F5 power amplifier

[bobodioulasso]

I agree with your post untill you say:

"At 100W output the dissipation has increased to approximately 60W. Much the same as it dissipates at the quiescent state."

So, according to you, the amp can produce more power than it draws from its psu?
This is impossible. I certainly misanderstand your words.

 

[AndrewT]

100W of output + 60W of dissipation = 160W of input.

No new matter/energy created in here.

 

[paulb]

Andrew was referring to the output stage dissipation, as he stated. Add this to the output power to get the total.

 

[Spooky Electric]

To everyone its own religion.

The Grail! The Grail!

 

[bobodioulasso]

100W of output + 60W of dissipation = 160W of input.

Thank you.
And compared to a class ab amp?

 

[npapp]

How do I calculate the biased current? I am currently running my F5 at 0.7v across r11 & r12, which brings the temperature on heatsinks a little closer to the touch test.

Also, I am using 3w panas for R11 & R12 which seem real hot. Should I replace with a higher wattage resistor to run a higher bias?

Thanks Nick

 

[MEGA_amp]

I'm assuming your using 0.47R for R11,12. 0.7V would be 1.5A, and 1W resistor dissipation. I'd step it up to at least 5W for R11,12 if you intend to increase bias.

0.7/0.47 = 1.489

 

[nigelwright7557]

How do I calculate the biased current? I am currently running my F5 at 0.7v across r11 & r12, which brings the temperature on heatsinks a little closer to the touch test.

Also, I am using 3w panas for R11 & R12 which seem real hot. Should I replace with a higher wattage resistor to run a higher bias?

Thanks Nick

I always set my bias with a signal generator and a scope.
I first set the bias to minimum.
I set the input to around 1 volt pk to pk and watch the output on the scope.
I turn the bias up slowly until crossover distortion goes.
This gives good results.

Less bias would give crossover distortion.
More bias is a waste of power turned inbto heat.

 

[npapp]

I am using 0.47, so this is where I use ohm's law.

If I stay at 1.5A, would 3w be enough of a safety margin for 1w of dissipation?

 

[MEGA_amp]

I am using 0.47, so this is where I use ohm's law.

If I stay at 1.5A, would 3w be enough of a safety margin for 1w of dissipation?

certainly

 

[npapp]

I don't have a scope to measure. I increased the bias to hoping to get a little more class A rating into a 4 ohm load. Also, to bring the IC temp up based on the F5 reco.

N

 

[bobodioulasso]

I turn the bias up slowly until crossover distortion goes.

This way is for class AB, not class A. There is no crossover in classA

 

[tinitus]

Isnt it also a question whether its pure classA up to the full 25watt, or say to just 15-20watt?

 

[nigelwright7557]

This way is for class AB, not class A. There is no crossover in classA

What about the currents going in and out of the capacitor ?

 

[Melon Head]

I think what he is trying to do is set the bias for his speakers knowing how much power he needs for his listening requirements.
If he never uses more than 5W there is no need for 20W class A.
Then he could reduce the bias to give Class A up to 5W or maybe 8W just to be sure.

 

[tinitus]

Then he could reduce the bias to give Class A up to 5W or maybe 8W just to be sure.

And the peaks/headroom would still have crossover distortion
And probably where its the worst

 

[Nelson Pass]

Isnt it also a question whether its pure classA up to the full 25watt, or say to just 15-20watt?

25 watts Class A into 8 ohms implies a 1.25 A bias current.

 

[npapp]

My speakers are 4 ohms. My understanding is with a 4 Ohm load I am only in Class A for 12.5W. I am hoping to extend the Class A rating by increasing the bias for the max that my heatsinks can dissipate. If I move to 1.5A bias doesn't this give me up 3A output in Class A increasing the rating a few watts before crossover to AB?

I am a noobie and usually just paint by numbers so please educate me if I am wrong or messing this wonderful amp.

 

[npapp]

If I did this right 1.5A bias would give me 18W class A into 4 Ohms & 1.7A bias yields 23W. Will the increase bias change the character of the amp?

 

[MEGA_amp]

I initially set it for 1.3A, then slowly set it to 2A over a period of a few days....whats next? 3A...why not. I think it sounds better with higher bias, there's a difference, however subtle it may seem. Listen in the dark, it'll keep the tree huggers happy.