Transformers to Record on Cassette Tape

Discussion in 'The Projects Forum' started by BarryBozeman, Oct 26, 2018.

  1. BarryBozeman

    Thread Starter Member

    Apr 1, 2016
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    Hi all-


    I am attempting to drive a tape head to record onto tape.
    For this process, I am sending an audio signal into a transformer - 8 Ohms to 1k (42TM013-RC for more information).

    While this does record onto tape, it does so with an extreme amount of distortion. I can attenuate the output of the audio signal, and I can attenuate the input to the transformer - but the only option seems to be distortion or OFF.

    Can anyone suggest a different ratio? Or have any thoughts about why this might not work?
     
  2. AlbertHall

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  3. Bordodynov

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    Alternative: You can do pulse width modulation and send this new signal to the transformer. You can use the TPA2000D1PWR chip.
     
  4. AnalogKid

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    Getting audio on and off tape cleanly is way more complex than you might imagine, primarily because othe way the magnetic particles in the tape respond to the magnetic field from the head is *very* non-linear. You are in for a lot of reading about bias, B-H curves, magnetic hysteresis, etc.

    @bertus might have some PDF links on this.

    ak
     
  5. bertus

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  6. MrChips

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    You need a bias signal typically from 40kHz to 200kHz depending on the speed of the tape.
    Note that the bias and audio are added linearly. This is not amplitude modulation (AM).
     
  7. BarryBozeman

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    Apr 1, 2016
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    What kind of system are you using?
    Is it a tapedeck or as cassettedeck?

    I am using 1/4" tape heads, but I don't know the specs.
    Is it just a matter of plugging in a sine oscillator with some range and messing with tuning and volume until it sounds good?

    Thanks for all the input, so far, everyone.
     
  8. MrChips

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  9. AlbertHall

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    You will not get good playback unless you use bias when recording. Please study the links which have been given above.
     
  10. bertus

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    Hello,

    Here I found an other old book about tape recording.

    Bertus
     
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  11. AnalogKid

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    There it is. I knew you'd come though.

    ak
     
  12. BarryBozeman

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    Apr 1, 2016
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    Hi all -

    Thanks for all of the great links.

    I've gone through all of them, but I can't say with any certainty that I've understood everything I've read. I am not an engineer, just an avid hobbyist.

    I was hoping to ask a few more questions here, so as to refine my understanding.

    I should say that what I am doing is essentially building a record head from scratch for a machine that does not have one, perhaps that helps better orient the difficulty of the task I'm endeavoring.

    To test the record head, I have a signal from an iPod and a sinewave generator. They are fed into the following stages:
    Summing Op Amp -> Inverting Attenuator Op Amp -> 1k:8r transformer -> Tapehead.

    I have tried a variety of things:
    * Different frequencies and amplitudes for the bias.
    * Different levels for the op amp
    * Different gain-staging (op amps/buffers/summers after the transformer, instead of before)

    So far, I haven't had much luck. I have a wider range of distortions.. there is an extreme oversaturation, but as I change the variables, I can diminish it to more of a crackle, and I can diminish the crackle until nothing is transferred to the tape.

    There is never a point where the audio sounds close to being good!

    To record, I am pressing the record head against tape that is being reeled on a spindle. I realize that this doesn't give particularly stable speed, but it has been stable enough to recognize the source material, even though it's heavily blown-out.

    Is it possible that simply the positioning of the record head is enough to cause all of this trouble? I've tried a few things to distance the head from the tape, but the signal becomes too faint almost immediately.

    Is it possible that the biasing only works within a very fine range - and that without certain mathematical calculations that I don't currently understand, will not be possible?

    Has anyone done a project like this? Are there standard schematics for it to look at?
     
  13. AlbertHall

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    If you mean actually constructing a tape head, what width gap do you have?
     
  14. MrChips

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    Distance of tape from the head? Zero

    On a reel to reel tape deck, the tape transport curves the tape against the head.
    On a cassette tape deck, the tape is pressed against the head with a spring loaded felt pad.

    Why don't you go and find a cassette player at a second hand store?

    [​IMG]
     
  15. AlbertHall

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  16. BarryBozeman

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    I'm interested in seeing if it's possible to do this with my limited resources. Maybe it's not! I'm also interested in understanding the process from the most basic standpoint, and I'm making progress on that front.

    I'm using a reel-to-reel tape head with a ferrite core. I can try to measure the gap width later today.

    Another thought I had re: biasing:

    MrChips' initial post shows biasing as a two-stage process: first, the tape is erased, using the bias frequency and a magnet. Then, the tape is inscribed using a summation of the bias and the input signal.
    I've been erasing the tape, applying the bias frequency, and then recording(bias+input) - all in separate passes. I'm wondering if this would potentially impact the recording.
     
  17. AlbertHall

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    If it is a head built for this purpose the gap will be suitable. I was concerned because it sounded like it was a head you had fabricated and that is not an easy job.
     
  18. MrChips

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    Erasing the tape as a separate step is not necessary.

    On cheaper tape recorders, the tape was erased with a simple permanent magnet that was brought up against the tape ahead of the recording head.

    AC bias applied to the recording head will also erase previous recordings.
     
  19. AnalogKid

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  20. BarryBozeman

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    Apr 1, 2016
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    So, if I am sending a bias tone to the record head and running it along old tape - it should be erasing?
     
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