Power output with 1kHz test signal
- 8-ohm load at 1% THD: 0.75W
- 8-ohm load at 10% THD: 14.6W
- 4-ohm load at 1% THD: 0.17W
- 4-ohm load at 10% THD: 8.7W
The Audiopax Stereo Eighty Eight is a low-/medium-powered
single-ended tube amplifier utilizing a single KT88 output tube in each channel. The
design is unusual in that it has a control for adjusting the "timbre" of the
sound for different speakers and situations. This control appears to vary the quiescent
current in the output tube. It turns out that one measurable effect of this control is to
vary the damping factor of the output. Gain and distortion characteristics are also
affected. Gain of the unit is somewhat lower than usual for power amplifiers. Rated power
is 15W into an 8-ohm load.
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 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 on the order
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 load, puts out about the same power
into 4- or 8-ohm loads, although the distortion is greater with 4-ohm loading.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. The amount of rise in
distortion at high frequencies is admirably low, but the increase in distortion at low
frequencies is quite pronounced.
As an illustration of one of the parameters that are
affected by the timbre control, damping factor is plotted in Chart 4 as a function of the
timbre control set at counterclockwise, mid-rotation, and clockwise positions. This
measurement was done at a much lower current (.125A) than the usual 1A in order to
maintain signal linearity at the lowest timbre setting.
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 is
very low, and the signal harmonic spectrum has the desirable characteristic of tapering
off as the harmonic number increases.