100 watts Symasym5 Power Amplifier

Symasym5, is a "cute" power amplifier, designed with quality but still low price in mind. This resulted in a ClassAB BJT amplifier, using only TO92 transistors for input and VAS, with a reasonable part count. The topology used is well known and consist of a single diffamp for input, plus a 2nd diffamp with current mirror for VAS. This is followed by normal darlington EF outputstage using modern high beta devices. The circuit uses large amounts of feedback over the whole audioband and an unconventional feedback compensation scheme.  Right now symasym is designed to be driven directly from a CD/DVD-player, simply place a 22k poslog stereo pot between player and symasym. (as voltage divider)

The at least change is to reduce C14 from 22pF to 10pF, but i recommend to follow Pavel's advice. I recommend C14 with 10pF also for MJLs, this increases safety margin. Do not forget to read Pavel's Review! A very promising evaluation of the qualities to be expected from symasym. Thanks Pavel ! Another update are the resistors R31/32 to be increased from 22ohm to 47ohm.

Symasym5_3 is an update of v5.2, with an improved board layout concerning power gnd, resulting in lower thd for high frequencies, giving more clarity in sound. The Page for symasym5_2 still exists.

If the gain is too high because symasym is driven from preamp, R30 can be increased from 499ohms to 1k, but in this case R16/19 (22 or 33 ohms, not on schematic, REs to Q1/2) are required to keep feedback at same level and
100 watts Symasym5 Power Amplifier
100 watts Symasym5 Power Amplifier

Some Specs :
  • THD: ~0.005% (measured) sim'd: 0.002%
  • Power into 8ohm: 60 watts
  • Power into 4ohm: 100 watts
  • Gain: 32dB (~1:40)  full output at 0.7v input (0.5v rms)
  • Feedback: 57dB
  • GainBandWidth: ~400Mhz
  • Slewrate: ~20v/us (symetrical)
  • Supply voltage: +/- 36v
  • Biasing: 55ma, ~12mv across a single 0.22 ohm
  • Measurings: RMAA Symasym5  The measuring setup itself is far from perfect, but gives a good idea !
  • Frequency response: 3.2hz to 145khz (-1db) using 4.7uf input cap
  • Phaseshift at 10khz: <3°
  • More will follow !