25-30 W Class A Power Amplifier (Mimir v2)

Introduction

This amplifier is inspired by, and is also somewhat similar to, the relatively well-known 20 W Hiraga amplifier with bipolar transistors in the input stage. Compared to the original Hiraga amplifier, this power amplifier reduces distortion and output resistance. The output stage is, however, relatively similar to the Hiraga amplifier, while the output power has been increased somewhat. For lower output power, the original Mimir amplifier is recommended

The input stage

The input stage is shown in the figure below. It is completely symmetrical and is supplied with the voltages VR+ (+22 V) and VR- (-22 V) from the zener diodes D5 and D6 when the voltages V+ and V- is approximately +25V/-25 V. The quiescent currents of the two transistors Q13 and Q14 are trimmed to approximately 1 mA using the potentiometers RV7 and RV8. However, these potentiometers will also trim the quiescent current in the output transistors, as we will see later. The two transistors are connected as emitter followers and thus have a voltage gain of just under 1 time (0 dB). The transistors are complementary types with fairly good specifications at 1 mA.

The resistors R1 + R2 determine the input impedance. R1 has a relatively low value while the value of R2 should not be too high since there is a small quiescent current at the input (equal to the difference between the base currents of Q13 and Q14) passing through this resistor.

The intermediate stage

The intermediate stage and the entry stage are shown in the figure below. Transistors Q21 and Q22 are connected as a common-emitter stage. The quiescent current in transistors Q21 and Q22 is about 1 mA as for Q13 and Q14 with the values shown. This is despite the emitter resistances of Q21 and Q22 being much smaller than the value of 220 ohms for R11 and R12. This is due to the fact that we use R17 and R18 to supply the current that is needed. Note that the connection between Q13 and Q21 as well as between Q14 and Q22 is relatively temperature stable since the base-emitter voltage in the complementary pairs will follow each other quite well since the transistors have the same quiescent current. The resistors R25 and R26 are connected to the amplifier's output and thus ensure the voltage feedback. This should then approximately give a (closed-loop) gain equal to:

A_V = R25/R23 = R26/R24 = 1200/56 = 20 times (26 dB)

The gain can then be reduced by reducing R25 and R26.

The gain in each common-emitter stage is approximately:

A₁ = R19/(r+R23) where r equals intrinsic emitter resistance given as the ratio between thermal voltage (25 mV at room temperature) and collector current (here 1 mA).

The gain in each common-emitter stage will then be:

A₁ = 1500/(25+56)=18 times (25 dB)

R19 and R20 are actually loaded with the output stage, consequently the gain is reduced from this value.

The output stage

The output stage is shown below. Transistors Q29/Q34 in the upper half and Q30/Q35 in the lower half form Sziklai pairs (complementary Darlington pairs). Since the output signal is taken from the emitter of Q34/Q35, these Sziklai pairs will constitute common emitter amplifiers. This is very unusual, since most audio power amplifiers have outputs that run in common collector (they are emitter followers).

The gain for each Sziklai pair is approximately given as the ratio:

A₂ = R_L/R32, where R_L is the load on the output (actually the speaker). This gain is then given as:

A₂ = 8/0.5 = 16 times (24 dB) for a purely resistive load of 8 ohms. Since the emitters of Q34 and Q35 are summed at the output, the gain is doubled, here equal to 32 times (30 dB).

The linearity of Sziklai pairs is very high, and the resistor R31 gives a further moderate increase in the quiescent current in the drivers Q29 and Q30 with resulting lower distortion in them. Both the drivers Q29/Q30 and the output transistors Q34/Q35 are mutually complementary pairs. The drivers are smaller power transistors and can be selected as fast types. These transistors can benefit from their own small cooling fins. The power transistors Q34 and Q35 must be mounted on a large cooling fin. The drivers must not be mounted on the same cooling fin as this will result in greater temperature drift.

Complete schematic

The complete schematic of the amplifier is shown in the figure below. Please note the capacitors C27 and C28. They stabilize the amplifier and provide sufficient phase and gain margin.

If we want a 25 W into 8 ohms class A amplifier , the RMS voltage must be:

V=√ P·R =√ 25·8 =14.1 V

This corresponds to a peak value of:

V_p = 14.1√ 2 =20.0 V

For a load of R_L = 8 ohms this corresponds to a peak value for the current of:

I_p = V_p/R_L = 20.0/8 = 2.50 A

Since the output operates in Push-Pull, the quiescent current of Q35/Q36 must be at least:

I_C = I_p/2 =2.50/2 = 1.25 A

For a 30 W into 8 ohms class A amplifier , the RMS voltage is:

V=√ P·R =√ 30·8 =15.5 V

This corresponds to a peak value of:

V_p =15.5√ 2 =21.9 V

For a load of R_L = 8 ohm this corresponds to a peak value for the current of:

I_p = V_p/R_L = 21.9/8 = 2.74 A

Since the output operates in Push-Pull, the quiescent current of Q35/Q36 must be at least:

I_C = I_p/2 =2.74/2 = 1.37 A

With a supply voltage of 25V, it is possible to get 30W out of this amplifier before clipping, so an open-circuit current of 1.4A in the warmed-up state may be a reasonable target. From a cold amplifier to working temperature, the bias current rises by approx. 0.2-0.3 A. This is mainly due to the base-emitter voltage in Q29/Q30 falling with temperature. Also note that the temperature in the output transistors Q34/Q35 in itself does not affect the bias current. This is unlike most other audio power amplifiers.

The quiescent current is adjusted with the potentiometers RV7 and RV8. Offset on the output is thus trimmed using these potentiometers.

With the feedback resistors R25 and R26 equal to 1.1 kohm, the closed-loop gain is equal to 20 times (26 dB). If this is too high, it is possible on the circuit board itself to insert a resistor between terminals TP1 and TP2 and between terminals TP3 and TP4. If, for example, these additional resistors are also chosen equal to 1.1 kohm, the amplification is reduced to 10 times (20 dB).

Q21 and Q22 were found to have a voltage gain from base to collector of about 18 times (25 dB). Since R19 and R20 are loaded by the input resistance of the Sziklai pairs Q29/Q34 and Q30/Q35, respectively, the voltage gain in Q21 and Q22 becomes approximately equal to:

A₁ = 15 times (23 dB).

The gain for each Sziklai pair for a purely resistive load of 8 ohms was given as:

A₂ = 16 times (24 dB).

The total open-loop gain is then approximately given as:

A₀ = 15∙2∙16 times = 480 times (54 dB).

With a closed-loop gain of 20 dB or 26 dB, the feedback is then 34 and 28 dB respectively.

Performance

The prototype was built with a closed loop gain of 20 times (26 dB). With a load resistance of 8 ohms and an idle current of 1.4 A, an open-loop amplification of approx. 52 dB is obtained. This is a value from the simulations. The fact that the open-loop gain is smaller than the calculated values is because the assumed gain in Q21/Q22 and in the Sziklai pair is optimistic. In addition, the voltage gain in emitter followers Q13/Q14 will be less than 1. The amplifier's open-loop bandwidth according to the simulator is in excess of 50 kHz with capacitors C28 and C29 of 68 pF. This gives a phase margin of about 85 degrees. A Slew Rate of about 30 V/us with the selected fast drivers has been achieved. The bandwidth of the amplifier is approximately 1.4 MHz. If C28 and C29 are not fitted or have values that are too low, stability problems must be expected. The amplifier can drive capacitive loads without problems, since such loads automatically lead to reduced open-loop bandwidth. The output resistance is about 0.4 ohms. The distortion at 1 kHz and 10 kHz is about 0.08% at half output power (15 W), and it is dominated by 2nd and 3rd harmonics with rapidly falling higher harmonics. With a supply voltage of +/- 25 V, the amplifier cuts out at approx. 22.5 V in peak value.

A closed loop gain of 10 times (20 dB) is achieved by connecting resistors of 1.1 kohm in parallel with R25 and R26 (TP1-2 and TP3-4), optionally choosing R25 and R26 equal to 560 ohms. With this amplification, a phase margin of approximately 77 degrees is obtained. If a larger phase margin is desired, capacitors C28 and C29 can be increased, for example to 100 pF. The Slew Rate will then be reduced somewhat, but the amplifier will still be considered fast. The bandwidth of the amplifier with a closed-loop gain of 20 dB is about 3 MHz. The output resistance is about 0.2 ohms while the distortion at 1 kHz and 10 kHz is about 0.04% at half output power (15 W).

Printed Circuit Board (PCB)

The layout of the amplifier board is shown in the figure below. This has the dimensions 99x52 mm. The components in the diagram above are all located on the same circuit board. J1-J6 are the connectors on the board. The values shown for the components in the diagram are suitable for a 25-30 W amplifier. The driver transistors Q29 and Q30 can be mounted with their own heat sink, a thermal resistance of 35 K/W or less can be considered suitable. The power transistors Q34 and Q35 must be mounted on a large heat sink, a thermal resistance of 0.4 K/W or less may be considered suitable. Resistor R38 distinguishes between signal ground (GND) and power ground (Earth). This resistance value will typically lie in the range 4-10 ohms.

With 3D in KiCad, the printed circuit board looks like this:

In the prototype, the power transistors are attached horizontally to the large cooling fin. The picture below shows this before soldering.

Power supply

The 22 V reference voltages, determined by the zener diodes D5 and D6, and the values of the dropping resistors R15 and R16 must be chosen in relation to the voltage supply so that there is enough current to flow through both zener diodes, transistors Q13 and Q14 and resistors R17 and R18. The voltage supply should be at least +24/-24 V for 25 W (into 8 ohms) class A operation.

As an example, the power supply can consist of, among other things, a 2x18 V 500 VA transformer (T1), common to both channels, see the figure below. Separate rectifiers (D1 and D2) can be used for positive and negative voltage. 47000 μF capacitors and 0.47 ohm power resistors for the filtering can be considered reasonable. A fuse (F1) on the primary side is a requirement. A circuit breaker is usually in series with this fuse.

In the prototype, the CRC components 1-6 are placed on a separate circuit board, but this is of course not a requirement; see figure below. The size of the resistors can be increased for better ripple suppression, but the power dissipation must be taken into account. This also results in a reduced maximum output power for the amplifier.

With 3D in KiCad, the power supply board looks like this::

Build-up

The outputs from the transformer are fed to the rectifiers and on to the separate board with the CRC filtering (shown above). A connection is made from ground on the power supply board to common ground on the chassis. The ground of the signal input socket is connected to the screen of the phono cable and then connected to the amplifier board, to the point marked GND. The hot end of the phono cable is connected to the amplifier board marked IN. From the speaker output, the two wires are twisted and led to the amplifier board to the points marked OUT and GND. The latter is connected to the minus conductor. From the power supply board, connections are made for the power supply to the amplifier boards. All connections should be as short as possible. If any kind of instability, noise or hum should occur, it is highly likely that the cause is poor wiring (leading to, for example, earth loops).

It is recommended to use a variable mains transformer when starting the amplifier for the first time. As the voltage supply increases, the bias and offset is adjusted by using the potentiometers RV7 and RV8. If possible use an oscilloscope to look at the output, there should be nothing but noise here if everything is fine. As the temperature increases, it may be necessary to readjust the bias and offset.

In excess of 1 V peak value is required for full output power with a voltage gain of 20 times. Even with a voltage gain of 10 times, the sensitivity of just over 2 V peak will be sufficient for most modern signal sources.

Bill of Materials (BOM) is shown below. The amplifier is well suited for personal adaptations. For replacements, remember to take into account changed physical dimensions and pin configurations, especially for the use of other transistor types when mounting on the circuit boards.

Bill of Material for one PCB

Metal film resistors with 1% tolerance have been used. The power resistors are 3 W while the other resistors are 0.6 W. Other types are of course also possible as long as they fit in the card. The driver transistors Q 29 and Q30 are fast types with very low Cob (collector-base capacitance). The pair 2SC5200/2SA1943 from Toshiba was used for the output transistors Q34/Q35. These come in a plastic housing and were mounted directly on a large cooling fin. For a 30 W amplifier, the power dissipation for each of these transistors is about 35 W, so that one should not underestimate the cooling requirement.

R1 680 ohm
R2 33 kohm
R9, R10 18 kohm
R11, R12 220 ohm
R15, R16 220 ohm
R17, R18 7.5 kohm
R19, R20 1.5 kohm
R23, R24 56 ohm
R25, R26 1.1 kohm (see text)
R31 220 ohm
R32, R33 0.5 ohm 3 W

RV7, RV8 10 kohm Potentiometer Bourns 3296W

C3, C4 1 uF 63 V L 7.2 mm W 5.0 mm P 5.0 mm
C27, C28 68pF NP0/C0G P 5.0mm
C36, C37 10u Radial Film L18.0mm W9.0mm P15.0mm

D5, D6 22V 500mW Zener DO-35

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

Q29 KSA1381 TO-126
Q30 KSC3503 TO-126
Q34 2SC5200 TO-264
Q35 2SA1943 TO-264

J1 Screw terminal 01x02
J2-J6 Solder terminals
TP1-4 Soldering tower

Heatsink (2 pcs) Fischer SK95 or equivalent
Heatsink 0.3 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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