August 2005

Conrad-Johnson ACT2 Preamplifier: Measurements

All  preamplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test preamplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

• Measurements were made at 120V AC line voltage. Measurements made on the left channel unless otherwise noted.
• Gain, volume at maximum:
      IHF loading, Lch/Rch = 10.49X, 20.4dB / 10.77X, 20.6dB
      Instrument loading, Lch/Rch = 9.98X, 20.0dB / 10.24X, 20.1dB
• IHF sensitivity, input volts for standard IHF output of 0.5V:
      IHF loading, Lch/Rch: 50.1mV / 48.8mV
• Output noise versus bandwidth and volume-control position, wideband/A weighted:
      At maximum, gain @ 99 = 557.9µV / 215.6µV
      Worst case, gain @ 99 = 557.9µV / 215.6µV
      Unity gain, gain @ 76 = 309.4µV / 20.6µV
      Listening level, gain @ 40 = 449.9µV / 5.4µV
• AC line current draw at idle: 1.0A.
• Output impedance at 1kHz: 516 ohms.
• Input impedance at 1kHz:
      Gain at max (99), Lch/Rch = 67.1k ohms / 68.0k ohms
      Gain at 40, Lch/Rch = 12.0k ohms / 12.0k ohms
• This preamplifier inverts polarity.

General

The Conrad-Johnson ACT2 preamplifier utilizes two 6N30P tubes in parallel for each channel, forming a single amplifying stage for its signal circuitry. A switched resistor network is used for the volume control. Solid-state regulators are employed to supply high voltage and heater voltages for the tubes.

Chart 1 shows the frequency response of the ACT2 with the volume control set for unity gain (76) for 0.5V input and IHF loading. The two channels are tracking within 0.25dB at this point on the volume control. Also shown in Chart 1 is the frequency response with an instrument load. The bandwidth with the instrument load is about 200kHz.

In Charts 2a and 2b, the frequency response of the preamp is shown for the volume at maximum (99) and at a typical listening position (40). Volume control tracking at these settings is also within about 0.25dB.

Chart 3 shows how total harmonic distortion varies with input level and frequency for both IHF and instrument loading. Unique in my experience is the identical nature of the distortion curves for 20Hz, 1kHz, and 20kHz. This is superb performance.

A spectrum of the distortion and noise residual of a 1kHz test tone at 0.5V output with IHF loading is plotted in Chart 4. AC line hum harmonics here are rather numerous. The signal frequency harmonics consist of just the second harmonic.

IHF loading
Magenta line = left channel
Cyan line = right channel

Instrument loading
Red line = left channel
Blue line = right channel

Chart 2A - gain at maximum (99)

IHF loading
Magenta line = left channel
Cyan line = right channel

Instrument loading
Red line = left channel
Blue line = right channel

Chart 2B - gain at 40

IHF loading
Magenta line = left channel
Cyan line = right channel

Instrument loading
Red line = left channel
Blue line = right channel

IHF loading
Magenta line = 20Hz, 1kHz, 20kHz

Instrument loading
Red line = 20Hz, 1kHz, 20kHz

Red line = spectrum of 1kHz test-signal distortion and AC-line harmonics

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