Source Follower PCB

As a result of playing with DHTs, pre-amps and various circuits for many years, I realised that there was a particular need to have a flexible source follower circuit. This is nothing new and I will not even claim any proprietary design as the source follower is a universal circuit that has been out there for many years.

I wanted to make my own PCB for my designs and decided to make this available to the wider community. As with every design, there are some compromise decisions to be made to keep the complexity and cost down whilst providing the maximum flexibility.

I hope you will find this little board as useful as I had and please take the time to provide any feedback as is very helpful to all of us in the DIY Audio community.

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.

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)

PCB Features

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 devices which perform the best in this role.
  • The tail CCS is simple and leverage the option of using same MOSFETs if wanted.
  • The board takes into account the use of a bipolar supply up to 300V differential. To set a source follower board, you only need to set the value of 2 resistors (CCS current and operating voltage) depending on the supply voltage levels, desired current sink 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 top MOSFET when capacitors are charged or if accidentally the output is shorted. This circuit can be bypassed easily with a jumper.
  • PCB has been manufactured to high quality levels:
    • FR4, 1.4mm thickness, ENIG finish, 2 Oz. copper.
    • PCB Dimensions: 25mm x 66.04mm
    • M3 mounting holes spacing
      • Width: 20.32mm
      • Length: 60.32mm

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.

SF PCB REv03 

Ordering the PCB

Please fill out the form below and I will get back to you once you confirmed quantity and shipping method (e.g. Standard or Tracked postage).

Due to pandemic situation, USPS and other national post services have been a bit unreliable so I recommend tracked shipping where possible depending the value of the goods.

Upon completion of payment, I will provide BOM and build guide.

I don’t sell kits nor parts, appreciate your understanding. I may build and sell occasionally a few boards completed and tested, so ask me if you’re interested.

 

 

12 thoughts on “Source Follower PCB”

  1. Hi Ale. I’m glad to have those boards in hands! The documents are also helpful.
    However, I wish I could see an exemble of screen voltage stabiliser for push pull pentodes. 6P14P-ev tubes in my case (or EL84m). Usually I’d use a Maida type voltage regulator for G2 (one regulator for each pair). Also, can one board feed one pair of output tubes in push-pull ?
    Maybe this will be my “winter project” !
    Thanks.
    Pierre

    Reply

    1. Hi Pierre,
      The follower output provides a very low AC impedance to ground (1/gm). I haven’t tried it myself on a PP configuration but I see no issues of having one board feeding two screens.
      Ale

      Reply

  2. I left a few comments about my positive experience with Ale on the 01a tab. However, I didn’t really mention the sonics of the 01a preamp with Ales Gyrator, Source Follower PCBs and Coleman filament regs, as a whole, and despite it replacing a bit of slouch, the project really is fabulous! In addition, I have two other, IMO excellent preamps, and this project brings about an immediacy to the experience that maybe bests these amps. It is part of a “bedroom” system with a CD source and remaining tube based OTL mono amps and small DIY Seas speaker kit. It seems particularly engaging but doesn’t over due it. I think it offers a nicely balanced presentation. I guess that’s a fairly abstract description but clearly emotionally satisfying. Really what I care about most! However, a challenge to get perfectly quite if you don’t focus on proper layout, maybe a two chassis design is required, at least recommended. What I really want to say here is Ale provided consistent support in helping me via e-mails to complete the project successfully! Somewhat of a novice amp builder I clearly needed a bit of troubleshooting guidance, which not surprisingly, was unintentionally self imposed. . .Ale was extremely patient in helping me resolve my build issues! Thanks again!

    Reply

  3. Hi Ale, I would like to try to insert a Source Follower at the output of my Preamp, to hear if improving the Slew Rate and obtaining a lower output impedance, further improve the already excellent quality that I now get from my preamp. with VT25 and Gyrator.
        This preamplifier with a 2uF capacitor drives a load of about 8K Ohm, (a 10K Ohm active crossover and a 47K Ohm active subwoofer), also with a switch I switch the RCA output, on the primary of a 10K Ohm transformer and with 600 Ohm secondary, which I use with headphones.
    The quality I get is very high.
    The only problem with this setup is that when I switch to headphones, I have to set the volume control almost to maximum.
    If you can give me your opinion on the configuration described and if you recommend trying Source Follower, I would be grateful.
    Best regards
    John

    Reply

    1. Hi John
      Glad you like the VT25 preamp. It can drive lower impedance loads but distortion will be higher. With the step-down transformer should perform nicely. The Source Follower won’t introduce any benefit here, there is no issue with the output impedance as the gyrator has a very low one (in fact as low as the SF) and the VT-25 has enough current drive capability. You may be not have enough gain to drive the HP it looks like.
      Cheers,
      Ale

      Reply

  5. Hi Ale. Your SiC source followers are great. Have you ever made a singe PCB that combines the SiC source flower and a Gyrator or CCS?

    Reply

    1. HI Dan, I thought about it but but I found too many reasons for not doing it. Firstly, orientation of TO220 devices is critical to allow for heatsinks and secondly, I find that the modular approach gives the builder more flexibility. Not everyone has a lot of space so it’s good to have the ability to place boards in different spaces in the amp chassis.
      Hope that helps?
      cheers
      Ale

      Reply

  6. Hi Ale,

    I have browsed your site for a week and found a lot of interesting subjects that I would like to try in the future. I wonder my 300B PP driving by 6SN7 would get any benefit with the Source Follower even though the sound is beautiful at the moment. Since my bias supply is 90V which I can run voltage doubler for Fixed Bias and SF like your 300B SE.

    Reply

    1. Hi Minh,
      Thank you for your comments. Yes, it would perform better, however in my opinion I’d rather change the driver. the 6SN7 isn’t the best driver for a 300B. There are plenty of much better drivers (see my recommendations on the blog) and you would get much better sound performance from a change in the driver I believe.
      Good luck.
      Ale

      Reply

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