F4 power amplifier

[Zen Mod]

when touching heatsink - is it laying on the floor as pictured , or you have it properly positioned , to ensure natural airflow ?

asking , just in case .......

[neoinc]

As pictured. The heatsinks are resting on a aluminum plate laying on the floor.

[Zen Mod]

hardly representative situation .

put them as intended - vertical , then try

ensure proper distance to floor (raise them at least 5cm) - they need space to breath

[generg]

However the bias of some transistors especially the set of IRF240 were off by 10-40mV at around 200mV.

This value is not bad for the beginning.........!

Did you check that every pair has the 200mA......? Only to be sure.....

[neoinc]

Originally Posted by generg

This value is not bad for the beginning.........!

Did you check that every pair has the 200mA......? Only to be sure.....

Generg, I biased the minimum at 200mV on the lowest value source resistors but the other resistors will read more then 200mV up to 230mV.

Is it advisable to change a tighter set of Vgs for the transistors? i already matched them to +/-0,02V beforehand.

Originally Posted by Zen Mod

put them as intended - vertical , then try

I have placed the heatsink to vertical position and yes, the heat is now bearable. more than 5secs not a issue but still this is a hot amp.

[neoinc]

Originally Posted by generg

This value is not bad for the beginning.........!

Did you check that every pair has the 200mA......? Only to be sure.....

Generg, I biased the minimum at 200mV on the lowest value source resistors but the other resistors will read more then 200mV up to 230mV.

Is it advisable to change a tighter set of Vgs for the transistors? i already matched them to +/-0,02V beforehand.

Originally Posted by Zen Mod

put them as intended - vertical , then try

I have placed the heatsink to vertical position and yes, the heat is now bearable. more than 5secs not a issue but still this is a hot amp.

[generg]

Originally Posted by neoinc

Generg, I biased the minimum at 200mV on the lowest value source resistors but the other resistors will read more then 200mV up to 230mV.

Is it advisable to change a tighter set of Vgs for the transistors? i already matched them to +/-0,02V beforehand.


I have placed the heatsink to vertical position and yes, the heat is now bearable. more than 5secs not a issue but still this is a hot amp.

Here comes a quotation from an article Nelson wrote 1993

"By placing 1ohm source resistors on each transistor, we can assure adequate current sharing for a fairly wide range of VGS. In Class A bias, we will be operating at about 200mA/device, which will place 0.2V across each source resistor. A variation in VGS will cause the bias to be unequally distributed between the devices. For example, for a 4.6V device in parallel with a 4.5V device, the first will run at about 160mA at 6W and the second at about 240mA at 9W."

You see how big the changes are from 4.6 to 4.5V Vgs!

In this article he also describes the method to change the resistors to counteract to the differences. I think it is not necessary in your case. But if you have the will and the time....

http://www.firstwatt.com/pdf/art_matching.pdf

When I compared some FW amps to my clones and they sounded much worse it was mostly the matching effort that could bring them nearer to Nelson Originals. (but never more than 80%)

[generg]

Originally Posted by neoinc

Is it advisable to change a tighter set of Vgs for the transistors? i already matched them to +/-0,02V beforehand.

did you match them at 200mA like described in Nelson's article?
You could also follow his advice at look at one or two other points to get the more matched parts...

[neoinc]

Thank you Generg, I will look through the articles and see if i can match the devices better next time. I shall not proceed further until i find matched parts. No point building a F4 if i cannot get the result at least 80% near to the orignal thing. but how do you measure this aspect? It seems most F4 builder descibe the amp characteristic to that of water. Hope i can experience it too when i fine tune mine further.

[generg]

.....the 80% is only a guess from my side! Saying that I succeeded getting the sound close to Nelson Original after some weeks......

In the meantime I was a bit depressive and thought to buy an original from the Master.....

and mind the Input J-fets too, I would suggest in this place an Idss of 10mA for the K170 and J74.