Source Follower PCB

I’ve been posting about the use of source followers in the circuits with some interesting results from testing. Some interesting posts to read, If you haven’t read them so far:

  1. Slew Rate, Slew Rate (Part 2), Slew Rate (Part 3) and Slew Rate (Part 4).
  2. 6SF5 driver for 300B/GM70/813 SE Amps
  3. DHT Phono Stage Test

After several tests over a variety of circuits, I finalised the prototype for a Source Follower PCB. The circuit is incredibly useful. Some examples of uses cases are:

  • Amplifier output stage grid drive
  • Screen drive amplifiers
  • Screen voltage stabiliser for pentode stages
  • HT voltage stabiliser for preamps
  • Buffer stage for high-mu/high-anode resistance stages – either triodes or pentodes (e.g. Phono)

Some key aspects of the board are:

  • The PCB has been designed to accommodate all sorts of power MOSFETs (both TO-220 and TO-247), in particular the high transconductance and low Crss ones which perform the best in this role.
  • The tail CCS is simple and leverage the option of using same MOSFETs.   
  • The board takes into account the use of any bipolar supply up to 450V diferential. You only need to change a resistor depending on the supply voltage levels and make sure there is a sufficiently big heatsink on the MOSFETs.
  • There is a current limiter circuit built in to protect screen or grid from excessive current. This is also very useful when the board is used as a voltage stabiliser for a preamp. You can limit the peak current and avoid destroying the MOSFETs when capacitors are charged or if accidentally the output is shorted. This circuit can be bypassed easily with a jumper.

Here is one of the boards submitted to the usual abuse during testing:

This is a very useful PCB in my view which can be used extensively in preamps, line stages and amplifiers.

If there is sufficient interest, I will run a batch of PCBs for the DIY audio community:

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