Classic Audio by Michael Fidler

SPARTAN 30 Headphone and Line Amplifier by Michael Fidler

SPARTAN 30


Construction

This page is due to be re-written with a more thorough description.

Features

Available in limited quantity direct for £450, the SPARTAN 30 headphone and line preamplifier takes a new look at traditional potentiometer-based preamplifiers. It further increases practical performance through original circuit architecture, resulting in a compact device that can replace traditional integrated amplifiers when used with active speakers.


SPARTAN 30 headphone amplifier

Front panel, showing controls

Three high-impedance RCA line inputs ensure minimal insertion loss with modern and vintage equipment alike, with electromechanical selection being performed via Panasonic relays right at the input connectors. A new active balanced volume control circuit with an inherently balanced output eliminates the need for extra circuitry on the line outputs for a shorter signal path with lower noise. Decreasing the value of the potentiometer from the more conventional 10kΩ to 5kΩ also reduced the noise power by a factor of two.


SPARTAN 30 rear connections

Rear panel, showing line connections

As the volume control is the most essential control in any sound system, a great deal of time was spent sourcing the potentiometer. During development, all the conventional devices tested in the design failed to meet the project’s channel tracking and lifespan requirements. After many hours of searching, a custom part, rated for 5 million cycles and initially intended for industrial applications, was then commissioned to ensure excellent stereo consistency for years to come.


Inside the SPARTAN 30 headphone amplfier

Internal view, showing build quality

Professional quality NJM5532 op-amps, mounted in turned IC sockets, are used for the line input and volume control stages, leading to a dynamic range of over 128dB with less than 5ppm distortion at 0dB gain. Thanks to the ±17V internal supply rails, they can deliver over 21V RMS of undistorted output at the XLR connectors and tolerate 10.5V RMS maximum input, making overload with even the hottest source signals very unlikely.

The headphone output, designed to work best with the most critical high-impedance types, uses purpose-designed NJM4556 high-current opamps, found in some of the best Japanese equipment from the 1990s onwards. Unlike these products, separate ICs for each channel working in parallel pairs allow for over 700mW per channel to be delivered into a 150Ω load while halving the effective noise power of each amplifier section. In addition to this, their operation as unity gain buffers maintains maximum feedback to keep distortion below 10ppm at 500mW output, so excellent clarity is maintained even at the most ear-splitting levels.

SPARTAN 30 preamplifier

Tilted view, showing dimensions

All of this is powered by a regulated split supply, using positive and negative power rails to keep non-linear amplifier power currents out of the reference ground where they might add distortion. As the power is delivered via an external linear transformer, its magnetic fields stay well outside of the compact enclosure. The linear operation also precludes any high-frequency switching noise from getting into the audio path via the RCA inputs and outputs.

Finally, a start up muting circuit operates 3 output relays to prevent any unpleasant thuds and bangs from getting through the system when powered on and off.


Specification

Parameter Measurement
Gain control range 17.5dB to -86dB (RCA), 23.5dB to -80dB (XLR)
Channel balance ±0.3dB (-20dB to maximum gain)
Frequency response (0dB gain) 15Hz to 40kHz, ±0.1dB
Output noise, 20Hz to 22kHz, 100Ω line source -108.5dBV (0dB gain), -113.5dBV (-10dB gain)
THD – line output (6dB gain) <0.0005% at 21V RMS output, 20Hz to 22kHz
THD – headphone output (6dB gain) <0.001% at 500mW into 150Ω, 20Hz to 22kHz
Maximum line input level 10.5V RMS, 15Hz to 40kHz
Maximum line output level 21V RMS (XLR), 10.5V RMS (RCA)
Maximum headphone output power 700mW into 150Ω, <0.0016% THD at 5kHz
Line input impedance 220kΩ
Line output impedance 75Ω (RCA), 150Ω (XLR)
Minimum line load impedance 1kΩ (RCA), 2kΩ (XLR)
Headphone output impedance 2Ω, damping factor of 75 into rated 150Ω
Power supply Split linear ±17V
Power consumption 6W at full output, 0.2W powered off
Dimensions (W*H*D) 172*62*132mm

Reviews

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