Audio Research VS115 Stereo Power Amplifier
Tip #109
Dick Olsher (November 2024)

The VS115 was introduced in January 2008 as part of ARC’s new millennium foray into hybrid power amplification. Unlike the physically similar VS110 which featured all tube amplification, the VS115 uses a FET front end coupled to a pair of 6H30 triodes. One triode is used for gain while the other is configured as a cathode follower for driving the beam tetrode output stage. Originally a quad of 6550C power tubes was used per channel in push-pull Class AB1. In 2010 production shifted to the KT120 power tube and the retail price increased from $6,495 to $6,995.

I suspect that the open architecture was intended as a cost saving measure. But it actually makes a lot of sense from an engineering standpoint. At idle the VS115 dissipates 400 watts, and that’s a lot of heat generation to try and “bury” inside a chassis. Over the years ARC has had a lot of trouble with enclosed power amp designs and has resorted in some cases to fan cooling. Excessive heat reduces tube life, so I think that the most sensible approach is in fact an open layout that maximizes convective cooling.

To be sure, ARC’s design approach flies in the face of conventional wisdom. The traditional hybrid amplifier substitutes a solid-state output stage for the power tube counterpart, the main rational being to do away with the bulky, expensive, and bandwidth-limiting output transformer. That was Julius Futterman’s motivation for his tube OTL circuit as well as Harvey Rosenberg’s NYAL hybrid designs of the 1980s. To be honest, my skepticism of ARC’s “inverted” hybrid circuit is what kept me away from ARC’s hybrid line for many years in favor of all-tube power amps.

Eventually, I arrived at the teetering edge of a purchase decision. What finally convinced me to purchase a used VS115 a few months ago was the result of a more careful study of the schematic. I realized that the output stage follows William Z. Johnson’s original and by now, classic cathode-coupled topology, outlined in US Patent 3,566,236 granted in 1971. The gist of it is that half of the output voltage from the transformer’s secondary winding is fed back to each cathode, while the beam tetrode screen grids are held at a constant B+ voltage of 420V. The benefits are reduced distortion and output impedance, important considerations for what is a genuine pentode power amplifier.

What has changed over the years is the power supply complexity. As Jonson described it in a 2008 interview with Robert Harley: “Its importance simply can't be stressed enough. No matter how good the audio circuitry is, no matter how well-balanced, no matter how dynamically stable it's capable of being, if you don't have rock-solid power supplies that are totally decoupled one from another, you've done it all in vain. You probably have noticed, Bob, that the sophistication of our power supplies has increased over the years, whereas the audio circuity has really not changed that much. Indeed, it's actually become simpler.” It was this combination of a well-regulated power supply and pentode circuit that had put ARC on the map during the 1970s with the likes of the D76 amplifier and continues to showcase ARC’s amplification magic.

The VS115 offered other inducements as well: a much improved biasing scheme and ability to select either single-ended or balanced inputs. Bias test jacks are provided on the rear panel for each power tube. However, only one tube in each parallel-connected pair of output tubes can be adjusted. That’s obviously not as convenient as having individual bias pots for each power tube and means that as a minimum two matched quads of power tubes will be needed.

Having purchased a VS115 sans power tubes I decided on New Sensor’s Tung-Sol KT120 as the power tube of choice. However, sourcing tightly matched tube sets turned out to be bit of an adventure. My initial purchase of a factory “platinum matched” octet varied in emission by up to 20%. I then had to supplement my initial purchase with a matched quad of KT120’s from Tube Depot which were matched to within 10%, as measured with my Maximatcher II tube tester. By selecting from that dozen KT120’s I was able to meet ARC’s specified 65 mV DC average bias voltage per pair of power tubes within about 5%. And that’s darn good.

Note that the KT120’s maximum plate dissipation is 60W, which is significantly higher than the 42W of the 6550C. Therefore, it is possible to bias the amp for increased plate current and output power. I decided to leave the bias as originally recommended by ARC to maximize tube life rather than squeeze out a few more watts of output power.

The VS115 really sounds its best after about an hour of playtime, progressing from good to spectacular. Partnered by the Fyne Audio F703SP loudspeaker it won me over immediately with a wonderfully dimensional soundstage whose depth and width perspectives generated a convincing and stable spatial impression. Harmonic textures were pure and velvety, lacking glare, brashness or any other sort of annoying distortions. A low-noise floor facilitated low-level detail retrieval. And the superb power bandwidth was responsible for excellent extension at the frequency extremes. In particular, the bass range was powerful and capable of dishing out a potent punch, quite amazing for a tube output stage. Output impedance is about 1 Ohm, yielding a damping factor of 8 into an 8-Ohm load, quite decent for a tube output stage. Transient speed and clarity were also superb and helped resolve complex orchestral passages with great precision. Tonal authority was that of an all-tube power amp - not at all what I expected. On wide-scale orchestral music it navigated dynamic passages with intense conviction and without a shred of compression.

In summary, the VS115 is the best sounding hybrid amp I’ve auditioned to date. It really does sound like an exceptional all-tube amp with no trace of any sold-state assertiveness. It has by now become one of my favorite amps. Sadly, it disappeared from ARC’s catalog circa 2014. But it’s still widely available on the used market and well worth seeking out, especially in view of its current asking price of around $3K.