What if a palm-sized amplifier could outperform larger, more expensive modules while delivering serious sound quality? This compact, low-cost discrete 60-W amplifier from 2015 proves that impressive audio performance doesn’t have to come in a large or expensive package.
What if a palm-sized amplifier could outperform larger, more expensive modules while delivering serious sound quality? This compact, low-cost discrete 60-W amplifier from 2015 proves that impressive audio performance doesn’t have to come in a large or expensive package.
The Circuit
Measuring just 38 × 56 mm, Dr. Robert M. Carter’s compact 60-W amplifier was smaller and more affordable than many comparable IC-based modules while still delivering excellent sound quality. First presented in Elektor November/December 2015, its efficient footprint and strong performance made it a versatile solution for a wide range of applications, from in-car audio systems and domestic subwoofers to smartphone or tablet speakers and multichannel AV setups.
The compact 60-W audio amplifier.
The circuit followed a conventional layout and used a fully symmetrical design with single-pole frequency compensation, though several less common choices were made to reduce the overall component count. Rather than the typical non-inverting configuration, the amplifier operated in an inverting mode. This required reversing speaker polarity, but inverting amplifiers generally require less effort to correct open-loop phase shifts compared to non-inverting designs — an advantage for a simplified circuit. Additionally, the input capacitor performed multiple roles, providing AC coupling while also reducing DC gain to unity. All frequency-dependent shaping was handled outside the feedback loop, with the signal fully pre-conditioned by the passive input network, further minimizing the number of required components.
At only 38 × 56 mm, the discrete design is cheaper than IC modules of similar power.
“The basic amplifier, therefore, operates with high gain to frequencies beyond the audio band and it will be noticed that aside from the 39-pF frequency compensation capacitors and some basic decoupling, the design is not festooned with mysterious RC networks ‘for stability’," Dr. Carter wrote. "Despite this, the amplifier has a theoretical phase margin of 40° and the compact PCB layout keeps parasitics to a minimum — think of it as a discrete power op-amp."
Note that 60 W (65 W at 10% THD) from TO-220 output transistors drove these devices hard. As a result, proper heatsinking was important.
“In circuit terms, this means overload protection is not considered a luxury here,” Dr. Carter noted, adding that conventional single slope circuits were used.
The PCB.
The 60-W Amplifier Project
The original article, “Compact 60-Watt Amplifier,” appeared in Elektor November/December 2015. Check out the article.
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