homebrew variable load buffer

J.D.White · Post by **J.D.White** » 24 Nov 2017 2:23 pm

If you do a search for Tilman Preamp or Fetzer Valve (runoffgroove.com) - a number of layouts will show up & could easily be modified for this FET based circuit.

There's also a freeware program called DIY Layout Creator (Windows/Linux/MacOS). It allows you to do layouts (pcb, perfboard, veroboard,...) It's simple to use & how-to videos on youtube.

Scott Duckworth · Post by **Scott Duckworth** » 24 Nov 2017 2:40 pm

OK, I edited the schematic, it's in the original post I did.

Dave Hepworth · Post by **Dave Hepworth** » 5 Dec 2017 8:06 am

Hi again folks,
With regard to this circuit can ask a question .
How will the circuit run using 2 X 2.7 M ohm resistors instead of the 2.2 M ohm ones as per schematic values.I am finding it hard to get 2.2 M values and I have a couple of 2.7M ones.
Thanks for looking .
Regards Dave

Chris Reesor · Post by **Chris Reesor** » 5 Dec 2017 12:58 pm

Dave, 2.7 meg should be fine for that bias voltage divider in this circuit.

Dave Hepworth · Post by **Dave Hepworth** » 5 Dec 2017 4:13 pm

Thanks Chris.

Dave Hepworth · Post by **Dave Hepworth** » 7 Dec 2017 11:00 am

Hi Folks ,
Yet another question!
The 500 k pot ...is it a variable resistor in this schematic or just a normal potentiometer ?
Where does the 47 K resistor go relative to the pot lugs? I realise one end goes to ground which makes me think it is not a variable resistor .Can you explain please.I have built the rest of the circuit and now need to wire the pot up.
Thanks for looking Dave

Brad Sarno · Post by **Brad Sarno** » 7 Dec 2017 4:33 pm

Yes, it is a variable resistor. You'll likely want fully clockwise to be 500k and counter-clockwise to be zero. Then the fixed resistor in series sets the minimum z.

You can put the fixed resistor on either side of the pot (variable resistor). Doesn't matter.

B

Mark Fowler · Post by **Mark Fowler** » 7 Dec 2017 6:00 pm

That 47k resistor is a limiter resistor so the 500kA pot doesn't go lower then 47k ccw.

Dave Hepworth · Post by **Dave Hepworth** » 8 Dec 2017 9:19 am

Hi Brad and Mark,
Thanks for the help .So I take it that the 47k goes like this.....
1) one end to ground
2) the other end to one of the 2 lugs used as an in and out of the pot.
Regards Dave.

Brad Sarno · Post by **Brad Sarno** » 8 Dec 2017 1:30 pm

The resistor can go between ground and the CCW lug on the pot.

Then the center (wiper) lug goes to the input signal.

It's also a good idea to also tie the CW lug to the center lug. It's a variable resistor and even if the pot gets dirty, there will always be the 500k load present no matter what.

B

Mark Fowler · Post by **Mark Fowler** » 9 Dec 2017 8:07 am

Perhaps something like this.

Dave Hepworth · Post by **Dave Hepworth** » 9 Dec 2017 11:23 am

Thanks Brad thanks Mark .Can now finish off the project !
Regards Dave

Tony Glassman · Post by **Tony Glassman** » 9 Dec 2017 5:13 pm

bOb, SGF seems faster since the change-over. Thanks for all that you do

Terry Lovett · Post by **Terry Lovett** » 3 Jan 2018 12:20 pm

Hello Guys,
I really like the drawing Scott included on this Steel Buffer project. The reverse current battery protection is a plus, especially if no mechanical safeguard is built in place to prevent battery or polarity reversal. I always hate changing out batteries and using a standard diode really shortens the battery life. There are a couple of options that are quite useful: you could use a Schottky diode with a much lower forward voltage drop or even better would be to place a P-channel MOSFET parallel with the original diode and reference its gate back to the ground (neg) side. This would have a minimum voltage drop on the circuit and clamps shut with a reversal. MOSFET's have a very low resistance from Drain to Source. See: TI Application Report SLVA139, 2003

Alan Muir · Post by **Alan Muir** » 18 Jan 2021 3:02 pm

One of my Covid hobbies is learning about electronics. I bought a breadboard and the parts to try this circuit. Thanks for all of your efforts and input on this. I worked from the schematic and experienced a big volume loss across the range of the pot. I added the jumper across the lugs as in the napkin diagram and it works better but I don't understand why. Can somebody enlighten me? Thanks.