F5 power amplifier

[CBRworm]

isn't it 1.3 amps per device, or 2.6 amps bias per channel?

[Zen Mod]

Originally Posted by CBRworm

isn't it 1.3 amps per device, or 2.6 amps bias per channel?

that's the case for circlotron , where you have two PSUs , but F5 have just one .

1A3 , going from + to - , or vice versa , whatever

[CBRworm]

:-( now i'm confused again.

I guess it is (1.3 amps per device @ 24 volts) x2 which is 1.3 amps total at 48 volts? Class A operation up to 2.6 amps output?

I thought I understood but now. . .

Maybe it's the ambien

[bobodioulasso]

Originally Posted by bear

tinitus, my dear friend... please look at the date I joined and the date that you joined, get back to me on that?

15 watts, 25 watts, what's less than 3dB amongst friends anyhow??

Now let's do the math?
Mr. Pass says to bias the F5 at 1.3 amps. Ohms law says P = I ^2 * R. So, P = 1.3^2 x 8ohms = 13.52 watts

The article states that the amp will deliver more than that into an 8 or 4 ohm load, but clearly no longer in pure Class A, rather in AB.

Thank you.

_-_-bear

You forgot: a push pull can deliver twice its bias into the load. (Single ended delivers only once)

I max = 2x I bias into the load.
2.6*2.6*8 = 54.08 watts
There are 27 watts pure class A into 8 ohms. (54watts max)
Read the PDF and Zen articles again.

PP Class a amp works like this:
When idle the two halves run equal current: the bias.
Each half runs its current through the load but as they are equal and in opposite direction the load see no current.
When a signal is applied, current varies the same quantity into the two halves but opposite .One side increase while the other decrease.
This variation's amplitude is from bias value to twice this value and then to zero.
When one side is 2xbias, the other side is zero.
This explains why one leaves class A at 2xbias.

[bobodioulasso]

A short pic...

[AndrewT]

Originally Posted by bear

Now let's do the math?
Mr. Pass says to bias the F5 at 1.3 amps. Ohms law says P = I ^2 * R. So, P = 1.3^2 x 8ohms = 13.52 watts

the F5 is a push-pull ClassA amplifier that when biased to 1.3A can deliver ~2,6Apk of ClassA current to the load.

If the load is 8r0 then the maximum ClassA power is 27W ([2 * 1.3]^2 * 8 / 2)

[AndrewT]

Originally Posted by bear

being two devices each biased at 1.3amps.



So, to stay in class A the current available has to be constantly 1.3 amps (in this case), IF the current available was 2.6 amps quiescent, then when the amp was driven hard the available current would drop on peaks (as one side neared cutoff). The bias current is quiescent current. One would expect to see a non-linearity (let's assume we removed the feedback which would try to correct for it), which I'm not thinking I have ever seen or seen written about.


Perhaps I am thinking about this incorrectly...

there is no perhaps about it.
Your description confirms you do not understand how push-pull ClassA operates.

BTW,
27W is equivalent to 54Wpk. i.e. both are 20.8Vpk and 2.6Apk into 8r0.

[tinitus]

Originally Posted by Nelson Pass

It is biased at 1.3A, and leaves Class A at 2.6A.

Start at page one

[bear]

I was thinking about a situation where the amp goes into AB (without feedback) and the Pch and Nch devices do not have curves that perfectly overlap so the area where one goes into cutoff does not track exactly as cutoff is reached, the cutoff is reached and the driven transistor is the transfer curve that continues.

Note to self, good idea: don't post in the AM before being fully awake...

[bobodioulasso]

Originally Posted by bobodioulasso

This explains why one leaves class A at 2xbias.

...When one half runs more than 2.6A ,which can be the case with less than 8 ohms loads, or even 8 ohms reactive ones, the other half is cut off. Then that is classAB mode.

Thanks Andrew to repeat what i did just say.