TEST: SPL Auditor

 A Headphone Amplifier!

At first glance, many readers might shrug their shoulders and think, "So what? Every mixing console has a built-in headphone amp." While this is true, it's worth noting that every mixing console also has built-in equalizers, yet you'll find multiple outboard EQ units in any ambitious studio. So why has SPL introduced a standalone headphone amplifier like the Auditor, which not only features a single headphone output but also comes with a hefty price tag of nearly 900 euros? It's certainly not due to product suicide or boredom in the development department.

Construction:

Let's address this upfront - the Auditor has almost no relation to its high-end counterpart, the Phonitor, which generates a spatial listening situation through complex phase management. The Auditor is indeed "just" a pure headphone amplifier, but one with an extremely high-quality build.

As with many products from SPL, the Auditor employs 120-volt technology. The house-made discrete SPL-SUPRA operational amplifiers operate at 120 volts, approximately double the operating voltage used in competing products. These voltage values are particularly beneficial in terms of dynamic range and overload resistance. The Supra op-amps boast a signal-to-noise ratio of 116 dB and offer an additional 34 dB of overload resistance, resulting in a dynamic range of about 150 dB.

The amplifier is suitable for all headphones with an impedance of 30 ohms or higher, provided they use a standard stereo jack plug. The only switching option on the front panel is a mono switch, which allows for checking the mono compatibility of the phase position. If the signal becomes quieter or frequency cancellations occur when this switch is activated, the signals are not mono-compatible.

A blue LED on the front indicates the operating status. Weighing 2.65 kilograms and measuring 95 x 210 x 315 mm (H x W x D), the product can be considered relatively compact. The internal components are housed in brushed aluminum.

The Auditor rests on four sturdy, screwed-on feet with rubber pads on the underside to prevent slipping. Two foldable front feet allow the angle of the device to be adjusted upwards by a few degrees. The front panel is dominated by an oversized volume control. This knob can attenuate the input signal by -80 dB or boost it by 10 dB. The 0 dB position is at 2 o'clock.

On the rear panel, the connection options consist exclusively of lockable and balanced XLR male and female connectors. As always with SPL, the labels are readable from both above and below - a simple feature that cannot be overvalued in studio operations. How often have I crawled around in a tangle of cables with a flashlight and hand mirror because a cable needed to be swapped? An IEC connector with a fuse and a voltage selector switch complete the rear panel layout.

Practice:

Now, why use a headphone amplifier at all, when conventional wisdom suggests that you can't really mix a proper production using the extreme stereo function of a regular headphone, and that one should avoid falling into this trap? While this is true, headphones are still unbeatable when it comes to control functions.

As a complementary alternative to loudspeaker monitoring, the extremely high precision of headphones in terms of detail resolution can reveal significantly more than comparable speakers. Excluding room influences, acoustic pitfalls such as noise, clicks, or pops, which often occur in edits, are placed under an acoustic magnifying glass and are therefore easier to detect.

From the first notes, the high impulse fidelity of the Auditor is noticeable. There's no compression to speak of, let alone clipping that cuts off the half-waves. Even high dynamic jumps are cleanly transmitted without any clouding of the sound impression. Due to the neutral design, the individual tracks sometimes sound a touch more centered than with corresponding loudspeaker reproduction, but these are really only nuances that a trained ear can adjust to within a short time.

Looking at the frequency responses, phase response, and distortion factor of the Auditor, one can confidently speak of linear reproduction, with the first two diagrams seeming to be drawn with a ruler. According to the manufacturer, the product internally processes everything between 5 Hz up to 200 kilohertz. By the way, the headphones should be unplugged before switching off the device, as SPL has dispensed with any discharge protection circuits for sonic reasons.

Conclusion:

Admittedly, the Auditor's almost brash suggested retail price might make even ambitious sound engineers flinch briefly, but only until they can test the product in practice. Developed and built in Germany, the Auditor is sonically extremely neutral and high-resolution, very resistant to overload, and almost fatigue-free in continuous operation. In conjunction with loudspeaker monitoring, it offers a real enrichment for studio operations.

In times when headphone operation is becoming increasingly common due to sometimes acoustically questionable room constructions, the Auditor offers more security in terms of sound assessment, especially in the detailed editing area. Personally, I would like to see a 19-inch rack version of the product, as the standalone solution seems to struggle to find the right place in studio operations.

Technical Specifications:

- Frequency response: 5 Hz - 200 kHz (+0/-3 dB)
- THD+N: < 0.001% (1 kHz, +4 dBu)
- Dynamic range: > 150 dB
- Signal-to-noise ratio: 116 dB
- Crosstalk attenuation: > 90 dB
- Input impedance: 20 kΩ balanced
- Maximum input level: +24 dBu
- Gain range: -80 dB to +10 dB
- Output impedance: < 0.5 Ω
- Maximum output level: +24 dBu
- Maximum output power: 2 x 1.25 W into 32 Ω

Power Supply:
- Operating voltage: 230 V AC / 50 Hz (115 V AC / 60 Hz)
- Power consumption: max. 15 W
- Fuse: 230 V: 315 mA slow-blow, 115 V: 630 mA slow-blow

Dimensions and Weight:
- Width: 210 mm (8.27")
- Height: 95 mm (3.74")
- Depth: 315 mm (12.4")
- Weight: 2.65 kg (5.84 lbs)

The Auditor's 120-volt technology is a key feature that sets it apart from many other headphone amplifiers on the market. This high-voltage design allows for an exceptionally wide dynamic range and headroom, which is crucial for accurate monitoring in professional audio environments. The discrete SUPRA operational amplifiers, designed and manufactured by SPL, are at the heart of this technology.

These op-amps operate at voltages significantly higher than those found in typical audio equipment, which usually run at around 30 to 60 volts. The increased voltage allows for several benefits:

1. Increased Headroom: The higher voltage provides more room for signal peaks before clipping occurs, resulting in cleaner, more transparent audio reproduction.

2. Lower Noise Floor: The 120-volt design contributes to the Auditor's impressive signal-to-noise ratio of 116 dB, ensuring that even the quietest passages in a mix are reproduced accurately without being masked by system noise.

3. Improved Transient Response: The high-voltage design allows for faster slew rates, meaning the amplifier can respond more quickly to rapid changes in the audio signal. This results in more accurate reproduction of transients and improved overall clarity.

4. Enhanced Dynamic Range: With a dynamic range exceeding 150 dB, the Auditor can faithfully reproduce both the softest whispers and the loudest crescendos in a recording without compression or distortion.

The Auditor's mono switch is a valuable tool for checking phase coherence and mono compatibility of stereo mixes. When engaged, it sums the left and right channels, allowing the engineer to identify any phase cancellation issues that might occur when the mix is played back in mono. This is particularly important for ensuring that a mix translates well across various playback systems, including those that sum stereo to mono.

The oversized volume control on the front panel is not just for show - it's a high-precision potentiometer that allows for fine adjustments to the listening level. The wide range of adjustment (-80 dB to +10 dB) provides flexibility for different headphone sensitivities and personal listening preferences.

The balanced XLR inputs and outputs on the rear panel are another nod to professional use. Balanced connections offer better noise rejection over long cable runs, which is crucial in studio environments where the Auditor might be located some distance from the mixing console or audio interface.

In terms of build quality, the Auditor's all-metal chassis provides excellent shielding against electromagnetic interference, which is crucial for maintaining the unit's low noise floor. The adjustable front feet allow for ergonomic positioning, which can be particularly useful in cramped studio environments where space is at a premium.

While the Auditor doesn't offer the spatial simulation features of its more expensive sibling, the Phonitor, its focus on pure signal quality makes it an excellent choice for critical listening tasks. Its ability to drive a wide range of headphones, from low-impedance consumer models to high-impedance professional units, adds to its versatility in the studio.

The lack of a protective circuit for headphone disconnection, while potentially risky, is a deliberate choice by SPL to maintain the purest possible signal path. This decision underscores the company's commitment to audio quality above all else, even if it requires more careful operation by the user.

The dynamic range of 150 dB, as specified for the SPL Auditor headphone amplifier, is an exceptionally high value that manifests itself in several aspects in everyday use:

1. Noise-free operation: Such a high dynamic range means that even during very quiet passages, practically no inherent noise from the amplifier is perceptible. This allows for extremely detailed reproduction even at low volumes.

2. Rich in detail: The large dynamic range allows for the perception of the finest nuances in music, from the quietest to the loudest parts. This is particularly important for classical music or audiophile recordings that often have a very wide dynamic range.

3. Overload resistance: With a 150 dB dynamic range, the amplifier can process very loud signal peaks without distortion, avoiding clipping or compression.

4. Realistic reproduction: The human auditory sense has a dynamic range of about 120-130 dB. At 150 dB, the Auditor significantly exceeds this range, allowing for a true-to-life reproduction of the entire audible spectrum.

5. Flexibility: The large dynamic range allows for optimal driving of various types of headphones with different sensitivities, without having to compromise on sound quality.

6. Precise control: For sound engineers and producers, this high dynamic range means they can hear even the smallest errors or artifacts in a recording, which is essential for precise mixing and mastering.

7. Future-proofing: Although current recordings rarely utilize the full 150 dB dynamic range, the amplifier is prepared for future high-resolution audio formats that might potentially use a larger dynamic range.

It's important to note that the full benefit of this enormous dynamic range only comes into play in conjunction with high-quality headphones and high-quality audio recordings. In everyday use with average recordings and headphones, one probably won't be able to exploit the full range of these 150 dB, but the reserves ensure consistently clean and distortion-free reproduction.

In conclusion, the SPL Auditor represents a no-compromise approach to headphone amplification. Its high-voltage design, meticulous attention to signal integrity, and robust construction make it a valuable tool for audio professionals who demand the utmost accuracy in their monitoring chain. While its price point may put it out of reach for some, those who invest in the Auditor will find a device capable of revealing nuances in their mixes that might otherwise go unnoticed, potentially leading to better-sounding final products.