F5T with 56V rails?

[AndrewT]

then you must prevent blowing up your input stage transistors.

 

[Leify]

I bought the pcb's here at the store and apperently you can cascode them. So can it handle the +- 52 V rails?

Appreciate all answers,

Br

 

[Andersonix]

F5Turbo Article says Yes. Can you handle the heat?

 

[Leify]

I have not bought the cases and heatsinks yet so that problem can be solved. However that I wrote in the beginning of this thread is that I want to have a lower bias for aiming to about 30W class A and the remaining in class AB. That could work right?

 

[AudioSan]

i say no. you need more then 2 output pairs to handle the heat. 52V*0.9A bias pr device=46.8W dissipation pr device. that is to high.
you can however run it with alot lower bias. like 0.5A pr device (26W dissipation). but it will only give you 16W class A at 8ohm.

 

[AndrewT]

30W of ClassA needs ~1.4A of total bias.
A 2pair output stage with each passing 0.7A is ~35W /device.

That allows upto 2.8A of ClassA output and then it transitions to ClassAB for the higher currents.

 

[AudioSan]

0.9A was for the 50W v2. a general answer regarding if one can build a v2 with 52V rails.
v2= 2pair output 50W class A amp.
and one would need to change source resistors so that the diodes dont kick in way too soon. or drop them all together. but then its not a v2 anymore.

 

[Leify]

Thanks guys,
So if I understand it right it's not possible to have +-52V rails and 30W class A and remaining in class AB what it now will be. What do I need to change specifically to make that happen. Sorry for all these maybe "stupid" questions but I'm new at this and need help.

Br Leif

 

[AudioSan]

it is possible. but i would drop the diodes. and cascode the front end.
it will be alot of extra heat doh. 0.7A pr device and 52V rails give you those 30W class A. with a dissipation of just over 140W pr ch(36W pr device). so you will need some monster sinks

 

[buzzforb]

Thanks guys,
So if I understand it right it's not possible to have +-52V rails and 30W class A and remaining in class AB what it now will be. What do I need to change specifically to make that happen. Sorry for all these maybe "stupid" questions but I'm new at this and need help.

Br Leif

It is possible but you will have to refigure the source resistor value to drop the apporopriate voltage needed to have the diodes near turn on. The easier way is balanced. You get voltage on the cheap.

 

[bear]

Buzzforb, he already has the PS iron, so he is trying to use it.

My suggestion was to use choke input...

_-_-

 

[buzzforb]

Sorry. missed that part. Of course it can still be done. Duncan Amps PSUD II would be a worthwhile download, i believe, for the PSU and then it is a matter of cascoding the Fe and increasing Rs on the output.

 

[AndrewT]

.............. apporopriate voltage needed to have the diodes near turn on. .............

I have previously explained that there are different ways to use these diodes for enhanced current ability (= less voltage droop at high output).

You do not NEED "to have the diodes near turn on".

 

[Leify]

I have just ordered another set of boards so I can build a F5T V3 with 4 pairs of mosfets per channel. Then should the problem be solved. I should have no problem to have +-52V rails and to set the bias for around 30W class A and the remaining in class AB. Then I don't need monster heatsinks as well I guess. My intention is to build monoblocks with 2.2 KVA tr and about 0.5 F per channel. Should be a very nice amp.

Br

 

[AudioSan]

you will stil need some semi monster sinks 52V and 1.4A bias = 145W dissipation
but with 2.2KVA transformer, i would build a stereo amp, and save the other transformer for another project.

 

[Andersonix]

Monster transformer, monster heat sinks to cool 16 mosfets (stereo), and monster cap bank? Better put it on casters, so you can move it...