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Cassette tape delay

Discussion in 'General Electronics Chat' started by Rorut, Dec 19, 2017.

  1. Rorut

    Rorut Member

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    Thank you very much Dr pepper, I will try the idea with the pot, that sounds like a good choice.

    I also find this image of the circuit with a modification but no explanation of what it does?
    I can se ground is moved away from the pot in the middle but dont understand the rest
    4B17BE49-61E3-443B-B9AA-BA4679E86325.jpeg
     
    Last edited: Dec 26, 2017
  2. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    I've eaten too much christmas cake so I might get this wrong.
    It looks like the old circuit always introduces an amount of 'clean' unecho'd signal into the output, the mod allows you to twiddle the pot R11 from just the echo'd signal to just the clean signal.
    So if you want you can have just clean or just delayed instead of having a fixed amount of clean + mixed in adjustable level delay.
    Hope that makes sense.
     
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  3. Rorut

    Rorut Member

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    Yes I also ate to much food, do not feel healthy at the moment :)
    Would it be difficult to have a new pot for dry signal allowing 100% "wet"?
    Someone tried "dry signal attenuation" by adding a pot in series with R14 but that did not work.
    Any ideas adding that to the circuit?
    (Circuit again)
    Image 2017-12-27 at 1.14.36 em.png
    thanks!
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Look at the components feeding R12 (C10 and R11), copy that for feeding R14.
     
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  6. Rorut

    Rorut Member

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    I dont know if this approach could be corrected somehow to what you suggested :)
    709BA1B4-C85D-4DC6-84C2-1AB72D912F36.jpeg
     
  7. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    The mod in post #21 makes pot R11 adjust from 100% wet to 100% dry.

    Your idea is #25 might work if you swap the pot connections going to pin 8 of the op amp and R14, the pot would need to be a low resistance like 1k, 10k would work but the linearity would be squiffy.
     
    Last edited: Dec 27, 2017
  8. schmitt trigger

    schmitt trigger Active Member

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    A 100% wet signal would cause howling that would last indefinitely.
     
  9. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    You sure about that scmitty, wouldnt a 100% wet signal just delay the audio.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The pot is the wrong way round, the slider (and the extra capacitor) should go to R14.
     
  11. Rorut

    Rorut Member

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    What I want to do is:
    Dry Out
    Determines the level of original, unprocessed audio.

    Wet Out
    Determines the level of delayed, processed audio.

    Delay(echo)
    Specifies the delay length.

    Feedback
    Creates repeating echoes by resending delayed audio through the delay line

    Gain

    5pots or 4 if wet and dry is on the same
     
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The new pot that I suggested in posts #24 and #29 does exactly that.

    Existing pot R11.

    Not possible, you need to adjust the length of tape between the record and play heads to do that, although altering the tape speed will give a crude limited similar effect, but will lower quality.

    Existing pot R10 - set it too high, and the entire system will oscillate (this is normal for echo units).

    Existing pot R16 (output level, rather than gain).
     
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  13. Rorut

    Rorut Member

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    Thank you so much! Really appreciate it and cant wait to get this done.
    I understand I need to modify the unit to put in a pot that makes it possible to adjust the motor. Think I remember reading about just that mod somewhere.
    Is this looking better?
    schematic.png
    And should all pots be log pots? Or just the output level? Can I buy 1K log pots?
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why suddenly 1K?, it should be 10K like the other one. Also you have C14 the wrong way round, it need to be the same as the other capacitor.

    All pots need to be log - value isn't criticial - 10K would be fine for all, as would 4.7K or 22K.
     
  15. Rorut

    Rorut Member

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    Thanks, I got the 1K advise earlier in the thread.
    Image is now corrected.
    schematic new.png
    Would this one "MC33079P DIP-14 quad op-amp low-noise" work a a replacement for the "MC33174 Quad opamp" ?
    https://www.electrokit.com/mc33079p.44016
     
  16. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    I advised the 1k as the impedance the output of the pot goes to a 22k load, 10k will work but it wouldnt be linear.

    No disrespect to nige, however some change in delay time would be possible if you change the speed of the tape motor, though that would mean serious mods to the motor if the speed control is built into it.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Not really a problem, and if it concerned you simply alter the 22K resistors and associated feedback resistors - but the design already uses 10K in the exact same way.

    I already suggested changing the speed, but it alters quality as well - and as you say, if the controller is built-in the motor it makes it more difficult.

    The problem with using a cassette deck is that the heads are very close together - if you could somehow pull a loop out between the two heads you could greatly increase delay time.

    For those who remember?, such tape delay systems (back in the day - and I repaired lot's of them) commonly used multiple heads, for different switched delay times. I'm also fairly sure I remember some have variable motor speeds?, but I wouldn't swear to it.
     
  18. Rorut

    Rorut Member

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    A bit confused. Should I go for 10k or 1k :)
    Here is a mod for adding a pot that changes the motor speed.
    [​IMG]
     
  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    10K is fine, it's already used as R11 in exactly the same configuration.
     
  20. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    Hi Nige, have a look at post #3, copycat is one of the machines your talking about, I repaired a few too back in my apprentiod days.
    If you have 10k rorut try it it'll probably work fine I might have been a bit too picky.
    Looks like the motor speed control is onboard rather than in the motor, some tape players had a variable speed control fitted.
    Have you considered modding a tape to an endless loop, or even using 2 decks with 1 tape?
     
    Last edited: Dec 29, 2017
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  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The Watkins Copycat was the most famous (and popular) of the echo systems, nice and easy to repair as well.

    Not something I've ever been involved in, but I remember the BBC Radiophonic Workshop used to make long delays with two reel to reel machines at either sides of the room and the tape fed across between them. But their 'real' echo system was the best - from the Radiophonic workshop they had cables down to a large room 'down in the depths' of the BBC - they had speakers and multiple microphones in this large room, and used it as an echo chamber.
     

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