Integrated Amp Reviews

I measured the Octave Audio V 80 SE using my top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). Measurements were taken with the Octave's rear-panel switch set to High Power, as recommended for the KT150 output-stage tubes. (I didn't use the Super Black Box power-supply capacitance booster.) After warming up the amplifier for an hour, I checked the bias for each of the four output tubes, using the central LED array and the right-hand rotary switch on the front panel. All tubes were correctly biased for KT150s, with the red and green LEDs illuminated for each tube.

The maximum voltage gain at 1kHz from the speaker terminals into 8 ohms measured 38.75dB for both the unbalanced and balanced inputs. Both inputs preserved absolute polarity (ie, were non-inverting), the XLRs being wired with pin 2 hot. When I switched the amplifier to Extern mode, which separates the V 80 SE's preamplifier and power-amplifier sections, the gain at the preamplifier output jacks was 0dB rather than the specified 12dB; ie, all the voltage gain was being supplied by the power-amplifier stage, which also includes the volume control. At the rear-panel headphone jack, the maximum gain was 24.6dB. The balanced input impedance was a high 26k ohms across the audioband; the unbalanced input impedance was 39k ohms at low and middle frequencies, dropping inconsequentially to 33k ohms at 20kHz.

The preamplifier section's output impedance was a low 5 ohms across the audioband; that from the headphone jack was 19 ohms, which will be too high for low-impedance 'phones. Measured at the speaker terminals, the output impedance was fairly high, measuring 1.84 ohms at 20Hz and 1kHz, rising to 3.1 ohms at 20kHz. As a result, the variation in the V 80 SE's frequency response with our standard simulated loudspeaker was ±1.1dB (fig.1, gray trace). The other traces in this graph show the response into 8, 4, and 2 ohms. Note the excellent matching between channels but also the slight peak just above 40kHz. With a 10kHz squarewave into 8 ohms (fig.2), this peak correlates with a small degree of overshoot on the waveform's leading edges, followed by a single cycle of damped ringing. This graph was taken with the volume control set to its maximum; there was no difference in response at lower settings, and the excellent channel balance was preserved. From the headphone output, the frequency response was flat to 30kHz, and down by 3dB at 140kHz (not shown).

Channel separation (not shown) was good rather than great, at 69dB R–L and 74dB L–R at 2kHz, decreasing to 48 and 57dB, respectively, at 20kHz. The unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms with the input shorted but the volume control set to its maximum—the worst-case situation—was very good, at 79.1dB left and 78.25dB right. Restricting the measurement bandwidth to the audioband improved these ratios to 82.6 and 81.7dB, respectively, while switching in an A-weighting filter gave respective ratios of 87.3 and 87.9dB. Fig.3 shows the low-frequency spectrum of the V 80 SE's output as it drove a 1kHz sinewave at 1W into 8 ohms. AC-supply–related components can be seen in both channels, though all the spuriae lie at a low level in absolute terms, and the 120Hz-related components are lower in the left channel than in the right.

Figs. 4 and 5 show how the THD+noise percentage in the V 80 SE's output varied with power into 8 and 4 ohms, respectively. Octave specifies the maximum power into 4 ohms with KT150 tubes as 120Wpc (17.8dBW), and fig.5 reveals that, with both channels operating, the amplifier just reached this power at 3% distortion. At our usual definition of clipping (1% THD), the amplifier gave 43.3Wpc into 4 ohms (13.35dBW) and 79Wpc into 8 ohms (19dBW). Though the THD+N is very low below 1W into either load, the steady rise in THD with power in both graphs suggests that only a modest degree of loop negative feedback is used. The headphone output clipped at 9V into 300 ohms, which will be more than sufficient drive capability with typical high-impedance cans.

I didn't test the Octave amplifier's clipping power into 2 ohms, as the V 80 SE was not comfortable driving a load that low. This can be seen in fig.6, which shows how the THD+N percentage varies with frequency: at 6.33V, which is equivalent to 5W into 8 ohms (blue and red traces), 10W into 4 ohms (cyan and magenta), and 20W into 2 ohms (gray). However, into higher impedances, the rise in THD at low frequencies is modest, implying that the V 80 SE's output transformer has a hefty core. The rise in THD at the other end of the audioband is also relatively modest for a push-pull amplifier with a single pair of output tubes and limited loop negative feedback.

Fortunately, the nature of the distortion is heavily third-harmonic (fig.7), which will be relatively innocuous. At low frequencies at moderate powers, the second harmonic is almost as high as the third (fig.8): the second lies at –60dB (0.1%), the third at –54dB (0.2%). At moderate powers, the V 80 SE produced a low level of intermodulation products when driving an equal mix of 19 and 20kHz tones (fig.9), the 1kHz difference product lying at –69dB (0.011%); still, the higher-order products are higher in level, particularly in the left channel (blue trace).

Although the rear-panel phono input jacks were marked "MC Phono," our review sample of the V 80 SE was fitted with a moving-magnet stage. The gain at 1kHz, measured at the preamplifier output jacks (as were all the phono-stage tests), was appropriate for MM cartridges at 40.4dB, as was the input impedance, which ranged from 45k ohms at 20Hz and 1kHz to 21.5k ohms at 20kHz. The phono stage preserved absolute polarity. The RIAA error is shown in fig.10; the two channels match closely, but there is a small peak 0.75dB high between 30 and 40Hz before the output rapidly rolls off, reaching –3dB at 18Hz. At the other end of the spectrum, the response is down by 3dB at 63kHz.

Channel separation via the phono input (not shown) was very good, at 80dB R–L and 90dB L–R at 2kHz, decreasing to 65 and 60dB, respectively, at 20kHz. The phono signal/noise ratios were also very good, at 70dB (unweighted wideband), 75dB (unweighted audioband), and 82.4dB (A-weighted), all ref.1kHz at 5mV and taken with the inputs shorted to ground.

The Octave's phono stage also featured very low distortion. I examined the spectrum with a 1kHz tone at 20mV input, 12dB higher than the standard MM level of 5mV and equivalent to an output at the preamplifier jacks of 2V. Even so, as can be seen in fig.11, the only distortion harmonic that can be seen above the low noise floor is the second, at –110dB (0.0003%)! Intermodulation distortion (not shown) was higher in level; nevertheless, the difference product at 1kHz resulting from an equal mix of 19 and 20kHz tones lay at –66dB (0.05%). The phono overload margins, ref. 1kHz at 5mV, were superb, at 28dB at 20Hz, 26dB at 1kHz, and still 14dB at 20kHz.

As I was with the other Octave Audio amplifiers we have reviewed (footnote 1), I was impressed by the V 80 SE's measured performance.—John Atkinson