F4 Channel Problem - Running Cold

[2 picoDumbs]

List of things you can do

1) use jfets with Idss of 7mA or lower
2) reduce the value of R6 and R7 to 5K
3) Increase R8 to 33k, leave R9 at 10K, Increase trimpot to 10k
4) Use TLVH431 (only requires around 100uA)
5) Use Mosfets with low Vgs threshold.
You don't need to implement all the changes just what ever gets the job done

Another one for the list as Zen Mod suggested:
6) Reduce value of R5/R22(+P2) to around 680 Ohms to 750 Ohms

You don't need to do all of them, in fact just replacing TL431 with TLVH431 would most likely fix the problem.

 

[2 picoDumbs]

I have added the resistor mods on the circuit so it is easier to see.


1) Jfet Idss not higher than 7mA
2) Use Mosfets with lowest Vgs threshold
3) Use TLVH431
4) If you're a fearless amplifier builder (actually doesn't require any courage) reduce source resistors from 0.47 Ohms to 0.22 Ohms

All these things will help.
Reducing source resistors to 0.22 could be seen as an upgrade if you're driving reactive speakers.

You can decide which choices you like best

 

[vdi_nenna]

Thank for the help.

How important is the voltage reference "initial accuracy" rating of the TLVH431 in this circuit?

TLVH431 Texas Instruments TO-92-3 Voltage References | Mouser

 

[2 picoDumbs]

Initial accuracy doesn't matter at all.
That's the purpose of the trimpot.

 

[2 picoDumbs]

With a 1.24V reference you want something like, 10k trimpot, 10k resistor, and 80k resistor
Or 5k trimpot, 5k resistor, 40k resistor.

 

[vdi_nenna]

Ok cool. Had like 20 to choose from.

 

[vdi_nenna]

Fixed the channel in no heroic manner.
Purchased new boards, jfets and matched mosfets from the store.
Its works great now.

I did listen with one channel with Toshiba and the other with Linear System (LS) jfets and it sounded fine, nothing stood out as spectrally or tonally aberrant. That's a power buffer for you. Still I couldn't let it run with different brand jfets. Used the LS in both channels.
Only made minor adjustment to offset and bias when dropping the LS jfets into the board.

Anyone need a single F4 board, matched mosfets and TL431 for a single channel? 😀

 

[2 picoDumbs]

Nice to hear everything is up and running again.