Class AB BJT 100 Watt Hi-End audio power amplifier circuit

Sym5 Class AB BJT 100 Watt Hi-End audio power amplifier circuit topSymasym5
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
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
  • More will follow !

Sym5 Class AB BJT 100 Watt Hi-End audio power amplifier eagle
Sym5 Class AB BJT 100 Watt Hi-End audio power amplifier parts
Sym5 Class AB BJT 100 Watt Hi-End audio power amplifier pcb

Part List
Device Qty Value Notes
Q1,Q2 2 MPSA18 can be substituted
by BC550C (pins reversed !)
Q7,Q8 2 BC546B or maybe 2n5551 (pins reversed
Q3,Q9 2 2N5551 OnSemi/Fairchild/Philips
Q4,Q5,Q12 3 2N5401 OnSemi/Fairchild/Philips
T1 1 BD139 or bd135, bd135-16
U$5 1 MJE15030 OnSemi
U$6 1 MJE15031 OnSemi
U$3 1 MJL3281A OnSemi
U$4 1 MJL1302A OnSemi
Device Qty Value Notes
C14 1 10pF (has been 22pF) Mica
C2,C7 2 100pF Mica
C3,C4 2 330pF Mica
C18 1 47nF Wima MKS2
C5, C6, C10, C11, C16, C17, C20 7 100nF Wima MKS2
C1 1 10uF (4.7uF also fits) Wima MKS2
C8,C9 2 100uF Electrolytic 63v (at least 40v)
C19 1 470uF Electrolytic 16v
C12,C13 2 1000uF Electrolytic 63v (at least 40v)
Device Qty Value Notes
R27,R28 2 0R22 5Watts
R1, R3 2 1R2 2Watts metal film
R4 1 4R7 2Watts metal film
R7 1 10 2Watts metal film
R2 1 10 250mW metal film
R8, R9 2 22 250mW metal film
R31, R32 2 47 (have been 22) 250mW metal film
R26 1 33 250mW metal film
R10 1 68 250mW metal film
R15,R17 2 150 250mW metal film
R11 1 220 250mW metal film
R24,R30 2 499 (or 500) 250mW metal film
R5,R6 2 680 250mW metal film
R13,R23 2 2k 250mW metal film
R12,R14,R29 3 22k 250mW metal film
R18,R20 2 47k 250mW metal film
R22 1 1k pot Piher, small (470ohm might be
Device Qty Value Notes
F1,F2 2 2.5T Fuse Slow blow
F1,F2 2 --- Fuse holder
L1 1 --- 0.6mm isolated (enamelled) copper
wire wounded around R7 forming the output coil.

Recomended Part
  • All resistors are standard metal film 250mW except:
    R1/3/4/7, these are 2W metal film, and the 0.22ohm beeing 5W.

    Around R7 is wounded a 0.6mm isolated
    (enamelled) copper wire forming the output coil. (~12 windings)

    For c19 i used 470uf/16v, all other
    electrolytics 63v. The 10/100/330pF should be mica-caps.

    The 100nf and 47nf is recommended to be
    Wima MKS2 (or better), also for C1 i suggest Wima MKS2, 4.7uf is enough.

    For Trimpot i use a Piher. The MPSA18
    can be substituted by BC550C, for all other parts i do not recommend changes,
    especially the feedback network (r29/30) should be kept unchanged, feedback
    compensation is very delicate for this circuit !

  • Be careful when substituting the MPSA18 with BC550C,
    the pinout is reversed between these 2 transistors !!!
  • The bias is adjusted via the trimpot (R22).
    Recommended bias is 55ma, resulting in 12mv across a single 0.22ohms or 24mv
    across both 0.22ohms. Connect a DMM to the upper wires of these resistors and
    adjust trimpot until DMM reads wanted voltage.
Symasym5 biasing
Symasym5 biasing: (for debugging)

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