Biasing F2 and F2J

[kinku]

Wondering if I can get a step by step instruction guide on how to Bias F2 and F2j ?

 

[Zen Mod]

as Papa sez, when making F2J :

The F2's that I have updated with the JFETs need the
following changes:

R6 = 47k

C1 = 10 uF

Z1 removed

The potentiometer must be re-adjusted for minimum
distortion at 4.5 watts (symmetric clipping) and readjusted
after an hour.

The distortion results are typically a reduction from about 1%
at 1 watt to .25%.

pretty much the same applies for regular F2 ,regarding pot position

 

[Zen Mod]

this thread is useful : DIY F2 clone

 

[kinku]

Zen Mod from post #51
DIY F2 clone
adjust P1 to get a reading of one half, of the DC-supply voltage just in front of the outputcap! Connect your DVM to GND and in front of the outputcap and adjust P1 to get the correct reading.

 

[Zen Mod]

that's if you don't have equipment to measure THD and/or to observe symmetrical clipping ..... or if you simply don't know how to do that

in any case half of DC supply voltage is starting point

 

[kinku]

I dont have any other devices to measure than a Multimeter.

 

[kinku]

how do I set the current Bias?

 

[kinku]

For F2j


DIY F2 clone
Measure at Drain pin (center) to ground point on schematic.
If bias is too low here <10v, I would replace pot with 10k, instead of 5k. Or replace R4 with a higher value.

 

[OllBoll]

The end result you want is that the top CCS and the bottom part should present an equal resistance at ~ 2.8A such that they together burn ~ 67 watts.

And as the performance point we want is nicely enough at that point in the middle which is the red arrow with a dot in Zen Mods picture should have half the voltage of the rail. Or in short with 24V rails the half point should be 12V.

 

[kinku]

The end result you want is that the top CCS and the bottom part should present an equal resistance at ~ 2.8A such that they together burn ~ 67 watts.

How can achieve that end result?

 

[El Reino]

How can achieve that end result?

Lower your ccs source resistor.

 

[Zen Mod]

How can achieve that end result?

first - knowing where to put black and red voltmeter probe

 

[kinku]

ZM , that is why I am here. Else I would have been a Zen Mod.

 

[Zen Mod]

for basic things , there are already established and better places , than this , or any other (slightly advanced ?) electronic forum

for instance , here : Basic Car Audio Electronics

look at right side menu

 

[kinku]

I am trying to see if there is a shortcut rather than reading about the the whole website( will do it eventually).It should be something easy like put red prob at this point and black here kind of...
I know ZM does not want to give up that info, but someone else may.

 

[Zen Mod]

give me proper photograph of your amp pcb , and I'll draw arrows where and what

 

[kinku]

Thanks ZM.

 

[Zen Mod]

set your DVM to range of more than 20Vdc

connect black probe permanently for this test- with alligator clip anywhere on GND potential ....... say on Out+ wire or terminal

put red probe on positive side of +24V terminal (other one is GND) , write down exact voltage , let's call it Ub

now put red probe to mid pin of Q1 (be careful not to slip with probe , it's easy to make short ) or even better , with small alligator clip to C2-P. and fiddle with P1 to have voltage exactly of Ub/2


repeat the same for other channel

that's it


edit: going in electronics assembly without basic knowledge is almost as doing things blindfolded ; you need all the luck , or it will end in smoke and pain

 

[kinku]

Will assemble it over weekend. Yes I need a lot of luck.
Awaiting stuff from mouser and Antek. I ordered a 400Va 18V tranny. I was told 350 is what papa uses but 300 is too small.

 

[kinku]

ZM, completed one channel,no smoke.Input Voltage 22.66V and Biased at 11.33V.Adjusted after 20 minutes to 11.12to 11.33V
Thanks for all your help.