back to my home page and other projects

OTL = Output Trasformer Less Tube Amplifiers

My consideration on problems of each design

  • Ciclotron balanced design problems (type 1):
       1] More high output resistence
       2] the duplication of the power supply (2 for each channel)
       3] impossible to use with earphone (because no ground on outputterminal)
       4] you can destroy the loadspeakers if you don't put a protection from DC
       (a simple variation of the bias of one output tube will give high DC on output)

  • Technics design problems (type 2)
       1] Changing the load the first feedback cannot to compensate asimmetry of the output stage.
       (In real case the loudspeakers is not a constant impedence)

  • SRPP (type3)
       1] Very high Output resistence
       2] Work only in Class A so very short life for tubes
       3] Many power lost in cathode resistence
       4] Very low efficiency
       5] High distortion
       6] You need 4 or 6 output tubes for channel

  • Futerman design (type 4)
       At this time I don't have found problems.

    Circuits gallery

    New design circuits
  • OTL by A. Ciuffoli
  • OTL ideas by Takuji Yamamoto (japan)
  • OTL by Morikawa Chuyu from MJ 1993/4
  • OTL by unknow Japan company
  • Yozo Matsuoka - from MJ 01/97
  • OTL by Yozo Matsuoka from MJ 1995/11
  • OTL by ? from MJ 1997/7
  • The COVI Mark II 100 W OTL Amplifier
  • mirror on my web lee.html
  • Kaneda ibrid OTL - 40W 1% thd 18db nfb
  • Kaneda ibrid OTL - power supply
  • New Kaneda ibrid OTL
  • ibrid 6C33C OTL by Shinichi Kamijo
  • EL509 OTL by Shinichi Kamijo
  • SRPP 20 watt Class A OTL by Fabio Camorani from Sound Practices (I have complete artile)
  • SRPP 20 watt Class A OTL by Fabio Camorani from Costruire Hi-Fi (I have complete artile)
  • Transcendent Sound, Inc.   complete patent with circuit (I have all pages)
  • Graaf 20 schema and photo

    Old design circuits
  • Futterman OTL
  • original Futterman H1 OTL
  • Technics 20A - 1965 - 75watt on 16 ohm
  • Schematic by P. Dickie Jr & A. Macovski (June 1954)
  • 6336A PP (Taki OTL) reg.
  • 6AS7 OTL
  • OTL by ? from Japan
  • Old OTL design by ? 25W 0.4% thd with 40db of nfb 
  • A design note: I would say that the power supply solution chosen to drive the output tubes in the "OTL by unknow Japanese company" and in the "OTL by Yozo Matsuoka from MJ 1995/11" looks a bit strange to me, because it is designed around is a voltage duplicator where there is a secondary with half voltage and 2 diodes. After a spice 3f4 simulation I have got "terrible" results. The bridge solution is better ! (nevertheless the output current problem ...)

    Frequently adopted OTL Output Tubes

    6C33C-B tube characteristic
  • page 1
  • page 2
  • photo
  • filament connections
  • size
  • Original specifications
  • actual plot by Sofia on one of my tubes
  • 6C33C-B by audiomatica


  • EL509/6KG6 tube chatacteristic
  • complete data sheet Adobe Acrobat format


  • 6AS7 tube chatacteristic
  • complete data sheet Adobe Acrobat format


  • PL504 tube chatacteristics
  • page 1
  • page 2
  • page 3
  • page 4
  • page 5
  • page 6

    Understanding OTL design

    Background and OTL Circuits by aria
  • mirrored page
  • official pages


  • Understanding Basic Circuits by Atma-Sphere
  • mirrored page
  • official pages

    Commercial OTL

    Graaf GM-200
  • photo1
  • photo2
  • specification
  • distortion at 12w 8ohm
  • Claudio Dolci comment
  • mirror of officail pages
  • official pages

    Fourier Panthere
  • photo1
  • photo2
  • specification

    Atma-Sphere
  • official pages

    Trascendent Sound inc
  • Many answers about OTL
  • Interview with Bruce Rozenblit, Transcendent Sound Inc.
  • official pages