Designer Michael Fidler creates under the name Classic Audio Ltd. compact but high-quality audio components. The Spartan 30 is a headphone amplifier and preamplifier with three stereo cinch inputs. According to the company, a lot of attention has been paid to the volume control and the Spartan 30 has an XLR output and a cinch output for connecting a power amplifier (or a set of active speakers).
Spartan 30 by Michael Fidler
The Spartan 30 is hand built in Kent, Great Britain. It is a small headphone amplifier and preamplifier with three stereo RCA line inputs. These three high impedance cinch line inputs ensure minimal input loss in both modern and vintage equipment, with electromechanical selection carried out via Panasonic relays directly at the input connectors. A new active balanced volume control circuit with an inherently balanced output eliminates the need for additional circuitry on the line outputs for a shorter signal path with less noise. Lowering the potentiometer value from the more conventional 10 kOhm to 5 kOhm also reduced the noise level by a factor of two, according to Michael Fidler’s explanation.
Spartan 30: Built with traditional components for long life
The Spartan has been consciously designed to deliver excellent overall performance through the use of more traditional components that will be available for a long time. That’s nice in case something breaks in the future. So a long lifespan lies ahead and any repairs should be simple.
Volume control
Since the volume control is the most essential control in any sound system, a lot of time was spent finding the potentiometer. High demands were placed on channel equality and potentiometer lifespan. After extensive research and testing, a custom-made part, rated for 5 million turns and initially intended for industrial applications, was put into service to ensure excellent stereo consistency for years to come.
Input circuit
Professional-quality NJM5532 op-amps, mounted in twisted IC sockets, are used for the line input and volume control stages, delivering a dynamic range of over 128 dB with less than 5 ppm distortion at 0 dB gain. Thanks to the ±17V internal power rails, they can deliver more than 21V RMS undistorted output on the XLR connectors and tolerate a maximum input of 10.5V RMS, making overload from even the hottest source signals highly unlikely.
Headphone output
Designed to work best with the most critical high impedance types, the headphone output uses specially designed high current NJM4556 op amps, found in some of the best Japanese equipment from the 1990s onwards. Unlike these products, separate ICs for each channel operating in parallel pairs allow more than 700 mW per channel to be delivered into a 150 Ohm load, while halving the effective noise level of each amplifier section. Additionally, their operation as unity gain buffers ensures maximum feedback to keep distortion below 10 ppm at 500 mW output power, ensuring excellent clarity is maintained even at the most deafening levels.
Power supply
All this is powered by a regulated split power supply, using positive and negative power rails to keep non-linear amplifier power currents out of the reference ground, preventing them from causing distortion. Because the power is supplied via an external linear transformer, the magnetic fields remain far outside the compact housing. The linear operation also prevents high-frequency switching sounds from entering the audio path via the RCA inputs and outputs.
Finally, a start-up damping circuit operates three output relays to prevent potential loud switch clicks from permeating the system when it is turned on and off. When a jack plug is inserted into the headphone socket, the two line output relays automatically mute the line output, eliminating the need to switch off active speakers when the user wants to use headphones.
Amplifier specifications
The manufacturer has opted for a wide gain control range design with a higher than usual upper limit of 17.5 dB to boost even the weakest line levels below 150 mV of older consumer equipment to the maximum level for most power amplifiers and active speakers ( 1 Volt at the cinch output and 2 Volt via XLR). The high-precision potentiometer that controls this gain has been hand-selected for each unit to meet specifications and ensures that the channel balance does not differ by more than 0.3 dB at the most commonly used settings from -20 dB to maximum gain, and another 0.5 dB. between -20dB and -30dbB as the minimum gain approaches.
The frequency response remains flat to 0.1 dB well outside the audio band (specified 15 Hz to 40 kHz +/- 0.1 dB), so no drop in response at either end of the spectrum can color the sound. The dynamic range is approximately 114 dB.
Distortion remains comfortably below 0.001% at normal listening levels and does not exceed this across the entire audio band when driving half a watt into a 150 Ohm load or 300 mW into a 300 Ohm load.
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