Due to some favorable impressions on the 'net, I bought the just-released Aiyima T20 Fully Balanced Tube Preamp. Attractions included the low price considering fully balanced input/output, tubes being used for actual gain not just buffer, remote control, and socketed opamps for rolling.
With the stock LME49720 opamps and Psvane 12AX7 tubes, the result was more than respectable but not quite worth inserting into the system when weighing the pros and cons. Thus follows...
[TUBE ROLLING (With stock opamps LME49720)]
Psvane 12AX7 (Stock tubes)
Balanced top to bottom without any one range sticking out. Neutralish and clean sound with good bass, but something feels missing.
That something is good NOS harmonic richness and Nth degree of low-level detail transparency, but still, this tube doesn't commit any significant sins. On the other hand, in this configuration, I would not feel the overwhelming need to keep it in the system, which is already well balanced.
Sylvania 7025/12AX7aProvides that NOS American Tube magic. Midrange fuller yet more detailed than Psvane. More lively, sparkly sound but not edgier. Images are grander and suffused with more air but probably larger than neutral. Still, now the T20 is sounding like a good preamp with tube virtues shining through.
Full Music 12AX7Even rounder and more air-suffused images than Sylvania. Full Music tube works great in my extremely-detailed horn/compression-driver setup where the extra airy forgiveness is more useful. With softer speakers, it can seem not as detailed as something like TungSol 12AT7.
Tungsol NOS 12AT7Lower gain than 12AX7 but works great in T20. This tube has more focused, less rounded/air-suffused images, especially for female vocals. Very detailed yet smooth upper midranges to treble, making female voices sound very nice. However, male vocals don't have as much thickness and weight as Sylvania or Full Music. If I listened much more to female vocals, I may prefer Tungsol.
Siemens Triple Mica ECC81/12AT7This is a super tube with Teutonic balance. Nothing is exaggerated or deficient, and balance is impeccable. Female and male vocals are represented with equal amount of great detail and weight. Overt air-suffused harmonics are not very noticeable, so those who want obvious tube harmonics may demand more.
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[OPAMP ROLLING (With Sylvania 12AX7a tubes)]
Sparkos SS3602 Given how highly Sparkos are regarded, the results in T20 were shockingly unimpressive. Sound did not really stand out as involving or inviting. All the notes were present and music played on, but tone was a bit pale and lean. Even the traditional Sparkos strengths of airy, extended top did not seem all that apparent. Very strange result in this circuit and NOT recommended. Yes, Sparkos works great in some other circuits!
Burson V7 VividAt least the Bursons did sound like itself in T20 as it does in other circuits. Vivid, detailed, and attention-grabbing as always, but some recordings did come across as bit more etched or naked sounding, not something we're after by inserting a tube preamp. Some of the sound signature reminds me a bit of the Siemens tube signature mentioned above actually. The resulting sound quality did not seem to me to justify the high price of Bursons relative to the price of Aiyima T20.
NewclassD DEXA DX2010 opampWhenever Sparkos or Bursons don't quite balance things out perfectly, the ancient DEXA DX2010 often works better, and this was also the case in T20. DEXA gently shaved off some of the etchy detailing of Burson and filled in tonal density a bit compared to Sparkos. This combination sounded quite neutral and uncolored, but then the question comes up. Why bother inserting an extra component like a tube preamp just to have similar sound?
At this point, I was ready to declare that Aiyima T20 was designed and tuned to work best with the stock LME49720 opamps and call it a day. LME49720 combined with Sylvania 12AX7a tubes sounds fantastic with air-suffused, sparkly tube bloom and that extra "magic." Only thing that needed some cleaning up was that images were perhaps too large and bloomy and that tube harmonics likely was slightly masking some resolution compared to no preamp in the chain.
OP2604Remembering how well the OP2604 worked in my 4x PCM1794 DAC, I popped in the OP2604 in Aiyima. Voila! This reigned in the image size a little and cleaned up the bloom some. The resulting sound still had super-pleasing tube harmonics and tone but with ear-grabbing detail and sparkle intact. In THIS system, OP2604 hit the right balance I'm looking for that makes me pull out recording after recording for just musical enjoyment. This result is definitely not universal since I have NOT liked OP2604 much in different circuits in the past. At its low cost compared to discrete opamps, highly recommended in Aiyima.
Addendum
Another T20 user mentioned on another forum he noticed background noise through speakers with T20. I had been using T20 in my "normal sensitivity speaker" setup, but I hooked up my high-sensitivity tweeter/supertweeter, and I can hear background noise with certain combination of older tubes.
So if one plans to use older/NOS tubes or in high sensitivity setup, I would consider these factors.
