SYMASYMFAB POWER AMP

Objective:

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).

Results:

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

Schematics: