Transistor Matching for Audio Amplifiers: Why It Matters and How to Do It Right

PUBLISHED BY IWISTAO · DIY Audio / Electronics

A practical guide to matching bipolar transistors in audio circuits — from differential input pairs to parallel output stages, with real-world examples from popular amplifier designs.

Contents

1. Introduction
2. Why Match Transistors in Audio Circuits?
   1. Differential Input Stages
   2. Complementary Pairs and VAS Stages
   3. Parallel Output Stages
3. Key Parameters to Match
   1. hFE (DC Current Gain)
   2. Vbe (Base-Emitter Voltage)
   3. Vgs (Gate-Source Voltage) — For MOSFETs
4. Popular Audio Transistors and Their Matching Considerations
5. Practical Matching Procedure
   1. Step 1: Group by hFE Rank
   2. Step 2: Low-Current Test
   3. Step 3: High-Current Test
   4. Step 4: Create Matched Pairs
6. Built-in Matched Pairs: A Convenient Alternative
7. FAQ
   1. Does matching improve sound quality?
   2. How close does the match need to be?
   3. Can I match NPN and PNP?
   4. Matching output transistors in Class-AB?
   5. What if I don’t match the input pair?
8. Conclusion

Introduction

Walk into any serious DIY audio forum and you will see builders swapping stories about transistor matching. Some treat it as a rite of passage; others dismiss it as audiophile voodoo. The reality lies somewhere in between. Matching transistors does not magically transform an average amplifier into a world-class design, nor does it improve frequency response or transient behavior. What it does do — when done correctly — is improve DC stability, reduce distortion in specific circuit topologies, ensure reliable current sharing in parallel output stages, and minimize offset voltage in differential input stages.

This article explains what transistor matching actually achieves, which parameters matter, and how to match transistors in practice. We use concrete examples from real audio circuits — differential input pairs built with 2SC2240/2SA970, VAS stages using 2N5551/2N5401, and output stages with MJL3281/MJL1302 power devices.

Why Match Transistors in Audio Circuits?

1. Differential Input Stages

The differential pair (also called a long-tailed pair) is the most common input stage in solid-state audio amplifiers. It consists of two identical transistors sharing a common emitter (or source) current. The difference between the two base voltages is amplified and passed to the next stage.

When the two transistors are not matched, the differential pair generates a DC offset voltage at its output. This offset propagates through the amplifier chain and appears as unwanted DC at the speaker terminals. More subtly, unmatched pairs produce higher even-order harmonic distortion because the transfer curves of the two devices differ [1].

In a typical power amplifier with a differential BJT input pair, matching the transistors to within 2 mV of Vbe and 10% of hFE reduces DC offset at the output from potentially hundreds of millivolts to well under 50 mV — without relying on a DC servo [2].

The most widely used transistor pair for audio differential input stages is the 2SC2240 (NPN) and 2SA970 (PNP) from Toshiba. These are low-noise audio transistors, but their noise figure depends strongly on source resistance and collector current. Typical datasheet NF values for the 2SC2240 are around 2–4 dB under specified test conditions (e.g., RG = 100 Ω, VCE = 6 V, IC = 100 µA, f = 1 kHz), while the 2SA970 is typically around 3 dB — not a universal 1 dB figure. Their transition frequency (fT) of 100 MHz ensures excellent linearity throughout the audio band.

 

Figure 1: A BJT differential pair (long-tailed pair), the most common input stage in audio power amplifiers. Matching Q1 and Q2 for Vbe and hFE minimizes DC offset at the output.

2. Complementary Pairs and VAS Stages

The voltage amplifier stage (VAS) of most power amplifiers uses a complementary pair of NPN and PNP transistors. Common choices include the 2N5551 (NPN) and 2N5401 (PNP) — high-voltage devices rated at Vceo = 160 V and Ic = 600 mA, with fT around 100 MHz. These are workhorse transistors found in countless amplifier designs.

In a push-pull VAS, mismatched NPN and PNP gain creates asymmetry in the drive signal delivered to the output stage. This asymmetry shows up as elevated second-harmonic distortion. Matching hFE between the complementary devices — at the actual operating current of the VAS, typically 5–20 mA — brings the positive and negative half-cycles into balance.

Some low-feedback amplifier designs absolutely require that NPN and PNP transistors be matched, because there is insufficient feedback to linearize the circuit unless the devices track each other closely [3].

3. Parallel Output Stages

High-power amplifiers routinely use multiple output transistors in parallel to handle the required current. If these transistors are not matched, the device with the highest gain (or lowest Vbe) hogs the current, runs hotter, and becomes even more conductive — a runaway condition that can destroy the output stage.

For parallel output transistors, both Vbe and hFE must be matched. A good target is ±10 mV for Vbe and within 10% for hFE at the quiescent current and at a current near the expected peak [3]. The popular MJL3281A (NPN) / MJL1302A (PNP) power pair from ON Semiconductor — rated at 260 V / 15 A / 200 W with fT = 30 MHz — is a common choice for high-end output stages and benefit significantly from matching.

 

Figure 2: Two matched MJL3281A output transistors in parallel. Matching Vbe and gain reduces the risk of current hogging, while emitter resistors, proper bias compensation, and common heatsinking provide the main protection against thermal runaway. The 0.1Ω emitter resistors provide additional current-sharing assistance.

Key Parameters to Match

hFE (DC Current Gain)

hFE is an important matching parameter, but it is not always the most important one. For differential input pairs, Vbe or collector current at the same bias condition often matters more for DC offset. For parallel output devices, Vbe, gain, emitter resistors, and thermal coupling all determine current sharing. hFE = Ic / Ib varies with collector current, temperature, and even between devices from the same production batch. Most transistor datasheets specify hFE at one or two current points, but real-world audio circuits operate across a wide range. A matching approach that tests only at a single current — say, 1 mA — overlooks gain differences that appear at 10 mA or 100 µA.

Many Japanese transistors, including the 2SC2240 and 2SA970, are sold in hFE classification ranks marked by a suffix letter on the package. The standard Toshiba ranks for these devices are:

hFE Rank	Gain Range (Vce = 6V, Ic = 1mA)	Typical Use
O	100 – 200	General purpose
Y	120 – 240	Standard audio
GR	200 – 400	Low-noise preamp / phono
BL	350 – 700	High-gain, low-noise input

Buying transistors from the same hFE rank is a good start, but even within the same rank, individual devices can vary by a factor of two. For critical differential pairs, further hand-matching is essential.

Vbe (Base-Emitter Voltage)

For BJTs operated in parallel, Vbe matching is as important as gain matching. Vbe has a temperature coefficient of approximately −2 mV/°C, so temperature differences between devices can easily overwhelm a close match. All devices under test must be at the same temperature, and for output transistors, mounting all devices on a single heat sink is mandatory [3].

Vgs (Gate-Source Voltage) — For MOSFETs

When using lateral or vertical MOSFETs in Class-A or Class-AB output stages, Vgs is the analogue of Vbe. Lateral audio MOSFETs generally have more benign current-sharing behavior at higher currents, but vertical MOSFETs still require careful biasing, source resistors, and thermal design. Vgs matching remains useful when devices are paralleled.

Popular Audio Transistors and Their Matching Considerations

Transistor	Type	Vceo	Ic max	fT	NF (typ)	Typical Role	Matching Priority
2SC2240	NPN	120 V	100 mA	100 MHz	2–4 dB typ., condition-dependent	Diff. input pair	hFE + Vbe (±2 mV)
2SA970	PNP	120 V	100 mA	100 MHz	~3 dB typ., condition-dependent	Complementary input	hFE + Vbe (±2 mV)
2N5551	NPN	160 V	600 mA	100 MHz	—	VAS, current source	hFE (±10%)
2N5401	PNP	160 V	600 mA	100 MHz	—	VAS complement	hFE (match to 2N5551)
BC550C	NPN	45 V	100 mA	150 MHz	1.0 dB	Phono preamp input	hFE (±5%)
BC560C	PNP	45 V	100 mA	150 MHz	1.0 dB	Phono preamp complement	hFE (match to BC550C)
MJL3281A	NPN	260 V	15 A	30 MHz	—	Output stage	hFE + Vbe (±5 mV)
MJL1302A	PNP	260 V	15 A	30 MHz	—	Output complement	hFE + Vbe (±5 mV)
2SC5200	NPN	230 V	15 A	30 MHz	—	Output stage	hFE + Vbe (±5 mV)
2SA1943	PNP	230 V	15 A	30 MHz	—	Output complement	hFE + Vbe (±5 mV)

Practical Matching Procedure

Step 1: Group by hFE Rank

Start by purchasing transistors from the same hFE rank. For 2SC2240, this means buying all "GR" rank devices for a given project. This immediately narrows the spread from a possible 10:1 range to roughly 2:1.

 

Figure 3: A basic constant-base-current test circuit for hFE matching. The emitter is grounded (Re shorted) for accuracy; adding an emitter resistor introduces local negative feedback that distorts the hFE calculation. The voltage across Rc (measured with a DMM) is proportional to collector current. For serious work, test at multiple current points by switching Rb values.

Tip for differential pairs: A constant-current Vbe test is often more useful than a simple hFE test. Force the same collector current through each transistor and record Vbe after thermal stabilization — this directly measures the offset that matters for DC performance.

Step 2: Low-Current Test (Quiescent Operating Point)

1. Pick one transistor from the batch as a reference.
2. Adjust the test circuit to produce the target quiescent current — for a differential pair using 2SC2240, this is typically 1–2 mA per device.
3. Record the Vbe and collector current (via voltage across Rc).
4. Without adjusting the circuit, swap in the next transistor. Record its readings.
5. Repeat for all devices. Devices whose current deviates by more than 10% from the reference are set aside.

Step 3: High-Current Test (Near Peak Ic)

From the surviving devices, run a second test at a higher current — 10–20 mA for small-signal transistors, or 1–5 A for output devices. Use a heat sink and limit the test duration to a consistent interval (e.g., 10 seconds per device). Allow the heat sink to return to the same starting temperature between measurements.

If you can get transistors that measure within 10% of each other for both the high and low current tests, this is a good result [3].

Step 4: Create Matched Pairs

Sort devices by their multi-point hFE and Vbe readings. The closest pairs become your differential input pair. The next-closest sets can be used for current mirrors, cascode stages, or other positions where matching is beneficial but less critical.

Built-in Matched Pairs: A Convenient Alternative

Several manufacturers produce monolithic matched transistor pairs — two transistors fabricated on the same silicon die. Because they share the same thermal environment and come from adjacent positions on the wafer, these offer far better matching than any hand-selected discrete pair. Popular options include:

• MAT02 / MAT03 (Analog Devices) — Ultra-low-noise matched NPN/PNP pairs with Vbe matching to ±50 µV
• SSM2210 / SSM2220 (Analog Devices) — Low-noise matched NPN/PNP pairs, Vbe matching to ±200 µV
• THAT 300 / THAT 320 (THAT Corporation) — Large-geometry, low-rbb' matched arrays designed for audio input stages
• LM394 / LM194 (National, now obsolete) — The classic "super-match" pair, still available as NOS

Monolithic pairs achieve Vbe matching of tens of microvolts — orders of magnitude better than hand-matched discretes. They also track temperature almost perfectly, since they share the same die. For the ultimate in DC precision, especially in DC-coupled preamplifiers and phono stages, these are the gold standard.

FAQ

Does matching transistors improve sound quality?

For most well-designed amplifiers with sufficient global negative feedback, matched transistors do not produce an audible improvement in sound quality. The primary benefit is DC stability (lower offset, better thermal tracking) and reliability (equal current sharing in parallel stages). In low-feedback or zero-feedback designs, matching becomes far more important because there is less feedback to linearize the circuit.

How close does the match need to be?

For differential input pairs, aim for Vbe within ±2 mV and hFE within 10% at the operating current. For parallel output devices, both Vbe and hFE should be within 10%. A 10% match across multiple current points is considered a good practical result for hand-matched discrete transistors [3].

Can I match NPN and PNP transistors to each other?

You can match their hFE values at a given current, but their Vbe values will always differ because of the fundamental physics of NPN versus PNP junctions. In well-designed complementary circuits, this Vbe difference is accounted for in the biasing arrangement, so hFE matching is the more useful goal between NPN and PNP pairs.

Is it worth matching the output transistors in a Class-AB amplifier?

Yes, if they are connected in parallel. For a single NPN/PNP pair in a standard emitter-follower output stage, matching between the NPN and PNP is less critical because feedback linearizes the stage. However, if you have multiple NPN devices in parallel (or multiple PNP devices), matching them to each other is essential for preventing current hogging. Emitter resistors, proper bias compensation, and common heatsinking provide the main protection against thermal runaway.

What happens if I don't match the differential input pair?

You will likely see higher DC offset at the amplifier output — potentially hundreds of millivolts. This offset heats up the speaker voice coil even with no music playing. Unmatched pairs also produce higher even-order harmonic distortion, though this is usually masked by the negative feedback loop in typical designs [1].

Conclusion

Transistor matching is not a magic bullet for better sound, but it is a disciplined engineering practice that pays off in measurable ways: lower DC offset, more reliable parallel operation, and reduced distortion in specific circuit topologies. For the DIY builder working with discrete audio circuits, understanding which transistors to match — and how to do it — is an essential skill.

Start with transistors from the same hFE rank. Test at the currents your circuit actually uses. Control temperature carefully. Accept that ±10% is a practical, useful match. And if you need the ultimate in precision, consider a monolithic matched pair — two transistors on one die, sharing the same temperature and process, will outperform any hand-matched discretes.

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References

1. Self, Douglas. Audio Power Amplifier Design, 6th Edition. Focal Press, 2013. Chapter 7: "The Input Stage."
2. Slone, G. Randy. High-Power Audio Amplifier Construction Manual. McGraw-Hill, 1999.
3. Elliott, Rod. "Matching Power and Driver Transistors." Elliott Sound Products (ESP), 2025. https://sound-au.com/transistor-matching.htm
4. Toshiba Semiconductor. "2SC2240 Datasheet: Silicon NPN Epitaxial Type (PCT Process)." https://handsontec.com/pdf_files/2SC2240.pdf
5. ON Semiconductor. "MJL3281A / MJL1302A Datasheet: Complementary Power Transistors."
6. Analog Devices. "MAT02: Low Noise, Matched Dual Monolithic NPN Transistor." https://www.analog.com/en/products/mat02.html
7. THAT Corporation. "THAT 300 Series: Low-Noise Matched Transistor Arrays." http://www.thatcorp.com/300-series_Matched_Transistor_Arrays.shtml
8. Diodes Incorporated. "Matched Pair Transistors." https://www.diodes.com/products/discrete-semiconductors/

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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

 

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

• [1] Linsley-Hood, J. L. (1969, April). "Simple Class A Amplifier." Wireless World. Available at: https://sound-au.com/tcaas/jlh1969.pdf
• [2] "Hood 1969 Class A 10W Power Amplifier Circuit Analysis)." (2024, May 27). CSDN. Available at: https://blog.csdn.net/qq_35240747/article/details/136416355
• [3] Linsley-Hood, J. L. (1969). "Simple Class A Amplifier" (annotated version). kaipanji.com.cn. Available at: http://kaipanji.com.cn/thread-80026-1-1.html
• [4] Dalmura, P. R. S. (2020, October 15). "The Williamson Amplifier: A brief history of the worlds first, true high fidelity amplifier." Available at: https://dalmura.com.au/static/The%20Williamson%20Amplifier%20History.pdf
• [5] Williamson, D. T. N. (1947). "Design for a High-Quality Amplifier." Wireless World.
• [6] "The Williamson Amplifier." (2012, January 5). r-type.org. Available at: https://www.r-type.org/pdfs/dtnw-amp.pdf
• [7] "High-Efficiency Power Amplifiers." (2019, February 19). Universitat Politècnica de Catalunya. Available at: https://upcommons.upc.edu/bitstreams/0922f39d-9fdb-4504-855d-2ba0bb04619d/download
• [8] "Class A amplifier." (2025, August 17). Analog Circuit Design. Available at: https://analogcircuitdesign.com/class-a-amplifier/
• [9] Image source from CSDN blog post. Available at: https://i-blog.csdnimg.cn/blog_migrate/04e0e554ccfb18de636a62302fc45dc2.png
• [10] "1969M Small Class A Amplifier Circuit Diagram and Analysis)." (2026, January 2). eeworld.com.cn. Available at: https://www.eeworld.com.cn/RDesigns_detail/77251
• [11] Image source from CSDN blog post. Available at: https://i-blog.csdnimg.cn/blog_migrate/c1f8381e9be01517f06d105699fb2b74.png
• [12] "BRZHIFI Power Amplifier Hood 1969 Class a Hifi Audio Close to Tube Amplifier." Alibaba.com. Available at: https://www.alibaba.com/product-detail/BRZHIFI-Power-Amplifier-Hood-1969-Class_1601214135434.htm
• [13] "DIY 1969 Small Class A Amplifier)." Quark Document. Available at: https://doc.quark.cn/preview/tongyongshenghuo-qita-qita/0A038AE06FFE756F5C50288FE841C163