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
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
amplifiers 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.