F5X from diyAudio Boards P-F5c-2V20e

[a.wayne]

Originally Posted by JBL4435

Design
Amp Boards X-ed as described at the beginning of this thread
Post #1

Amp Data
- Transformer 800VA, 2x18V per mono block
- PS CLC 2x 33.000uF 33.000uF 2.2mH/0.12Ohm 33.000uF; total 198.000uF per mono block
- Vrail +/- 21.8V
- Ibias 1.3A per output FET (gives 0.29V across R11/12), 2 output pairs! X-ed 4 output pairs
- output FETs IRFP240/IRFP9240
- input Fets 2SJ74BL/2SK170BL
- app. 70W into 8 Ohms, voltage clipping
- app. 130W into 4 Ohms, voltage clipping
- app. 175W into 2 Ohms, voltage clipping
- app. 200W into 1 Ohms, current clipping

Changes made to schematic
R10 = 33,2k
R21/22 = 22k
R17/R18 = 702
R117/R118 open
R11/R12/R111/R112 = 0.22

That’s it.

Hellol JBL4435,

With such a big psu, why would your amp not double power from 4-2 ohm..?,

[JBL4435]

Originally Posted by a.wayne

Hellol JBL4435,

With such a big psu, why would your amp not double power from 4-2 ohm..?,

You loose a lot of voltage at higher currents. Under 2 Ohms the current limiting kicks in.

[AudioSan]

Originally Posted by JBL4435

Yes, I know there is a thread giong on F5X EUVL Approach but the diy Store boards are available and why not use them?

I'm currently building 2 mono F5X with the P-F5c-2V20efrom the diyAudio Store.
those are the boards with cacode option and 2nd output pair option.
I'll build the 2nd output option and "X" 2 boards for a mono block.

AFAIK Smyslow did this with the 1 output pair boards but it was not further commented.

To "X" the boards two pcb connections need to be cutted. Not nice but the boards have no "X-Option". See picture.

Here are my first 2 questions to the knowledgeable:

1. Input impedance
If I go with the 1k input resistir plus 100k reference to ground as in the original F5 I'll end up with only 500 ohms input impedance in a X design. This is fairly low. I built my AX amps with 10k input impedance and 10k reference to ground.
Here I would like to go with 22.1k input impedance and 221.1k reference to ground.
Would this be ok?

2. Relative DC offset resistors output to ground
Would it be helpful to implement the output to ground resistors from other X designs (33 to 68 ohms plus/minus output to ground) also in this design? Or is it unneccessary? If yes, why?

hello. do you realy need to cut the connections? it look like you can just lift one side of the resistors R1, R2 and wire the X from there.

[JBL4435]

Originally Posted by AudioSan

hello. do you realy need to cut the connections? it look like you can just lift one side of the resistors R1, R2 and wire the X from there.

Of course you can do that. You'll have 4 resistors hanging around then.

[AudioSan]

Originally Posted by JBL4435

Of course you can do that. You'll have 4 resistors hanging around then.

not realy. they will be the tip of the wires you need to x the 2 PCB's.
then you don't need to damages the PCB's.