80W HiFi Audio Amplifer

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I'm building a fancy high performance amplifier based off the examples from the book "High Power Audio Amplifier Construction Manual" by G. Randy Slone. It's gong to be my first HiFi design. It is expected to have THD of 0.001% while delivering 80W to 8 Ohm speaker

The HiFi Amplifier consists of three stages: input stage, voltage amplifier and power stage.

Input stage

Input stage is a differential pair of Q1 and Q5. The pair balanced through the current mirror made of Q2 and Q4 and degeneration resistors R1, R2, R5, and R6. The current of 0.65/150 = 4.5mA is provided by the current sink Q3. Since differential branches split the current equally, there is 2.25 mA on each branch. It means that transconductance of the differential pair transistor is gm = Ic/UT = 2.25mA/25mV = 90 mS.

Voltage amplifier stage

The VA stage is a classical Darlington pair with a current limiter through Q7 set to 0.65/47 = 13.8 mA. The current is supplied by the current source Q9 and is set to 0.65/100 = 6.5 mA.

The VA stage contains a double pole compensation through RC network of C7, C9, and R13.

Output power stage

Output power stage is basic Class B amplifier made of high-power transistors Q16 and Q17. These transistors are pre-driven by lower power Q14 and Q15. Combined, they form something called Sziklai pair, that improves current gain of the output stage.

Power transistors are biased by collector follower Q10, R18, R19, R21, and RV1. Q10 has to be mounted on the heatsink together with power transistors to ensure thermal tracking. What? When the high power transistors warm up, their dynamic properties will change (i.e. output current). Bias must be able to follow this change, and not them slip out of control. So, when output current start increasing because of silicon warming up, Vce of bias will start dropping and reduce this current. This keeps transistors in the safe operating area.

Output current must be monitored and limited by power resistors R33 and R34 and transistors Q12 and Q13. Monitored voltage is scaled by voltage divider R26/R29 (or R27/R30) by factor of 0.3. When the current reaches the value of 0.65/0.22/0.3 = 9.6A, Q12 (or Q13 in negative half-period) shall tie the predriver transistors base to ground and keep the current limited.

R25 - R28 are here to provide something called single slope current limit. What is this? Our maximum allowed power dissipation is 150W, that means product of Vce and Ic must be 150 all the time. In the Ic - Vce diagram, this looks like the 1/x curve (Ic = 150/Vce). We're ignoring the secondary breakdown curve here. Current limit will keep transistor in the rectangular area under the Ilim. If Ilim is 9.6A then the maximum VCE allowed is  15.6 V.

But if VCE get's higher than 15.6, let's say that it climbs to the value of 20V, then max allowed current will be 7.5A, a value that current limiter will easily allow, and lead the transistor into the destruction. To avoid this, a bit of current is always fed to the bases of Q12/Q13 by resistors R25 and R28. This way, even for smaller output currents than 9.6V, the Q12/Q13 will start to shut down.

Rest of the circuit

To match the speaker's impedance, a Zobel network (R35, C12) and air coil inductor L1 was put.

Global feedback is fed from the output to the negative input of the differential pair via network of R10, R11, R12, C6, C8, and D1.

The HiFi Amplifier consists of all discrete, through-hole parts. It's basically a huge, high power, discrete operational amplifier.

  • 1 × 2SA1302 Discrete Semiconductors / Power Transistors and MOSFETs
  • 1 × 2SC3281, Discrete Semiconductors / Power Transistors and MOSFETs
  • 4 × 2SD669 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 2 × 2SB649 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 5 × 2N5401 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs

View all 6 components

  • Power calculation

    Pero2 days ago 0 comments

    Let's see what kind of trafo and heatsink to I need for this amplifier.

    • As stated before, the average output power is expected to be 80W at 8 Ohm load. During one cycle, power will be delivered from positive supply rail half the time, and from negative supply the other half, meaning that each rail will have to deliver at least 40W.
    • This implies effective current of 3.16 A, or 4.5 A peak , if we pretend that signal is pure sine (which is not). Power supply at 42V delivering 4.5 A peak half the time, gives away on average 42*3.16/3.14 = 60.2 W.
    • Transistors will, therefore dissipate 60.2 - 40 = 20.2 W each.
    • We re talking about efficiency of 66%, what is reasonable for Class B/AB amplifier
    • Knowing that maximum junction temperature is 150°C, the maximum allowable thermal resistance is (150-25)/20.2 = 6.18 K/W. This means, that Rth of junction-to-case, case-to-heatsink and heatsink-to-ambient combined must be less than 6.18 K/W.
    • Transformer I need for this amplifier is then 2x60.2 = 121 VA minimum, with a +/- 45V secondary voltage and a center tap.

  • Orders placed

    Pero01/21/2019 at 14:57 0 comments

    Finished layout and ordered a PCB from the JLCPCB. The PCBs are two layered, 10x16 cm and 5 pieces cost about 13€ all together.

    The power BJTs are not available from standard distributors, so I ordered them from e-bay.

View all 2 project logs

Enjoy this project?



roelh wrote a day ago point

Hi Pero, in the schematic in your gallery (main project picture), it looks if Q9 has its collector and emitter swapped ! But perhaps it is an older version ?

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Richard Dudley wrote 01/26/2019 at 13:51 point

When using two pole compensation its worth checking out the clipping behaviour. You might find a need for some kind of clamp to minimize 'sticking'.

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Ted Yapo wrote 01/24/2019 at 03:34 point

I'm watching this with great interest! I've made a few designs based on Douglas Self's books. It's great to see people still designing amps like this in our current class-D dystopia :-)

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Pero wrote 01/24/2019 at 21:01 point

Thanks! I hope it'll turn out good!

  Are you sure? yes | no

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