True Bypass for Fender (Clapton) Mid Boost

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by gitarzan1, Jan 15, 2015.

  1. gitarzan1

    gitarzan1 Member

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    The following wiring diagram shows a mod I'm planning for my Strat Plus. In addition to providing new pickup switching capabilities and active mid boost, it should also bypass the mid boost circuit completely (true bypass) and allow pure passive operation.

    The mod uses two S-1 switches, a Fender Superswitch, and a TBX tone control. The first S-1 controls master volume and the switch selects between standard switching and Hermetico's American Deluxe SSS mod. The TBX is a master tone control for both active and passive modes with Doc's TBX mod. The second S-1 controls the mid boost level in active mode. When the switch is up, the active circuitry is completely bypassed and the battery is switched off. When the switch is down, the active circuitry is engaged and a 68K ohm resistor is placed across the master volume control to simulate a 50K ohm MV control (for the mid-boost circuit). (Actually, the correct resistor should be 62K ohms, but 68K is a standard size and should be close enough.)

    I've been looking for a good circuit description for the mid boost with pinouts to verify that all of the bypass connections make sense, but haven't found one yet. This is as close as I've been able to get based upon the generally available mid boost wiring and circuit information. Does anyone have a description of the mid boost circuit that shows both colors and functionality on each of the I/O connections? I haven't built it yet, but will as soon as I'm sure it will work. Does anyone see anything that won't work? Thanks.

    By the way, huge thanks to Andybighair, Hermetico, Doc, Alan0354 and WalterW (true bypass info), and others who have contributed to my education on how to do this.

    [​IMG]
     
    Last edited: Jan 15, 2015
  2. Tone_Terrific

    Tone_Terrific Member

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    I have done a similar mod but without the s-1 and tone mods so YMMV. But..

    There is no need to switch the 68k vol pot replacement resistor.
    Keep it engaged and run the volume control from the standard vol pot.
    Just view the whole thing as a booster with variable gain on the mids and preset overall gain (volume).
    Engage/disengage with a DPDT, just like a pedal.
     
  3. DeanIversen

    DeanIversen Member

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    dear Sirs, noob here and fairly new at modding strat electronics, entry level at soldering but seem to be doing very nice work even on the smallest of caps and posts, anyway I recently picked up this same kit but find it truly lacks any real key functions as in an "active" indicator LED, an on/off switch other than removing the jack, thats a "jack move" IMO are they trying to push batteries with this mod? lol

    anyway and IMO the most crucial feature of them all is the bypass, how could this possibly be overlooked knowing that a battery could begin to fail mid stream then what? blow the set and go install a new battery or plug in another instrument?

    don't get that at all?

    I bought a Lyle Superstrat brand new in 76 in Seattle near Pioneer square and it had 3 built in effects, pre amp, phase and cant remember the other echo if I remember right, it all ran off a 9 volt battery with no low battery indicator, just a bypass and an on LED, however you knew when the battery was going to bin it, the crackle would begin then the intermittent till suddenly just a whisper now and again but you had the ability to shut it down and bypass running.

    I never got into the electronics too deep before it was stolen but I may have been like a blower mod bypass but I really don't remember been a while, I miss that guitar for sure, a brother lost it at his girlfriends apartment, thats the story anyway..


    curious how your bypasses have gone and if you have any tips how one might do a simple bypass even if its just a one pickup blower style with a DPDT submini toggle switch?

    seems this preamp could be the bee's knees if just a little more thought went into it, even when I see places sell loaded pick guards they don't add a few simple mods and want to stick costumers with unplugging their guitars to turn it off and not giving them any sort of indicator its on other that it works? is the battery dead? sheesh my $400 acoustic has a low battery indicator light and the option of bypassing the effects and going full passive..

    also, wondering since they do have it wired for the on/off to be activated by the jack could one just wire the negative of the battery terminal clip straight to the the black wire coming out of the board itself?

    reason is I have a 9 volt battery holder with an on off switch that I could locate under the pick guard and just have the on/off switch revealed with a little dremel work, the would certainly be the right direction, they even have them with small tiny LED on indicators and a low battery is easy enough to wire in. just a thought again I don't even qualify as an amatuer electrician so I need to ask

    Thanks man and Thanks for the accept to the board!

    Rock on Fellas
     
    Last edited: Aug 2, 2015
  4. swiveltung

    swiveltung Member

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    Well....... I'm waiting to see how this works out, often wondered if it was doable. Never logic'd out a scheme though....
    My mid boost batteries last so long I never can remember how long it's been. But you do need to unplug. You could put a simple switch like the S1 push button on to disconnect the battery I suppose.
     
  5. walterw

    walterw Gold Supporting Member

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    here's why:

    an LED is just an additional battery sucker; if you can't tell the circuit is on, you probably shouldn't bother using it.

    battery on-off switches are problematic because they create hideous power-up noise through the system! the switching jack is also simpler and more reliable. just don't leave the guitar plugged in when you're not using it, same as any pedal, or any acoustic guitar with onboard electronics for that matter.

    the true-bypass switch might be the one useful thing here! mostly for tonal choice reasons though; in all of these systems, the battery lasts a long time (like years) and you get lots of notice that it's due to be changed, as the sound slowly begins to get less clear and more grungy. it's not gonna work perfectly and then stop in the middle of a gig.
     
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  6. DeanIversen

    DeanIversen Member

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    its not that I can't tell if the circuit is on. IF the battery is good and the board is working proper, theres just no way to tell if the battery is dead or the unit is malfunctioning if that time ever comes? LEDs get pretty small and the energy they use is negligible, there should at least be an indicator that the board is getting power or a low volt light lights up, something like acoustic preamps have only better or more warning, especially for troubleshooting, if the board light is on you know its getting power but if the pre-amp is still not functioning proper then one needs to look elsewhere, wiring? instrument? basically the same logic as to why amplifiers have on lights..


    one tiny little LED wouldn't use much at all, FACT is smoke detectors work for years on one tiny 9 volt and actually have a small nickel sized battery for backup that runs a siren and a battery dead (red) LED so battery drain isn't really an issue if you think about it

    also a TRUE bypass switch is what I was referring to here, what I'm looking for I'm sure there are many ways to wire one in there are other reasons one may want to not run the preamp aside from a dead battery

    the reality is it should have had one or the other, I owned a guitar that had a bypass with 3 effects and it worked out just fine and Fender made one pretty much just like the Clapton that did have a bypass, it was another signature Sambora was it? also on my journey I saw a Fender schematic how to install the S1 switch on this preamp for a bypass so they clearly know its something users want.

    dunno why it lacks one but every board I run into owners wish it had a bypass or are looking just like me least like to have had the option to install a bypass without having to go offline and search for parts and ask questions, at this point I'm working on taking this kit to the next level, perhaps not an LED on light but perhaps a battery low light like the acoustics have, its a matter of one more resistor and 2 more caps then a board I just need to determine at what voltage the performance is useless then have the light come on at about a 1/2 volt before that, one could even wire in a piezo siren like smoke detectors have beeps every so often as a warning then they get closer and closer..

    what would be best is something like the fishman rechargeable for these onboard effects and preamps, a lot more juice maybe a charge could go couple months or so?

    http://www.musiciansfriend.com/onboard-preamps-eq/fishman-9-volt-rechargeable-battery-pack-for-strat

    or maybe something that can be hidden yet charged easy enough thats a larger capacity

    I was just fishing to see if anyone modded this preamp for more efficiency or at least to get it to bypass and still run off the pickup(s) might just wire in a simple blower switch for playability?

    hoping the OP or the other member who wired in a bypass chimes in for some feedback, how it works and if it was the best way to go?

    oh well Thanks for the help maybe they do chime in at some point, its just wire and solder :)
     
  7. RicOkc

    RicOkc Member

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  8. '59_Standard

    '59_Standard Member

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    Fixed, Walter.



    /Humor.
     
  9. walterw

    walterw Gold Supporting Member

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    Looks like that at least is a thing.

    Anyway yeah, a DPDT true-bypass switch is pretty straightforward, you just wire the preamp like it was a pedal. Throw a 50k resistor across the leads that were for the (active) volume to simulate it being on "10" and go to regular passive volume and tone pots before hitting the preamp switch. You wanna use the bypass diagram that shorts the preamp input to ground when bypassed to prevent noise and switch popping.

    here's what you want:

    [​IMG]
     
    Last edited: Aug 3, 2017
  10. gitarzan1

    gitarzan1 Member

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    I know this is an old post, but I'm still getting some PMs about it, so I thought I'd provide an update. I still haven't built this mod, but I'm getting ready to do so. I was waiting to find some information about the pinouts and operation of the mid boost circuit, which I finally found. There is one (hopefully, just one) potential problem. The Buffer In (Yellow) wire for the MB circuit should ideally be switched to ground in passive mode for true bypass. I don't have a way to do that. It currently just floats, but the power to the MB circuit is also switched off in passive mode, so I don't know what the effect will be. It may result is some kind of undesirable noise when the switch is activated. The only alternative I can see is to remove the 68K resistor that is intended to make the 250K pot look like a 50K pot in active mode and use that pole of the switch to tie the Buffer In to ground in passive mode. Let me know if anyone has other ideas. I'm going to build it anyway and see what happens.

    I have updated my schematic to reflect some necessary changes. I also included some of the S1 and Super Switch functional information I used to verify the PU switching and bypass circuits.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
    Last edited: Aug 2, 2017
  11. walterw

    walterw Gold Supporting Member

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    why not? you can do just that with a standard DPDT push-pull or whatever.
    yes, which is why nobody ever does it that way, with either guitar preamps or pedal effects.

    you want to keep the system powered up (via TRS jack to make the 9V- connection to ground) and just use true bypass switching to kick the effect in or out.
     
  12. gitarzan1

    gitarzan1 Member

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    Hi Walter, I was hoping you'd chime in. I haven't been able to get the image to load (imgur). Still working on it.

    True, but I would like to keep the guitar as stock as possible in appearance, thus the two S1 switches.

    Yes, that would work, but I was hoping to preserve battery life in passive mode. I tend to forget to unplug some of my battery operated stomp boxes (no DC in), which runs the batteries down. I would probably do the same with the guitar. Leaving the MB circuit on while plugged in and switching the Buffer In to ground may be the best solution, though. Thanks for the thought.

    Larry
     
  13. walterw

    walterw Gold Supporting Member

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    why two? a single S1 could do it all, it's essentially two DPDT switches on one push button. one DPDT to handle the true bypass (including shorting the input to ground) and the other to disconnect the battery.

    still don't recommend it, you'd likely get that loud "zipper" noise into your amp when turning it back on again.
     
  14. gitarzan1

    gitarzan1 Member

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    If you look at my original schematic above, the top S1 switches between some interesting pickup configurations. The 2nd S1 switches the 68K resistor across the volume pot in and out to simulate a 50K pot in active mode, disconnects the power, switches the signal from the pickups between Buffer In for active and the volume pot for passive, and finally determines whether the tip gets signal from the MB out or the volume pot. The wiring for the 2nd S1 is not correct in the previous picture, which is why I'm trying to upload an update.
     
  15. gitarzan1

    gitarzan1 Member

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    Woohoo! Just figured out how to add the schematic image. For those having similar problems loading images, at least on Imgur, the key is getting the "Linked BBCode" URL.

    I think the interesting feature of this design that may allow it to work is that both the input and and the output of the MB circuit are disconnected when switching to passive mode, as well as the power, even though the input is not tied to ground. What do you think, Walter?
     
    Last edited: Aug 2, 2017
  16. walterw

    walterw Gold Supporting Member

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    not grasping the entire circuit idea, but this part is pointless.

    use a regular 250k volume pot to run the pickups passively, then send the output of that passive volume either through the preamp or not. no need for the "active volume" control at all.
     
  17. gitarzan1

    gitarzan1 Member

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    As I recall, some people were experiencing problems with MB circuit oscillation using a 250K pot in the master volume position. So, to prevent that problem, I added the 68K resistor in parallel with the 250K pot in active mode to make it look closer to a 50K pot. The goal was to make the active mode function like the standard EC strat circuit and the passive mode function like a standard strat (well, except for the fancy pickup selections on the MV S1 switch and the super switch).

    I want to be able to dial in as much mid boost as I need and still be able to control the overall volume level with the master volume (the way the EC Strat works), so I think I need both pots. I can use the buffer as a clean boost with the mid boost at minimum, or dial in as as much MB gain as I need. You are correct, that I could modify the MB circuit to hardwire the buffer to the mid boost with a fixed resistor in between (as in the Richie Sambora circuit) and feed it with the 250K pot signal, but that is somewhat less flexible in that the buffer gain is fixed. Functionally, it may not matter, but I would prefer not to modify the MB circuit board.

    The original concept of the circuit idea, which I hadn't seen anyone else try, was to try to switch the whole MB circuit in and out. You have given me a good fallback option, though, that would allow me to ground the buffer input. I could hardwire the output connections and use that pole of the MB S1 to tie the Buffer In to ground.

    I typically either sit in with bands or play at jams. It would be much easier and quicker to be able to just plug straight into an amp without worrying about pedals. In addition, I don't always know what tones will be required, so having one guitar that can handle most Strat, Tele, and Les Paul tones is a big advantage for me.

    Thank you so much your help, Walter!
     
  18. walterw

    walterw Gold Supporting Member

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    seems likely, which is why you don't do that.

    you wire in the 50k resistor to the three wires that were going to the "active" master volume (to simulate the stock 50k pot being on "10") then put those wires away.

    the pickup switch feeds the regular 250k volume, which goes "outside" the preamp, either feeding the preamp input or bypassing it. that volume will then work passively whether the preamp is engaged or not, just like if the preamp was housed in a pedal at your feet.
     
    Last edited: Aug 3, 2017
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  19. Tone_Terrific

    Tone_Terrific Member

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    Yep.
    To summarize, again:
    -remove the vol control from the MB with resistor substitution
    -bypass using a dpdt switch
    -power remains on when cord is plugged in
    -vol function is moved to a normal Strat pot as is switching...all pre the MB
    Any wiring pre MB is irrelevant to the MB function.
    Don't make it complicated.o_O
     
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