ln76d

Re: AKG Perception P220 to Neumann u87 5 min mod ( p200, p100, p400, p420? )
« Reply #20 on: November 19, 2017, 07:01:08 AM »
    • It would cost nothing to the manufacturer to change this cap; then, why don't they do it? I guess there are enough people who like the added sizzle...

    Because it's typical chinese factory circuit, modified only to use dc converter. No one bothers how it sounds ;)

    2. In this case they just followed blindly u87 schematics, even with some blank spots. Like Piotr said, didnt even bother to bias the fet, so even with 220pF doesnt work as it should.

    Matija, to be clear - one thing is that unbiased fet makes a nasty repsonse - other thing is that the whole bias circuit don't fit deemphasis network - that's why it only works with higher capacitance.
    If you will change biasing to the same as in U87, then it will work with 220pF.
    Left - chinese biasing, right - U87



    Khron

    Re: AKG Perception P220 to Neumann u87 5 min mod ( p200, p100, p400, p420? )
    « Reply #21 on: November 19, 2017, 07:23:56 AM »
    Ok, feedback values affect the response differently between the two arrangements, but what about the max undistorted output?

    I recall the Schoeps JFET gate biasing being, in essence, the equivalent of a "split source resistor", and adjusted to get the max unclipped level. The KM84 / U87 one has the 1G straight to ground, and you just mess with the source resistor.
    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

    KrIVIUM2323

    Re: AKG Perception P220 to Neumann u87 5 min mod ( p200, p100, p400, p420? )
    « Reply #22 on: November 19, 2017, 07:36:55 AM »
    You mess with the source resistor or you define a stric idss range for the fet, select them fix the resistor value and all is good.
    But, this is time consumming as jfet are all over the map (even for a given range) and time is money, easier to load an amno box or a reel and let robot do his work. ;)
    « Last Edit: November 19, 2017, 07:52:09 AM by KrIVIUM2323 »

    Re: AKG Perception P220 to Neumann u87 5 min mod ( p200, p100, p400, p420? )
    « Reply #23 on: November 19, 2017, 08:27:12 AM »
    Have to try it out. Thanks Piotr.