This audio power amplifier in principle has an output stage similar to my a la Hiraga amplifier. That amplifier used Toshiba complementary JFETs in the input. These transistors are discontinued, but the american company Linear Systems has made some very good replacements, although the price is not low... I have therefore looked at the 20 W Hiraga amplifier with bipolar transistors at the input. So, say hello to Mimir, which ended up be an amplifier similar to the 20 W Hiraga, but with a twist (or two) to lower the output impedance and distortion.
The initial input stage (before a slight modification) is shown in
the figure below. It is completely symmetric, powered from the voltages
V+ and V-. Two zener diodes (D5/D6) are used to give stable bias
currents to the two emitter followers Q13/Q14. The offset and bias
adjustment of this amplifier is made up by the two potentiometers
RV7/RV8, since they set the current in the amplifying stage made up by
Q21/Q22. These are common emitter amplifiers with a gain from base to
collector given approximately by the relation R19(loaded)/R23 and
R20(loaded)/R24. In this amplifier, the bias current of all transistors
is set to about 1 mA.
The resistors R1 + R2 determines the input impedance. R1, with a
quite low value, together with a source resistance (from e.g. an
preamplifier), give a slight badndwidth limitation on the input
together with the input capacitance of the amplifier. The value of R2
should not be too high, since there is a small input bias current (the
difference between base current of Q13 and Q14) flowing through this
The output stage, together with the common emitter stages (Q21/Q22),
is shown below. The transistors Q29/Q34 in the upper leg and Q30/Q35 in
the lower leg form Sziklai pairs - complementary feedback pairs. Since
the output signal is taken from emitter of Q34/Q35, these pairs in fact
act as common emitter amplifiers, where the gain for each pair is set
by the relation RL/R32 (and RL/R33), where RL is the load at the output
- in reality the loudspeaker. The linearity of the Sziklai pair is very
high, and the resistor R31 increases the linearity slightly.
The amplifier without the power supply is shown in the figure below. The components are all placed on the same PCB. J1-J5 are the connectors on the PCB. There are two "grounds" on the PCB, one noisy (EARTH) and one quiet (GND). These are separated by the resistor R38. It may help against ground loops, if this should be a problem. In the prototype this was chosen to be 10 ohm, but can be shorted if unnecessary. The values shown for the components in the schematic are suitable for a 10-30 W amplifier. The feedback from the output is fed via the two resistors R18 and R19. The potentiometers RV7 and RV8 are used both to set the bias current in Q34/Q35 and to cancel the offset voltage at the output.
The input impedance of the amplifier is about 33 kohm in parallel with a very low input capacitance, less than 10 pF. For a 25 W class A amplifier with a nominal load of 8 ohms, the bias current in Q34/Q35 should be at least 1.25 A. This follows from the fact that 20 V peak (14.14 V RMS) is necessary for 25 W into 8 ohms. The current is then 20/8 = 2.5 A peak; the push-pull design makes it possible to halve this current. For a 20 W class A amplifier with a nominal load of 8 ohms, the bias current should be at least 1.12 A. The bias current of Q29 and Q30 is mostly set by the current in Q34 and 35, but is about 20 mA for 25 W output.
Comparing this schematic with that presented in the first figure, one
can observe two additional resistors: R17 and R18. These are adding
about 1 mA extra to the current through R23 and R24. This implies that
R23 and R24 can be about half the value of R11 and R12 and still
maintaining good thermal tracking. In this way we can increase the gain
in the common emitter amplifiers Q21 and Q22 (and increase the open
loop gain). The gain of this stages are about R19(loaded)/R23 (and
R20(loaded)/R24), where the collector loads of Q21 and Q22 are about 1
kohm, giving a gain of about 22 dB.
The 12 V references set by the zener diodes D5 and D6 and the values
of the fall resistors R15 and R16 should be chosen in proportion to V+
and V-, allowing enough current to flow in both the zeners, the
transistors Q13 and Q14 and the resistors R17 and R18. The voltages V+
and V- should be at least about +20/-20 V for 20 W (into 8 ohms) class
A operation and about +23/-23 V for
25 W class A operation. For lower supply voltages, R15 and R16 should
be reduced. About 390 ohm is appropriate for +15/-15 V.
In the figure below is shown a transformer (T1) and rectifier (D1) to supply a filter bank. A fuse (F1) on the primary side is mandatory. A mains switch is nomally in series with this fuse. In the prototype amplifier, a transformer for each channel was used. A common transformer for two channels is of course an option. It is however advisable to use separate rectifier and capacitor bank for the two channels.
A capacitor bank is placed on a separate PCB, as shown below.
Instead of using only two capacitors, this power supply is of the type
CRC, giving a better ripple rejection. The size of the resistors can be
increased for better ripple rejection, but the power handling must be
taken into account. It is advisable to use a separate power bank for
each channel. The capacitor values were 33000 uF in the 10 W prototype.
On the PCB, the diameter of the capacitors is limited to about 25 mm,
thus also restricting the maximum capacitance value.
One may also argue for a common supply for the amplifier. Since this is a class A amplifier with global feedback, a common supply could be tempting, but it has not been tried for this amplifier.
The prototype was an 2x10 W amplifier, where the power supply was
about +15/-15 V. The distortion at 10 W was about 0.05 %. This implies
that a 25 W amplifier could deliver more than 15 W with this distortion
Less than about 2 V Peak input voltage is required for full output power for 25 W RMS. This should be sufficient for the most modern signal sources without being forced to use a preamplifier. If higher gain is wanted, increase R25 and R26. Please note that the output impedance and distortion increases for the higher gain.
The BOM is shown below. This amplifier is well suited for tweaking. However, for all replacements, be sure that the size of the components and pinning is correct, especially for other transistor types, when mounting the components on the circuit boards.
With the exception of the power resistors, 0.6 W metal film resistors with 1 % tolerance was used. The power resistors are wirewound 3 W with 1% tolerance. Other types are of cause possible.
The driver transistors Q29/Q30 are fast devices with a very low Cob. However, the difference to the traditional pair BD139/BD140 was insignificant, when tried. The output transistors used for Q34/Q35, are the couple KSC5200/KSA1943 from OnSemi. They come in a plastic housing, and is mounted directly on the large heatsink. For a 25 W amplifier, the power dissipation for these transistors is about 30 W per transistor, so do not underestimate the cooling demand.R1 680