April 2005

CR Developments Romulus Integrated Amplifier: Measurements

All integrated-amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test integrated amplifiers 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 with both channels being driven.
• Measurements made on left channel unless otherwise noted.
• Input sensitivity for 1W output into 8 ohms: 12.1mV
• Gain, output voltage divided by input voltage, volume at maximum: 233.6X, 47.4dB
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 1.46mV, -65.7dBW; A weighted 0.367mV, -77.7dBW.
• AC line current draw at idle: 1.2A.
• Output impedance at 50Hz: 1.6 ohms.
• This integrated amplifier inverts polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 22.0W
• 8-ohm load at 10% THD: 35.7W
  
  
• 4-ohm load at 1% THD: 21.0W
• 4-ohm load at 10% THD: 25.0W

General

The CR Developments Romulus is a low-/medium-power integrated tube amplifier utilizing a pair of 6L6WGC output tubes in each channel. Gain of the unit is somewhat higher than necessary for typical line-level sources that will likely cause the volume control to be turned way down for normal listening levels.

Chart 1 shows the frequency response of the amp with varying loads. The high-frequency response is not very extended in this design as the 3dB down point is about 20kHz. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open circuit, 8-ohm, and 4-ohm loading, is of a typical value for tube amplifiers. The variation with the NHT dummy load in the audio range is of the order of +1/-3dB. The frequency response was quite independent of volume-control setting. This plot was made with the reference volume control position as set for 0.5V input to produce 5W output into an 8-ohm load.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. This design, with its single output connection for speaker loads, is more optimized for 8 ohms rather than 4 ohms. As can be seen, the power attainable is greater for 8-ohm loading for a given distortion amount.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Amount of rise in distortion at low and high frequencies is reasonable for a modest-powered integrated amplifier such as the Romulus. The distortion falls off above 10kHz because of the amplifier’s limited high-frequency bandwidth.

Damping factor vs. frequency is shown in Chart 4 and is of a value typical of many tube amplifiers.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the AC-line harmonics are quite numerous and intermodulation components of line harmonics with signal harmonics are also very numerous and visible. Indicative of good push-pull balance, the test-signal harmonics are dominantly odd order and tail off fairly rapidly with frequency.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load

(line up at 5W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

8-ohm output loading
Cyan line: 30W
Blue line: 10W
Magenta line: 5W
Red line: 2W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 8-ohm load

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