Power output with 1kHz test signal
- 8-ohm load at 1% THD: 105W
- 8-ohm load at 10% THD: 133W
- 4-ohm load at 1% THD: 103W
- 4-ohm load at 10% THD: 163W
General
The Convergent Audio Technology JL2 Signature Mk 2 is a
medium-power stereo push-pull tube amplifier utilizing four triode-connected pairs of 6550
output tubes per channel. Having the output tubes connected as triodes means that the
resulting output stage will be more tolerant of load-impedance changes than if a smaller
number of output tubes were connected in ultralinear or pentode modes. Furthermore, the
larger number of output tubes means that the primary impedance of the output transformer
can be lower, resulting in potentially better and more controlled high-frequency response.
Chart 1 shows the frequency response of the amp with
varying loads. The output impedance, as judged by the closeness of spacing between the
curves of open-circuit, 8-ohm, and 4-ohm loading, is reasonably and acceptably low for a
tube power amplifier. With the NHT dummy speaker load, the variation is better than
+/-0.6dB. Of particular interest is the beautifully controlled and consistent way the
high-frequency response retains its shape with the different load impedances -- not a
usual quality of most power amplifiers, tube or solid state.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. Of
interest, this amplifier’s general distortion characteristic is typical of many tube
amplifiers and is one of continuously increasing distortion with increasing power. As is
the case with most power amplifiers, this one puts out more power into the 4-ohm load.
Total harmonic distortion plus noise as a function of
frequency at four different power levels is plotted in Chart 3. The amount of rise in
distortion at high frequencies is reasonably low, and there is the typical rise in
distortion at low frequencies due to the characteristics of output transformers.
Damping factor vs. frequency is shown in Chart 4. Notable
here is the reasonably flat characteristic with frequency over most of the audio range.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The principal signal harmonics are the
second and third, with all higher harmonics some 34dB or so lower. AC-line harmonics are
low, although there are some line-harmonic-related sidebands around the nulled-out 1kHz
test signal fundamental -– something I have seen from quite a few of the power
amplifiers that I have tested, but not all of them.
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