Power output with 1kHz test signal
- 8-ohm load at 1% THD: 4.5W
- 8-ohm load at 10% THD: 11.8W
- 4-ohm load at 1% THD: 2.2W
- 4-ohm load at 10% THD: 9.9W
The Opera Consonance Cyber-10 Signature is a low-power
integrated tube amplifier utilizing a great complement of tubes, including a 12AX7 first
tube, a 6SN7 phase-inverter driver, and most interesting of all, a pair of 2A3 directly
heated triodes operated in push-pull for output. These 2A3s are of the single-plate
variety and made in Russia. I, being interested in tubes for a long time, was more
familiar with the more common, dual-plate variety of 2A3s.
When I received the amp for measurement, one of the four
output tubes had an open filament. Being determined to listen to this amp, I dug into my
old bag of tubes dating back to my high school days and found, to my amazement and relief,
exactly one pair of ancient 2A3 tubes. The old tubes worked just fine, and the amp sounded
quite musical in my setup. Replacement tubes were obtained from the importer and the
measurements were made with the intended tubes installed.
Directly heated triode output tubes frequently have hum
balance controls. In the case of the Cyber-10 Signature, I had adjusted these for minimum
hum, and the right channel had somewhat more hum, reflected in the wideband number.
Normally, if the channels are more or less similar, only one channel's data is shown.
Chart 1 shows the frequency response of the amp with
varying loads. As can be seen, the output impedance, as judged by the closeness of spacing
between the curves of the open circuit as well as 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/-1.5dB. Frequency response as a function of volume control setting was
plotted over a range of 0dB down to -60dB, and the response was found to be quite constant
over this range. The level tracking of the two channels stayed within about 1.5dB over
this range, with the right channel usually being the higher output of the two.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. The
amp is well matched to produce the best result with tap loading -- meaning with an 8-ohm
load on the 8-ohm output and a 4-ohm load on the 4-ohm output. However, power and
distortion are still reasonable with loads of half and double the output-tap value.
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 very respectable for a low-power integrated
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 the signal
fundamental and harmonics are also very numerous and visible. The standard test level
normally used of 10W is into clipping for this amp. Lower-power spectrum results showed a
nice, quickly decaying spectrum of harmonics.