60W Korrekti

60W Ultra-Simple Audio Power Amplifier


Highlighted features

  • Simple but powerful 60W amplifier
  • Uses easily available (junkyard) components
  • Overload protection
  • No turn on-thump (no need for output relay)
  • Low parts count, low-cost BOM
  • Low hum and noise


  • Input impedance: >30kohm
  • Bandwidth @40W: 10Hz-30kHz (-1.5dB)
  • SNR: >94dB
  • Gain: +34dB
  • Damping factor: >50
  • THD@1kHz: <0.05% @ 40W into 6 ohm

Fig.1 Complete and compact 4-channel version built into an aluminum casted box.

Main Specifications

This audio power amplifier is simple and robust. The used components give a good compromise between performance and cost. The overall performance is suitable for serious home use. All used components are simple garden variety types that can probably be found in your component junkyard. The amp is dimensioned for 8-ohm loads, in which it can feed 40W continuous power with a distortion level of less than 0.05%. 6-ohm loads are also OK. This amp is not suitable for loads less than 6 ohms. Original instructions are in Finnish. I have translated here the most important parts.

Design background
This design has many similarities with the old and popular Texan amplifier from 1972. The output stage consists of a clever Darlington stage with voltage gain. It uses complementary, feedback or "compound" - or Sziklai pair - Darlington transistors. The output transistors'' Q6 and Q7 Vbe voltage (and its variation due to temperature changes) have a minuscule effect on the idling current. The control of the output stage idle current is fairly simple since temperature feedback from the power transistor heatsink is not needed. Electronic short term overload protection is also added, and the maximum output current is reduced by limiting the maximum base current to the power transistors.

The Texan design had a fuse at the output. This caused increased distortion at lower frequencies. Electronic protection circuits also add some distortion but have some advantages over fuses.  The overload protection in Texan was not fast enough to protect the output transistors during brief overloads. Let's look at the schematic (TULO means input):

Fig.2 Complete schematic of one channel. 

Output Stage

The output stage voltage gain is determined by the ratio of R17 to R13 (and R16 to R12). The overall resistance at the driver transistor (Q2, Q4) emitters must be fairly low since transistors Q2 and Q4 supplies the base current to the output transistors (Q6, Q7). Excess resistance would increase distortion, too small resistance would increase the power dissipation in the gain setting resistors. The shown values are a good compromise. R14 and R15 limit the maximum base current to the output transistors during clipping, and also keeps the current and power dissipation in Q2/Q4 at a safe level when the overcurrent protection transistors (Q5, Q3) goes into action.

Since the output stage has a voltage gain of about 11, the op-amp output only needs to swing a few volts for a full output swing of ±35V. The design can thus tolerate a relatively slow op-amp. The current gain in the output stage is also relatively high  (about 2000) due to the compound Darlington transistors. The op-amp "sees" a relatively easy load to drive.

The output RC (Zobel) network (C7, R20) is essential for stability, guaranteeing proper loading of the output at high frequencies. The traditional output choke is not required since this design is stable with moderate capacitive loads. However, do NOT drive electrostatic speakers with this amp, this will certainly cause oscillations and fast destruction.

Biasing and quiescent current

The idle current in the power transistors is determined by the dc-voltage between the Q2 and Q4 bases. This voltage controls how much current is fed at idle to the power transistor bases. This bias voltage is controlled by Q1 that is mounted in thermal contact with Q4. The voltage drop across Q1 is set by R9. R5 and R6 generate a 5mA current to properly bias Q1 to work as a shunt regulator. The opamp receives also well regulated and stable voltage from the two Zener-diode shunt regulators. Since the voltage to the bias circuitry is taken from these same regulated and smoothed op-amp power supply lines, the output stage is inherently very quiet and totally hum-free. Due to the symmetry and lack of bigger (electrolytic) capacitances, the dc-operation point is immediately correct when the power supply lines are still rising. Therefore there is no output thump (voltage spike) during turn-on, and there is no need for a speaker disconnecting relay.

Closing The Loop

The feedback loop is closed by the (in the year 1978) high quality bipolar low noise externally compensated uA748 op-amp. The op-amp's task is to fix the overall DC-balance and close the feedback loop by suitable AC-feedback. The open-loop linearity of the output stage is not very good (especially with reactive loads), neither is the open-loop output impedance something to write home about in this current driven design. The performance relies very much on the op-amp. TL071 can be used as a replacement. The ±18V supply lines are too high for TL071 so the Zeners must be replaced with 15V types (1N4744A). 

R4 ensures stability by isolating the op-amp output from parasitic capacitances. The input and feedback chains are more or less traditional stuff. For best performance, it is good to use tantalum capacitors in locations C1 and C2. However, they can be substituted by similar valued and rated electrolytic caps. Tantalum capacitors work well with zero or even a small negative voltage. The correct (prudent) capacitor polarity is determined by the op-amps input leakage current direction.

Construction

Start with the assembly of the resistors, then capacitors, and last the semiconductors. Fig. 7 shows the component placement. Be very careful with the polarity of the polarized components. Q1 and Q4 should be mounted back to back and in contact with each other. A small drop of thermal compound is recommended, alternatively, a drop of Cyanoacrylatic glue works also well. Glue the transistors back together before soldering them.

Remember to assemble the one and only jumper wire on this single-sided PCB, if you use this layout as a basis for your design. The jumper is close to where the power supply ground wire enters the PCB. Leaving out the jumper will kill the transistors. Make a final check for shorts or bad soldering and proper component placement.

Output transistor assembly

The output power transistors can be assembled directly to the PCB if a suitable and at least 6mm thick aluminum profile is mounted between them and the PCB. This aluminum profile needs to be mechanically and thermally connected to a bigger heatsink.

Either TO-3 (metal) or TO-247 (epoxy) encapsulated transistors can be used with this PCB design. The power transistors collectors must be isolated from the heatsink by mica washers or low thermal coefficient insulators (SilPad). See Fig3333 as an example. Don't forget to use a thermal compound.

Alternatively, you can assembly the transistors on a separate heatsink and using flexible wires (AWG16 or thicker).  The wires should not be longer than 30cm (one foot). The insulation requirement is the same.

Independent of the mounting, make sure that the power transistor collectors are in contact with the PCB's speaker output copper traces. Insulating washers on the mounting screws cannot be used with TO-3 metal cased transistors, the mounting screw needs to contact both the transistor case  - AND the PCB. But not the heatsink. It sounds complicated but is not. Just be careful.

The schematic shows that the collectors of the transistors are connected together. Mounting the transistors directly to the heat sink - without insulators - is a tempting idea because they are allowed to be in electrical contact. This would technically be possible - assuming you then electrically float the heatsink from the rest of the system. However, since this amplifier is non-inverting (as amps usually are) the risk of inducing capacitively positive feedback into the input is overwhelming. A floating heatsink could not use the chassis of the amp for additional cooling since it must be isolated. I strongly advice to always isolate the power transistors from the heatsink, and then properly groud the heatsink to the power supply ground.

Last but not least, make sure by using an ohm-meter that the isolation and the power transistor attachment are properly done.
Power supply and wiring

The amplifier needs a power supply of maximum ±35VDC. Fig.5 is an example of a simple and working PSU. An unregulated voltage is OK as long as there is enough filtering capacitance in the supply lines. At least 4700uF for a stereo amplifier. 100VA transformer power is recommended per channel. Fuses in the power lines after the filter capacitors and before the amplifier are required for safety. You can use a rectifying bride instead of the four MR501 diodes. At least a bridge with 100V and 6A rating is needed.

The wiring is shown in Fig 6. Use flexible and shortest possible wires. Keep the input shielded coax away from the speaker wires. The heatsink and metallic enclosure must be grounded to the power supply ground.

Power up and idle current adjustment

  1. Double-check that you have installed the one and only wire-jumper on the PCB.
  2. Double-check once again the correctness of all component placements and wired connections.
  3. Remove the secondary fuses for all amplifiers. Check the power supply voltages, they should be symmetrical and below ±40VDC. 
  4. Do not yet connect an input or the speaker.
  5. Turn off the power supply and wait until the capacitors have discharged. You can speed things up by loading the capacitors with a suitable resistor. Do NOT short circuit them.
  6. Put the fuses in place for the amplifier you will next test (repeat from step 5 forward for each channel - if your system is stereo or multichannel).
  7. Turn the trimmer R9 fully counterclockwise so that its resistance is at its minimum. If unsure - measure the resistance across the trimmer (or Q1 base-collector), it should be below 10 ohms.
  8. Connect a DC-voltage meter on the speaker output.
  9. Turn on the power supply and check that the DC-voltage at the speaker output is less than ±25mV. If the fuse(s) blow(s) or if this voltage is greater - then turn off the power - there is somewhere an error or mistake. Find and fix the problem before continuing.
  10. Next, the idle current must be set due to component tolerances.
  11. Still, without an input signal or speaker connected, measure the voltage across R18 (or R19) with a sensitive DC-voltage meter (must be able to resolve 1mV or better).
  12. Adjust the trimmer R9 slowly until the reading on the meter is as close to 5mV as possible. 
  13. Turn off the power, connect the input and speaker and enjoy your new amp.
  14. It is advisable to recheck and readjust the idle current after running the amplifier warm (idling or in use for at least 15...30 minutes). Repeat steps 10 to 13 to do this - for each amplifier.



Part

Value

Note

Op-Amp

uA748

DIP-8

Q1, Q2, Q3

BC337-25

TO-92

Q4, Q5

BC327-15

TO-92

Q6

MJ2955 or 2N2955

TO-3 / TO-247

Q7

MJ3055 or 2N3055

TO-3 / TO-247

Z1, Z2

1N4746A

Zener 18V/1W

R1, R2

33kΩ/5%

1/4W or 1/2W

R3

680Ω/5%

1/4W or 1/2W

R4, R12, R13

27Ω/5%

1/2W

R5, R6

3.3kΩ/5%

1/2W

R7

470Ω/5%

1/4W or 1/2W

R8

390Ω/5%

1/4W or 1/2W

R9

2.2kΩ/5%

1-turn trimmer

R14, R15, R20

100Ω/5%

1/2W

R10, R11

820Ω/5%

1W

R16, R17

270Ω/5%

1W

R18, R19

150mΩ/5%

4W (low ind.)

C1

1µF/35V

Tantalum, radial

C2

100µF/6V

Tantalum, radial

C3, C4

10pF/50V

Ceramic

C5, C6, C7

100nF/250V

Polyester, radial

Table.1 Parts list for one channel. 

Fig.3 The original construction article, as published in 1978 in a Finnish electronics magazine. The article targeted absolute beginners in the art of electronics. Click the upper right corner to open.

Fig.4 Single-sided PCB layout seen from below (solder side). Note that the PCB accepts both TO-3 and TO-247 encapsulated power transistors. This picture in not in scale and is shown here for reference only, if you plan to design your own PCB.

Fig.5 Suggestion for the power supply. This example is good for one channel. For stereo, you need to double the transformer power rating,, double the filer capacitors capacitance (of double their count) and use separate fuse pairs for each channel.

Fig.6 Wiring of the power transistors (if located remotedly), speaker output, power supply connections, and the input connection.

Parameter

Value

Input resistance

>30kΩ

Bandwith

 

10Hz...30KHz @ 40w
(-1.5dB)

Signal to Noise Ratio

>93dB

Voltage gain

+34dB

Damping factor

>50

THD 1kHz

0.09% @ 100mW

0.06% @ 10W

0.05% @ 40W

0.12% @ 60W

Measuerement conditions

1kHz into 6 ohm resistive load.

Table.2 Some basic but real-world measured parameters. 

Final words

I made some crude measurements on the amplifier in 1978, they are gathered in Table 2. If you plan to design your own PCB or build this amplifier from scratch, I strongly recommend the following modifications:

  1. Place the ±35V fuses directly on the amplifier board. This makes the wiring easier and enables individual fuses for each amp. It also enables the following change.
  2. Connect the R10 and R11 resistor power supply ends directly to the power supply lines - before the fuses. In other words, R10 and R11 should always power the Zener diodes even if the fuses are blown or missing. This totally safe to do. The benefit of this modification is huge. The modification will keep the output voltage close to the ground, in the case of only one of the fuses has blown. The original design has the flaw that one blown fuse will produce a dc-voltage on the speaker terminals that could blow your elements. In this sense the original design does not differ too much from other amplifiers, many amps suffer from the same problem

  3. Add a 100nF/250VAC polyester capacitor across the hot ends of the power transformer secondary wires (between the rectifying bride AC input terminals). This prevents the diodes to generate high-frequency noise when turning off every AC-cycle.


Happy tinkering!

.

Fig.7 Component placement seen from the component side. Apologizes for the poor image quality.

Fig.8 Stereo amplifier with the transistors mounted straight to the PCB using an aluminum bracket heatsink. Never cross the input signals (must be shielded cable) with the speaker output wiring. Keep these a separate as possible.

Fig.2 Stereo amplifier with the transistors mounted straight to the PCB using an aluminum bracket heatsink. Here the PCB's are slightly shortened to make them fit.

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