10-25-30 W Class A Power Amplifier (ASK)

Introduction

This amplifier is a simplified version of the Mimir amplifiers (Mimir and Mimir v.2). It is inspired by, and is also somewhat similar to, the relatively well-known 20 W Hiraga amplifier. The output stage is relatively similar to the Hiraga amplifier, while the output power has been increased somewhat. It is also no problem to downscale this amplifier to a lower output power.

Complete schematic

The supply voltages VPOS and VNEG is approximately +25V/-25 V. The input stage is formed by two symmetrical emitter followers, made by the transistors Q9 and Q10, as shown in the figure below. The quiescent currents of these two transistors are trimmed to approximately 1 mA using the potentiometers RV5 and RV6. However, these potentiometers will also trim the quiescent current in the output transistors and the offset voltage on the output. Since the two transistors are connected as emitter followers, they 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 Q9 and Q10) passing through this resistor.

Transistors Q13 and Q14 are connected as a common-emitter stage and makes the intermediate stage. The quiescent current in these transistors is about 1 mA as for Q9 and Q10 with the values shown. Note that the connection between Q9 and Q13 as well as between Q10 and Q14 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 R17 and R18 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 = R17/R15 = R18/R16 = 1000/100 = 10 times (20 dB)

The gain in each common-emitter stage is approximately:

A₁ = R11/(r+R15) 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 approximately be:

A₁ = 1500/(25+100)=12 times (21 dB)

R11 and R12 are actually loaded with the output stage, consequently the gain is reduced from this value.

The output stage is made of transistors Q21/Q25 in the upper half and Q22/Q26 in the lower half. They form Sziklai pairs (complementary Darlington pairs). Since the output signal is taken from the emitter of Q25/Q26, 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/R23, 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 Q25 and Q26 are summed at the output, the gain is doubled, here equal to 32 times (30 dB).

The linearity of Sziklai pairs is very high. Both the drivers Q21/Q22 and the output transistors Q25/Q26 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 heatsink. The power transistors Q25 and Q26 must be mounted on a large heatsink. The drivers must not be mounted on the same heatsink as this will result in greater temperature drift.

Please note the capacitors C19 and C20. 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 Q25/Q26 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 Q25/Q26 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 30 W out of this amplifier before clipping, so a bias current of 1.4 A 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 A. This is mainly due to the base-emitter voltage in Q21/Q22 falling with temperature. Also note that the temperature in the output transistors Q25/Q26 in itself does not affect the bias current. This is unlike most other audio power amplifiers.

The quiescent current is adjusted with the potentiometers RV5 and RV6. Offset on the output is thus trimmed using these potentiometers.

Q13 and Q14 were found to have a voltage gain from base to collector of about 12 times (21 dB). Since R11 and R12 are loaded by the input resistance of the Sziklai pairs Q21/Q25 and Q22/Q26, the voltage gain in Q13 and Q14 becomes approximately equal to:

A₁ = 10 times (20 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 = 10∙2∙16 times = 320 times (50 dB).

With a closed-loop gain of 20 dB, the feedback is then 30 dB.

Simulation Results

The data in the following assumes a closed loop gain of 10 times (20 dB). With a load resistance of 8 ohms and an idle current of 1.4 A, an open-loop gain of approx. 48 dB is obtained. The fact that the open-loop gain is smaller than the calculated values, is because the assumed gain in Q13/Q14 and in the Sziklai pair is too optimistic. In addition, the voltage gain in the emitter followers Q9/Q10 will be less than 1. The amplifier's open-loop bandwidth according to the simulation is about 60 kHz with capacitors C19 and C20 of 68 pF. This gives a phase margin of about 85 degrees. A Slew Rate of about 25 V/us with the selected fast drivers has been achieved. The bandwidth of the amplifier is approximately 1.8 MHz. If C19 and C20 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 in excess of 0.3 ohms. The distortion at 1 kHz and 10 kHz is about 0.07 % 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 at approx. 22.5 V in peak value.

10 W version

The only thing to be done, referring to the schematic, is to reduce the value of the resistors R3 and R4 to 13 kohm and the resistors R11 and R12 to 1.3 kohm. If we want 10 W into 8 ohms operating in class A, the RMS voltage must be:

V=√ P·R =√ 10·8 = 8.9 V

This corresponds to a peak value of:

V_p = 8.9√ 2 = 12.6 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 = 12.6/8 = 1.60 A

Since the output operates in Push-Pull, the quiescent current of Q25/Q26 must be at least:

I_C = I_p/2 = 1.60/2 = 0.80 A

A supply voltage of approx. +/- 16 V will then be suitable. Then a 2x12 V toroid transformer in the power supply described below may be suitable.

Printed Circuit Board (PCB)

The layout of the amplifier board is shown in the figure below. This has the dimensions 100x52 mm. The components in the schematic 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 schematic are suitable for a 25-30 W amplifier. The driver transistors Q21 and Q22 can be mounted with their own heat sink, a thermal resistance of 35 K/W or less can be considered suitable. The power transistors Q25 and Q26 must be mounted on a large heat sink, a thermal resistance of 0.4 K/W or less may be considered suitable. Resistor R0 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:

And a nearly fully populated board looks like this:

Power supply

The Supply voltage 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 300-500 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. VPOS in the schematic refer to V+R and V+L in the figure below, while VNEG in the schematic refer to V-R and V-L in the figure below (R for Right channel and L for left channel).

The CRC components 1-6 may be placed on a separate circuit board, but this is of course not a requirement; see the 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 10 W version of this amplifier has been built. The outputs from the 2x12 V transformer were fed to the rectifiers and on to the separate power supply board with the CRC filtering. A connection was made from Earth on the power supply board to common earth 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 connected to the amplifier board to the points marked OUT and EARTH. The latter is connected to the minus conductor. From the power supply board, connections are made from 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. 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 RV5 and RV6. 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.

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 board.

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 on the PCB. The driver transistors Q21 and Q22 are fast types with very low Cob (collector-base capacitance). The pair 2SC5200/2SA1943 from Toshiba was used for the output transistors Q25/Q26. 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
R3, R4 22 kohm (13 kohm for 10 W, see text)
R7, R8 100 ohm
R11, R12 1.5 kohm (1.3 kohm for 10 W, see text)
R15, R16 100 ohm
R17, R18 1 kohm
R23, R24 0.5 ohm 3 W

RV5, RV6 10 kohm Potentiometer Bourns 3386F

C19, C20 68pF NP0/C0G P 5.0mm
C27, C28 10u Radial Film L 18.0mm W 9.0mm P 15.0mm

Q9, Q14 KSA992 TO-92
Q10, Q13 KSC1845 TO-92

Q21 KSA1381 TO-126
Q22 KSC3503 TO-126
Q25 2SC5200 TO-264
Q26 2SA1943 TO-264

J1 Screw terminal 01x02
J2-J6 Solder terminals

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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