Thanks. I did find some little Chinese Ebay boards without power supplies when searching for "small tube buffer circuit board", but I'm not sure I trust these designs. And I don't know if they would integrate well with the PH12 output.
So in the spirit of learning something new I did a little reading and found tons of info on cathode followers. It seems the most basic circuits use just 3 or 4 resistors and 2 capacitors per channel, and common tube choices are 12au7, 12AX7, 6DT8, 6SN7, among others. I like the idea of a single twin triode handling both channels. I am attracted to the 12au7/12ax7 since I have a lot of them.
If anyone has a favorite design using one of these tubes I would appreciate it.
I found the following link very interesting, but it may be over complicating things for my needs. It points out a lot of potential pitfalls in design, and quickly moves on to designs using two triodes per channel (White Cathode Follower). I was hoping to stick to a single tube design.http://www.tubecad.com/october99/index.html
A good introduction is at merlin blencove's site : http://www.valvewizard.co.uk/accf.html
In addition i tested a FET-based buffer intended to be mounted inside a PAS, getting the
minuscule power needed from the filament power. It's still mounted in a PAS !
( similar : https://www.ebay.com/itm/Dynaco-PAS-Preamp-Impedance-Matching-Module-New/303180358355?hash=item4696f522d3:g:2aIAAOSwTm9c-X5E
I decoded the schematics, and it's really nothing special. But it works and according to my ears
it has no negative drawback. As i reverse engineered this i consider this MY work and feel
free to share on request.
Cheaper fet buffers are available at ebay.( but many has push-pull , not class A types) I think that
a buffer should be minimalistic, using only one class-A device and a corresponding resistor.
Last edited by peterh on Wed Jun 12, 2019 3:28 pm; edited 1 time in total (Reason for editing : Spelling)