient power stage.

Only thing I don't know is what common amplifier rails are. I have a schematic of a very very large sound reinforcement amplifier which are using something around 80V per channel, but I doubt 100W receivers use rails that high. My guess is that its around 50V where 10,000uF only give 25W average power for 500mS.

I am interested in the Yamaha Rx-750 verse the Rx-1500. Crashguy noted above that the 1500 uses 10000uF caps. Anyone know what size caps the Rx-v750 is using?

 

[Polkfan]

Don't forget that if you are using a powered sub there is also some additional capacitance there. My onk receiver uses 2x12000 uF caps, but there is an additional 2x15000 uF in my subwoofer (two amps with a total of 54000 uF of filter capacitance). A powered sub can really help a receiver especially with speakers set to small.

 

[Votrax]

Only thing I don't know is what common amplifier rails are. I have a schematic of a very very large sound reinforcement amplifier which are using something around 80V per channel, but I doubt 100W receivers use rails that high. My guess is that its around 50V where 10,000uF only give 25W average power for 500mS.

Typically the average voltage is +/-50VDC on the rail at idle for most 100W systems and closer to +/-60VDC on 120W systems. The rail voltage will drop as the current increases. The rail will drop less as you increase the filter capacitance.