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Thread: Reverb unit BDTR-2H

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    Reverb unit BDTR-2H

    Hello Im trying to understand how does the BDTR-2H chip works.
    I understand that there are 3 delay units that make the reverb, but I dont understand how the circuit works, what is that Osc and how the 3 PT2399 are connected to each other. Can anyone help me ?

    Thanks!
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    Old Timer J M Fahey's Avatar
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    In a very simplified way, you have 3 PT2399 with very short delays, set by the resistor from pin 6 to ground, so 4.99k (longest), 3.24k(intermediate) and 1.82k(shortest) , I *guess* to simulate a 3 spring reverb, the most complex they make.
    Fine but itīs way too "accurate" and does not sound that much like a mechanical spring but more some Digital thingie.
    So they modulate the middle one with an oscillator.
    I donīt know the internal PT2399 timing, although I guess its clock must run quite high.

    I guess the external oscillator modulates the internal one (or beats against it) and it introduces some inaccuracy ... which is good and more like mechanical springs do.

    beware: lots of caveats and "what if" involved here.

    In any casel "they" do not explain it.

    They somewhat show "how" (in the patent) but information on "why" is scarce.

    I guessed more based on what is being tried to accomplish than on Belton supplied data.

    Others may explain it better, of course.

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    Juan Manuel Fahey

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    Though it is successfully used to replace spring pans in many amps and pedals, I think one of its weaknesses is the use of identical cap values from pins 7 through 12. In the real world, later reflections have more top end eaten up by non-reflecting surfaces, yet the 3 delay chips set for different delay times have identical lowpass filtering. One can certainly impose additional lowpass filtering on top of the output of the unit, but it would impact on the entire delay/reverb signal, and not differentially on the simulated early and later reflections.

    When some manufacturers used the old Panasonic MN3011 for solid-state reverb in their pedals or amps, they committed the same error. All 6 staggered taps of the chip were mixed - often in equal proportion - with no differential filtering of early and later reflections. As you might imagine, as "pans" they tanked (or was it "as tanks they were panned"?). One has to be an obsessive seeker of such pedals to even know they ever existed, such was the teensy incursion they made into the solid-state reverb realm.

    I would have to get the details, but perhaps the 3H Belton module improves upon those shortcomings of the 2H.

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    The multiple PT2399 chips is an attempt to increase the number of delayed signals to get closer to a natural reverb sound. The circuit shown uses two devices in parallel, then feeds the signal into a third device to increase the number of 'reflections' - reverb on reverb.

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    There seems to be a mistake in there, All looks kinda right except the output of the of the middle one feeds back into it's timing input? What? Hows that going to work? Methinks that's a mistake as in that where the oscillator should go if it indeed supplies that point with a changing resistance. As for the out of the middle shouldn't it join the input to the bottom one?

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    Maybe this gives clearer picture and more closely aligns with the actual device. Turns out i got it wrong;

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