Micro 25 Tube Preamp

The Croft Acoustics Micro 25 is a valve preamplifier featuring a single moving-magnet disc input, as well as CD, tuner, and tape inputs. The internal construction is hardwired using single-strand wire. Audiophile-grade capacitors are used in the RIAA disc equalisation and coupling networks.

Each disc input uses a double triode with shunt feedback equalisation. The line stage has unity gain and is implemented as a cathode-follower buffer.

The disc input is suitable for both moving-magnet cartridges and higher-output moving-coil cartridges, provided the power amplifier's sensitivity is sufficient. The auxiliary input gain may be too low for some sources, so compatibility with other equipment should be checked.

Measured Performance

• Total Harmonic Distortion (aux input):
  • 20Hz: -76.0dB
  • 1kHz: -78.3dB
  • 20kHz: -76.5dB
• Intermodulation Distortion (aux input, 19/20kHz, rated power): -93.1dB
• Intermodulation Distortion (disc input, 19/20kHz, 0dBW): -41.0dB
• Noise (IHF, CCIR weighted):
  • Disc (mm) input: -73.5dB
  • Aux/CD input: -92.0dB
  • Residual, unweighted (volume at min): -86.5dB
• DC Offset (pre-amp): Left 0 mV, Right 0 mV
• Input Overload (IHF):
  • Disc (mm) input: 22.8dB (20Hz), 24.0dB (1kHz), 5.9dB (20kHz)
  • Aux/CD input: >20dB (20Hz, 1kHz, 20kHz)
• Stereo Separation:
  • Disc input: 61.2dB (20Hz), 62.8dB (1kHz), 36.0dB (20kHz)
  • Aux input: 101.3dB (20Hz), 82.0dB (1kHz), 57.3dB (20kHz)
• Channel Balance (disc, 1kHz): 0.37dB
• Input Data:
  • Disc (mm) input: Phono socket, 2.92mV sensitivity, 47kΩ loading, 110pF
  • Aux input: Phono socket, 550mV sensitivity, 480kΩ loading, 45pF
• Output (pre-amp, tape): 11.1V max, 500Ω impedance
• Disc Equalisation Error: +0.26dB (30Hz), -0.16dB (15kHz)
• RIAA Equalisation Accuracy: ±0.2dB (30Hz–15kHz), ±2.1dB (100kHz)

See later Model Micro 25R with a regulated power supply.

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