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OTL, Output Trasformer Less Tube Amplifier

Designed by: Andrea Ciuffoli.

contact me for any problems or question: webmaster@audiodesignguide.com

Specifications:
overall NFB less than 10db (usually kept as low as 4.5db with 90 dB/m loadspeakers).
Gain about 26db in close loop mode.
Rout about 1 ohm Vout =  20Vpeak on 4 ohm load.
Very long life bias for 6C33C-B (only 0.2A).

I use such a setup with normal loudspeakers and with my Beyerdinamic DT990 earphone. It could be a good choice to drive an ESL loudspeaker.

The power stage has been designed to work in a SEPP (Single Ended Push Pull) configuration.

Design Issues

In my opinion the bigger problem encountred with the SEPP configuration is due to the unbalance of the amplification factor between the output tubes (upper and lower).

To overcome such a trouble, my choice was for the Technics configuration:

  • Technics simplified schematics (email by Takuji Yamamoto)

    • until version 8.0a but now I have choices the VARIATION ON THE FUTTERMAN
    as show in a good article on
  • OTL Amplifier Background

    I don't like the Circlotron (some problems here)


    Now online the last versions:

Double power only duplicate output tubes!
Better drive to any load (loadspeakers or earphone)
Lower  output resistence
Lower  distortion
Lower  power supply stages
Lower  components
Lower  local feedback
Zero   noise
Only   tubes on signal (no FET)
Output capacitor less design possible


  • Picture of the version 8.2 cheap & easy but same sound of the others


  • Picture of the version 8.2 OCL(8/8/98)


  • Picture of the version 8.2 (6/8/98)

to increase gain put a capacitor 220uF 16v
ELNA Cerafine only on 680 ohm of 2 stage


  • Is the OCL (Output Capacitor Less ) design real ? 



old versions

  • version 1.0

  • version 6.0

  • version 7.0


Some plot by simulation on Version 5.0

  • frequency response with nfb = 5db - ft(-3db) = 135KHz
    • 

  • frequency response with nfb = 13db - ft(-3db) = 222KHz
    • 

  • Rout with nfb = 5db - 2.5 ohm
    • 

  • Rout with nfb = 13db - 1.47 ohm
    • 

Some photo

  • photo1
    • 

  • photo2
    • 

For tubes datasheet ask me or search on Internet.

  To see other OTL designs
    •   To look about an ultra cheap power supply

    Important email messages related with this project:

  • from Kevin Kennedy
  • from Takuji Yamamoto
  • from Sovtek
  • from Roman Efimov (Odessa, Ukraine)  

    • Single Ended Pure Class A Power Follower Amplifier

    see new webpage on: http://www.audiodesignguide.com/PowerFollower/

    The End Millennium

    by L C Audio


    This is not a my design and I don't like assemble the kit of other people but
    when I have seen this project.....

    This is a complete push-pull amplifier in class AB operation without any feedback.

    To get low distortion by the transistors in the voltage stage a simple but good
    solution has been used. Infact an emitter follower is used after any voltage
    amplifier to have low load for this one.

    Other characteristics of this amplifier are:

  • the signal street on the board is very short
  • output device are the best Sanken transistors
  • high capacitors value used in power supply 120.000uF
  • Separated voltage stage power supply
  • Balanced or unbalanced input is allowed
  • Many components are preasseblated (all the SMD)
  • Top quality bypass capacitors are used in power supply (Music cap)
  • Very large band DC to 500.000 Hz
  • High output current 46 Ampere for 1ms
  • Output power from 120watt to 250watt (depend by version)
  • DC servo control included
  • Short circuit protection included
  • You can buy only some parts of the kits
  • You need only a volume control as the CT1 of DACT to connect to a CDPlayer

    I have tested this amplifier on some loudspeakers, also a type without caps. in the
    filter with very low impedance and the result is in any case very good, never there
    is distortion,the bass frequency are fast and controlled, the middle and high frequency
    are clear on any kind of music from classical to hard rosk


    to get more information go on L C Audio or contact by email: lcaudio@post5.tele.dk

    • Push Pull Pure Class A "Hot Follower 99" Amplifier

    Designed by: Andrea Ciuffoli.





    The target of this project is a 50W RMS on 8 ohm and 100W RMS on 4 ohm in Class A


    Both input/driver stage work with the same bias so the Vbe distortion should be compensated!


    The simpler design of the Coll Follower could be used!

    Ofcourse also a normal capacitor/bridge only power supply could be used!

    Must be driven by a voltage stage with min bias 30mA

    WARNING: designing phase so some parts could be change in the next days!

    Ofcourse this design cannot give the same sound of the reference: "Power Follower 99"

    Push Pull Pure Class AB "Cool Follower 99" Amplifier

    Designed by: Andrea Ciuffoli.




    This is a 100W RMS (8 ohm) Pure Class AB Amplifier with original design never seen!

    Both input/driver stage work in optimal Class AB (100mA for each devive)

    Ofcourse also a normal capacitor/bridge only power supply could be used!

    Must be driven by a voltage stage with min bias 40mA

    WARNING: designing phase so some parts could be change in the next days!

    Ofcourse this design cannot give the same sound of the reference: "Power Follower 99"


  • Different level distortion simulation
    This plot has been created with a my custom version of spice3f4.
  • Different frequency distortion simulation
    This plot has been created with a my custom version of spice3f4.
  • Limitation of Cool Follower
    This plot show the limitation of the Cool Follower!
  • Input current


    Winsurfing, my preferred sport

  • photo 1
  • photo 2