Erik Mandaville Posted January 2, 2005 Share Posted January 2, 2005 The Moth 2A3 amplifier is probably the most involved SET amp I have built in terms of wiring, but it went together fairly quickly because the chassis was of course already finished. The Electra Print OPTs were already installed, as was some of the other hardware. Moth Audio CEO, Craig U. said it would be better to ship the amp with parts like the transformers already installed, but he helped us out by leaving all the wiring and connections for me to do. It is an outstanding design, I think, and the first 2A3 amplifier I have built that uses a regulated DC heater supply on the 2A3 filaments. As such, This amp doesn't have hum-balance controls. I have also found that with the center channel, the directly AC heated filaments on my own amps and Moondog center channel seem to combine together, resulting in a little more hum than I would like. The DC filaments on the Moth should very much improve this. It is a direct coupled design, using paralleled sections of a 6SL7 driver into the 2A3. No coupling capacitors are used anywhere. I ordered a combination of silver plated copper (Teflon insulation) and basic 18AWG 1015 type hookup wire for the power supply connections. I have three more connections to make in each channel, and I'll post three pictures of wiring and the finished product later today. Oh, for signal ground connections, channel and power supply separation was made by using two CL-60 Thermitors, where two legs of the current limiter are common/ground, and the other two are connected to the R and L channels, respectively. As I have mentioned, this amplifier was something I have looked at for years and always really, really liked. Marie surprised me with this as a very special gift! (for you, my dear!) Erik Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted January 2, 2005 Share Posted January 2, 2005 Erik, You sure picked a good one! Marie, I mean. The amp is a good one, too. That is a very nice surprise gift. When you have a chance to post some pics and say more about the sound, I'll sure be interested to hear. Quote Link to comment Share on other sites More sharing options...
Born2RockU Posted January 2, 2005 Share Posted January 2, 2005 ERIK : I am so happy for you. I know you have waited so long to finally get this amp. I can't wait to have the chance to come over to your place to hear this "bad boy" in action ! Please inform us of what your first impression was when you "light it up" . ~ Show us some pics !!! ~ Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted January 2, 2005 Share Posted January 2, 2005 http://www.enjoythemusic.com/magazine/equipment/0600/mothsi2a3.htm I'd remembered seeing this on the net. This is it, right? Looks fab with those AVVT tubes. Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted January 2, 2005 Share Posted January 2, 2005 Ah, yes...Moth Audio also makes a 300B version with practically the same chassis as the si2A3. It reminds me alot of those ol' '20s Philco AM radios in those long rectangular metal cabinets and those big black bakelite knobs (except the radio's tubes are inside the cabinet). Very retro! Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 2, 2005 Author Share Posted January 2, 2005 That's the one, Dee. I will be using KR Enterprise 2A3s, however. I will be using this amp as the L/R, the Horus for side channels (or possibly still for L/R, depending on what we find with the Moth), and the Moondogs for the rears. The surround aspect of this is something I'm not in the position to do now. There have been a couple of DC-1 Lexicons (the model Shawn let me borrow) on ebay, and I was SOOOOO tempted to place a bid. Just can't do it right now, though. Hey, Craig! I'm glad those cables are working out for ya! Home Depot has some decent stuff at very low cost. Erik Quote Link to comment Share on other sites More sharing options...
Born2RockU Posted January 2, 2005 Share Posted January 2, 2005 ERIK: ...thanx for the lead on those Gold RCA to RCA 6 footers at Home Depot. I am just amazed at their design and they come with a lifetime warranty, to make the inexpensive investment more worthy !!! ....so have ya "fired" the amp up yet ? Quote Link to comment Share on other sites More sharing options...
leok Posted January 2, 2005 Share Posted January 2, 2005 Erik, The Moth sounds like a fun amp in several ways: visual appeal, DC heater, and the absolute best in interstage cap technology: none at all. I liik forward to your impressions. Leo Quote Link to comment Share on other sites More sharing options...
Dflip Posted January 2, 2005 Share Posted January 2, 2005 Moth Si2A3 http://www.mothaudio.com/products2.html 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: Custom manufactured for Moth Audio Corporation. User selectable: 2,4,8 & 16 Ohms. TUBES: Output optional. One 5AR4 Rectifier Tube "Sino" Pair 6SL7 "Sovtek" LINE CORD: 16 gauge, 3 conductor IEC cord. VOLUME CONTROL: Alps potentiometer. SELECTOR SWITCH: Three input with coin silver contacts. POWER OUTPUT: 3 watts, 5% THD @ 1kHz. TOTAL HARMONIC DISTORTION: Less than 1% 40 - 20 kHz @ 1 watt. POWER SUPPLY: Is a low-flux leakage design using R.C.L.C.R.C. design. FREQUENCY RESPONSE: (-3dB points at 1 watt) 5Hz - 32 kHz. INPUT SENSITIVITY: 500 mV for 1 watt out INPUT IMPEDANCE: 100 K ohms. OUTPUT: 2,4,8 & 16 ohms. User select. CARDAS: Speaker jacks. Hand wired point to point for all signal circuits. OVERALL NEGATIVE FEEDBACK: None HUM & NOISE: .15 mV full bandwidth. TEST MEASUREMENTS: Made with KR2A3 output tubes. POWER REQUIREMENTS: 110 - 125 VAC 60Hz (210 - 250 VAC 50Hz) 75 Watts DIMENSIONS: 4" H x 14 1/4" W x 15 ½" L WEIGHT: 40 Lbs. HEADPHONE RECOMMENDATIONS: Sennheiser HD600, RECOMMENDED SPEAKER SENSITIVITY : 96 dB or better. WARRANTY: All components carry a 3 year parts and labor warranty. Tubes carry the manufacturers' warranties. Shipping and handling is not included. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 4, 2005 Author Share Posted January 4, 2005 The amp has been mostly done for a couple of days, but there were a couple of small glitches along the way. A few corrections (really updates) needed to be made to the schematic, but all has worked out very well. This amplifier was built without a manual, where I was relying on the schematic, alone. As such, I am working out some grounding issues, which become all the more complex when working with an integrated stereo amplifier. Here is one case, I think, where the comparative somplicity of monoblocks is nice. The designer also told me Moth had to do some troubleshooting as far as the best, most quiet grounding scheme was concerned. I am now at the point where there is just the slightest bit of distant buzz with my ear next to the speaker. It's good, but I think I can do better, still. This is a completely direct-coupled, single driver stage amplifier, and so has no DC blocking capacitors in the signal path. In fact there, are no capacitors in the signal path at all. There are the cathode resistor bypass caps in both driver and output, which are something, after tons of experimenting with and without, I have decided I prefer. Anyway, shown here are a couple of in-progress shots of the wiring -- far more involved than any other SET amp I have worked on due to the point-to-point nature. There are actually three separate levels of wiring, with the majority of the work on the very bottom of the amp. BTW: This amplifier has its own volume control and source switch, so a preamp isn't needed. The interconnects I am using (now) are 6 feet long, and there wasn't even a hint of capacitance induced roll-off. Very, very nice! The wiring shown is in progress, with zip ties used to help combine and indicate signal and power supply. I am still working on it, and will need to get another bias meter for the top -- the one in the right channel is bypassed by the yellow lead shown until the replacement arrives. But is it better than the others!? Depends on what one likes. The character of the sound is very much like the SE OTL, the Horus, and probably most of all like the 45 DRDs, but is a little more powerful sounding due to the stronger output tube. I will not be installing the KR 2A3s until I have it as good as I can get it -- which should be by this weekend. Erik Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 4, 2005 Author Share Posted January 4, 2005 Ok, I'm having trouble loading the last picture showing bottom wiring -- that's where the majority of the work is. What is shown above is bascially power supply related, including the regulated heater supplies for the 6SL7s and both 2A3s. This amp is really completely free of filament hum because of the use of DC instead of 2.5 volts of AC. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 4, 2005 Author Share Posted January 4, 2005 Amazing what a new battery will do! Typical... This is a picture of the underside wiring of the driver stage, some of the power supply, and ground wiring. I got an email from Moth Audio explaining the point they eventually found works best for the chassis ground. The paint was removed from that place, and the chassis ground relocated to that spot, and the slight buzz became inaudible. Not even tweeter hiss, which is something I have almost always heard from other amps. I am really happy with it, and will be installing the new bias meter when it arrives. Thanks for responding, Mark! Erik Quote Link to comment Share on other sites More sharing options...
Born2RockU Posted January 4, 2005 Share Posted January 4, 2005 Yahoo...no more noise. I was nervous there for a while, ERIK !!! Congrats ! Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 4, 2005 Author Share Posted January 4, 2005 Hey, Craig: Building stuff like this often involves a little head-scratching. I seriously thought the outputs had suddenly gone south, or something, because with the relocation of the chassis ground, ANY and ALL noise truly just vanished. The Klipschorns, which have always given some hint of being awake and ready for work -- as I said, usually in the form of slight hiss from the tweeter -- were inert. Nothing. I stuck my ear firmly against the tweeeter (remember, I took the grille cloth off my Klipschorns) and I could tell the amp was on by a very faint whisper of hiss. It's really satisfying after quite a bit of wiring. There is much more than enough gain from the CD player directly into the amp, and I am not missing my preamp at all. I was thinking of an excuse to use it, which I could do, but why add to the the complexity if it isn't needed? It's really NOT needed. So, I think I can now button everything thing up! I need a nice rack for this thing, the Denon CDP, my Scott tuner ...with one shelf saved for a good processor of some sort. I haven't added the center channel permanently yet, because I wanted to get the Moth sounding the best that I could on its own. Erik Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted January 4, 2005 Share Posted January 4, 2005 Impressive work as always, Erik...I envy your skills (and your patience). Bravo! Quote Link to comment Share on other sites More sharing options...
Born2RockU Posted January 4, 2005 Share Posted January 4, 2005 Sounds like it won't be long until you are ready to show it off. Just let me know...for I have a full tank of gas in my car, and am waiting the invite !! Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted January 5, 2005 Author Share Posted January 5, 2005 Hi, Jim: Working out bugs is half the fun for me. Grounding is always a strange thing, and what works well in one case might not and probably won't in another. Many of the 'educational mistakes' I have made over the years, where I work was meth with rather poor performance, very often had to do with grounding issues. I have heard it referred to more than once as a sort of 'black art' of audio design. It's elusive, and sometimes really hard to pin down. Thanks, Erik Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.