Amplifier with jfets and laterals

Discussion in 'The Projects Forum' started by Experimentonomen, Jan 27, 2012.

  1. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    Heres another project of mine from last year, that i dug out, dusted off and fixed up.

    A amplifier using jfets in the input stage and laterals in the output stage.

    This amplifier is unique from my other projects in the sense that its dc coupled, which means there are no capacitors in the signal path to cause low frequency rolloff, instead this amplifier has frequency response down to DC.

    The overall design is based off of several commercial amplifier topologies including D Self's "blameless" mixed together, and with some of my own ideas mixed in.

    The overall schematic:
    http://i.imgur.com/XSIEd.png

    And a picture of the actual unit:
    http://i.imgur.com/uEp0M.jpg
     
  2. PaulEE

    Member

    Dec 23, 2011
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    I think you meant to post this in the completed projects section? Cool circuit, BTW.
     
  3. Experimentonomen

    Thread Starter Member

    Feb 16, 2011
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    Tecnically its not done, i just cannot proceed any further due to lack of tools for metalwork.
     
  4. Ron H

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    Do you select the JFETs for matching?
     
  5. Experimentonomen

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    Feb 16, 2011
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    No need to, the jfet is one of those dual ones, two in same package.
     
  6. Ron H

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    I can find spice models for the J2SK332, but no datasheet, and no vendors. Can you post links to these?
     
  7. Experimentonomen

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    Feb 16, 2011
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    These Jfets are obsolete since decades ago, i was lucky to find three on ebay.

    I think you can still get the 2SK389 from digikey, mouser or farnell, its in a different package but should still be a suitable replacement.
     
  8. atferrari

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    Jan 6, 2004
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    Hola Ron,

    In this site there are some listed:

    http://www.rfcandy.biz/shop/

    I do not know if they will be of use for you.

    Look in the left side menu
     
  9. Ron H

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    I couldn't find them at that site.
    Thanks for looking, Agustin. I'm not in the market for them. I was just curious to see a datasheet, and that's when I discovered that neither the part nor its datasheet is readily available.
     
  10. Experimentonomen

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  11. Ron H

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    I don't see any matching specs.
    Matching may not be critical. With a closed loop gain of 28.5, you'll get 28.5mV of output offset per mV of input offset (if your current mirror is perfect - it won't be). If the transistors are matched within 20mV (my arbitrary guess), that's 590mV of output offset. with a 4 ohm load, you'll have 150mA of current imbalance in your output drivers. Will that cause significant distortion? I have no idea. Does a current bias cause a problem with speakers? I don't know.

    EDIT: What's the purpose of the RC network on the input?
     
    Last edited: Jan 29, 2012
  12. Experimentonomen

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    Feb 16, 2011
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    To remove unwanted rf from the input, without it you can sometimes hear radio stations when using long audio cables, i've heard stories of guitar amplifiers picking up radio stations from other countries.

    This amplifier with no signal is dead silent and measured dc offset on the speaker outputs are 10-25mV.

    It has strangely also has better psrr than my bjt amps which audibly hum on idle.
     
  13. Experimentonomen

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  14. atferrari

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    Sorry Ron, there are some dual FETs. Not the ones you were interested on.
     
  15. Experimentonomen

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    Feb 16, 2011
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