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June 2000

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Moth Audio si2A3 Vacuum Tube Integrated Amplifier Review
Like a moth to a flame...
Review By Steven R. Rochlin


Moth Audio si2A3 Vacuum Tube Integratd Amplifier Review


  How do you make a great product even better? How do you make it less expensive? How do you offer one of the most cost-effective ways to reach your audience? If you said "the internet" than you could very well be right. Moth Audio first grabbed the eye of the industry at the Hi-Fi '97 show in San Francisco. In fact i personally found their gear to be very refreshing as the visual appeal of retro-looking solid construction was a well needed relief from the usual black or silver box.


Compare And Save!
Here we have the second generation of this Moth Audio amplifier now called the si2A3. The si2A3 is a single-ended zero-feedback "Class A" three watt per channel tube amplifier. This new model is a good evolution as the customers have asked for more than one input and the rectifier tube to be more accessible... and Moth delivered this and more! The following is true of the new unit as compared to the previous version:

1) The older unit had most of the wiring on top with a few wires below for the 6SL7's. According to Moth Audio, this made it difficult to assemble. The new version has all the wiring under the bottom pan and below the tube bracket. This has better shielding and saves in labor expenses.

2) The power supplies of the older s2A3 and newer si2A3 are essentially the same with slightly higher ratings on the capacitors used in the newer model.

3) Because Moth Audio added a three input switch there is more signal wire in the new amp. Of course there is now three inputs whereas the original unit only has one. The new si2A3 has the same volume control as the older unit. So now there is no need for an upstream preamplifier in many systems.

4) The old unit had the rectifier tube inside the unit while the new si2A3 has the rectifier tube top and center (see photo at top of page). This allowed better heat dissipation plus the ease of "tube rolling" which is a sport unto itself.

5) Though the public at large seemed to prefer the headphones disconnecting the speakers, Moth Audio has opted to keep the signal path cleaner by getting rid of the switch. Therefore the headphones do not disconnect the speakers when plugged into the si2A3.

6) Moth no longer has US dealers and therefore the retail price has come down significantly. In my previous review on April 1998 it retailed at $3,450, the newer more flexible version is sold direct for only $1,695 plus the cost of output tubes. Sovtek 2A3 single plates are $48.00 per pair at the time the amp is purchased and the AVVT 2A3 mesh plates are 10% below current US price. As of the time of this writing it would be $445.50 per pair with the amplifier (though might be available for less through other discounters).

During this review i opted to use the AVVT Mesh Plate 2A3 and also the excellent Sovtek single-plate units which my friends seem to be raving about. For rectification duties i used my brown base Mullard CV1377/5AR4 (military grade expensive jobbies). Of course those looking to sacrifice a bit of dynamics for a more lush and harmonically rich sound should try the Mullard CV378. Enough with the technicalities, it is time for the music!


Magic Bus
Initially the new Moth with the AVVT Mesh Plate 2A3 tubes sounded like the bass had become more pronounced while the upper frequencies more extended. After a few hundred hours the bass settled in nicely as the highs became still extended yet more refined. i can easily understand why the Moth Audio si2A3 is a fave rave among the tube heads. While fairly inexpensive, this is one solidly built and flexible unit. Those of the not full DIY but big time tweaker and parts changer will easily find this amplifier a dream. Without any tweaks this unit offered sound quality that belied it's under $2000 price. While not having the ultimate pace and rhythm like my reference VAC Avatar in this regard, it did equal that of other fine amplifiers.

i can easily understand why there is a huge underground rave about the AVVT Mesh Plate 2A3. While expensive, it delivers the goods! Virtually no microphonics, tight and tuneful bass, glorious midrange and extended and refined highs. The Sovtek have been rated as the best value for the dollar. It also offered very impressive, if not as bottom of the Earth pipe organ deep, the midrange is about as glorious as the AVVT while the highs are close to the AVVT as well yet not quite as extended. Of course this is all in regards to how they sound through the Moth si2A3. If you can not afford the AVVT immediately, the Sovtek make a very good argument for high value and bang for the buck.

Imaging was just beyond the right of the right loudspeaker and slightly left of the left loudspeaker. Depth was good yet slightly shortened. The music did come into the room. With some speaker placement experimentation i could get more depth but then the music did not seem to come forward into the room plus it closed down a bit of the width. Ah, the dilemma and that of personal preference. If there is one thing really good single0ended tube amplifiers have in common is the music's speed and agility. Here the Moth Audio si2A3 had it is spades. Especially with the AVVT Mesh Plate 2A3.

Harmonics actually seemed a bit richer with the Sovtek. Of course using the Mullard CV378 with the AVVT could be considered a great combination for those looking for that incredible harmonically rich sound yet still have all the speed and good accuracy. One of my friends has been busy using Black Gate power capacitors instead of the stock jobbies, though if i talked about al the different tweaks here in this review we would have over five different reviews! Hmmm... i better get back keep to how the unit sounds stock.

Of course with any low powered amplifier, this three watt unit needs to be mated with proper loudspeakers. With seemingly strong drive and good bass, the Moth si2A3 should do better than lesser units in that the extra grunt helps to really take hold of the music with loudspeakers that are less than optimum. Of course you can change the output impedance matching of the output transformers to find which setting sounds best (2, 4, 8, or 16 Ohms).

During this review i did get a feeling of a slight stripping away of the harmonics. If i only had some Black Gate power capacitors here... Ooops, there i go again wanting to get off stock as i desire to tweak the Moth Audio si2A3 to more of my liking. And there is the truth of the matter. In stock form the si2A3 is a great value for the dollar. Then guys like me seem to take this great design and tweak a little of this, a little of that. Even the dudes at Moth know that guys all around the world are doing this with excellent results. This is not to say it is bad in stock form. In fact it is very good indeed. Maybe it is so good that you just want to make it that little extra better for ultimate audio exxxstasy. It is like my friends who own a Porsche and love the fact they can easily add aftermarket product to further enhance the performance (shameless plug for Performance Products here).

While i felt it obvious, there was no need to use a preamplifier with the Moth Audio si2A3 as the three inputs were used with my DAC and my MC preamplifier/turntable combo. The third input was not needed. So as a system i saves money on not just the elimination of a preamplifier but also the obligatory interconnect cable as well.


I Call That a Bargain
For those looking for single-ended magic and whose system can integrate with a three-watt-wonder, this could be your ticket to audio exxxstacy. The possible elimination of your preamplifier and the possibility of tweaking to your heart's content make this not just a set amplifier, but one that can grow as you tube roll and change out parts and wire to the ever tweakier audiophile ones. With the now lower price than the previous version and added flexibility it is hard for me to imagine a better deal other than a full-blown DIY amplifier kit. Looking for three really good watts? Tired of the same hum drum non-tweakable solid state amplifier or if you have a 300B-based amplifier and looking for that famed 2A3 purity than it almost seems natural to me to audition the new Moth Audio si2A3. It saves you from fully DIY kits. Audition highly recommended.




Sub-bass (10 Hz - 60 Hz)


Mid-bass (80 Hz - 200 Hz)


Midrange (200 Hz - 3,000 Hz)


High-frequencies (3,000 Hz on up)






Inner Resolution


Soundscape width front


Soundscape width rear


Soundscape depth behind speakers


Soundscape extension into the room




Fit and Finish


Self Noise


Value for the Money


Chassis: Zinc plated 18 gauge cold rolled steel. Top and bottom are 18 gauge cold rolled steel, brass plaque logo on top plate. The chassis is satin Iron Glimmer powder coat. Black wrinkle powder coat on top and bottom. Machined aluminum feet with rubber inserts. Glass port holes are Pyrex glass with machined aluminum rings.

Output Transformers: Manufactured by Electra-Print Audio Company
User selectable: 2, 4, 8, and 16 Ohms

Tube Compliment: Output optional
One 5AR4 Rectifier Tube "Sino"
Pair 6SL7 Jan Phillips

Line Cord: 16 gauge, 3 conductor IEC cord

Volume Control: Alps potentiometer

Power Output: 3 watts, 5% THD @ 1kHz

THD: Less than 1% 40 - 20 kHz @ 1 watt

Power Supply: Low-flux leakage design using toroidal power transformers as well as toroidal inductors

Frequency Response: 5Hz to 32kHz (-3dB points at 1 watt)

Input Sensitivity: 500 mV for 1 watt out

Input Impedance: 100kOhms

Output: 2, 4, 8 and 16 Ohms.

Cardas: Speaker jacks
Hand wired point to point for all signal circuits

Dimensions: 4" x 14.25"  x 15.5 (HxWxD)

Weight: 40 Lbs.

Warranty: All components carry a three year parts and labor warranty. Tubes carry the manufacturers' warranties. Shipping and handling is not included.

Price: $1695 plus the cost of output vacuum tubes. Sovtek 2A3 single plates are $48.00 per pair at the time the amp is purchased and the AVVT 2A3 mesh plates are 10% below current US price. As of the time of this writing it would be $445.50 per pair with the amplifier (though might be available for less through other discounters).




Moth Audio Corporation
1746 Ivar Avenue
Hollywood, CA 90028

Voice (323) 467-4300
Fax (323) 464-9100

Website MothAudio.com
















































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