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Fæpbœx II: 2-Band Preamp

Discussion in 'Effects [BG]' started by EricssonB, Jul 30, 2012.

  1. EricssonB

    EricssonB

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    While planning for the Bœx series of pedals, my friend requested to build and older-style preamp to contrast the power of the more modern 3-band that I was also building. Found schematics to an older MM preamp and built to that. Glad he did, as the frequency range on this one is phenomenal! Not quite as powerful as the 3-band that I built, but it makes up for that in tone.

    This thing is great for pushing an overdrive pedal -- wow.

    Like FBIII, these'll be cracked in half, thrown into 1590 enclosures with a bunch of rad LEDs and a 3PDT switch. I just primed the enclosures and am waiting to buy some rattle-cans to finish them up when I get payed again.

    I don't have money to buy electronics for all of my instruments (2 guitars, 2 basses), so building what would normally be internal preamps and sticking them into enclosures is a damned-good compromise.

    Hopefully soon I can shutter the factory and get my studio back into full summer production; finish the ol'album that's going on seven years.

    http://www.talkbass.com/forum/f36/sp%E6kerb%9Cx-27-2hz-high-pass-filter-896616/
    http://www.talkbass.com/forum/f36/f%E6pb%9Cx-iii-3-band-preamp-901439/#post12936643

    [​IMG]
    [​IMG]
    [​IMG]
    ...how romantic. Not only do they share boards, but currently share a power supply!

    Edit: Currently! Hah! Electricity pun!
  2. Pollinator95

    Pollinator95

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    :p

    Are you going to leave the board like that or snap it for a smaller enclosure?
  3. EricssonB

    EricssonB

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    1590s, sir. Standard generic pedal size. Definitely be updating when I get them some. Heck, I got payed today! To the paint store!@

  4. Pollinator95

    Pollinator95

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    *Facepalm* Must have skipped that paragraph to the link or something.
  5. EricssonB

    EricssonB

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    Working on the layout. Any suggestions?

    Yeah, it's going to be polished aluminum under black burlap. Yeah. It's going to be rad. Red and orange LEDs, sort of a Halloween theme.

    [​IMG]
    [​IMG]
  6. Stealth

    Stealth Supporting Member

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    I like the perfboard layout a lot, and I applaud your effort with the boexes. :D

    There are a couple of things I'd suggest, though. I just went through my files and found a schematic of the old '76 MM preamp (or, rather, Bajaman's schematic of it and SGD's corrections) which yours is based on, and I found a few things that differ.

    In the SGD corrected schematic:
    R4 = 220kΩ (bass pot bias resistor)
    R6 = 2.2MΩ (quiescent current resistor)
    C9 = 10μF (output cap)
    His schematic didn't account for the LED on the R6 - is the LED the reason you dropped the R6 so low?
  7. EricssonB

    EricssonB

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    Bassically. I plan on having a few always-on LEDs, so I figured I'd put a test LED on the board to make sure that it's on. I'll probably update the drawing when I figure out how many will be plugged in around the enclosure and how much throttling they require.
  8. EricssonB

    EricssonB

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    Two months later and a lot of life happening between the first two of the Fæpbœxes are done, with two more to go.

    Now, a problem I'm having with Fæpbœx II is that there's a gigantic pop when switched on and off. I'll place this on it being designed for on/being used or off/not being used, as it's a design from active basses.

    I've tried pulldown resistors all the way down to 20k Ω, on both the input and output sides of the circuit and it doesn't help.

    Any ideas? Kinda fed up with it and may just say "DEAL WITH IT" as it's a preamp and it likely won't be switched very regularly.

    [​IMG]
    [​IMG]
  9. alec

    alec

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    Isn't that normal for a 3PDT switch?

    From Iron Ether's website:
  10. Skrogh

    Skrogh

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    Nahh if things are wired correctly, the 3pdt should not make any loud pops. maby a slight "puf" ´, but no pops. How is the switch wired up? Are you turning on a lot of LED's directly at the switch? That could lead to a pop.
  11. EricssonB

    EricssonB

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    Added a 33k Ω resistor and between the internal side of the input capacitor and ground.

    All unwanted electrical noise is gone.

    -- That's the fix if you want to run full voltage (18V) with bright LEDs.
    -- If not, lower the voltage. 9 or below works.
  12. SGD Lutherie

    SGD Lutherie Supporting Member

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    Disclosures:
    Owner, SGD Music Products
    Use a 1megohm resistor to the input side of the first coupling cap, and output side of the last coupling cap. That's a pull down resistor, and bleeds off the stored charge on the coupling cap which is what causes the pop.

    33k will set too low of an impedance to the front of the circuit and dull the tone.

    Shown in blue:

    http://www.muzique.com/news/pulldown-resistors/
    [​IMG]
  13. SGD Lutherie

    SGD Lutherie Supporting Member

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    The coupling caps are popping because the circuit was not made to be bypassed.
  14. EricssonB

    EricssonB

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    Yeah that's what I ended up figuring out. I tried pulldown resistors, but they really weren't doing it until I moved them to the inside of the coupling cap. That did the trick.
  15. SGD Lutherie

    SGD Lutherie Supporting Member

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    You have to keep in mind that putting a 33k resistor inside the front coupling cap is going to affect the treble control. You are lowering the inout impedance of the preamp, and loading down the treble control which feeds into the input of the op amp.

    When I do these with bypasses I use the pull downs on the other side of the caps.
  16. EricssonB

    EricssonB

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    Yeah, and I actually did some experimenting with the values and where the pull down resistors were placed. I'm not terribly scientific about this sort of thing. I really needed to find a balance between tone loss and that house-rumbling pop of switching this thing on and off. It was BAD.

    33k didn't seem to affect tone, and I'm overwhelmingly glad that the pop is going. The treble could afford to be rolled down a bit.

    It's screwed closed and I don't want to open it up again. Ever. Ever! EVER!

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