====== AEL-2026 50W POWER AMPLIFIER ======

This amplifier will happily produce 50W in to 8 ohms from a +/-35V DC supply. This was designed from scratch in TINA-TI to tweak the performance and a PCB has been designed for it. The gerber files for this project can be purchased from my [[https://www.patreon.com/posts/158435522|Patreon]].

The schematic is shown below:

{{:2026:schematic.png}}

**Fig. 1: the complete schematic of the AEL-2026 power amplifier**

===== Circuit Description =====

This circuit is a discrete class-AB audio power amplifier capable of delivering around 50 watts of output power into a speaker load. The design follows the same basic structure used in many traditional solid-state audio amplifiers: an input differential stage, a voltage amplification stage, a bias network, and a complementary push-pull output stage.

The audio signal enters through connector J1 and passes through coupling capacitor C1, which blocks any DC voltage from the source equipment. Resistor R1 provides input biasing and defines the input impedance, while R3, R4, and C3 help with stability and high-frequency filtering to reduce unwanted oscillation or noise. R2 and C2 are a "ground-lift" circuit which attempts to prevent mains-hum ground loops from interconnected equipment.

The first active stage is formed by Q2 and Q4, which act as a differential amplifier. This stage compares the input signal with the feedback signal returned from the amplifier output through resistor R13. Negative feedback is extremely important because it stabilizes gain, reduces distortion, improves bandwidth, and lowers DC offset at the output.

Q3 and Q5 form a current mirror load for the differential pair. A current mirror helps balance the differential stage and greatly increases gain and linearity compared to a simple resistor load. Q1 provides current source biasing for the differential stage, ensuring more stable operation independent of supply voltage variations.

The amplified signal from the differential stage drives Q7 and Q9, which form the voltage amplification stage (VAS). This stage provides most of the amplifier’s voltage gain. Q7 is an emitter-follower which improves the linearity of the VAS. Capacitor C6 is the Miller compensation capacitor, used to stabilize the amplifier and prevent high-frequency oscillation.

Q6 and Q8 form an active current source and driver arrangement that improves linearity and allows the VAS stage to swing voltage more effectively. The output from this stage then drives the bias and driver section.

The bias network consists of RV1 along with diodes D1 and D2 and Vbe multiplier transistor Q11. This section sets the idle bias current for the output transistors. Proper biasing is critical in a class-AB amplifier because it minimizes crossover distortion that would otherwise occur when the output waveform transitions between the positive and negative halves. RV1 allows adjustment of the quiescent current through the output stage.

Q10 and Q12 act as driver transistors, supplying sufficient current to control the large output transistors Q13 and Q14. The final output stage uses a complementary feedback pair (CFP), also known as a Sziklai pair, configuration. This arrangement provides high current gain, low distortion, and improved thermal stability while allowing the amplifier to efficiently drive the loudspeaker with a low output impedance.

Emitter resistors R22 and R23 help equalize current flow and improve thermal stability of the output transistors. The output network consisting of R25 and L1 forms a Zobel/output inductor network that helps maintain stability when driving reactive speaker loads and long speaker cables. R24 and C11 form an additional output stabilization network for high-frequency behavior.

The power supply section uses dual supply rails (Vcc and Vee), allowing the amplifier output to swing both positive and negative around ground without requiring a large output coupling capacitor. Capacitors C10 through C14 provide local supply filtering and decoupling to reduce noise and maintain stable operation during large current demands.

Overall, this amplifier is a fairly classic discrete transistor audio amplifier design. It combines differential input amplification, current mirrors, voltage amplification, adjustable biasing, and a complementary emitter follower output stage to produce relatively high power with good linearity and low distortion.

===== Simulation Results =====

Simulation of the circuit in TINA-TI shows a total harmonic distortion (THD) at near full power of 0.019% and a DC offset at the output of -1.25mV. This is with "ideal" transistors. In the real world, the DC offset was closer to 5mV.

Test results show an output power of 52.5W into 8 ohms, and 72.4W into 4 ohms at the rated supply voltage of +/-35V.

{{:2026:fr.jpg}}

**Fig. 2: frequency response curve of the amplifier**

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**Fig. 3: total noise vs. frequency**

===== Conclusion =====

The amplifier performs extremely well; in fact I have a couple of the PCB modules in a complete build with my [[discrete_preamp|discrete preamplifier]] and it's very low noise. There obviously could be a lot of improvements done to lower the THD.

I have included the TINI-TI v9 {{:2026:ael2026-v6-compound-pair.zip|simulation file}} for those that wish to play with it further :-P!