Mimir - A Class A Power Amplifier (Updated)

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 lower output power and a twist (or two) to reduce distortion and output resistance. If a higher output power is desired, Mimir v.2 can be recommended.

Input stage

The initial input stage (before a slight modification) is shown in the figure below. It is completely symmetric, powered from the voltages VPOS and VNEG. Two zener diodes (D5/D6) are used to give stable bias currents to the two emitter followers Q12/Q13. The offset adjustment of this amplifier is made up by the potentiometer RV7. The quiescent current for Q12/Q13 is set at 1 mA. When R10 = R22 and R11 = R23, the quiescent current for Q21/Q22 will be the same. Q21/Q22 are common emitter amplifiers.

The resistor R1 determines the input impedance. R2, 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 R1 should not be too high, since there is a small input bias current (the difference between base current of Q12 and Q13) flowing through this resistor.

Output stage

The output stage, together with the common emitter stages (Q20/Q21), is shown below. The transistors Q28/Q32 in the upper leg and Q29/Q33 in the lower leg form Sziklai pairs - complementary feedback pairs. Since the output signal is taken from emitter of Q32/Q335, these pairs in fact act as common emitter amplifiers, where the gain for each pair is set by the relation RL/R30 = RL/R31 = 8/0.5 =16 times, where RL is the load at the output - in reality the loudspeaker. Since the emitters of Q32 and Q33 are summed on the output, the gain is doubled, here 32 times (30 dB).The linearity of the Sziklai pair is very high.

The drivers Q28 and Q29 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 Q32 and Q33 should be mounted on a large heatsink.

Complete amplifier schematic, no 1

The amplifier without the power supply is shown in the figure below. The components are all placed on the same PCB. J1-J6 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 R0. 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 W amplifier with a power supply with +/- 15-16 V. The feedback from the output is fed via the two resistors R24 and R25. The potentiometer RV7 is used to nulling 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 10 W class A amplifier with a nominal load of 8 ohms, the bias current in Q32/Q33 should be at least 0.8 A. This follows from the fact that 12.6 V peak (8.9 V RMS) is necessary for 10 W into 8 ohms. The current is then 12.6/8 = 1.6 A peak; the push-pull design makes it possible to halve this current. The quiescent current of Q28 and Q29 is approximately 10 mA (for 10 W output power) and is determined by the base current in Q32 and Q33. The bias current for Q12/Q13 and Q20/Q21 is as mentioned about 1 mA.

The gain for the common-emitter-stage Q20 (and Q21) is about R18L/(r+R22//R24), where r is intrinsic emitter resistance, given as the relation between thermal voltage (25 mV at room temperature) and the collector current (here 1 mA). R19L is R19 loaded with the output stage. If the current gain for the Sziklai pair is about 10000, R19L is 1.2 kohm in parallel with 5 kohm, i.e. 1 kohm. With r = 25 ohm, the gain in each common emitter stage is:

1000/(25+200//2k) = 4.8 times (13.7 dB).

With a gain in the output stage of 30 dB, the total open loop gain is therefore about 43.7 dB.

The feedback path of this amplifier is divided into two: via the resistors R24 and R25. Thus the closed loop gain of this amplifier is approximately R24/R22 (or R25/R23). 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 about 90 degrees. The open loop bandwidth is about 60 kHz and the closed loop bandwidth is about 1 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 0.6 ohm througout the whole audio range. It is not advisable to put capacitors in parallel with R24 and R25 to limit the bandwidth. Since the output stage runs in common emitter, this amplifier has no problem with large capacitive loads either.

Note that when the pairs Q12/Q20 and Q13/Q21 are complementary with approximately the same base-emitter voltage, they will follow each other to maintain a quiescent current of about 1 mA if the temperature changes. This means that the quiescent current in the power transistors Q32/Q33 will increase only about 0.2 A even with a fairly large temperature increase. This change will mainly come from the reduced base-emitter voltage for Q28/Q29 at the temperature increase.

Complete amplifier schematic, no 2

The second version of the amplifier is shown in the figure below. If one compares this schematic with the first one, one can see that two extra resistors have been added: R16 and R17. These increase the current through R22 and R23 by just over 2 mA. This means that R22 and R23 can be less than half of the value of R10 and R11, while maintaining good thermal tracking. In this way, we can increase the gain in the common emitter amplifiers Q20 and Q21 (and increase the open-loop gain). With the same assumptions as before, the gain in each common emitter stage will therefore be:

1000/(25+68//680) = 11.5 times (21.2 dB).

With a gain in the output stage of 30 dB, the total open loop gain is therefore about 51.2 dB. The phase margin is more than 80 degrees. The open loop bandwidth is about 60 kHz as before while and the closed loop bandwidth now is about 2.5 MHz. The output impedance is nearly resistive at about 1/4 ohm througout the whole audio range. This is pretty much the only reason to introduce this modification. However, as a side effect, the distortion will also be less.

Power supply

The 13 V references set by the zener diodes D3 and D4 and the values of the fall resistors R14 and R15 should be chosen in proportion to VPOS and VNEG, allowing enough current to flow in both the zeners, the transistors Q12 and Q13 and the resistors R16 and R17. The voltages VPOS and VNEG should be at least about +15/-15 V for 10 W (into 8 ohms) class A operation. For R14 and R15, about 330 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 normally 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.

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.

Layout

The outputs from the transformers are connected 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.

From the ground on the power supply boards, a connection is made to a common ground on the chassis. The ground of the phono socket is connected to the screen of the phono cable and then connected to the amplifier board, to the point marked GND (Signal Ground). The hot end of the phono cable is connected to the amplifier board marked IN. From the speaker output, the two conductors are twisted and led to the amplifier board to the points marked OUT and EARTH. The latter is connected to the negative conductor. 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 in the figure below. Remember that R16 and R17 are not used for Version 1 and that resistors R22-R25 have different values for the two versions.

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 offset voltage by means of the potentiometer RV7. 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 may be necessary to re-adjust the offset voltage. The offset voltage at the output varies, but should not exceed 50 mV. The quiescent current in the output transistors is measured as the voltage across R30/R31. If the quiescent current is too high (relative to the capacity of the heat sinks), it is easiest to reduce the size of the resistors R10/R11. For Version 2, the size of the resistors R16/R17 can also be reduced. If the quiescent current is too low, it is easiest to reduce the size of the resistors R8/R9.

The prototype was an 2x10 W amplifier, no 2, where the power supply was about +15/-15 V with a quiescent current of about 1 A (suitable for class A down to 6 ohms). The distortion at 10 W was about 0.05 %.About 1.25 V peak value is required for full 10 W RMS output power. 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 2SC5200/2SA1943 from Toshiba. They come in a plastic housing, and is mounted directly on the large heatsink.

R0 10 ohm
R1 33 kohm
R2 680 ohm
R8 8.2 kohm
R9 12 kohm
R10, R11 180 ohm
R14, R15 330 ohm
R16, R17 6.2 kohm NB! Version 2 only
R18, R19 1.2 kohm
R22, R23 200 ohm for Version 1
R22, R23 68 ohm for Version 2
R24, R25 2 kohm for Version 1
R24, R25 680 ohm for Version 2
R30, R31 0.5 ohm 3 W

RV7, RV8 10 kohm Potentiometer Bourns 3386F or equivalent

C5, C6 100 nF Axial Film L12.0 mm D6.5 mm P15.0 mm
C26, C27 68 pF NP0/C0G P5.0 mm
C34, C35 10uF Radial Film L18.0 mm W9.0 mm P15.0 mm

D3, D4 13V 500mW Zener

Q12, Q21 KSA992
Q13, Q20 KSC1845

Q28 KSA1381/KTA1381
Q29 KSC3503/KTC3503
Q32 2SC5200
Q33 2SA1943

J1 Screw Terminal 01x02
J2-J6 Solder Wire Pad

Heatsink (2 pcs) Fischer SK95 or equivalent
Heatsink (2 pcs) 0,5 K/W or equivalent

Please notice:
This project description is for non-commercial use, only. Using this document on a site and charging a fee for download is vialation of non-commercial use and prone to demand for payment. So, for commercial use, contact me for agreement of terms. This page, however, can be downloaded for own use, and linked to, not violating term of non-commercial use.

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