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Sunday, January 4, 2026

A Comprehensive Guide to Audio Power Amplifier Design

A Comprehensive Guide to Audio Power Amplifier Design


Published by IWISTAO

The audio power amplifier is the heart of any high-fidelity sound system. It performs the critical task of taking a low-power audio signal from a source like a DAC or preamplifier and boosting it to a level sufficient to drive loudspeakers [2]. Designing a great amplifier is a multifaceted discipline, touching on everything from fundamental device physics to advanced feedback theory, thermal management, and PCB layout [6].

This article provides a comprehensive overview of audio power amplifier design, covering the fundamental principles, common architectures, key design considerations, and modern trends. Whether you are an electronics student, a seasoned professional, or an audio hobbyist, this guide will help you understand the science and art behind creating powerful, clear sound.


Understanding Amplifier Classes

Amplifier "class" refers to the design of the output stage and how its active devices (transistors or vacuum tubes) are biased. This choice fundamentally dictates the trade-off between **efficiency** and **linearity** (sound fidelity). The most common classes in audio are A, B, AB, and D [14].

Class A

In a **Class A** amplifier, the output transistors are always conducting, regardless of the audio signal. This means they conduct through the full 360 degrees of the signal waveform. This "always-on" state eliminates the distortion that occurs when transistors switch on and off, resulting in the purest sound and highest linearity. However, this comes at a steep price: extremely low efficiency (typically 20-30%) and massive heat generation, as the amplifier dissipates maximum power even at idle [48][49].

IWISTAO Power Amplifier 2x25W ClassA FET Single-ended Stereo Whole Aluminum Casing

 

Class B

A **Class B** amplifier uses two transistors in a "push-pull" arrangement, where one handles the positive half of the waveform and the other handles the negative half. Each transistor is active for only 180 degrees of the signal. This dramatically improves efficiency (up to ~78.5%) but introduces **crossover distortion**—a nonlinearity that occurs at the zero-crossing point where one transistor turns off and the other turns on [47].

Class AB

As a hybrid of the two, **Class AB** is the most common design for high-fidelity amplifiers. It is essentially a Class B design but with a small amount of **quiescent (idle) current** flowing through both output transistors at all times. This small bias is enough to keep the transistors "on" through the crossover region, significantly reducing crossover distortion while maintaining much of Class B's efficiency (typically 50-70%) [13][47]. It represents a well-balanced compromise between performance and efficiency.

IWISTAO 2x100w HIFI Amplifier Stereo Discrete-Component HDAM Music Box A1 2SC5200/2SA1943

 

Class D

Often mistakenly called "digital amplifiers," **Class D** amplifiers are highly efficient switching amplifiers. They work by converting the analog audio signal into a high-frequency **Pulse Width Modulated (PWM)** signal. This PWM signal rapidly switches the output transistors fully on or off—states where they dissipate very little power. A passive low-pass filter at the output then reconstructs the amplified analog audio signal and removes the high-frequency switching noise [47]. Class D amplifiers can achieve efficiencies exceeding 90%, making them ideal for compact, high-power applications like car audio, portable speakers, and professional sound systems [48][21].

IWISTAO TPA3116 Class D Digital Power Amplifier for Subwoofer 100W DC18V to DC24V Black

Class G & H

Class G and H are advanced variations that improve efficiency over Class AB by using multiple or tracking power supply rails. A **Class H** amplifier, for instance, switches between different fixed supply voltage rails depending on the signal level, while a **Class G** amplifier modulates the supply voltage to track the signal envelope. This reduces power dissipation by providing just enough voltage for the required output swing, making them suitable for high-performance systems where power management is critical [48][56].

 

Class T 

Class T amplifiers are a special type of audio amplifier that combine elements of Class D switching amplifiers with digital signal processing (DSP) to achieve high efficiency and excellent sound quality. Unlike traditional Class D amplifiers that rely solely on fixed-frequency pulse-width modulation (PWM), Class T uses advanced modulation schemes and feedback control to optimize performance in real time. One well-known example of a Class T amplifier is the Tripath TA2020 chip, which was highly regarded for delivering transparent and dynamic sound in a tiny power module.

IWISTAO TA2022 Class T Amplifier 2x90W Toroidal Transformer NE5532 Preamplifier Stereo HIFI

 

Amplifier Class Comparison [48]
Amplifier Class Efficiency Sound Quality Heat Generation Ideal Use
Class A Low Best (Purest Sound) High Audiophile home systems
Class B High Good (Slight Distortion) Moderate General use (less common now)
Class AB Moderate Very Good Moderate Home audio, car audio, balanced choice
Class D Very High Good to Excellent Low Portable systems, subwoofers, pro audio
Class H/G High Good Low High-performance, power-sensitive systems


Core Architecture of a Solid-State Power Amplifier

Most modern solid-state power amplifiers, particularly Class AB designs, follow a well-established three-stage architecture. This structure has evolved to effectively address performance deficiencies and optimize linearity [4].

1. Input Stage (IPS)

The input stage is typically a **differential amplifier** (often called a "long-tailed pair"). Its primary functions are:

  • To receive the incoming audio signal at one input and the negative feedback signal from the amplifier's output at the other.
  • To subtract these two signals, producing an error signal that the rest of the amplifier works to minimize.
  • To provide some initial voltage gain and set the overall DC operating conditions.

The choice of transistors for the input stage—**Bipolar Junction Transistors (BJTs)** or **Junction Field-Effect Transistors (JFETs)**—has significant implications. BJTs offer better matching for low DC offset, while JFETs provide a very high input impedance and eliminate input bias current issues, which can simplify DC servo design [4].

2. Voltage Amplification Stage (VAS)

The VAS is responsible for providing the majority of the amplifier's **voltage gain**. It is typically a common-emitter stage that takes the error signal from the input stage and amplifies it to a level high enough to drive the output stage. The VAS has a high-impedance output, making it sensitive to loading. Therefore, it must be effectively buffered by the output stage [4].

3. Output Stage (OPS)

The output stage is a **current amplifier**. It has a voltage gain of slightly less than unity but provides the high current necessary to drive low-impedance loudspeaker loads. It acts as a buffer, isolating the high-impedance VAS from the speaker.

Common configurations include the **Darlington pair** (a "Double") or, for better performance, the **Triple Emitter Follower** (also known as a "Triple" or Locanthi T circuit). The Triple provides much higher current gain, which better isolates the VAS from the load, especially at high output currents and with low-impedance speakers. This increased buffering significantly reduces distortion caused by effects like beta droop in the output transistors [4].


Key Design Principles and Calculations

A successful amplifier design relies on careful calculation and consideration of several key parameters.

Gain Calculation

One of the first questions in amplifier design is, "What gain should I use?" The goal is to set the gain just high enough to achieve the desired maximum output power from the maximum input signal level. Setting the gain too high unnecessarily amplifies the noise floor of the source (e.g., a DAC or CODEC), reducing the system's overall **Signal-to-Noise Ratio (SNR)** and dynamic range [18].

The process involves:

  1. Calculate Required Output Voltage (VO(RMS)): Based on the target output power (PO) and speaker impedance (RL).
    VO(RMS) = sqrt(PO * RL)
  2. Determine Input Voltage (VI(RMS)): Convert the peak-to-peak voltage from the source (VI(PP)) to RMS.
    VI(RMS) = VI(PP) / (2 * sqrt(2))
  3. Calculate Required Gain: The required voltage gain (Av) is the ratio of the required output RMS voltage to the available input RMS voltage.
    Gain (Av) = VO(RMS) / VI(RMS)

For a typical inverting amplifier configuration, the gain is set by the ratio of the feedback resistor (RF) to the input resistor (RI). You choose standard resistor values to approximate this calculated gain [18].


Power Dissipation and Thermal Management

Thermal management is arguably the most critical aspect of ensuring an amplifier's reliability and long-term performance [25]. Inefficient designs, like Class A and AB, convert a significant portion of input power into heat. This heat must be effectively removed to keep the output transistor junction temperatures within their safe operating limits (typically below 150°C) [63].

Calculating Power Dissipation (Class AB)

The power dissipated in a Class AB output stage (PD) is the difference between the power drawn from the supply (PS) and the power delivered to the load (PL). The dissipation varies with the output signal level, reaching its maximum not at full power, but at a specific output voltage.

Formula for power dissipated in a Class AB output stage.
Power dissipated in the output stage. Source: [64]

The peak power dissipation occurs when the output voltage swing is VOUTpeak = (2/π) * VCC. The maximum power dissipation can be calculated as:

Formula for peak power dissipation in a Class AB amplifier, including bias current.
Peak power dissipation calculation, including quiescent bias dissipation. Source: [64]

For example, for an amplifier with ±25V supplies, a 4Ω load, and 50mA bias current, the peak dissipation is approximately 34.2W [64].

Example calculation of peak power dissipation for an LM3886 amplifier.
Example calculation of peak power dissipation. Source: [64]

Heatsink Selection

Once the maximum power dissipation is known, a suitable **heatsink** can be selected. The heatsink's ability to dissipate heat is defined by its **thermal resistance (θSA)**, measured in °C/W. This value indicates how many degrees Celsius the heatsink's temperature will rise above ambient for every watt of dissipated power.

The total thermal resistance from the transistor's internal junction to the ambient air is the sum of several resistances in series:

  • θJC (Junction-to-Case): The thermal resistance from the silicon die to the transistor's package.
  • θCS (Case-to-Sink): The resistance of the thermal interface material (e.g., mica washer or silicone pad) between the transistor and the heatsink.
  • θSA (Sink-to-Ambient): The thermal resistance of the heatsink itself.

The final junction temperature can be calculated as: Tjunction = Tambient + Pdissipated * (θJC + θCS + θSA) [64]. The goal is to choose a heatsink with a low enough θSA to keep Tjunction well below the component's maximum rating.

Modern Thermal Solutions

To combat thermal instability and distortion caused by temperature fluctuations, modern components like ON Semiconductor's **ThermalTrak™** power transistors have been introduced. These devices integrate a temperature-sensing diode directly on the transistor die. This allows for much faster and more accurate tracking of the power transistor's junction temperature, enabling the bias circuit to adjust almost instantaneously and greatly reducing thermal lag distortion [4].


Advanced Design Topics & Modern Trends

The field of amplifier design is constantly evolving, with new technologies and techniques pushing the boundaries of performance and efficiency.

Class D Amplifiers: Efficiency and Fidelity

While early Class D designs were relegated to low-fidelity applications like subwoofers, modern Class D amplifiers offer performance that rivals or even exceeds traditional Class AB designs. Key innovations include:

  • Higher Switching Frequencies: Increasing the PWM switching frequency (e.g., to 2.1 MHz, well above the AM radio band) pushes the switching artifacts and noise much further out of the audio band. This allows for simpler output filters and reduces distortion within the audible range, as there is more loop gain available for error correction [45].
  • Advanced Modulation and Feedback: Sophisticated modulation schemes and post-filter feedback loops help to linearize the amplifier's output, correct for power supply variations, and make the performance less dependent on speaker impedance [58].
  • Gallium Nitride (GaN) Transistors: The adoption of **GaN** power transistors is a major trend. Compared to traditional silicon MOSFETs, GaN devices offer lower on-resistance, faster switching speeds, and zero reverse recovery charge. This leads to higher efficiency, less heat, smaller form factors, and a significant reduction in distortion, enabling a more precise and detailed sound [32][34].

Vacuum Tube Amplifiers and the Output Transformer

Vacuum tube amplifiers remain cherished by many audiophiles for their characteristic "warm" sound. A key component that distinguishes them from solid-state designs is the **output transformer (OPT)**.

Tubes are high-voltage, low-current devices with a very high output impedance (thousands of ohms). In contrast, loudspeakers are low-impedance devices (typically 4 or 8 ohms). The OPT is an impedance-matching device that steps down the high voltage/impedance from the tubes to the low voltage/impedance required by the speaker, allowing for efficient power transfer [68].

The design of the OPT is critical to the amplifier's performance, influencing its frequency response, distortion, and damping factor. Key design parameters include:

  • Primary Inductance (Lp): Determines the low-frequency response. A higher inductance is needed to extend bass response without saturation.
  • Turns Ratio (n): Sets the impedance matching between the tubes' plate-to-plate resistance (Raa) and the speaker load (RL). n = sqrt(Raa / RL).
  • Leakage Inductance and Winding Capacitance: These parasitic elements determine the high-frequency response. Careful winding techniques, such as **interleaving** (alternating primary and secondary windings), are used to minimize them and extend the bandwidth.

 

Engineering diagram summarizing the key design parameters for a 200W push-pull output transformer. Source: [68]


300B Single-ended Class A 2X8W Tube Amplifier British Amorphous 8C Advanced Core Output Transformer

Integrated Circuit (IC) Power Amplifiers

For many applications, integrated circuit (IC) amplifiers offer a compact, reliable, and high-performance solution. Chips like the **TDA7294** and **LM3886** have been staples of the DIY audio community and commercial products for years. These ICs integrate the entire amplifier—input stage, VAS, output stage, and protection circuitry—into a single package [35].

 

IC TDA7294
IC LM3886
Popular power amplifier ICs: the TDA7294 and LM3886. Sources: [73], [74]

 

The TDA7294, for example, is a DMOS power amplifier capable of delivering up to 100W. It includes features like mute and standby functions, short-circuit protection, and thermal shutdown [36]. A typical mono application circuit is relatively simple, requiring only a handful of external components for gain setting, feedback, and power supply filtering.

 

A typical mono amplifier circuit using the TDA7294 IC. Source: [36]

For even more power, two ICs can be used in a **Bridge-Tied Load (BTL)** configuration. In this setup, one amplifier drives the speaker's positive terminal and a second, identical amplifier drives the negative terminal with an inverted signal. This doubles the voltage swing across the load, theoretically quadrupling the output power [37].

IWISTAO 2X30W HIFI Amplifier Stereo LM1875 Power Amp Desktop With Preamp OP TL084 Independent Rectifier

 

Practical Implementation: PCB Layout and Protection

A brilliant circuit schematic can be ruined by poor physical implementation. Proper **Printed Circuit Board (PCB) layout** and robust **protection circuits** are essential for a stable, quiet, and reliable amplifier.

PCB Layout Best Practices

Good PCB layout is crucial for minimizing noise, hum, and distortion.

  • Grounding: A well-designed ground system is paramount. A **star ground** topology, where all ground connections meet at a single point (usually at the power supply capacitors), is a classic approach to prevent ground loops. Alternatively, carefully implemented ground planes can also be effective, especially in multi-layer boards [27].
  • Trace Separation: Keep sensitive, low-level analog input traces physically separated from high-current power supply and output traces. Routing them on different layers with a ground plane in between provides excellent shielding [27].
  • Power Supply Decoupling: Place small ceramic capacitors (e.g., 0.1µF) as close as possible to the power pins of ICs and transistors. These provide a local reservoir of charge for high-frequency currents, preventing noise from propagating through the power supply rails. Larger bulk electrolytic capacitors (10µF or more) should be placed nearby for lower-frequency transient demands [27].
  • Thermal Layout: Place high-power components like output transistors and heatsinks near the edge of the board to improve airflow. Use **thermal vias** under surface-mount power devices to conduct heat to large copper planes on other layers, effectively turning the PCB itself into part of the heatsink [25].


Protection Circuits

A good protection system serves two purposes: protecting the amplifier from fault conditions and protecting the loudspeakers from amplifier failure [50].

  • Overload Protection: This circuitry monitors the output voltage and current to ensure the output transistors remain within their **Safe Operating Area (SOA)**. If a short circuit or excessive load is detected, the circuit limits the drive current to prevent the transistors from being destroyed [4].
  • DC Fault Protection: If an amplifier fails, it can output a large DC voltage, which can quickly destroy a loudspeaker's voice coil. A DC protection circuit monitors the output for DC and, if detected, uses a relay to disconnect the speaker [52].
  • DC Servo: To eliminate DC offset at the output during normal operation, a **DC servo** is often used. This is an auxiliary feedback loop using an integrator (typically an op-amp) that senses the DC level at the output and injects a small correction current into the input stage to force the output DC to zero. This is a more elegant solution than using a large, sound-degrading DC-blocking capacitor in the signal path [4].

 

Simulation and Measurement

Before committing a design to hardware, modern engineers rely heavily on circuit simulation to verify and optimize performance.

SPICE Simulation

SPICE (Simulation Program with Integrated Circuit Emphasis)** is an invaluable tool that allows designers to build a virtual prototype of their amplifier. Software like the free and powerful **LTspice** from Analog Devices (formerly Linear Technology) can be used to perform various analyses [4][60]:

  • .OP (DC Operating Point): Verifies bias voltages and currents throughout the circuit.
  • .AC (AC Analysis): Plots the frequency and phase response to check bandwidth and stability (gain/phase margins).
  • .TRAN (Transient Analysis): Simulates the circuit's response to a time-varying signal, allowing for the analysis of waveforms and distortion.

SPICE simulation can save countless hours of lab work by helping to evaluate different circuit topologies, optimize component values, and identify potential issues like instability before a single component is soldered [4].

Key Performance Measurements

Once a prototype is built, its performance must be verified through measurement. Common audio amplifier tests include [30]:

  • Total Harmonic Distortion + Noise (THD+N): Measures the unwanted harmonics and noise added by the amplifier. Lower is better.
  • Frequency Response: Measures the amplifier's gain across the audio spectrum (typically 20Hz to 20kHz). A flat response is desired.
  • Signal-to-Noise Ratio (SNR): The ratio of the maximum signal level to the residual noise floor. Higher is better.
  • Damping Factor: The ratio of the speaker's impedance to the amplifier's output impedance. A high damping factor indicates good control over the speaker cone's movement.


Conclusion

Audio power amplifier design is a rich and rewarding field that blends scientific principles with engineering artistry. From the fundamental trade-offs of amplifier classes to the intricate details of feedback, thermal management, and PCB layout, every decision impacts the final sound quality. The journey from a simple three-stage concept to a high-performance, reliable product requires a holistic approach that considers the circuit, its physical implementation, and its interaction with the real world.

As technology marches forward, the trend is toward ever-greater efficiency and integration. The rise of advanced Class D amplifiers, powered by cutting-edge technologies like GaN transistors and sophisticated digital control, is redefining what is possible in terms of performance, size, and power consumption [21]. Yet, the classic principles of analog design remain as relevant as ever, forming the foundation upon which all great amplifiers are built.


References

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Building a High-Performance FM Tuner: A Deep Dive into a Classic IC-Based Design

Building a High-Performance FM Tuner: A Deep Dive into a Classic IC-Based Design

Published by IWISTAO

Introduction

In an age of streaming services and digital audio, there remains a unique and enduring magic to FM radio. The thrill of tuning across the dial, the serendipity of discovering a new song, and the pristine quality of a strong stereo broadcast are experiences that continue to captivate audio enthusiasts and hobbyists. For those who appreciate not just the listening but also the building, constructing a high-quality FM tuner from discrete components is a deeply rewarding journey.

This article details the design and construction of a high-performance FM tuner, built around a carefully selected set of specialized integrated circuits (ICs). Our design philosophy is to leverage the strengths of each component to create a modular, robust, and audiophile-grade receiver. We will start with a highly sensitive automotive high-frequency tuner for the front-end, followed by a dedicated IF amplification stage using the TA7302P. The core demodulation is handled by the legendary LA1235 IF system, and finally, the composite signal is decoded into glorious stereo by the LA3401 MPX decoder. Let's embark on a detailed exploration of each stage, from the antenna to the final audio output.


Component Breakdown: The Heart of the Tuner

The performance of any radio receiver is determined by the quality of its constituent parts. Our design relies on a quartet of key components, each chosen for its specific role and proven performance in the signal chain.

The Front-End: Automotive High-Frequency Tuner

The journey of an FM signal begins at the front-end. This stage is arguably the most critical, as it's responsible for plucking a single, faint station out of a sea of powerful broadcasts. For this task, we've chosen an automotive high-frequency tuner module. These modules, designed for the harsh and demanding automotive environment, offer several distinct advantages for the hobbyist.

Firstly, they exhibit exceptional sensitivity, capable of receiving weak, distant stations. Secondly, their design prioritizes strong signal handling and selectivity, meaning they are less prone to overload and interference from powerful local stations—a common problem known as intermodulation distortion. Internally, these modules typically contain an RF amplifier, a local oscillator, and a mixer, all carefully shielded in a metal can. Their job is to receive the incoming RF signal (88-108 MHz), mix it with a signal from the local oscillator, and produce a fixed Intermediate Frequency (IF), which is standardized at 10.7 MHz for FM broadcasting. The tuning is accomplished by applying a variable DC voltage (often labeled V_T) to a varactor diode, which changes the capacitance in the oscillator and RF amplifier tuning circuits.


The IF Amplifier: Toshiba TA7302P

Once the front-end has converted our desired station to the 10.7 MHz IF, this signal is still relatively weak. It needs significant amplification before it can be demodulated. While our primary demodulator IC, the LA1235, has its own IF amplifiers, we are adding a dedicated pre-amplifier stage using the Toshiba TA7302P.

The TA7302P is an FM IF amplifier and detector IC. In our specific design, we are leveraging its excellent front-end gain and limiting capabilities. By placing it before the LA1235, we ensure the signal is robustly amplified and, crucially, "limited." Limiting is the process of stripping away any amplitude variations from the FM signal. Since information in FM is encoded in frequency changes, not amplitude changes, any amplitude variations are considered noise (often from atmospheric interference or multi-path distortion). The TA7302P excels at this, providing a clean, constant-amplitude IF signal, which is the ideal input for the next stage. This two-stage IF approach contributes significantly to a high signal-to-noise ratio (SNR) and excellent AM rejection (AMR).


The Demodulator: Sanyo LA1235

The Sanyo LA1235 is a cornerstone of high-quality FM tuner design, a highly regarded and versatile FM IF system IC. After being amplified and limited by the TA7302P, the 10.7 MHz signal enters the LA1235 for the critical task of demodulation—converting the frequency variations back into an audio signal.

The LA1235 performs this magic using a quadrature detector. This circuit requires an external resonant component, typically a tunable coil or a ceramic resonator, to create a reference signal that is 90 degrees out of phase with the incoming IF signal. By comparing the phase of the incoming signal against this reference, the detector produces a voltage output that is directly proportional to the frequency deviation of the original FM signal. The quality of this demodulation is paramount for low distortion audio.

Beyond demodulation, the LA1235 offers a suite of indispensable features. It includes further IF amplification stages, a signal strength meter driver (for a tuning meter), an Automatic Frequency Control (AFC) output to help lock onto stations, and a sophisticated muting circuit to silence the output between stations, eliminating inter-station noise. Its reputation for low distortion and high SNR makes it a perfect choice for this project.


The Stereo Decoder: Sanyo LA3401

The audio signal that emerges from the LA1235 is not yet stereo. It's a composite or multiplex (MPX) signal. To understand what the LA3401 does, we must first understand how stereo is broadcast. The composite signal contains three main components:

  1. The main audio channel, which is the sum of the Left and Right channels (L+R). This ensures compatibility with mono radios.
  2. A 19 kHz pilot tone. This acts as a flag to the receiver, indicating that a stereo broadcast is present.
  3. A sub-channel containing the difference between the Left and Right channels (L-R). This signal is modulated onto a 38 kHz subcarrier, which is suppressed at the transmitter to save bandwidth.

The Sanyo LA3401 is an FM MPX stereo decoder designed specifically to unravel this composite signal. It uses a Phase-Locked Loop (PLL) circuit to lock onto the 19 kHz pilot tone. From this stable reference, it internally generates a precise 38 kHz carrier signal. This regenerated carrier is then used to demodulate the L-R sub-channel.

With both the L+R (mono) and L-R (difference) signals now available, the LA3401 uses an internal matrix circuit to algebraically combine them and recover the original, separate Left and Right audio channels. The LA3401** is known for its excellent channel separation, low distortion, and includes a built-in driver for a stereo indicator LED, providing visual confirmation of a successful stereo lock.

 

Circuit Analysis and Integration

With an understanding of the individual components, we can now examine how they work together in a cohesive system. The proper integration, power supply design, and alignment are what transform a collection of parts into a functional, high-performance tuner.

Signal Flow: From Antenna to Audio

The signal path through our tuner is a logical progression of amplification, filtering, and decoding:

  1. Antenna & Front-End: The signal is captured by the antenna and fed into the automotive tuner module. The module selects the desired station frequency and converts it down to the 10.7 MHz IF.
  2. Ceramic Filtering: The output of the front-end is passed through one or more 10.7 MHz ceramic filters. These passive components are crucial for selectivity, sharply filtering the IF signal to reject adjacent channels.
  3. IF Pre-Amplification: The filtered signal enters the TA7302P, where it is significantly amplified and limited, preparing it for demodulation.
  4. Main IF & Demodulation: The output of the TA7302P is fed, often through another ceramic filter for even greater selectivity, into the LA1235. This IC provides final IF gain, performs the quadrature detection to recover the composite audio, and generates control signals for muting and metering.
  5. Stereo Decoding: The raw composite (MPX) audio from the LA1235 is passed to the LA3401. The decoder locks onto the 19 kHz pilot tone, decodes the L-R signal, and reconstructs the separate Left and Right audio channels.
  6. De-emphasis & Output: The final L and R outputs from the LA3401 pass through a de-emphasis network (typically a simple RC filter) to restore the correct tonal balance, as FM broadcasts use pre-emphasis to improve the signal-to-noise ratio at higher frequencies. The signals are then ready to be fed to an external amplifier.


Power Supply Considerations

A clean and stable power supply is non-negotiable for a high-performance audio project. RF and IF circuits are particularly sensitive to power supply noise. A well-regulated DC voltage, typically between 9V and 12V, is required. It is best practice to use a dedicated voltage regulator (e.g., a 78xx series IC) with adequate heat sinking. Furthermore, each IC (TA7302P, LA1235, LA3401) must have its own local decoupling capacitors (e.g., a 10-100µF electrolytic capacitor in parallel with a 0.1µF ceramic capacitor) placed as close to its power pins as possible. This shunts high-frequency noise to ground, preventing instability and inter-stage interference.


Alignment and Tuning

Building the circuit is only half the battle; proper alignment is what unlocks its full potential.

  • Front-End Tuning: Manual tuning is achieved by supplying a stable, variable DC voltage to the V_T pin of the automotive tuner. A 10-turn precision potentiometer connected as a voltage divider provides fine control over the tuning range.
  • IF and Demodulator Alignment: This is the most critical alignment step. The quadrature coil connected to the LA1235 must be tuned precisely. The goal is to center its resonant frequency exactly at 10.7 MHz, This is best done by tuning to a strong station and adjusting the coil's slug for minimum audio distortion and maximum stereo separation. A distortion analyzer is the professional tool for this, but careful listening can also yield excellent results. The point of lowest distortion often corresponds to the peak reading on a signal strength meter connected to the LA1235.
  • Stereo Decoder Alignment: The LA3401 has a VCO (Voltage-Controlled Oscillator) that must be set to its free-running frequency of 76 kHz. This is done by adjusting a small variable resistor. The procedure is to tune to a known stereo station and adjust the resistor until the stereo indicator LED lights up brightly and stably. The correct adjustment point is typically in the middle of the range where the LED remains lit.


IWISTAO LA1235 FM Stereo Radio Tuner PCBA High Frequency FAE352 IF 3 Stages TA7302P Decoder LA3401

Conclusion and Further Thoughts

This project represents a beautiful synthesis of classic analog design and specialized integrated circuits. By combining a robust automotive tuner front-end with the proven performance of the TA7302P, LA1235, and LA3401 ICs, we can construct an FM tuner that rivals many commercial units in sensitivity, selectivity, and audio fidelity. The modular nature of the design allows the builder to understand the function of each stage and appreciate the intricate process of receiving and decoding an FM broadcast.

For the aspiring builder, remember that a good PCB layout with a solid ground plane is essential for stability and low noise. Shielding the front-end and keeping signal paths short will pay dividends in performance. While the alignment process requires patience, the reward is the immense satisfaction of hearing crystal-clear stereo sound from a device you built with your own hands. This design serves as both a fantastic learning experience and a gateway to high-fidelity radio listening, proving that the art of analog receiver design is very much alive and well.

 

References

  1. TA7302P Datasheet – Toshiba Corporation
    Detailed specifications and application notes for the TA7302P IF amplifier.
    https://www.datasheetarchive.com/TA7302P-datasheet.html

  2. LA1235 Datasheet – Sanyo Semiconductor
    Information on the LA1235 FM discriminator and related FM tuning functions.
    https://www.datasheetarchive.com/LA1235-datasheet.html

  3. LA3401 Datasheet – Sanyo Semiconductor
    Data and application information for the LA3401 FM stereo decoder IC.
    https://www.datasheetarchive.com/LA3401-datasheet.html

  4. Automotive FM Tuner Design Principles – Electronics Textbook and Application Notes
    Typical design approaches and technical background for automotive FM tuner circuits.  https://www.electronics-notes.com/articles/radio/radio-receivers/fm-receiver-tuner.php

 

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Friday, January 2, 2026

The Enduring Legacy of the 1969 JLH Class A Amplifier

The Enduring Legacy of the 1969 JLH Class A Amplifier


Published by IWISTAO

Introduction: A Landmark in Hi-Fi History

In the annals of high-fidelity audio, few designs have achieved the legendary status and enduring popularity of the 1969 Class A amplifier by John Linsley-Hood. First published in the April 1969 issue of Wireless World, this elegant and deceptively simple circuit emerged during a pivotal era of audio technology—the transition from vacuum tubes to solid-state transistors [1, 2]. While transistor amplifiers offered advantages in size, cost, and power, the prevalent Class B and quasi-complementary designs of the time were often plagued by audible imperfections, most notably crossover distortion [3].

Audiophiles of the day held tube amplifiers, particularly designs like D.T.N. Williamson's landmark 1947 amplifier, as the benchmark for sonic purity [4, 5]. The Williamson amplifier set a high standard, aiming for less than 0.1% total harmonic distortion (THD) at full power, a feat achieved through a complex four-stage, push-pull triode design with a massive, high-quality output transformer [3, 6]. Linsley-Hood's objective was audacious: to create a solid-state amplifier that could meet or exceed this standard of performance, but with a dramatically simpler, more accessible, and transformerless design [1]. The result was a masterpiece of minimalist engineering that continues to be built, modified, and cherished by DIY enthusiasts and audiophiles over half a century later.


The Design Philosophy: Simplicity and Purity

Why Class A?

Linsley-Hood's fundamental design choice was to operate the output stage in Class A. In a Class A amplifier, the amplifying devices (transistors, in this case) are always conducting current, regardless of the input signal. This means they operate over the full 360° of the signal waveform, ensuring the highest possible linearity [7, 8]. This approach directly addressed the primary sonic flaw of contemporary Class B designs. In Class B, two transistors work in a push-pull arrangement, with each handling one half (180°) of the waveform. The transition between the two transistors can be imperfect, creating crossover distortion, which is particularly noticeable at low volumes [3].

 


Figure 1: A basic Class A amplifier with a resistive load. This simple configuration suffers from very low efficiency, typically around 12% [1, 8].

By choosing Class A, Linsley-Hood deliberately prioritized sonic purity over efficiency. This configuration inherently avoids crossover distortion, asymmetry issues found in quasi-complementary circuits, and signal-dependent variations in power supply current demand [1]. However, this choice comes with significant trade-offs. Because the transistors are always on and biased to handle the maximum signal swing, they dissipate a large amount of power as heat, even with no signal present. This results in very low power-conversion efficiency—theoretically a maximum of 25% with a simple resistive load, and often only 10-20% in practice [7, 8]. Consequently, the JLH 1969 requires substantial heat sinks to prevent the output transistors from overheating, a characteristic that has become a visual signature of the design [1].


An Elegant, Minimalist Circuit

The genius of the JLH 1969 lies in its "less is more" philosophy. The entire amplifier consists of only four transistors arranged in three stages, a stark contrast to the increasingly complex Class B designs of the era [2, 3]. Each component has a clear and vital role, creating a circuit that is both simple to understand and remarkably effective.

 

Figure 2: The complete schematic of the original JLH 1969 amplifier, as published in Wireless World. Note the use of only four transistors [1, 2].

The circuit can be broken down as follows [2]:

  • Input Stage (Tr1): A single PNP transistor (originally a 2N3906) acts as the input voltage amplifier. It receives the audio signal and also serves as the injection point for the negative feedback loop, which is crucial for lowering distortion and stabilizing the circuit.
  • Driver/Phase Splitter Stage (Tr2): This NPN transistor (originally a 2N1613 or similar) is arguably the most clever part of the design. It serves two functions simultaneously: it provides further voltage amplification (driving the output stage) and acts as a phase splitter. It delivers an in-phase signal to the upper output transistor (Tr3) and an out-of-phase signal to the lower output transistor (Tr4), enabling push-pull operation.
  • Output Stage (Tr3 & Tr4): This is a single-ended push-pull (SEPP) output stage using two identical NPN power transistors (originally MJ480/481, with 2N3055 being a common modern substitute). Tr3 acts as an emitter follower, while Tr4 acts as a constant current source whose current is modulated by the signal from Tr2. This configuration allows the load to be driven effectively in both directions of the waveform [1, 2].


Deep Dive into the Circuit's Key Features

The Unique Output Stage

Unlike conventional push-pull amplifiers that use complementary pairs of NPN and PNP transistors, the JLH 1969 uses two identical NPN power transistors for the output [2]. This was a practical choice, as high-quality silicon NPN power transistors were more readily available and offered better performance and lower cost than their PNP counterparts in the 1960s [1].

Figure 3: Typical OTL output stages. The JLH 1969's use of two identical NPN transistors (left, quasi-complementary style) was a departure from the fully complementary (NPN/PNP) approach (right) that would later become standard [9].

To make this work, the driver stage (Tr2) must provide two signals of opposite phase. Linsley-Hood achieved this elegantly by taking one output from Tr2's collector and the other from its emitter [2]. The upper transistor (Tr3) functions as a collector load for the lower transistor (Tr4). This arrangement not only simplifies the parts list but also contributes to the amplifier's low distortion. As Linsley-Hood noted, the non-linearities of the two output transistors tend to cancel each other out, as one is turning full on while the other approaches cutoff [1].

Gain and Negative Feedback

A cornerstone of high-fidelity amplifier design is the use of negative feedback to reduce distortion, lower output impedance, and stabilize performance. The JLH 1969 employs a significant amount of feedback. The open-loop gain of the circuit is approximately 600 (or 55 dB) [1, 2]. The output signal is taken from the junction of the two output emitters and fed back to the emitter of the input transistor (Tr1) via resistor R5.

The closed-loop gain is determined by the ratio of the feedback resistors, specifically (R4 + R5) / R4. With the original component values, this results in a gain of about 13 (or 22 dB) [1, 2]. The difference between the open-loop and closed-loop gain gives a feedback factor of approximately 33-34 dB, a substantial amount that is responsible for the amplifier's impressively low distortion and an output impedance of about 160 milliohms [1].

The Bootstrap and Soft-Start Mechanisms

The circuit includes several other clever design elements. Capacitor C3, connected from the output to the junction of R1 and R2, forms a bootstrap circuit. This technique feeds a portion of the output signal back to the bias network of the driver stage. Its purpose is to make the driver transistor's collector load appear much larger than it actually is, allowing for a larger voltage swing and improving linearity and efficiency [2]. While effective, this is a feature some modern builders replace with a constant current source (CCS) for theoretically better performance [2, 10].


Figure 4: The unique biasing network for Tr1, featuring a 39k resistor and 100µF capacitor, creates a soft-start effect to protect the loudspeaker [2, 11].

Another subtle but important feature is the biasing network for the input transistor Tr1. The inclusion of a 39k resistor and a 100µF capacitor creates a slow-charging circuit at power-on. This causes the DC voltage at the output to rise gradually to its target of half the supply voltage over several seconds. This "soft-start" minimizes the turn-on "thump" that could otherwise damage the connected loudspeaker [2].

IWISTAO HIFI Power Amplifier Pure Class A 2X8W Combined Headphone Amp 1969 Circuit Audio


Performance and Sound Characteristics

Distortion and Power Output

Linsley-Hood's design goal was to achieve a distortion level of less than 0.05% at full power, a very ambitious target for its time [3]. The published measurements show that the amplifier comfortably met this goal. The total harmonic distortion (THD) at 9 watts into a 15-ohm load was measured at just 0.06% with matched output transistors [1]. A key characteristic of this Class A circuit is that distortion decreases linearly as the output power is reduced. This is in contrast to Class B amplifiers, where crossover artifacts can cause distortion to rise at lower levels [1]. The residual distortion is predominantly benign second-order harmonic, which many listeners find to be musically pleasant and is often described as adding "warmth" and "richness" to the sound, reminiscent of tube amplifiers [1, 12].

The original design was specified for 10 watts of output power. The required supply voltage and quiescent current depend on the speaker's load impedance. For example, driving a 15Ω load to 10W requires a 36V supply and 0.9A of quiescent current. For an 8Ω load, it requires a 27V supply and 1.2A, and for a 3Ω load, a 17V supply and 2.0A [1]. This highlights the critical relationship between power supply, load, and thermal management in a Class A design. While 10 watts may seem low by modern standards, it is more than sufficient for high-efficiency speakers or for bi-amping systems where it might drive tweeters [3].

The Subjective "JLH Sound"

Beyond the specifications, the JLH 1969 is renowned for its subjective sound quality. It is often described as having a smooth, warm, and detailed character, frequently compared to that of a high-quality single-ended triode (SET) tube amplifier [12]. This is attributed to its simple signal path, lack of crossover distortion, and the nature of its harmonic distortion profile. The sound is often perceived as being very musical and non-fatiguing, making it a favorite for long listening sessions. Its ability to render vocals and acoustic instruments with a natural, lifelike texture is particularly praised by its proponents.

IWISTAO 1969 Ultralinear Class A Headphone Amplifier 16 to 150 Ohms K601 K701 for Low-impedance and Low sensitivity Headphone

IWISTAO 1969 Ultralinear Class A Headphone Amplifier 16 to 150 Ohms K601 K701 for Low-impedance and Low sensitivity Headphone

 

Building and Modifying the JLH 1969 Today

The enduring appeal of the JLH 1969 is fueled by the vibrant DIY audio community. Countless kits, PCBs, and fully assembled versions are available from vendors worldwide . For many, building a 1969 amplifier is a rite of passage, offering a rewarding project that results in a genuinely high-performance amplifier [15].

Over the decades, numerous modifications and improvements have been proposed. Common mods include:

  • Upgrading Components: Using modern, high-quality transistors, metal-film resistors, and audiophile-grade capacitors (especially for the input and output coupling capacitors) can yield significant sonic improvements.
  • Constant Current Source (CCS): Replacing the bootstrap circuit (C3 and associated resistors) with a dedicated CCS can improve linearity and power supply rejection [2].
  • DC-Coupled Output (OCL): Eliminating the large output coupling capacitor (C4) to create a direct-coupled (OCL) design. This requires adding a DC servo circuit to protect the speakers from DC offset but can improve bass response and transparency.
  • MOSFET Output Stage: Substituting the bipolar junction transistors (BJTs) in the output stage with MOSFETs. This creates a "1969M" variant, which can offer a different sonic signature and potentially operate in Class AB for higher efficiency [10].
  • Power Supply Enhancements: Using regulated power supplies or capacitor multipliers (electronic filters) can reduce noise and ripple, further cleaning up the sound [1].


Conclusion

The John Linsley-Hood 1969 Class A amplifier is more than just a vintage circuit; it is a testament to the power of elegant, thoughtful design. By prioritizing sonic purity and embracing simplicity, Linsley-Hood created an amplifier that not only challenged the performance of complex tube and solid-state designs of its day but also captured a musical quality that continues to resonate with listeners. Its legacy lives on not just in the yellowed pages of a 1969 magazine, but in the countless workshops and listening rooms around the world where enthusiasts continue to build, tweak, and enjoy its uniquely beautiful sound. It stands as a timeless reminder that in the pursuit of high fidelity, sometimes the simplest path is the most rewarding.


References

 

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