Beyond Power Ratings: A Qualitative Benchmark of Output Stage Topologies in Armorly Reference Amps

When comparing reference headphone amplifiers, the first number that catches the eye is often the power rating—watts into 32 ohms, milliwatts into 300 ohms. But seasoned listeners and engineers know that raw power figures can be misleading. Two amplifiers with identical output power can sound radically different, driving the same headphones, because of their output stage topology. At Armorly, we've spent countless hours listening and measuring across dozens of designs, and we've found that the topology—how the output stage is configured—shapes the listening experience more than any single specification. In this guide, we move beyond the wattage race to explore what truly matters: the qualitative behavior of output stages in reference-grade headphone amps.

Why Topology Matters More Than Wattage

The Limits of Power Specifications

Power ratings are measured under ideal conditions: a pure resistive load, a single frequency, and a fixed impedance. Real headphones present complex loads with reactive elements—capacitance from cables and inductance from voice coils. A topology that delivers clean power into a resistor may struggle with the back-EMF from a planar magnetic driver or the impedance swing of a multi-driver IEM. We've seen amplifiers rated at 2W into 32 ohms that sound thin and compressed with demanding planars, while a 500mW class-A design drives the same headphones with authority and finesse. The reason lies in the output stage's ability to maintain linearity under dynamic load changes.

What Topology Determines

The output stage topology governs several key behaviors: output impedance, damping factor, distortion profile, and how the amplifier handles reactive loads. A single-ended class-A stage offers low distortion at the cost of heat and efficiency. A push-pull stage can deliver more current but may introduce crossover distortion. An OTL (output transformerless) design eliminates transformer coloration but requires careful impedance matching. Each topology imparts a sonic signature that interacts with headphone characteristics—planars crave current, dynamics need voltage swing, and IEMs demand low noise floors. Understanding these interactions is the first step to choosing an amplifier that complements your headphones, not just one that meets a power target.

Class-A Single-Ended: The Gold Standard for Transparency

How It Works

In a class-A single-ended output stage, the output device (transistor or tube) conducts current for the entire signal cycle. This eliminates crossover distortion entirely, as there is no 'handoff' between complementary devices. The trade-off is efficiency: the device dissipates maximum power even at idle, generating significant heat. But for headphone listening, where power demands are modest (typically under 1W), the thermal penalty is acceptable in exchange for pristine linearity.

Sonic Character and Load Interaction

Class-A stages are known for a liquid, natural midrange with exceptional micro-detail retrieval. The absence of crossover artifacts means that low-level information—ambient cues, reverb tails, and harmonic textures—is preserved without grain. However, the output impedance of many class-A designs can be relatively high (often 10–50 ohms), which can alter the frequency response of headphones with varying impedance curves. For example, a 300-ohm Sennheiser HD 600 is relatively unaffected, but a 32-ohm planar like the Audeze LCD-2 may see a slight bass roll-off if the amp's output impedance exceeds 1 ohm. We recommend pairing class-A amps with high-impedance dynamic headphones or using an impedance adapter to flatten the response with low-impedance loads.

Composite Scenario: The DIY Builder's Choice

One builder we corresponded with built a pure class-A headphone amp around a single 2SK170 JFET and a MOSFET follower. With a 24V supply and a bias current of 300mA, the amp delivered only 300mW into 32 ohms but drove his HD 800 S with stunning clarity. He noted that the amp sounded 'effortless' compared to a 2W push-pull design he had previously built. The key was that his headphones never needed the full 2W; what mattered was the first few milliwatts, where the class-A stage excelled.

Push-Pull Topologies: Power and Efficiency with Caveats

Balanced and Complementary Designs

Push-pull output stages use two devices (NPN/PNP transistors or a pair of tubes) that handle opposite halves of the waveform. This arrangement cancels even-order harmonics and doubles the available current swing, making push-pull designs more efficient and capable of higher output power than single-ended class-A. Most commercial solid-state headphone amps use a push-pull topology, often with a class-AB bias to reduce crossover distortion at low signal levels.

Crossover Distortion and Bias Trade-offs

The Achilles' heel of push-pull is crossover distortion, which occurs at the zero-crossing point where one device turns off and the other turns on. Increasing the bias current (moving from class-B toward class-A) reduces this distortion but increases heat and reduces efficiency. Many reference amps bias their output stage into class-A for the first few hundred milliwatts—a 'class-A region' that covers typical listening levels—then transition to class-AB for higher power. This hybrid approach offers the best of both worlds: low distortion at low levels and high power when needed.

Load Handling and Damping Factor

Push-pull stages typically have low output impedance (under 1 ohm), yielding a high damping factor that gives tight control over driver motion. This is ideal for planars that need current delivery without frequency response shifts. However, some listeners find push-pull amps to sound slightly 'dry' or 'analytical' compared to class-A designs, possibly due to residual crossover artifacts or higher feedback levels. We've observed that well-designed push-pull amps with generous class-A bias can approach the transparency of single-ended designs while offering more power and versatility.

OTL and Hybrid Stages: Unique Characters for Specialized Needs

Output Transformerless (OTL) Tube Amps

OTL amplifiers use tubes in a push-pull configuration without an output transformer. This eliminates transformer distortion and bandwidth limitations, but it also means the amp's output impedance is inherently high—often 30–100 ohms or more. OTLs are best suited for high-impedance headphones (200 ohms and above), where the high output impedance has minimal effect on frequency response. With low-impedance planars, an OTL will drastically alter the sound, rolling off bass and boosting treble. Many OTL enthusiasts use them with Sennheiser HD 600/650/800 series or Beyerdynamic DT 880/990, where the high impedance allows the amp to deliver its full voltage swing without current limiting.

Hybrid Designs: Blending Tube and Solid-State

Hybrid amplifiers use a tube voltage gain stage followed by a solid-state output stage. This combines the harmonic richness of tubes with the low output impedance and high current capability of transistors. Hybrids are versatile: they can drive a wide range of impedances while offering a 'tube-like' midrange. The challenge is that the solid-state output stage must be carefully designed to avoid introducing its own artifacts. A poorly implemented hybrid can sound 'neither fish nor fowl'—lacking the transparency of a pure tube amp and the speed of a solid-state design. We've found that hybrids with a class-A MOSFET output stage tend to be the most successful, preserving tube warmth while maintaining control.

Composite Scenario: The IEM Listener's Dilemma

A reader shared his experience using a high-power push-pull solid-state amp with his 16-ohm IEMs. The amp's low output impedance was ideal, but the noise floor was audible as a faint hiss between tracks. Switching to a hybrid amp with a tube input and a class-A output stage reduced the noise floor to inaudible levels while adding a touch of harmonic richness that made his IEMs sound more 'musical.' The trade-off was that the hybrid amp's maximum output was lower, but for IEM listening, it was more than sufficient.

Output Impedance and Damping Factor: The Hidden Specs

Why Output Impedance Matters

Output impedance interacts with headphone impedance to create a voltage divider that alters frequency response. The rule of thumb is that output impedance should be less than 1/8th of the headphone's nominal impedance to keep frequency response variations below 0.5 dB. For a 32-ohm headphone, that means output impedance under 4 ohms. For a 300-ohm headphone, under 37.5 ohms. Many OTL and class-A amps exceed this threshold, which can be used intentionally to 'voice' the sound—for example, a high output impedance can boost the upper mids of a low-impedance headphone, adding presence. But this is a double-edged sword: it can also cause bass roll-off and inconsistent treble.

Damping Factor and Transient Control

Damping factor is the ratio of load impedance to output impedance. A high damping factor (low output impedance) means the amplifier can control the driver's motion after the signal stops, reducing ringing and overhang. For planars, which have low electrical damping, a high damping factor is beneficial to prevent 'sloppy' bass. For high-impedance dynamics, the driver's own mechanical damping is often sufficient, and a lower damping factor can add a pleasing 'bloom' to the sound. We've observed that many listeners prefer a damping factor between 10 and 50 for dynamic headphones, giving a balance of control and natural decay.

Practical Decision Framework: Matching Topology to Your Headphones

Step 1: Identify Your Headphone Type and Impedance

Start by knowing your headphone's nominal impedance and sensitivity. High-impedance dynamics (150–600 ohms) are forgiving of high output impedance and benefit from voltage swing—OTL and class-A tube amps are excellent choices. Low-impedance dynamics (16–80 ohms) need low output impedance and current—solid-state push-pull or hybrid amps work best. Planar magnetics (typically 20–50 ohms) are current-hungry and need low output impedance—a high-current push-pull or class-A solid-state amp with low output impedance is ideal. IEMs (8–32 ohms) demand low noise floor and low output impedance—a class-A solid-state amp with a quiet power supply is often the best match.

Step 2: Choose Your Sonic Priorities

If you value micro-detail and a natural midrange above all else, a class-A single-ended topology (tube or solid-state) is hard to beat, even if it limits your headphone choices. If you need versatility to drive multiple headphone types, a push-pull solid-state amp with class-A bias offers broad compatibility. If you want a 'tube sound' without the impedance constraints, a hybrid amp can be a great compromise. We recommend auditioning amps with your own headphones before committing, as the interaction between topology and load is highly personal.

Step 3: Consider Real-World Listening Levels

Most listening occurs at average levels of 70–85 dB SPL, requiring only a few milliwatts to tens of milliwatts. In this region, the amplifier's behavior at low power—its class-A region, noise floor, and distortion profile—matters far more than its maximum output. A 500mW class-A amp may sound better at normal volumes than a 5W class-AB amp that operates in class-B at low levels. Don't be seduced by high power ratings if you never use them.

Common Pitfalls and How to Avoid Them

Mismatched Output Impedance

The most common mistake is pairing a high-output-impedance amp with low-impedance headphones, resulting in a bass-light, treble-forward sound that many find fatiguing. Always check the output impedance of your amp and compare it to your headphone's impedance. If the ratio is greater than 1:8, expect audible frequency response shifts.

Ignoring Noise Floor

High-gain tube amps and some class-A designs can have elevated noise floors that are audible with sensitive IEMs. Listen for hiss between tracks or during quiet passages. If noise is an issue, consider a solid-state amp with a lower noise floor or a hybrid with a tube input but solid-state output.

Overlooking Thermal Management

Class-A amps run hot—often 50–60°C at the heatsink. Ensure adequate ventilation and avoid stacking components. A hot amp not only risks component failure but can also drift in bias over time, affecting sound quality. We recommend placing class-A amps on a separate shelf with at least 10 cm of clearance above and around the heatsink.

Conclusion: Listen Beyond the Numbers

Power ratings are a useful starting point, but they are only one piece of the puzzle. The output stage topology determines how an amplifier interacts with your headphones, shaping the sound in ways that no single number can capture. By understanding the strengths and limitations of class-A, push-pull, OTL, and hybrid designs, you can make an informed choice that prioritizes sonic synergy over raw specifications. At Armorly, we believe that the best amplifier is the one that disappears into the music—and that often comes from a topology chosen with care, not a spec sheet chosen by wattage. We encourage you to audition, compare, and trust your ears. The numbers will follow.