------------------------------ From: X90BALLAST@wmich.edu Date: Fri, 22 Jul 1994 11:22:43 -0400 (EDT) Subject: Construct your own SE amp Path: wmichgw!x90ballast From: x90ballast@wmich.edu Newsgroups: rec.audio.high-end Subject: Construct your own SE amp Message-ID: <1994Jul22.112219.19840@wmichgw> Date: 22 Jul 94 11:22:19 EDT Organization: Western Michigan University News-Moderator: Approval required for posting to rec.audio.high-end Lines: 125 With all the talk about SE amps... ------------------------------------------------------------------------------ Here are two tubed single ended designs. The first design uses a directly heated triode, while the second uses a pentode wired as a triode. These designs would be suitable for a Voight, Altec 604, Klipsch, or any high sensitivity speaker (96dB+), since the power out is only 3.5W for both designs. Transformers are available from Magnequest or Audionote. AMP #1: --------------------------------------------------------+300V | | | 2 | }|| 5 | }||{--- 0 | 2.5K primary }||{ speaker out K \ }||{--- |---------------.1uf/400V--------- }|| plate / | | grid3 | | plate grid2-----1.5M |----------grid in----- grid1 | | /\ cathode 1 4 | | /| u 7 | ______ / 2 F 0 | }||{--- 2 0 | K | -- }||{ 117V 5 0 | | | \ }||{--- u 0 | | --------- F | | | / \ | | | /| \| | | 4 7 5W | | | 7 5 | | | u 0 | | | F | | | | | | ----------------------------------------------------------ground 6J7/6C6 2A3,6A3,6B4G for 2A3, a 2.5V/2A filament transformer must be used. For a 6A3/6B4G amp, use a 6.3/1A filament transformer. Chinese 2A3s are available for $12-16 and are of good general quality. They don't seem to stack up well against the original RCAs, but they are cheap for experimenters. I plan to use Chinese 2A3s in the push-pull amp I am currently working on. This design is from the Radiotron Designer's Handbook. AMP #2: -----------------------------------------225/60mA | | | }|| | }||{---- | 5K primary }||{ speaker out 1 }||{---- 0 }|| 0 | K plate-------- |----------.1uF/400V----- -grid3 | plate | / grid2----100- in---10K---grid |-------grid1 | cathode | \-cathode | | | | 4 6 2 /| 7 8 2 / | 0 0 0 1 | K | K 0 2 5W | | | 0 7 | | | 0 0 | | | u | | | | F | | | | | | -------------------------------------------ground 1/2 12AT7 EL34/6CA7 Grid 3 of the EL34 is connected to the cathode internally. Grid 2 is tied to the plate via a 100 ohm resistor. One could use one 12AT7 shared between both channels of a stereo amp. With only some 210V on the plate, this EL34 should last a long time. The Sovtek EL34s are supposed to be of a rather high quality. Not exactly a Mullard, but in these days only the lucky and the wealthy can afford to be choosy about their tubes. This design is from Haden Boardman of Audio Classics in Lancashire. Power supplies: If you are lucky you can use a regulated power supply from Lambda, Tektronix, etc. These will allow you to dial in the voltage you need while providing great regulation. Or you can build a power supply using a transformer and a nice PI filter. Use a tube rectifier for a slow warmup. I like 5V4s. Be original...power supplies have a great effect on the sound so do some experimentation. I prefer polypropylene power supply caps for reliability. SCR makes some nice 630VDC caps...or Solen Fast capacitors. A good choke is mandatory. Conclusion: Give these circuits a try... remove all preconceptions you have about circuit design and give them a listen. The past holds some great ideas ... I am not only interested in well measuring products, but also have an interest in the history of amplification and speakers. Single Ended designs do not always measure well, but they do provide a different glimpse of the music than most people are used to. The music breaths with life. Performers sound like they are actually THERE, instead of the canned music we mostly suffer through. The high content of 2nd harmonic is gift for those of you w/o Harry Pearson's record collection... almost all recordings sound damn fine! I think this 2nd harmonic also removes most of the harshness associated with horn speakers, so the strengths of horn reproduction come to light - power, warmth, dynamics, and realism. ----------------------------------------------------------------------------- Kevin Ballast x90ballast@wmich.edu VRA