MEASUREMENTS

Imeasured a different sample of the Octave Audio V 70 Class A than that auditioned by RS. Mine had the serial number 22018130 and was not fitted with the optional phono module. As RS primarily used the balanced line input and KT120 output tubes with the Power switch set to High and the amplifier connected to the Super Black Box external supply, I performed a full set of measurements in that condition. I then repeated some of the tests with the single-ended inputs, then without the Super Black Box,1 and finally with KT88 tubes replacing the KT120s and the Power switch set to Low, as advised in the manual.

The four KT120s were not marked, but I followed the guide in their box to install them correctly. (Unlike the KT120s, the KT88s were marked V1–V4.) I let the amplifier warm up for an hour before starting the testing with my Audio Precision SYS2722 system.2

The amplifier preserved absolute polarity with all its inputs. The maximum gain into 8 ohms from the single pair of output terminals was 36dB for the unbalanced inputs with both types of output tube, and 6dB lower for the balanced inputs rather than the usual 6dB higher. The balanced input impedance is specified as 25k ohms, which my measurements confirmed. The single-ended input’s impedance was the specified 50k ohms at 20Hz and 1kHz, dropping inconsequentially to 39k ohms at the top of the audioband.

The amplifier’s output impedance with the KT120s, including 6' of spaced-pair speaker cable, was a high 1.86 ohms in the bass and midrange, rising to 2.14 ohms at 20kHz. (It was 0.4 ohms greater across the audioband with the KT88s.) Consequently, the modulation of the V 70’s frequency response due to the Ohm’s law interac

measurements, continued

tion between this impedance and that of our standard simulated loudspeaker 3 was an audible ±1.1dB (fig.1, gray trace). The amplifier’s response into resistive loads was flat in the audioband but rose slightly above 20kHz into 8 ohms (blue and red traces), peaking by 0.5dB at 50kHz before rolling off sharply. Fig.1 was taken with the balanced inputs and with the volume control set to its maximum; the excellent channel matching and the small ultrasonic peak were identical at lower volume control settings and with the unbalanced inputs, as well as with the KT88 tubes. The ultrasonic peak correlated with a small amount of overshoot and one cycle of damped ringing in the V 70’s reproduction of a 10kHz squarewave (fig.2).

Channel separation was >60dB below 1kHz but decreased to 37dB at 20kHz, presumably due to capacitive coupling. The wideband, unweighted signal/noise ratio, taken with either the balanced input or the unbalanced input shorted to ground and the volume control set to its maximum, was an excellent 81.3dB in the left channel, 82.3dB in the right channel, both ref. 2.83V, equivalent to 1W into 8 ohms. These ratios improved to 84.7dB left and 87dB right when the measurement bandwidth was restricted to the audioband, and to 92.7dB

in both channels when A-weighted. The S/N ratios were very similar with the KT88 output tubes and without the Super Black Box.

The blue and red traces in fig.3 show the spectrum of the amplifier’s lowfrequency noisefloor at 1Wpc into 8 ohms with KT120s, the Super Black Box, and the volume control set to its maximum. The level of random noise is low, but even- and odd-order harmonics of the AC supply frequency are present. Reducing the volume by 20dB and increasing the input signal by the same 20dB so that the output power remains at 1W (green and gray traces) reduced the level of the higher-order spuriae by around 10dB and that of the random noise by 5dB. The noisefloor spectra were identical with the KT88s, but removing the Super Black Box introduced low-level sidebands at ±120Hz and ±240Hz around the 1kHz tone.

Octave specifies the maximum power with KT120s and the High power setting as 50Wpc into 4 ohms, 20% lower into 8 ohms. Fig.4 plots how the THD+noise percentage with KT120s and the Super Black

Box varies with output power into 8 ohms with both channels driven. The THD+N rises with increasing power above 0.5W, reaching 1%, which is our usual definition of clipping, first at 14.8Wpc (11.7dBW) then again at 36.4Wpc (15.6dBW). The power into 8 ohms at 3% THD+N was 38Wpc (15.8dBW) and was not affected when I disconnected the external supply. Fig.5 repeats the THD+N measurement into 4 ohms. The THD+N reaches 1% at 17.5Wpc (9.4dBW) and 3% at 57Wpc (14.55dBW). The maximum powers were not significantly af

fected when I repeated these tests without the Super Black Box.

With KT88 tubes and the Power switch set to Low, the V 70’s maximum power is specified as 25W into 4 ohms. In this condition, the V 70 clipped at 27Wpc into 8 ohms (14.3dBW, fig.6) and 13.5Wpc into 4 ohms (8.3dBW, fig.7). Relaxing the clipping definition to 3% THD+N, the 4 ohm clipping power was 28Wpc (11.46dBW).

Out of curiosity I examined how the 8 ohm THD+N with KT88s varied with power with the Power switch set to High. The result is shown in fig.8. The V 70 now clips at 34Wpc into 8 ohms (15.3dBW), and the THD+N is generally lower than with the Low switch setting. It was still high into 4 ohms, however (not shown).

Fig.9 shows how the THD+N percentage with KT120s changed with frequency at 6.33V, which is equivalent to 5W into 8 ohms and 10W into 4 ohms. The distortion into 8 ohms (blue and red traces) is around 0.2% over most of the audioband, with small increases below 50Hz and above 4kHz. The THD+N percentage almost doubles into 4 ohms (green and gray traces). Fig.10 repeats this test with the KT88 tubes and Low Power. The THD+N in the right channel (red trace) is very much lower than it was with KT120s, and though it is higher in the left channel (blue trace), it is still lower than with KT120s. However, as expected from the shape of the trace in fig.7, the THD+N rises precipitously into 4 ohms (green and gray traces).

The THD+N waveform at 5W into 8 ohms with KT120s was primarily the third harmonic (fig.11), though the second harmonic is also present at a slightly lower level (fig.12).4 This graph was taken with the Super Black Box connected. Without it (fig.13), the supply-related sidebands I noted in fig.3 were 20dB higher in level than in fig.12, though still low in absolute terms. The distortion spectrum with KT88s (not shown) indicated that the increased distortion in the left channel was due to the second harmonic being dominant, which suggests that the output tubes in this channel were not well-matched. Intermodulation distortion with KT120s and an equal mix of 19kHz and 20kHz tones was relatively low (fig.14), with the secondorder difference product at 1kHz lying at –67dB ref. a peak level of 5W into 8 ohms. The higher-order products at 18kHz and 21kHz lay at –60dB (0.1%).

The Octave Audio V 70’s measured performance is typical of a tubed amplifier that doesn’t use a large amount of corrective loop negative feedback. The lowest distortion at moderate powers is obtained with the KT88 output tubes driving 8 ohms with the Power switch set to High, but I don’t recommend this condition for loads below 8 ohms. The KT120 tubes cope better with lower impedances.—John Atkinson

1 I powered down the V70 before disconnecting the Super Black Box, then waited for 60 minutes after powering up the amplifier before repeating the testing.

2 See stereophile.com/content/measurements-mapsprecision.

3 See stereophile.com/content/real-life-measurements-page-2.

4 The signal-related class-A bias topology means that while higher-order harmonics can be seen in this graph, they are 25dB lower in level than the lowerorder harmonics.

Fig.2 Octave V70, KT120 tubes, small-signal, 10kHz squarewave into 8 ohms.

Fig.1 Octave V70, KT120 tubes, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (1dB/vertical div.).

Fig.5 Octave V70, KT120 tubes (High), Super Black Box, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.3 Octave V70, KT120 tubes, Super Black Box, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms with volume control set to the maximum (left channel blue, right red), and to –20dB (left green, right gray) (linear frequency scale).

Fig.4 Octave V70, KT120 tubes (High), Super Black Box, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.8 Octave V70, KT88 tubes (High), Super Black Box, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.6 Octave V70, KT88 tubes (Low), Super Black Box, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.7 Octave V70, KT88 tubes (Low), Super Black Box, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.11 Octave V70, KT120 tubes, 1kHz waveform at 5W into 8 ohms, 0.19% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.9 Octave V70, KT120 tubes, Super Black Box, THD+N (%) vs frequency at 6.33V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).

Fig.10 Octave V70, KT88 tubes, Super Black Box, THD+N (%) vs frequency at 6.33V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).

Fig.14 Octave V70, KT120 tubes, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 5Wpc peak into 8 ohms (left channel blue, right red, linear frequency scale).

Fig.12 Octave V70, KT120 tubes, Super Black Box, spectrum of 1kHz sinewave, DC–10kHz, at 5Wpc into 8 ohms (left channel blue, right red, linear frequency scale).

Fig.13 Octave V70, KT120 tubes, internal supply, spectrum of 1kHz sinewave, DC–10kHz, at 5Wpc into 8 ohms (left channel blue, right red, linear frequency scale).