Octave Audio V110 Integrated Amplifier

Boasting a distinctly musical name, Octave Audio, headquartered in Karlsbad, Germany, has established a fine reputation for high-end tube electronics in the company’s home market since its founding in the mid-1980s. Its recent transition to the U.S. market has been facilitated by Dynaudio North America. I met up with Andreas Hofmann, Octave’s executive director and chief designer, during the 2013 Consumer Electronics Show. He explained that the V110 is based on Octave’s V70SE integrated amp and has been optimized specifically for the KT120 beam power tube. He pointed out that the V110’s 110Wpc power delivery (into a 4-ohm load) from a pair of push-pull beam power tubes in fixed-bias pentode mode would be impossible were it not for the Russian KT120, basically a KT88 on steroids.

It’s difficult not to be impressed with Octave’s parts-quality, manufacturing excellence, and attention to reliability and stability. As Octave was originally a transformer-winding firm, the company has considerable know-how in the development of transformers and produces proprietary units specially tailored in-house for each model. Its output transformers are wideband designs that feature a silicon-steel PMX core, high-temperature-enameled copper wire, and magnetic shields. To increase long-term stability, transformers are soaked and dried in a vacuum chamber. Octave also employs top-quality components, such as relays by NAIS, capacitors by EPCOS, WIMA, Solen, and ROE, and switches by ELMA. After the pre-production phase, the amplifier is extensively tested—up to one hundred measurement points are checked and compared to reference values.

It is perfectly safe to operate this amplifier without a load or with the speaker cables shorted out, as Andreas Hofmann demonstrated for me. A soft-start circuit limits in-rush current and ramps up to full idle voltage in about 50 seconds. Overcurrent sensing-and-protection circuitry is provided for the power tubes. In fact, all tube heaters and high-voltage rails are monitored and controlled by a “power management system” to protect tubes, rectifier, electrolytic caps, and switches against excessive current. The power tubes are burned-in and matched at the factory. I should mention that the amp ships with a set of five KT120s—a spare is included just in case. An “eco-mode” may be selected on the rear panel, which comes in handy if you forget to turn off the amplifier or simply want to keep it idling with low power consumption for extended periods. In this mode, after a nine-minute period with no input signal, the V110 goes into “sleep” mode and only draws 20W of idle current. Once a music signal is detected, the unit is powered back on with about a 35-second delay.

Key Features

A key theme of the V110 is flexibility. It can be operated as a stereo amp by setting the input selector to “Front Channel” and hooking up an external preamp to the line input labeled “Front Channel.” Doing so bypasses the volume control and feeds the preamp signal directly to the first voltage amplifier stage. In total there are five line-level unbalanced RCA inputs, and a single balanced XLR input. Optionally, one of the line-level inputs can be dedicated for either a moving-magnet or moving-coil internal solid-state phono board. A buffered preamp output is provided, which is handy for driving a powered subwoofer.

A unique feature is Octave’s Black Box technology, which provides the capability to attach external capacitance modules via a connector on the rear panel. One of the sure fire ways to enhance an amplifier’s sound quality is by increasing its power supply’s storage capacity. This is especially critical for a Class AB pentode amp driving a low-impedance load since the power supply gets hit hard during musical peaks as current demand rises dramatically above quiescent levels. Two external upgrade options are available: The Black Box increases storage capacity by a factor of four, while the Super Black Box ups the capacity by a factor of ten. I had the chance to experiment with the Super Black Box (SBB), and I’ll have more to say about that experience later. As a safety feature, the SBB is equipped with a rapid-discharge circuit, which discharges its electrolytic capacitors when the amp is powered down or in the event the connecting cable is accidentally removed.

A Power Selector slide-switch on the rear panel makes it possible to use a variety of power tubes by adjusting the plate voltage. The High position is dedicated to the KT120. However, if you’d like to try KT88, 6550, KT90, or KT100 beam power tubes, slide the selector to the Low position, but only do so with the amp powered off. An EL34 could also be used in the Low position, but only with impedance loads in excess of 4 ohms. In the Low position, power delivery is limited to 70Wpc to avoid over-dissipating the power tubes. Note that tube types 6L6, KT66, 5881, EL519, and EL156 are not suitable for use in the V110.

Biasing of the power tubes is done manually via four trim pots and corresponding LED indicators reminiscent of the scheme implemented by conrad-johnson. The indicators run from yellow (too low) to red (too high) with green being OK and green+red still being OK and providing a slightly higher bias suitable for the KT120. I opted for the latter setting as it slightly increases the damping factor.

From the Source

In its quest for a stable and wideband output stage, Octave detoured from the classical tube-amp design path. Hofmann’s research and development led him to conclude that in order to optimize output the circuit needed to include a solid-state gain stage. The end result, and a feature unique to Octave audio, is a hybrid topology in which the first voltage gain stage after the volume control is an integrated chip that provides 10dB of gain. One half of a 12AX7 makes up the second gain stage. A 12AT7 twin-triode, connected in parallel, is configured as a cathodyne phase-splitter. Any pentode-connected output stage worth a damn needs a regulated screen supply.

The V110 uses a MOFFET-based regulator to provide a fixed 340V screen voltage. The overall global negative feedback (NFB) is very low at 5dB. As a consequence, the measured output impedance is rather high at 5 ohms, a figure which Octave confirmed to be correct. Most push-pull tube amps fall in the range of 1 to 2 ohms by using a fairly large amount of negative feedback, which gives a damping factor of four to eight when driving an 8-ohm load.

Hofmann acknowledges that 5 ohms is a rather high output-impedance but that for him “the absolute stability of an amplifier design is of greater importance than having a relatively low output impedance with the drawbacks of high NFB.” And what about amplifier-load interactions? Hofmann argues that for a 4-ohm nominal impedance loudspeaker, the resultant frequency-response deviations would be no worse than the sort of response anomalies introduced by room modes and therefore, would likely be perceptually acceptable. Well, that depends, of course, on both the specific load and the room in question. To test that notion, I performed comparative frequency-response measurements in my own room with the Basszilla Platinum Edition mk2 DIY loudspeaker. A baseline measurement was performed with the PrimaLuna Dialogue Premium in triode mode, which has a much lower source impedance of 1.2 ohms. Relative to the PrimaLuna, and with SPL levels matched in the treble, the V110 exhibited about 2-to-3dB more midbass but about 2dB less upper-bass output.

These tonal balance deviations were definitely noticeable, but to be fair, they weren’t severe enough to be particularly objectionable. Still, a careful audition with any prospective speaker load would be advisable in order to ensure compatibility.

First Impressions

My first listening impressions were through the Front Channel input, bypassing the volume control, and driving high-efficiency speakers. The V110 followed the Triode Corporation TRX-M300 monoblocks in the reference system, and thus had big shoes to fill. But I was in for a surprise as the Octave managed to keep up fairly well with what is undeniably a superlative SET amplifier in soundstaging—that’s high praise for any push-pull amp. The depth perspective was darn good and image outlines were tightly focused. The reverberant signature of a particular recording venue was readily resolvable. The V110 was able to dig into a complex mix with confidence, power, and finesse. Dynamic nuances burst from the fabric of the music with startling impact.

In particular, female voice was treated with tender loving care. Its presentation seemed to be a marriage between Beauty and the Beast—powerful with plenty of orchestral weight, yet possessing a purity of tone and naturally warm and liquid midrange. Though I should clarify that at no time was the tonal balance tilted towards the romantic side of tube amplification. This amp avoided euphonic editorializing and was rather obsessive about the plain truth and nothing but the truth. Steely string tone had no place to hide. Removing the Pass Lab XP-30 line preamp from the chain and operating the V110 as an integrated amp resulted in a slight loss of immediacy and a livelier treble; all of its other sonic virtues remained intact.

The stock preamp tubes are Russian new-production: a pair of 12AT7W Tung-Sol and a Sovtek 12AX7LPS. In an effort to nudge the V110 toward a somewhat more romantic sound, I substituted a Raytheon 12AX7 black plate for the Sovtek. The sonic outcome was no big surprise. The Raytheon, as is typically the case, fulfilled its sonic potential and made for a sweeter and more liquid midrange that helped beautify string tone on less-than-ideal recordings. While driving the Acoustic Zen Crescendo, I also experimented with 12AT7 substitutions, finally settling on the GE 6201 for its refined textural liquidity and superb tonal color saturation.

The V110 and Crescendo loudspeaker made for an impressive coupling—one which was fully capable of reproducing an orchestra’s macrodynamic peaks. In this context, the V110 appeared to hold its own dynamically with the Bob Carver Cherry 180, and even bettered the latter when it came to image focus and soundstage transparency.

Quick Fix

I’ve saved the best for last. My early impression of adding power-supply storage capacitance via the optional SBB module (mentioned above) was quite positive—that is until the SSB failed with a loud pop when hooked up to the MartinLogan Summit X loudspeaker. It turned out not to be an issue of compatibility, since the replacement unit performed flawlessly. Octave subsequently traced the problem to a faulty switching FET in the discharge circuit. With high-resolution loudspeakers, it became obvious that the SBB made for a considerable improvement in all sonic aspects. Textures became more velvety and the music appeared to flow out of a blacker background. The spatial impression was more solid—in particular, image outlines had greater presence while depth perspective was better resolved. Transients were facilitated with enhanced clarity, and a slight upper-midrange crispness characteristic of the stock unit was removed by the SBB.

In audio as in human affairs, small details add up to big things. In the case of the Octave Audio V110, various circuit refinements and superior output transformers translate to world-class status, especially when the amp is aided by the optional SBB. That Super Black Box allows the V110 to challenge far more expensive designs. If your system is in need of around 100Wpc, the V110 should be near the top of your shopping list.

SPECS & PRICING

Power output: 110Wpc into 4 ohms, 88Wpc into 8 ohms (power selector set to High)
Frequency response: 20Hz–70kHz at 90W, -1/-3dB
THD: 0.1% at 10W and 4 ohms
Signal-to-noise ratio: -110dB/90W
Gain: 38dB
Inputs: Five RCA unbalanced, one XLR balanced
Input sensitivity (line-level inputs): 220mV
Input impedance: 50k ohms
Tube complement: One 12AX7/ ECC 83, two 12AT7/ECC 81, four KT120
Power consumption: < 20W in Ecomode, 160W idle, 500W at full power
Weight: 48 lbs.
Dimensions: 451 x 170 x 415mm
Price: $8300 line version; $8900 w/mc or mm phono; Black Box capacitance module, $1200; Super Black Box, $3500

Octave Audio
Industriestr. 13
76307 Karlsbad, Germany
octave.de

Associated Equipment:

MartinLogan Summit X and Acoustic Zen Crescendo loudspeakers, Basszilla Platinum mk2 DIY loudspeaker; April Music Eximus DP1 DAC/Pre and Stello U3 digital data converter, Sony XA-5400 SACD player with ModWright Truth modification; Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Clearaudio da Vinci V2 phono cartridge; Pass Labs XP-25 phono stage; FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Acoustic Zen Hologram II speaker cable; Sound Application power line conditioners