Onboard EQ/preamp bypass

Posted by Don Tucker on February 10, 2013 at 5:47pm

I'm looking to build a CBG using Gitty's EQ-7545R EQ/preamp. I want to add a bypass switch to send the piezo signal straight to the jack in case the battery goes dead or the preamp craps out in the middle of a gig or jam. I'm thinking a simple SPDT switch would work, but I'm a little fuzzy on the details and hope some of you with a better understanding could provide some input or maybe an illustration of how to wire this. I'm also thinking the switch could be used as a sort of tone selector by switching back and forth. Any thoughts on this?

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Replies

  • BrianQ. February 12, 2013 at 8:02pm

    If yer using a DPDT this is how U could bypass it?305971403?profile=RESIZE_1024x1024

    • David L. > BrianQ. February 13, 2013 at 4:47am

      You can do the same using a SPDT

      305971357?profile=RESIZE_1024x1024

       

      But from the preamps I have seen most are wired like this (not always in this order)

      305972368?profile=RESIZE_1024x1024

       

      so if you don't do something to act as a manual on/off switch the inserted jack plug joins the 2 grounds of the negative battery wire and the preamp is permanently on until the battery runs down or the jack plug is removed. The ones I have hacked came from a batch of "spares" that had the sockets removed (shop display) so I just hacked into the positive lead of the battery when I rewired them.

      305973589?profile=RESIZE_1024x1024

       

      If the jack is prewired rather then do this I go for the switchable jack as it is easier.

  • David L. February 12, 2013 at 10:19am

    The easiest way is to use a switchable jack soldered into the piezo cable going from the rod piezo to the mini plug which plugs in the preamp -

    Cut the braided wire attached to the piezo rod partway along it's length. Pull some of the braid to one side to expose the inner wire and twist the braid to a point (to give  a ground wire) and gently strip the sleeve off the end of the small wire inside the braided cover (to give a hot wire). Repeat for the other cut end (which will still have the mini  plug that plugs into the preamp).

    305969542?profile=RESIZE_1024x1024

    These solder either side of a switchable jack socket (info about these is in this previous thread http://www.cigarboxnation.com/forum/topics/wiring-a-double-neck )

    the ones from the rod go to the side with the L shaped arm that goes over the jack plug and the ones going to the preamp mini plug solder to the other side.

    Ground wires go to the lugs nearest the open socket and hot wires go to the ones at the far end away from the open socket (which the tip of the jack plug will touch).

    With nothing plugged in the circuit flows as though it isn't there. With a jack inserted the signal is diverted to the amp cable plugged into it.

    You could add a switch instead but as well as diverting the signal you also need to hack into the power supply to the preamp as inserting the jack plug switches it on (the preamp battery negative wire is split by two ground lugs on the stero jack socket and inserting the jack plug connects these to complete the power circuit). So you could do it with a dpdt switch - either a stand alone switch or replace a volume pot with a push pull potentiometer (if you don't want to make more holes).

    • Don Tucker > David L. February 12, 2013 at 7:16pm

      Thanks so much David. Very useful info. This may be just what I need.

  • Charlotte Henry February 11, 2013 at 10:45pm

    I wonder if any of you have heard of the Mi Si pickup?  It is a little pricey, but it has a preamp and uses a capacitor instead of a battery. It takes 60 seconds to charge it and you get 16 hours of play time on that charge.  http://mi-si.com/products/

    • Don Tucker > Charlotte Henry February 12, 2013 at 7:15pm

      Hi Charlotte. Thanks for taking the time to reply, but like I explained to Brian this is for a customer and price is paramount. I will however check them out and maybe use it in the future for a build for myself.

  • BrianQ. February 11, 2013 at 2:19am

    Another thought if yer worried about the preamp crappin' out on ya, is to use an external

    preamp, if it craps out on ya, then just unplug it & go direct. my Fishman EQII is external,

    I don't like to cut big holes in my guitars for some reason? I've had instances where I forgot

    to charge my battery, so I just simply unplugged it & went straight to the amp.

    • Don Tucker > BrianQ. February 12, 2013 at 7:11pm

      Thanks for the replies Brian. I agree with you about using quality components, but I'm building this for a customer and cost is definitely an issue. He also wants the preamp onboard.

  • BrianQ. February 11, 2013 at 1:59am

    I don't think you'll have much trouble with that preamp, it looks reliable enough,

    just keep a fresh battery @ hand, I use rechargable 9V batteries to cut down on

    having to buy batteries all the time. I've got @ least 1 extra battery in my gear @ all times.

    If yer worried about it crapping out on you, then buy a Fishmann preamp,pro quality stuff.

    I always have piece of mind that my gear won't malfunction when i'm using professional

    quality gear.that's why it costs a little more than usual?

  • StarrCBGs(Donovan) February 10, 2013 at 6:03pm

    Ted might can help you with it as well. Most of the wiring diagrams here are his I think.

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