Best OPAMP and RC oscillator version for 70 kHz

Discussion in 'The Projects Forum' started by hifi, May 2, 2011.

  1. hifi

    Thread Starter New Member

    May 1, 2011
    3
    0
    Hi electronics friends :)

    I need build maybe Wien Oscillator OR Phase shift one. The result must be FIXED frequency 70 kHz very low distortion sine-wave output.

    At first I tried to do it with CA3140. I had a big problem with distortion and also with starting of it. For amplitude gain control I used JFET 2SK30 transistor.

    When I tested the method by test 1kHz - all is OK.

    It means - problem must be in lazy CA3140 ... I think.

    I want to buy two diferrent OPAMPs = TL071 and LM318. Not tested yet - I am only planning to do it.

    What You are thinking about it ?

    a) which is better for this - TL071 or LM318 ?
    b) what method : Wien Oscillator OR Phase shift Oscillator will be better ?
    c) what type of amplitude control is best (for minimal distortion) ?

    Thanks for advice.

    In case You have some "best" schematic - thanks in advance.

    Best regards - PETER.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    How about posting the schematic of your design that did not live up to your requriements.

    hgmjr
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    TL071 is better
     
  4. hifi

    Thread Starter New Member

    May 1, 2011
    3
    0
    Hi, of course ... now is already using TL071, but still big distortion ...

    see schematic - there is virtual ground ... because only single supply.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The AGC (automatic gain control) of the circuit (it must have something of this nature) sounds flawed.

    [​IMG]
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Unless the virtual ground is really good, there will be crosstalk all around the circuit, including the JFET AGC control element.

    Come to think of it, I don't see any supply decoupling caps. Presumably there are some?

    Another issue might be the slow LM358 handling the AGC rectification. Looking at the datasheet, its signal handling capacity seems to be only a couple of volts at 70kHz.
     
  8. hifi

    Thread Starter New Member

    May 1, 2011
    3
    0
    Thanks to all:

    Now I tried to insert between output and rectifiers a resistor M1 and add blocking capacitor 47M directly to virtual ground and supply ground. It seems to be manytimes better.

    But I´m thinking about little change - make symetrical power supply (two OpAmps 7812 and 7912 are cheap :)

    Maybe all circuit will be better and sine-wave also ...

    In that case I can after canceling of dual OpAmp use only one as rectifier and use there also TL071 (instead 1/2 of LM358).

    What do You think - is it good idea ?

    =========================================

    For Your complete imagine what I´m developing :

    Output of this Oscillator will be buffered - linked to small power amplifier (TDA2030 or different one - I don´t know, if TDA2030 will work at 70 kHz ...) and from it to step-up transformer. I need to make very high quality signal 70 kHz and voltage about 50 V for full track erasing head.

    It will be a special device for quality erasing of old tapes, which are :

    a) used and bought from people AND/OR
    b) old tapes recorded with faulty head setting of recorder.

    In these cases the best way is erase all width of the tape.

    Unfortunately traditional oscillators, where the head is used as working inductance of oscillator´s circuit, mostly have not big quality and/or use complicated transformer.

    This is a reason of my delvelopment. My first imagine was not use a transformer, but make end stage with 50 V output on 70 kHz seems to be problematic.
     
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