Audio Research D-115

Ever since the revelatory experience of playing music on his Dynaco ST-70, my friend Luigi had been scanning the market for an all-tube pre and amp combination capable of delivering abundant power into even the most demanding speakers. This proved to be a difficult task, because large tube amplifiers had been well out of fashion for at least 30 years. Then, one night, he called me to say that he had made a purchase. The units he had found were an Audio Research SP6 preamplifier and a D-115 tube amplifier, both in need of a massive technical overhaul. He consequently dropped the units off at his trusted technician, and for the next few months that was the last we heard of them.

After what seemed like an eternity, I was invited to Luigi’s house for a listening session and immediately noticed the large silver face plates that are so typical for the Audio Research products of the 80s. I could not help but grin with apprehension. So, they had finally come back from the shop, fitted with new quality capacitors, boasting re-soldered connections and a brand new set of the finest matched tubes. All the dust of 40 years had been cleaned out, and lots of love and energy had been invested to make this set of tube separates one of the best of its kind on the audio market today.

As I would find out, this type of gear certainly merits the effort. Audio Research has an excellent name amongst tube enthusiasts. In fact, the company can look back on a long history of audio excellence and is today the oldest remaining manufacturer of high-end audio products. Audio Research is considered to have given rise to the very notion of High End audio, and their SP-3 High Definition preamplifier was said to be the best preamplifier on the market over a period of many years. A whole range of products manufactured by the Audio Research Corporation (ARC) were declared the official benchmark of music by some of the most influential audio magazines of the 1980s, among them ‘The Absolute Sound’ and ‘Stereophile’. Read the full history of the Audio Research Corporation in my review of the SP-6.

Instead of listening to the preamplifier and amplifier combination as a set, Luigi and I agreed that we should take a step-by-step approach in writing reviews. It would be far more interesting to learn how each component performed when playing against and among the units that we were familiar with. I therefore first took the ARC SP-6 preamplifier for testing in December 2020. Despite its advanced age, the large and heavy preamplifier gave a stellar performance, making our otherwise excellent DB Systems DB1 preamp sound a little thin and analytical in direct comparison. And—while I had no difficulty readjusting my ear to the familiar sound of our DB1 following the test—I still hold the SP-6 in very high esteem. It actually inspired me to purchase our first tube preamp a few month later.

Having returned the SP-6 preamp to Luigi, I was hesitant to take the complimentary ARC D-115 out for testing. I suspected that dealing with the amp would not be quite as easy. One reason for this suspicion was the amp’s weight. With its 3+1 large transformers and thick sandwich base plate, it weighs just above 30kg. And with the speaker terminals protruding out the back, I could not simply pick it up by the handles, either. The D-115 rather needed to be carried horizontally, like a serving tray, which all the more accentuated the fact that this was, indeed, a very heavy piece of equipment. When I finally did bring it home, I arrived at our house in the middle of the night and telephoned my wife to give me a hand in carrying the D-115 up our long flight of stairs. I remember feeling quite embarrassed about it, like someone who had bitten off more than he could chew.

But weight was not the only consideration. The second factor was its operating temperature. When switched on, the D-115 draws close to 400 watts of idle power from the grid. That’s a lot of juice for a machine that is not yet playing music, and most of this energy is simply transformed into heat. When placing the amp in our rack, I first had to make some adjustments to provide it with the ventilation it needs. This included taking out one shelf board and dismantling the front door. Just out of curiosity, I then placed one of our children’s bathing thermometers on the grill above the tubes. It soon reached 50 centigrade, which was also the end of this particular thermometer’s scale. A reminder that an all-tube power amplifier is a serious piece of gear that requires some thought and attention before first operation.

The D-115 likes to be positioned close to the floor where it is relatively cool and vibrations coming from the transformers can be channelled directly to the ground. The amp’s three rubber feet assure that it will not wobble when placed on a stable surface. Thorough mechanical decoupling from source devices is advisable for an amp of this caliber. I have sometimes read that tube amplifiers prefer to play into close to linear 8 Ohm loads or higher. On the other hand, there are some reports of the D-115 being paired with Magnepan and Martin Logan speakers with some success. For this reason, I thought it safe to begin my exploration on our SL3 electrostatic speakers via the amp’s 4 Ohm output terminals, before attempting to pair it with our 8 Ohms Tannoy system. The amp actually has separate binding posts for 16, 8, and 4 Ohm speaker loads—another indication that 4 Ohm connections are possible. Connecting to the correct terminal is important in order to protect the D-115 from undesirable levels back current.

Martin Logan SL3 (DB Systems DB1)

After about 30 minutes of the tubes heating up without music playing, I started my session with “Turn up the quiet” by Diana Krall. The album has been my personal benchmark for studio recordings for some time now. Playing the album’s familiar songs, I first noticed the D-115’s thick and fruity midrange that manifested itself around Diana Krall’s vocals. The singer’s voice carried more weight than I was used to from our B&K ST-140 class-A MOSFET amp. There was greater focus on the voice itself, with the instruments being slightly set back. The sound was darker and tonally rich, as one would expect from a live performance. Where the ST-140 had dug out even the tiniest nuances of the studio recording, the Audio Research D-115 painted a slightly more homogenous and live-sounding image.

After listening to Diana Krall for some time, I changed to “All the Little Lights” by Passenger. This used to be my favourite non-audiophile recording. As far as singer-songwriter albums go, “All the Little Lights” is a decent recording, and yet with each advancement of technology on my side, I was also beginning to hear the limitations of the studio. To my delight, the D-115’s more forgiving approach to music worked rather well with this album. The amp was revealing enough for the music to be entertaining, but it did not reach as deeply into the spaces as some of its solid state competitors will. I found listening to Passenger tonally charming and occasionally magical. The D-115 was neither technical nor overly analytical sounding. There was no trace of harshness, and the whole experience proved to be non-fatiguing.

Although the D-115 perhaps did not offer as much presence at the frequency extremes as some of its more recent cousins, it did present its music with lots of slam and rhythm. This impression was supported by the fact that it created a huge and spacious soundstage with full-bodied yet soothingly intimate vocals. And—although bass contour was slightly limited, perhaps due to its relatively low damping factor—its overall bass performance did feel natural. Tube amps are special in their ability to interact with the listening room in a way that solid state amplifiers simply cannot. The effect on voices is magical and thought to be unique to tubes. With its strong centre focus, the D-115 without fail found and caressed the singers voice at an instance. And the same magic happened when strings were playing.

The final album I listened to was “Foot Tappin’ Boogie” by Jörg Hegemann. This is still a relatively new addition to my benchmark series, and yet, I enjoy every minute of it. Since Jörg’s album is mostly instrumental, I was able to focus on the D-115’s depiction of instruments. In this context, I felt that the Audio Research slightly favoured the double-bass performance over that of the piano, specifically in combination with the SL3 loudspeakers. While the double-bass was presented fully and elaborately, especially the higher piano keys did not have the same piercing authority that I had come to enjoy from our B&K ST-140. As it turns out, the D-115 is quite sensitive to power cord issues. I was able to improve piano playback by moving the amp’s power cord away from all other cords in a way that they would neither touch nor cross. A re-play of the same song showed some improvement to piano notes, although an audible difference to the solid state amp remained.

Tannoy XT8F (Dynaco PAS-4)

Listening to the D-115 on our Tannoy system with Dynaco tube preamplifier produced a more insightful and perhaps more widely applicable result. Similar to the first scenario, the all-tube setup created a huge and mesmerising soundstage with the music being lush, agile and forward sounding. As I was sitting only two meters from the speakers, I could literally feel the D-115 throwing music at me with a vengeance. Walking around the ample space and listening in different positions, it became clear to me that this amp could fill large venues with ease, exhibiting its full dynamics in every corner, even at low volumes. While imaging may have been crisper with our solid state amps, the Audio Research excelled in giving each sound full body and life-like dimension, with excellent piano and double-bass on Diana Krall’s “No Moon at All”.

The D-115 sounded non-aggressive, exhibiting a slightly mitigated top-end. In marked difference to our solid state amps, piano notes remained non-piercing throughout. I noticed a slight sibilance on Diana Krall’s voice that might have stemmed from the simpler power cord I used and that I had not noticed on our other system with the higher quality cord. In comparison with our solid state amplifiers, the tube amp’s bass notes did not extend quite as low. Diana Krall, for example, often finishes her songs with a bass thump, and so does Jörg Hegemann in “Foot Tappin’ Boogie”. The effect is especially highlighted when playing music via our silver cables. Listening with the D-115, this final thump was audible but it was not quite as pronounced. From my early experience with Hafler, I remembered that the real merits of an amp can often be judged when listening to it play from another room with the doors left open and asking myself the question “What is playing: an amplifier or a band?” For the D-115, the answer to this question clearly leans towards the latter.

In summary, it can be said that the Audio Research D-115 is a highly musical tube amplifier for experienced audio enthusiasts who prefer to be tonally caressed rather than impressed with extended range. It prefers to drive conventional speakers with higher and more linear Ohm loads. As far as tube amps are concerned, the D-115 offers a balanced and smooth sound with a slight emphasis on vocals which it plays in a lush and full manner. Rather than impressive bass contour or treble extension, the D-115 offers a huge and spacious sound that is highly engaging and non-fatiguing to listen to. Later models of the same manufacturer are said to perform in a more extended, analytical, and less charming manner. For audiophiles who have the necessary skills and resources to set up and maintain an all-tube power amp of this caliber, the D-115 is certainly an excellent pic.

I thank Luigi for this new opportunity of exploration. It is through his passion and good ear that I have learned more about HiFi in just a few years than I otherwise could have done in a lifetime. The above is a factual account of the subjective listening events with the D-115 in our two systems around the time from 5. - 9. July 2021. As always in HiFi, alternative listening setups may lead to different results. Readers familiar with the D-115 or related all-tube designs, please feel free to leave a comment below.

Specifications:

• Power output: 100 WPC (16 Ohms, 20Hz to 20kHz < 1% THD)
• Total harmonic distortion: <005%, 1 watt
• Power bandwidth: 10 Hz to 60 kHz, -3 dB
• Input sensitivity: 1.2V RMS
• Input impedance: 75K ohms
• Output regulation: 0.4dB, 16 ohm
• Negative feedback: 20dB
• Damping factor: 20
• Slew rate: 15 V/µs
• Rise time: 5 µs
• Hum: 90dB below output
• Power supply: 1000 watts, 280 joules; 400 watts idle
• Tube complements: 4 x matched pairs KT77; 1 x 6550, 1 x 12AT7; 7 x 6DJ8/ECC88
• Dimensions: 48 cm (W) x 18 cm (H) x 42 cm (D)
• Weight: 31 kg
• Year: 1983-1989

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