How to DIY High-Power Amp with 128 Parallel 5532s?

The 5532 is one of the most commonly used dual operational amplifiers in the audio field. To address the issue of insufficient drive current in standard op-amps, multiple units can be connected in parallel to increase the current capacity. When driving speakers, significant power is required, and thus, using 128 parallel 5532s to form a power amplifier is a viable design option. Though 128 units may sound excessive, modern surface-mount technology (SMT) and soldering techniques make it straightforward to have this assembly processed by a professional SMT manufacturer. The 5532's simplicity, requiring only power supply and current-sharing resistors, means the circuit is quite streamlined while maintaining the 5532’s excellent linearity, power supply rejection ratio, and built-in overload protection, minimizing the need for external circuitry.

While the 5532 is not a new design, its excellent cost-performance ratio, good load-driving ability, and superb noise performance have ensured its continued use. As the demand has increased, costs have decreased. A power amplifier made from 128 parallel 5532s is a typical Class B single-ended amplifier. If more power is needed, additional 5532s can be added in parallel, or a bridge-tied load (BTL) amplifier can be used, although this complicates the circuit. The power output of a parallel configuration is limited by the supply voltage, which is only ±18V, meaning more parallel op-amps cannot increase power beyond this voltage limit.

The entire unit consists of a voltage amplification stage, an output stage with parallel 5532s, a speaker protection board, and a power supply board. The voltage amplification stage provides a 25.9dB voltage gain, but due to the gain-bandwidth product limitation of the 5532, this gain cannot be achieved with a single amplifier. Therefore, a multi-stage amplification approach is used. U1A and U1B provide 13.9dB gain with 3dB noise suppression via R12 and R13. U2B provides 6dB gain, with U2A serving as an isolation stage to prevent overloading R17. U3B provides another 6dB gain, and U3A acts as an inverting amplifier to facilitate the BTL configuration. The voltage amplification stage is on a separate PCB, with through-hole op-amps, WIMA capacitors, and 1206 surface-mount resistors. The power and output connections use dual-row pins, and MCX connectors are used for input.

The output stage employs 64 pairs of 5532s in parallel per channel, resulting in 128 op-amps per channel. To address inconsistencies among the op-amps, each output is balanced with a 1-ohm resistor, making the output impedance about 0.008 ohms. For an 8-ohm load, this gives a damping factor of 1000. Given the extensive soldering required, a full surface-mount design is used for better heat dissipation. Each 5532 has an idle power consumption of 8mA, resulting in about 18W for 64 units, necessitating careful thermal management. The output stage is designed for flat installation, with thermal grease used to connect the op-amps to a heatsink, ensuring even heat distribution.

The speaker protection board is integrated into the power supply board and includes a startup delay circuit and a DC detection circuit to protect the speakers from amplifier failure. The power supply uses LT1083 chips for regulated positive and negative supplies. A locking mechanism ensures that a failure in one power rail cuts off both to protect the amplifier. The linear regulator design generates significant heat, so proper cooling is essential to minimize power loss. Initial rectification was done with standard diode bridges, which generated excessive heat, later resolved by using LT4320 ideal diodes. The toroidal transformer is sealed with epoxy within a shielded case, and the power input is filtered by an LC EMC circuit. Reliable grounding of the power input is crucial.

The entire assembly is housed in a chassis measuring 336mm wide, 75mm high, and 208mm deep. The input potentiometer is a custom-made 12-step unit. Initial audio connections lacked shielding, causing significant noise, which was resolved with MCX RF shielded cables. The front panel features two LEDs indicating standby (yellow) and operational/protection status (green). The design, assembly, and testing of the parallel 5532 amplifier demonstrate its feasibility and suitability for music playback with a CD player, even at 8W output.

Basic tests with a signal source and an AD725D show a frequency response from 10Hz to 100kHz, with THD+N below 0.001% at 8W and below 1% at 15W. Listening tests using an SACD source and JAMO C605 speakers, with the "Best of the Best" album by Faye Wong, revealed very clean sound, wide dynamic range, rich bass, and clear highs, fully showcasing the advantages of the op-amp. More performance metrics can be measured with an Audio Precision analyzer.

Due to the heat generated by the standard diode rectifier, a small rectifier board with LT4320 ideal diodes is recommended for better performance. 

Voltage amplification stage diagram: 

Power supply and speaker protection board diagram: 

Output stage with parallel 5532s diagram: