Thursday, 23 February 2012

Xotic EP Booster - Briggs Modded

Requested.  This is a modified version of the Xotic EP Booster, done by Fred Briggs and seen on his blog La Révolution Deux.  Have a look around there and you'll find some really interesting articles and information about many modern and classic effects.  This version includes the voltage converter to provide 18V from a 9V supply.  Cheers Fred.


48 comments:

  1. Wow, than't quick! Thanks man!
    I was already starting on the Lovepedal Super 6. When That one's finished, I'll know what to do...

    Greetings,

    M

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  2. IC7660SCPA where do you find that? Any subs?

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    1. Rapid Electronics in the UK sell them, but you can pick them up from a number of sellers on eBay for about £2 for 5:

      http://www.ebay.co.uk/itm/260823468782

      It's worth getting a few anyway because a number of these layouts use them. You could make a standalone 9 to 18V converter as well as a Katana Boost, Klon, Colorsound SupaSustain or Supreaux Deux.

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    2. purchased. thanks for the link :)

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  3. i have an EP Booster, will be good to compare with the modded clone (once i build it...)
    has anyone compared these with ClinchFX's EP-Pre?

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  4. Very interesting! I am going to built the Briggs Modded too. I have a Lumpy's '73 boost (echoplex and superlead booster) and it's gorgeous. People talk very well about the Clinch. I have found an schematic of the Clinch here:

    http://forum.metroamp.com/viewtopic.php?f=12&t=29331

    Maybe with the layout... ;-)

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    1. I know this is a late reply, but take a look at madbeans fatpants. From what I understand it has many similaries to the ep-pre.

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  5. I built this Brigg's modified EP booster last night, works like a charm. The tone controll "high rolloff" is interesting and usefull, lots of clean boost in this sucker. Thanks for the excellent layout!

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    1. That's excellent, thanks for verifing Michael

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  6. I'm trying to figure out if this could be transformed for bass. Only high pass filter i can see in the Briggs' schem is the one in the input. That cuts out everything below 4 hertz...

    Or could this be used with bass as it is?
    +m

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  7. Hi there,
    first let me thank you for all these fantastic layouts you are posting here m8!

    I built this Brigg's modified EP booster over the last couple' days, and it seems to work ok. I only tested it on my Beavis Board so far, and a few small things got me questioning my build:
    1. The Tone knob seems to work in reverse:
    Clockwise rotation cuts some rather high highs, full counter-clockwise seems to be the unaltered, original sound.
    Is that as intended? I used this (your) image as reference for the pot numbering (1-2-3): http://4.bp.blogspot.com/-Eu1vuXVH3UM/TzGNIkeLuAI/AAAAAAAAAlM/h7re0CGH6A8/s1600/Pot20numbering.jpg

    2. There's not a whole lotta Boost / Volume available:
    It is definiteley noticeably louder and the sound gets fuller / bass-heavier when cranking both controls, but i am used to more "push" from other boosts...
    Could i have made a specific mistake here, or does this circuit only have a rather mild boost?

    3. Regarding Fred Briggs write up here: http://revolutiondeux.blogspot.de/2012/01/xotic-ep-booster.html
    In the "[Edit 2]" part he adds a few suggestions.
    I'd like to change the "drain resistor on the first FET with a 10k trim pot".
    In your layout, this drain resistor would be the 8k2 resistor in column 9 row 8, right?
    And could you give me a hint on how to correctly measure the bias on the Jfet's drain after having added the trim pot?

    Thank you very much!

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  8. @shagadelik I'm having the exact same experience you are. My only diversion was using a Maxim Charge Pump instead of the IC7660SCPA. I'm not reading 18v from any pin though, so I'm wondering what might be going wrong... Either way, I know from messing around with the original that this pedal has way more boost on tap than what I'm getting now.

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  9. Alright, just kidding, I'm getting 18v now. I'm gonna say this one's back to not-quite-verified...

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  10. @Brandt Miller: Could you tell me, where exactly in the circuit you are getting the 18V? I do have a IC7660S(CPAZ), but i don't get 18V measured anywhere in my buid!? If i read the schematic correctly, the 9V going into pin 5 of the IC should be coming out as 18V on pin 2. But i only get about 4.2V measured across these two pins!?

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    1. "pin 5 of the IC" being the top right one obviously! ;-)=
      I don't know the official way of numbering these IC pins right now...
      Is it generally 1-4 from top left to bottom left and 5-8 from top right to bottom right?
      Or rather 1-8 going counter-clockwise around the IC?

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    2. 1---8
      2---7
      3---6
      4---5

      and you should be getting 18V on the row where the drain of Q2 sits

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    3. ok, good to know!

      i am getting about 15-16V measured there.. could be a dying battery, right? i will try and hook up a wall wart and check again...

      would you say that changing the "drain resistor on the first FET with a 10k trim pot" makes sense and might help regarding the questions of my comment above? Fred Briggs is stating that 5.5V on the drain would be the "sweet spot" for the first FET, i am currently measuring about 6.1V there...

      thanks for the fast reply! :-)=

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    4. Yes well you'll get the battery voltage doubled minus the forward voltage drop of the diodes and so 15-16V is understandable if the battery isn't full. And yes a trimmer would make it easier to achieve, or if you're getting 6.1V with an 8K2 then just try something like a 9K1 or 10K which will get you closer to the 5 - 5.5V.

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    5. @shaggadelik yeah, I have 18v coming off of Pin 2 of my Charge Pump with a wall wart.

      @IVIark this may be a noob question, but will adjusting the bias of said FET take care of the boost (or lack thereof) issue?

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    6. Also, I'm clearly doing something terribly wrong, because I'm still reading around 15-17v at the first drain, and 17-18v at the other.

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    7. I haven't built this so can't speak from a position of experience, but could anyone who has successfully built this post the JFET voltages you're getting. Cheers

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  11. This is somewhat strange:
    Measuring from pin 2 of the IC7660S to ground, i get about 4.5V. Shouldn't i be getting the full 18V (minus the diodes' forward voltage drop that is!) there?
    Measuring from the same pin 2 of the IC7660S to the drain of FET Q2, i get a little less than 12V. (Both from a stable 8.95V supply on the 9V input)
    Just out of curiosity i measured from pin 3 of the IC7660S to the drain of FET Q2, and i get about 16.3V there!? I guess that has nothing to do with anything though, right?
    Oh my, sorry for bothering you guys!
    Cheers!

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    1. You shouldn't get 18V on pin 2, you'll only get that after the two diodes in series from pin 1. So you should be able to measure around 17ish volts at the bottom of the diode or top of the 100R on the row above the input wire, or very slightly less than this on the row with Q2 drain on it.

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    2. I finally got it, thank you!

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    3. Is it working ok now then? What was the issue (just in case anyone else has similar problems).

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    4. I'm sorry but i didn't change anything yet, i just meant to say that i finally understood the 18V generating part of the circuit and how to measure this correctly! ;)
      My build is more or less working ok from "day one", i just thought that there should be somewhat more boost / volume output capability available in that circuit. (well, and the tone knob is more like a high-cut knob and works in reverse to my understanding: turning the knob clockwise cuts the highend - full counter-clockwise yields an unaffected signal...)
      I will definitely play around with the value of the drain resistor on the first FET though!

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    5. Hi all, sorry for still bugging you with this thing, but i just got around to checking my work-in-progress build again, and i am getting around 14V measured between both the first and second FET's drains to ground??
      If i understand correctly, there is supposed to be around 5.5V for optimum working conditions, right?
      So my next step would be to remove the 8k2 resistor connecting both FET's drains and instead add a 10k trimmer there and see, if i can get those drains to measure 5.5V...
      Could i be doing something else wrong in the first place?

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    6. You'll never get Q2 down to that sort of voltage, it's directly connected to the supply from the charge pump (which should be over 17V, are you using a weak battery?)

      An 8K2 resistor will also never get the voltage down to 5.5V on Q1. Have you checked out the thread about this on FSB or on Fred's blog?

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    7. Ok, i changed the 8k2 for a 10k resistor, and it didn't change a thing!? Now what's up with that? Am i measuring completely wrong?
      I put the multi-meter's red lead onto the FET's drain pin (or onto the row where it's soldered, makes no difference) and the black lead either onto the ground row (lowest one, nr. 13 from top) or the ground connection on my beavis board which i am using to test circuits, both show me a measurement of around 14V (13.91 ish) with the 8k2 and the 10k resistor. (placed from column 9 / row 8 to column 9 / row 12)

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  12. Hi, thanks for your reply!
    I just re-read both Fred Briggs blog post and the thread on FSB, but could'nt find any help there. :-(=

    So, do you think that there are never 5.5V to be possible between the drain of Q1 and ground?
    (That's what Fred Briggs wrote on his blogpost to be the sweet spot for Q1 though)

    I believe i am totally not understanding something here...

    Shouldn't the voltage at that point at least change somewhat, when i put higher resistance in there instead of the 8k2?

    Thanks for bearing with me!

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    1. I'll have a word with Fred and see what he says, but in my mind Q2 will always be around 17.5V from a 9V supply, that can't change, and I can't see how an 8K2 resistor could taje 17.5V down to 5.5V if that has been mentioned, I'd expect you to need a much bigger resistor.

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    2. This is the part of Fred's blog post that i'm referring to:
      "[EDIT2] - ... I'd also recommend one other small change - replace the drain resistor on the first FET with a 10k trim pot then use it to dial in a 5.5v bias on the Jfet's drain to make sure you've got it in the "sweet spot". Just thought I'd add that ;-)"

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    3. Still thinking about this: do i really measure the bias of a JFET between drain and ground?
      And: maybe the 5.5V "sweet spot" bias is only valid when using 9V power? Might it need to be doubled in this case (18V circuit), so being around 11V should be the "sweet spot"?

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    4. Reading up on the issue of "JFET biasing", the most often mentioned method is using a "source resistor to bias a JFET"? So the source resistor should be variable, not the drain's? I am officially confused now... ;-)=

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    5. As the ratio between the drain and source resistors changes, so does the bias point, gain and output impedance, so you could use a trimmer in either spot to perform the task. Some people swear that one way is better than the other but to me as long as it works I don't have the enthusiasm to look into it much more deeply. Any advantage one may have over the other isn't too important to me as long as the pedal still sounds good.

      I don't know for sure, but maybe drain trimmers are used more frequently by manufacturers simply because the drain resistor is usually larger and the source relatively low, and 20K, 50K, 100K trimmers are more readily available than 330 ohm, 470 ohm or 1K. But whatever the reason I think it preferable to do the layouts as it was done in the original pedal and not think too much about it :o)

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    6. I hear you Mark! I shall stop tinkering with this now, put the old 8k2 back in and call it a day.
      My constant nagging about this stems more from scientific interest i think. I wanted to fully understand how this thing works, ya know?
      And you explained that well!
      Thank you very much again! :-)=

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  13. Hi all, any ideas how to get back the treble in this circuit? It's sooooo bassy.
    On another note, I biased the q1 drain to 5.8v using a 100k trim pot. Ended up around 65k dial up. Listening through the amp, it sounded the loudest at that point. I don't know if this is good or bad but it sounds right. If I could only put some treble back.....
    Cheers for any help.

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  14. There is a radical solution for fixing this, I built the non modified version and it is brilliant! What a charmer :)

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  15. There is a radical solution for fixing this, I built the non modified version and it is brilliant! What a charmer :)

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  16. So can anyone recommend this highly enough to let go of the Madbean Fat Pants 2012 version?

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  17. Built this pedal and here is my impression:

    I am disappoint.

    The treble completely disappears. The layout *requires* a trimpot for the jfet. I used a 100k trimpot for mine. I also tried j201 jfet so along with the suggested ones and nothing. IMHO this pedal isn't very good at what it does at all. I am going to give the CAE line driver or the tc spark booster a shot instead. Don't waste your time with this one.

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    1. If you are losing all the high frequencies, i'm quite certain that you have an error in your build.
      +m

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  18. Hi all,

    Love the site! Just ordered the bits I needed for this and got 16v rated 100uF caps sent to me instead of 35v ones. Can anyone with more knowledge than me tell me if that's gonna be an issue for this build? I know it runs on 18v but not sure how to tell whether these specific caps will be exposed to that voltage. Thanks in advance for your help!

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    1. You're ok for the top right hand cap, but I wouldn't risk it with the bottom one. It will work without them both though so you can still test the effect is working. They are both filter caps and so are there to help prevent noise, but if it isn't noisy you may not even need them, plenty of effects don't have them anyway, and supplies normally have one built in anyway.

      The 16V rated caps will still be useful for future builds, because I often have a 100u in 9V pedals which they are perfect for.

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    2. Thanks for taking the time to reply lvlark; really appreciate it :)

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  19. Hey Mark, first of all, I'd like to thank you for making this layout (and a ton of others). I've built a decent amount of pedals based on your layouts, and I just wanted to let you know that your work is appreciated. Now, onto the bad news. I don't think my build is working properly. I mean, it works, but it doesn't have as much volume on tap as I thought it would. I think that the charge pump might be the problem. It seems like the effect is only getting 9v instead of 18v. You can hear a high pitched whine whenever the effect is engaged. It's not very loud, but it's there. I had a Klone a while ago that did that too, but I'm pretty sure that it was getting 18v because it had a massive amount of headroom, so maybe that's just the nature of the beast with charge pumps? I replaced the charge pump in that one and I think it reduced the noise a little bit, but it was still there. Anyways, the op-amp is extremely hot to the touch when the pedal is on. Is it possible that the op-amp is bad? I'm pretty sure I ordered it from Tayda, and I've had pots and switches from them fail on me before, so I wouldn't be surprised. Any help is greatly appreciated. Thanks!

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  20. Hey all

    Two questions:
    Are you guys substituting ICL7660SCPA for the IC7660SCPA (note the added "L")? Or using a different IC?
    Also, reading above it seems people have had issues with the Briggs mod.... has anyone had success with the Briggs as it's drawn above? Or should I stick with the original EP-B?

    Thanks

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    1. as long as the charge pump ic is a 7660s you're good, do not use one that is just a plain 7660. also, while i haven't built this, i can tell you that if it's verified it should be good to you, and have no issues if built correctly.

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