Ferbose
Headphoneus Supremus
- Joined
- Jun 27, 2004
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This is a first peek at a new single-ended tube headphone amp from SAC Thailand.
The Minute EL34 headamp is a brand-new model not yet advertised on the manufacturer's website and I am the first owner in the US. It is so amazing that I feel compelled to let everyone know about it before I am ready to write a real review.
Before I go into the details, here is how I would summarize its performance:
It is exceptionally powerful, and exceptionally quiet. It works great with sensitive cans like my Grado, and the most power-hungry beast like AKG K1000. It is more transparent, airy, full-sounding and bass-rich than any headphone amp that has passed through my system. The tone is highly adjustable through tube rolling. It has 10 Watts of pristine, class-A power to make my speakers sing like never before, again beating a bunch of low-powered speaker amps I have used. But even the first milliwatt sounds really great on my sensitive headphones. I bought it to take my K1000 to a new level from my previous SET amp, but it goes much further. This reasonably-priced amp ($850) uses affordable tubes (two 6N1P or 6CG7, and two EL34 or 6L6), and comes in a user-friendly package (small footprint, no excessive heat, tube grille). Using pentodes in a special cathode-feedback topology, its sound is smoother than triode-connected pentodes and more transparent than directed heated triodes that I have heard. I absolutely love it.
How does an affordable amp achieve such class-defying performance? The key is its unusual circuit topology called cathode feedback. I have previously heard amps with pentodes (or beam tetrodes) wired in triode, ultralinear linear, and pentode modes. I prefer the triode mode, which has the best tone but the least power and bass. So what is cathode feedback? Why is the sound in a (better) class of its own? You can check out the SAC Thailand website for an explanation of cathode feedback:
SAC Thaliland Minute EL34 SE Amp
According to SAC Thailand, cathode-feedback EL34 behaves like 300B and only produces 2nd and 3rd order harmonic distortions, but at much lower levels. This really got me interested. I love the smoothness of 300B but don't want its overly fat tone (excessively tubey to me), high tube cost, high heat, and high amp cost and weight. It seems that the 10W Minute speaker amp is aimed at competing with 9W 300B amps. Thanks to the recent popularity of low sensitivity orthodynamic headphones like Hifiman HE6, SAC Thailand has added a headphone circuit to the Minute amp. When I inquired about using the Minute speaker amp to drive K1000, I was informed of this new headphone amp and the first unit shipped to North America landed in my house three weeks ago.
Though I am still exploring its full potentials, I already think of it as something truly magical.
As you can see in the pictures below, it has five selectable RCA inputs and two speaker binding posts in the back. In the front we have a headphone jack with a low/high impedance switch (low for <100 ohms), and an on/off switch. There is a separate power supply unit (a hefty 10 pounds by itself). First-stage tubes are Russian 6N1P (high gain) or American/European 6CG7 (low gain). Power tubes are auto-biased, using EL34 family (KT77, 6CA7) or 6L6 family (KT66, WE350B, 5881, 6P3S).
Three things amaze me about this amp:
1) Super powerful and super quiet
It will drive my K1000 (130 ohms, 74 dB/mW) to hearing-damage levels, and yet I can also enjoy my Sennheiser HD595 (50 ohms, 104 dB/mW), which is 1000 times more sensitive. With HD595, the self-noise of the amp is inaudible at the high impedance setting and just audible at the low setting. It won't interfere with music, because many CDs made from analog maters have louder tape hiss than this. This is astonishing performance because I am essentially examining its noise with a microscope still not finding much. The Minute can properly drive all hi-end headphones except the ultra-sensitive IEM type. I attribute this stellar noise performance to a separate power supply. Low noise always means great inner resolution, more air, and spaciousness.
2) Measures well and sounds even better
For the purity of tone, true triodes sound better than pentodes in triode mode. Single-ended sound better than push-pull. To my ears, great triodes like 300B with only 2nd and 3rd order harmonic distortions sound very smooth. But having these distortions at several percent can still be too fat and euphonic, not entirely accurate. Graphs on the SAC's website show this amp with triode-like distortions, but several-fold lower than 300B at the same power output. Of course steady-state distortion measurements don't show what really happens with recorded music. My ear tells me that the Minute has the smooth tone of true power triodes and but not the excessive tube-ness. Simply said, it's the great sound of small signal triodes made 20 times more powerful. Amazing!
3) Its tonal color can be easily altered to give me THE TONE I WANT
One great thing about tube amps is that the tone changes with the tube. Although good tubes can make the noise lower and the bass stronger, it cannot cover up obvious flaws in these two departments. Fortunately, the Minute headamp is already stellar in both aspects. A test CD I have listened hundreds of times over 13 years on all kinds of speaker and headphone systems suddenly shows bass-heavy passages that I was unaware of. That's what happens when the bass is full and tuneful. This amp is a superb platform for tube rolling. Moreover, its tone changes with tubes more than several EL84-based tube amplifiers I have used in my system.
I noticed that tubes with the same electrical rating (Sovtek 12AX7 vs Amperex 12AX7) don't affect the tone as much as compatible tubes with different electrical ratings (12AX7 vs 12AT7). Take my Cayin HA-1A amp for example, a GE 12AX7 can sound smoother than a Sino 12AT7, but any 5751 will sound even smoother because of the lower gain. For the Minute, 6N1P (high gain, brighter) and 6CG7/6FQ7 (low gain, darker) will sound very different. The difference between the 6L6 family (darker) and EL34 family (brighter) are equally obvious. EL34 is a pentode and there are beam-tetrode near-equivalents (6CA7, KT77). 6L6 beam-tetrode family has a long evolutionary history of tubes with slightly different electrical ratings (6L6 G/ GA/ GB/ GC, 5881, 5932, EL37, KT66, WE350B) and even the Russian near-equivalent 6P3S. These are just tube types and not brands and factory variations. With so many choices, people can really find the tone they want based on their ears, system, and musical preference. Metal and classical just need different flavors of amplification and different headphones. I can confirm that EL34, 6CA7, 6L6G, and 6P3S all work well and give me very different tones. I am currently using the Tung-Sol reissue 6L6G, which has a handsome "Coke Bottle" shape (see pictures below). Combined with clear-top RCA 6CG7, I am hearing sonic bliss right now but it may get even better with more extensive tube rolling. The magic: I love the sound of violins. On the other hand, I can totally make this amp sound faster and three times more aggressive if I like screaming rock music.
I play the violin and I do research on them. A few months ago, I went to a private museum to record some of the finest violins in the world, including 4 Stradivaris, 3 Guarneris, and 2 Amatis. Spending a whole day going back and forth between different violins with a combined market value of $70M is the greatest "listening experience" of my life. Thanks to inserting the Minute between my Benchmark DAC1 and AKG K1000, I not only get violins to sound like violins but also the finest violins to sound like real treasures. Myself playing a $16K violin sounds pretty bad compared to the best violin recordings. I already know that the Minute is capable of producing my "dream tone," and I am still tweaking for more refinements. The good news is: old stock 6CG7 tubes are still very affordable on eBay and two dozens of EL34 and 6L6 tubes are in current production. It's going to be fun!
Quick comparisons:
Compared my Cayin HA-1A tube headphone amp (overrated as class A by Stereophile but still pretty nice), the Minute is more powerful and much quieter. The Minute clearly has better tone, spaciousness, and bass. There is simply no comparison. Even with sensitive cans like HD595, the Minute does not lose details compared to solid state headphone jacks on my Presonus Central Station (very good) and Benchmark DAC1 (decent). The warmth, air, and smoothness of tubes are just more musical for acoustic instruments. Even with pop music I still prefer to use my Grado with the Minute to get a fuller sound.
I have used several low-power speaker amps below $1K in my system: Jolida 301 hybrid (30W hybrid), modified Jolida 102 (20W EL84 class-AB PP), Almarro A205 (5W single-ended EL84 pentode), a Chinese clone amp (10W EL84 class-A PP), and Sonic Impact T amp (5W). While all of these amps try to achieve good tone at a reasonable price by sacrificing power, the Minute easily beats them all in terms of tone, resolution, and bass. Compared to every speaker and headphone amp I have owned, it is much easier to listen to music too loud with the Minute because its distortion and tone do not obviously worsen by turning the volume up. Combined with the soft-clipping nature of single-ended tube outputs, it can play much louder than other 10W amps.
Summary:
The Minute is designed to drive today's power-hungry hi-end headphones. It does its job so brilliantly that I wonder if cathode-feedback pentode is the best practical solution to tube amplification. My own test shows that the Minute is equally comfortable with more sensitive full-size headphones and real-world speakers. In addition to low noise, great transparency, and great bass, it can provide so many tonal colors by tube rolling. Ordering and communicating with SAC Thailand has been super easy by email and Paypal. This company has a real passion for developing unique audio products. As you can see from the pictures below, the build quality is excellent in every aspect:
[Edit] I forgot to mention that mine is 110/120V version. The stock version is 220/240V. It took several weeks to custom produce my amp.
With SED EL34 Tubes
With Tung-Sol Reissue 6L6G "Coke Bottle"
My K1000 loves the "Coke Bottle" 6L6G
The self-noise is so inaudible with K1000 that I switched to iPOD earbud (several thousand times more sensitive) to listen to the noise.
This is the look with the tube grille (less pretty, but safer).
My favorite picture, the Coke Bottle glow
