Bridging a class A amplifier

[NavnFugl]

Hi,

I've 2 Accuphase A-75 60w 8ohm class A amplifiers.
They can be bridged to 240w 8ohm which I want to experiment with.

I'm curious as to how many of the 240 watts are actually class A and thus how many are class A/B.
Is it still 60 watts class A? Is 120 watts class A?

In the end I will let my ears decide if it sounds good. However, I would like to if there is an easy way of calculating the amount of watts that are class A in a bridged class A amplifier.

Hope anyone has some wisdom to share.

 

[antcollinet]

Hi,

I've 2 Accuphase A-75 60w 8ohm class A amplifiers.
They can be bridged to 240w 8ohm which I want to experiment with.

I'm curious as to how many of the 240 watts are actually class A and thus how many are class A/B.
Is it still 60 watts class A? Is 120 watts class A?

In the end I will let my ears decide if it sounds good. However, I would like to if there is an easy way of calculating the amount of watts that are class A in a bridged class A amplifier.

Hope anyone has some wisdom to share.

As far as I can see, they are all going to be class A. The output stage of each side of the amp still operate in class A. Unless the amp changes its mode when bridged?

 

[voodooless]

If the claim of “pure Class A” is true, then all the Watts are Class A, no matter if it’s bridged or not.

 

[NavnFugl]

If that's the case I guess I can look forward to a lot more heat.

 

[antcollinet]

If that's the case I guess I can look forward to a lot more heat.

Winter is coming.

 

[DonH56]

If that's the case I guess I can look forward to a lot more heat.

No, it will not change if it is truly class A. Bias current stays the same, all the current all the time, for class A operation. Bridging means doubling the output voltage swing, since one amp channel goes "up" while the other goes "down", and doubling the voltage is four times the power output. That means bridging two 60 W channels will yield 240 W if the power supply and everything else can handle it.

P = V^2 / R so if V doubles for the same R (load) then power quadruples.

Output impedance also doubles, uncorrelated noise goes up about 40% (sqrt(2)), and a few other things happen as well when bridging.

HTH - Don

 

[voodooless]

If that's the case I guess I can look forward to a lot more heat.

Ehm, no, not from the single amp anyway. It will produce exactly as much heat as it ever did.

In any case, it’s a very inefficient way to generate heat, never mind the initial investment

 

[antcollinet]

No, it will not change if it is truly class A

Except now there is going to be two of them.

 

[DonH56]

Except now there is going to be two of them.

Good point, so twice the heat for a stereo pair even though the individual amps' dissipation won't change. Actually more than twice the heat since you now have two input and driver stages plus power supply losses.

 

[antcollinet]

In any case, it’s a very inefficient way to generate heat,

Its a very efficient way (ignoring investment) Power in = power out. 100% efficient.

Perhaps not as efficient as heat pumps, but they are only more efficient for electrical power input, not total power input.

 

[DVDdoug]

The idle current shouldn't be any different than when operating in stereo so it's probably not "pure class A".

In any case, it’s a very inefficient way to generate heat, never mind the initial investment

It's 100% efficient converting electricity to heat! Even the soundwaves are converted to heat.

 

[voodooless]

Its a very efficient way (ignoring investment) Power in = power out. 100% efficient.

Perhaps not as efficient as heat pumps, but they are only more efficient for electrical power input, not total power input.

Sure, but the electrical is the only thing you pay for. For the price of such an amp you caneasily get a heat pump

 

[sergeauckland]

It's how I justify my valve amps environmentally. As long as I have the gas heating on, they contribute to the heating with 100% efficiency.

With the added benefit of playing tunes.

S

 

[antcollinet]

It's how I justify my valve amps environmentally. As long as I have the gas heating on, they contribute to the heating with 100% efficiency.

With the added benefit of playing tunes.

S

Oh, that is a sticky road to go down. You find you have to start considering the overall efficiency of the electricity generation and distribution compared to gas

 

[NavnFugl]

Thank you for all the information, much appreciated.

Sorry if it went over my head, byt what is the basic clear answer in regards to the amount of the 240 watts in 8 ohms that will be class A in my case?

 

[antcollinet]

Thank you for all the information, much appreciated.

Sorry if it went over my head, byt what is the basic clear answer in regards to the amount of the 240 watts in 8 ohms that will be class A in my case?

The basic clear answer is:

All of it. All 240W into 8ohm is class A. You have a class A amp. It only operates in class A. Even bridged, all the power is from a class A amp operating in class A.