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dbx/ hx pro stuff

Discussion in 'Electronic Repair' started by [email protected], Jun 13, 2007.

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  1. Guest

    i have a couple of broken cassette decks. one with dbx, one with hx
    pro. was wanting to experiment with tapping into both the dbx and hx
    pro circuits (in/out), making them independent processing units. i
    would like to use these on other tape based recorders that don't have
    them, but could, hopefully, benefit from using their noise reduction/
    headroom extension.

    i am not a fully knowledgable tech. i can solder and diagnose/
    locate minor things, and can repair/ replace parts very well with
    direction. my ideas exceed my education. mainly, if someone points
    to something or says "this does that, and here's what you need to
    do..", i can do it. i don't necessarily know why, but am learning.

    i'm really wanting to turn this into a useable project. could someone
    with specific technical knowledge let me know (before i tear into my
    cassette decks) if it is technically possible to tap into the hx pro/
    dbx circuits (through rewiring, ect). i'd like to be able to go into
    stereo rca/quarter inch inputs, through the hx pro/ dbx circuit
    processing, then out through rca/ quarter inch outs to a separate tape

    with hx pro, if i'm not mistaken, there is a bias adjustment signal
    applied to the tapehead to allow for less high end distortion. would
    this change things with the wiring? in other words, would the bias
    adjustment signal not be able to transfer through rca/ quarter inch
    jacks, but rather have to be hard wired to the circuit of the separate
    tape recorder, going straight to the heads (i hope not)?

    with dbx, i realize that it's a two way process needing to be recorded
    onto tape then used to process during playback. so, would i need two
    sets of in/out jacks for compressing (in)/ monitoring (out), and
    recorded signal (in)/ expanding (out)?

    i feel more interested in the hx pro method, than dbx, but would like
    to know about both. i know that there are pro audio independent dbx
    processors, but am interested in knowing if a broken cassette deck
    with dbx can be turned into an independent processor itself.

    specifically, i want to use the hx pro with recording onto 8 track
    cartridges. i'd like to extend their headroom, lowering the noise
    floor. is this technically possible? does hx pro have some cassette-
    specific eq applied to it, that wouldn't work with the 1/4" tape in 8
    tracks? seems like it should be able to work, somehow, because - 1)
    isn't a circuit, a circuit?! as long as you do it right, can't you
    send something through it, getting something different on the other
    end...good or bad? 2) even though 8 tracks use 1/4 inch tape, the
    track sizes are the same as cassette. the difference being that they
    are recorded at 3 3/4 inches per second vs. 1 7/8.

    secondarily, i want to use this with vhs and reel to reel technology,
    but realize that it may not be necessary since they both have
    relatively lower noise floors.

  2. Hi!

    How broken are the cassette decks you have?

    A really cheap and easy solution would be to simply buy a cassette shell
    adapter (like the kind use to play portable devices into car radios with
    cassette players), put it into the tape well on your deck and then either
    play into the deck or record through it. The cassette transport doesn't have
    to be able to do any more than raise the play/record head assembly to meet
    the head inside the adapter.

    You could play a source through the deck, turn on the noise reduction
    circuit you wish to use and then listen to the line outputs on the deck. Or,
    assuming the HX/dbx circuits are functional in recording mode, play a source
    into the deck's inputs, set the recording level on the deck (if needed or
    possible) and then plug the shell adapter into the device that will be
    receiving the final signal.

    I suppose audio quality would suffer in some way by doing this, but the
    price is certainly right.

  3. Zebra

    Zebra Guest

    Chord_pop & William,

    How about a fusion of 2 deck? By connecting the mechanical good deck's
    playback/recording head and the erasing head's wirings to the the deck with
    dbx/hx pro togther with the play and record triggering wirings. I think this
    might work.
  4. Guest

    i like this idea. i think it's a good one for getting a "scratch"
    sense of how it will work. yeah, seems like the quality would suffer,
    but definitely a fast and cheap way to check it out. who knows, maybe
    it'll be an unexpectedly good sounding method. the deck isn't THAT
    broken, just needs new belts and a definite azimuth
    adjustment.....hmmm, that azimuth factor might affect things a bit. i
    don't have an alignment tape, just do it by ear, hoping it's
    accurate. thank you very much for the idea.
  5. Guest

    yes, this is what i was getting at. was wondering if i could avoid
    hard wiring them together, though, tapping into the circuit on the
    board, not the head leads. i guess i could just run wires from where
    the signal leaves the board going to the head, and run them to the rca
    jacks/ vice versa. thanks alot for your input.
  6. TMI

    TMI Guest

    dbx is a 2 to 1 compander, that is, a 10 db change is recorded as a
    5db change, then expanded back to a 10db change.

    Dolby a, b, c are and SR band limited compressors.

    HX PRO senses high frequency content in the program signal and reduces
    the bias level dynamically to compensate for "self bias". The only
    reel to reel deck ever equipped with HX PRO is a Studer A-80 MK IV 24
    track with the HX-PRO option.

    If you are not Willi Studer and Bruno Hockstrasser, I doubt you will
    get it to work. You would have to take bias frequency, record head
    gap, tape formula saturation spectrum into consideration and insert a
    high speed, low distortion VCA into the bias circuit, coupled to the
    appropriate detector and control circuitry, none of which can be taken
    from a cassette deck.

    Nothing about HX PRO is an external circuit. It IS a wonderful idea
    for a balls to the wall deck but must be built application specific
    for the record head, bias frequency, tape width and formula to be
    used. It would make a big difference on an 8 track compared to 20 year
    old high speed duplicated commercially produced tapes.

    Tom Maguire
    TMI Engineering
  7. Gary Tait

    Gary Tait Guest

    wrote in
    I am thinking you might need head level access. If you can get data on the
    NR chips, you might be able to use line level access.
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