Mimir - A Class A Power Amplifier

Introduction

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.


Input stage

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.
1

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 resistor.


Output stage

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.

2

The drivers Q29 and Q30 are medium power transistors, and can be selected to be fast devices. For a high power amplifier, it might be necessary to fit a small heatsink on each of these two transistors. The power transistors Q34 and Q35 should be mounted on a large heatsink.

Complete amplifier draft

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.

3

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 gain of one Sziklai pair is about 24 dB for 8 ohm load, so the total gain for these pairs are about 30 dB due to the doubling at the output. The total open loop gain is therefore about 50 dB (due to some loss in the input stage).

The feedback path of this amplifier is divided into two: via the resistors R25 and R26. Thus the closed loop gain of this amplifier is approximately R25/R23 (or R26/R24). With the values shown, the closed loop gain thus is about 20 dB.

As can be seen from the schematic, it is also added two capacitors, C27 and C28. These set the open loop and closed loop bandwidth and ensures the amplifier to be absolutely stable. The phase margin is more than 80 degrees. The open loop bandwidth is about 60 kHz and the closed loop bandwidth is about 2.5 MHz. The high open loop frequency range ensures a nearly equal amount of distortion and the same output impedance over the whole audible range. The output impedance is nearly resistive at about 1/4 ohm througout the whole audio range. It is not advisable to put capacitors in parallel with R25 and R26 to limit the bandwidth. Since the output stage runs in common emitter, this amplifier has no problem with large capacitive loads either.

Power supply


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.

4

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.

5

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.


Complete Amplifier


The outputs from the transformers are carried to the rectifiers and to chassis ground as shown in the power supply schematics. From the rectifiers, make connections for the positive and negative voltages to the power supply PCBs. The layout of these are shown below.
7 

From the ground on the power supply PCBs, also make a connection to the common ground on the chassis. The phono socket ground terminal is connected to chassis (near the input) and the (ground) shield of the phono cable is connected to the circuit board, at the point marked GND (Ground). The inner conductor of the phono cable is connected to the amplifier board point marked with 'IN'. From the loudspeaker output the two conductors are twisted and fastened to the circuit board in the two points marked with OUT and EARTH. The last one is connected to the minus conductor. The loudspeaker minus output is connected to chassis. From the power supply PCBs, make connections to the amplifier PCBs. All connections should be as short as possible. If some sort of instability or noise should occur, the probability is high that the reason is bad wiring (e.g. earth loops).

The layout of the amplifier board is shown below.
6

It is recommended to use a variable mains transformer the first time the amplifier is started up. When the power supply voltage is increased, adjust the bias and offset voltage by means of the potentiometers RV7 and RV8. If possible, look at the output with an oscilloscope, there should not be anything but noise here if everything is OK. When the temperature is increasing, it is necessary to re-adjust both offset voltage and quiescent current. The offset voltage at the output varies, but should not exceed 100 mV.

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 figure.

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.


BOM

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
R2 33k
R9, R10 10k
R11, R12 120
R15, R16 680 (390 for 10-20 W output power)
R17, R18 12k
R19, R20 1.2k
R23, R24 68
R25, R26 680
R31 220
R32, R33 0.5
R38 10

RV7, RV8 5k Potentiometer Bourns 3296W

C3, C4 100n Axial Film L12.0mm D6.5mm P15.0mm
C27, C28 68p NP0/C0G P5.0mm
C36, C37 10u Radial Film L18.0mm W9.0mm P15.0mm

D5, D6 12V 500mW Zener DO-35

Q13, Q22 KSA992 TO-92
Q14, Q21 KSC1845 TO-92

Q29 KSA1381 TO-126
Q30 KSC3503 TO-126
Q34 KSC5200 TO-247
Q35 KSA1943 TO-247

J1 Screw Terminal 01x02
J2-J5 Solder Wire Pad

Heatsinks

 
[Home]

Copyrightę2020
Knut Harald Nygaard