Passive bass roll off?

[mr.adambeck]

This is actually not for a pedal, but I figured thisw would be a good place for a complete and utter newbie to look for advice...
I built a little electro-shaker thing by throwing a contact mic in a homemade shaker.  I basically just needed something so that we wouldn't need sound guys to throw up an extra mic live.
However, the thing has a very low and "mid-rangey" (is that a word?) sound, which doesn't really lend itself to a replacement for a maraca.
Does anyone know of a simple way to cut off some of the bass and mids?  I don't need a tone control, just completely cutting the out would probably be fine.
Is this as simple as just adding a cap into the signal path?  Or am I mistaken?  And if so, any suggestions on what size to use?

[Meanderthal]

Is this as simple as just adding a cap into the signal path?

Yeah. Start with .01uf or so, should do the trick, but you may have to experiment with higher or lower values to get what you have in mind.

[Gus]

Is the contact microphone a piezo? 

[mr.adambeck]

Yes it is, it's the (what seems like a pretty standard beginner project) DIY contact mic made from a Radio Shack piezo element.  I still haven't found the right Cap to put in it, I think these microphone's get so little high end that it might be impossible to get the sound I'm looking for.

[Gus]

I would build a fet source follower with a 10 meg gate resistor and mount it as close to the pickup as possible.  Piezos need a high input resistance.

google , ask etc DIY piezo preamp you might want to make it a source follower if running from a 9 V battery

make the 1meg a 4.7 meg to 10 meg and you don't need the input cap
http://web.telia.com/~u31617586/#simple%20FET%20impedance%20converter

[mr.adambeck]

I want to keep it passive and not running off of a battery, but thanks for the suggestion!

[Gus]

A piezo often does not sound good at lower than a few meg input resistance. 

What preamp or amp is the piezo plugged into and what is it's input resistance?

Think about piezo guitars and the preamps inside them.

  The circuit I linked is about the simplest one you can build that might help you.

[soulsonic]

I want to keep it passive and not running off of a battery...

You should re-think that.
Like Gus said, piezo pickups don't work well with passive circuits. They need a good high impedance preamp or impedance converter. The schematic he linked to is perfect if you you'll be using the sound board's mic input, or you can check out my beginner's JFET preamp project HERE for a super simple circuit which can easily be tailored to work best with piezos and give a strong line level output. Just do like Gus suggested for the other circuit... change the 1M to 4.7M or 10M and you'll be good to go! Sometimes, the 2SK117 JFET is hard to find, but you can substitute any of the common ones there and get good results.

[mr.adambeck]

Would something like an LPB2 work?  I could just plug it into the stompbox.  I've already made on of those.  I really don't want a battery inside of it.  Every time we used to tour or play  a big show one of the battery's would die mid set.  I can't afford to replace them every time we're going to play, that would be ridiculous. I have everything rigged on a pedal board through a power supply now, and if this is something that's going to need a 9 volt, I'd honestly rather ditch the project than start relying on batteries again.  I'm still unclear though, are these pre-amps going to help me get more highs or not?  Because otherwise just getting more volume really isn't going to change that it sounds like crap.

[soulsonic]

A Linear Power Booster isn't really appropriate because the input impedance isn't high enough, so it won't be much of a help with this problem. Using a high impedance preamp like my JFET boost WILL help with the highs. Or even just the buffer Gus linked to.
Without the high impedance interface, the piezo will sound bad - end of story.

No one is forcing you to use batteries... there's nothing stopping you from running on the power supply with the other pedals!!!

This is seriously, the easiest thing in the world to take care of. Stop thinking about batteries and start thinking about where you're going to buy a JFET!

[mr.adambeck]

I think I'm going to do it, but keep the pre-amp separate from the shaker, so that I can wire a power supply into it.  Thanks for all the help.