I already had my "small" class A/B power amp (see Dual classes Power AMP) but my speaker had provisions for bi-wiring thus for bi-amping. Then I saw several threads DIYAUDIO forum on the "symasym" power amp (see Mike Bittner's Symasym 5.3). This other web site Ampslab also sells kits based on this topology. Recently I was playing with fully symetrical designs but the "symasym" is based on another topology using 2 differential stages in series (VAS stage is differential). I had already built this configuration but for a quasi-complementary V-mosfet power amp (40W, class B mainly) and I used it for about 10 years but needed to move to something different for a change. I then decided that I would build my own design based on symasym topology but with these main differences:

  • Lateral MOSFET output stage (3 pairs)
  • Differential stages (input and VAS) using "Low memory transistors blocks"
  • High local feedback for input and VAS stages
  • No miller cap use
  • Enhanced VBE multiplier (for bias current setting)
  • Class A operation until 5 to 10 Wrms (8 ohms)

Constructions pictures:

 Notes: You will notice that I use lots of wires which requires more manual work. However, the benefits are:

  • individual modules (driver circuit boards, output stage boards, medium power supply boards, in-rush/dc-protection board) can be modified and replaced easily for upgrades. It also helps to pin point more easily on possible problems/mistakes.
  • what size of a pcb trace equals a 14 AWG wire ?...

I use a "star grounding" scheme as you can see at the screw located in the middle of the main power supply electrolytic caps (for each channel since isolated ground).


T.H.D, square wave and damping factor are similar to my "small" other Dual Mono Power Amp measurements.