Although it made an analog junkie out of me, I have to admit that I’ve always been aware of the slightly resonant signature of Oto’s phono stage. To my ears, there has always been a sonic disparity between its line-level and phono performance, which manifested itself as a kind of exaggerated euphonic quality in the latter.
At first, I was inclined to think these attributes were intrinsic to the analog medium itself and to the way the audio signal is extracted from the grooves through the stylus–tonearm interface. But the more I listened, the more I realized that the issue was likely to stem not from the source, but from the amplification of the tiny signal occurring in the phono stage.
The phono stage itself is exquisitely made: a variation on the anode-follower theme, comprised of an ECC83 (12AX7) and an ECC88 (6DJ8) double triode, along with a passive RIAA equalization network. The board is made of epoxy resin and serves only as a mounting platform for the components, which are point-to-point wired on the back side. Despite the fact that it is beautifully made, over time I became even more aware of the artificial liveliness that was distracting me.
It was as if a slightly delayed and reduced-level copy of the signal was superimposed over the original coming from the input. These observations slowly crystallized into the idea that the issue probably had something to do with vibrations and resonances transmitted through the aluminium chassis to the phono board, thus compromising its performance.
That might be one of the reasons why we don’t often see integrated tube amplifiers featuring a phono stage: it can be quite a challenge to isolate it from sources of vibration and electromagnetic radiation. The usual suspects generating enough mechanical energy to be of concern are the output transformers, with their oversized permalloy cores typically used in single-ended tube amplifiers.
With my conviction growing, I started to look for a solution that could, if not completely solve the issue, at least mitigate it to a degree that would allow me to fully enjoy my records without the aforementioned reverberant coloration. I first experimented with the shields of the two double triodes, trying to understand the differences they made and the exact role they played sonically.
Without the shields, the sound became easier and more open, much like a kitchen radio, but it lacked the foundation and definition that characterize modern sound. After trying different configurations, I eventually preferred the way these shields/dampers are fitted originally: one compressed spring is located inside the metal can and presses against the top of the tube, while two additional stretched springs located outside the shield can pull it down toward the socket.
These tube shields are obviously useful and appropriate, and although they do their job of damping the tubes’ own resonances and shielding them against external sources of electromagnetic radiation, they alone cannot deal with vibration entering the phono board from the chassis.
What I saw other manufacturers doing was either isolating the tube sockets from the board by using rubber grommets, or keeping the sockets hard-coupled to the board and isolating the board itself. Why Audio Note hadn’t taken any of these measures was unclear to me. What I could tell was that the white plastic legs used to support the board were rigid enough to effectively couple it mechanically to the chassis. So I decided to check how the phono stage would perform if its main board were supported by rubber feet.
I keep a small stash of old turntables in case I need a rubber grommet, spring, bolt, or whatnot, because they are great sources of all kinds of parts and I never throw them away. Luckily, inside one old Dual, I spotted four rubber absorbers used to decouple its steel sub-chassis from the plastic enclosure. They have plastic tube sockets and conical rubber inserts with small mushroom-shaped tops for securing the connection.
Installing them was straightforward and easy. I didn’t expect much to happen, but I was curious to hear the result of the intervention.
And I was shocked. It was one of those happy moments when my whole being rejoices; it was a big “Wow!” One can instantly tell when a tweak has succeeded simply because the focus of one’s attention shifts from sonic observation to the music itself. Any thought of judging the reproduction is instantly forgotten. And this is exactly what happened after replacing the stock plastic footers with the rubber ones.
It was not a small improvement, but a substantial refinement of Oto’s phono stage performance. It dried the sound, cleaned up the mids, brought the bass further down into the cellar, and simply made me entirely happy — even proud of myself.
I have no explanation for why Audio Note has overlooked this detail. After all, this amplifier has been in production for decades, and the improvement I hear from the modification is not subtle. For me, it is a real game changer that makes me wish for no other amplifier than Oto — this eternal source of joy during my days of self-isolation.
I can finally concentrate on records. In these uncertain times, marked by the spreading fear of infection, I find myself acting irrationally. Parcels containing records ordered on Discogs and dispatched by random people arrive every week, keeping delivery workers busy and exposing me to danger, but I can’t help it.
I take some precautionary measures by keeping the plastic sleeves quarantined for a few days, disinfecting the record covers with a soft cloth, and washing my hands. All that hassle has made me realize that the main advantage of streaming, aside from its convenience and accessibility, is that there is no hygiene issue. It’s antiseptic, but that’s pretty much it.
Take care, guys, and stay healthy.
Dimitar