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So, Did I Go Too Far?

I finished my second build last week. A three-string fretted CBG. I went to Gitty for piezo disk. I saw that they had 12 mm, 20 mm and 27 mm. I figured "What the heck, isn't bigger better?" so I opted for the big 27 mm pieces.I bored a countersink in the side of the neck (inside the box) and mounted the piezo in a bed of silicon seal top and bottom.

I'm feeding the piezo into one of the small 2.5 watt amplifiers I built into a spare cigar box. When I play my original frettless CBT into the amp, the sound is good (not too loud) and not too sensitive to my movements around the box (even with everything turned to the max). This one has a single 20 mm piezo.

When I crank up the volume knobs on my amp and new CBG (the one with the 27 mm disk) the amp picks up every stray movement of my body and hands. I slide my hand across the body of the box and it is amplified noticeably. I can turn down the pot on the CBG to reduce this unwanted sound, but, of course, give up string volume.

My question is: Is the 27 mm disc noticeably more sensitive than a 20 mm unit? Should I look at adding more damping around the Piezo? Maybe encase it in soft leather or something? Should I remove it from its attachment to the neck and mount it in a less sensitive area?

I like the string volume of this CBG, but think I may have taken a wrong turn on either selecting the piezo or in my mounting scheme.

Thanks for any suggestion or insights you may have.

Tom T

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Permalink Reply by Ron "Oily" Sprague on November 13, 2014 at 9:59am

More surface area to interact with vibration means more current generated piezoelectrically. Also means greater sensitivity, all other things being equal. So, go smaller.

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Permalink Reply by Tom T on November 13, 2014 at 12:14pm

Thanks Ron.

I have some smaller disc and may swap them out to see what happens.

Tom T

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Permalink Reply by Tom T on November 13, 2014 at 12:17pm

Thanks for responding.

I'm not sure I follow where you position the disc. Is it to the inside top of the box? When you talk about "groove" do you mean the gap between the neck and the top of the box?

I can move the disc pretty easily, but need to figure out where to move it.

Thanks

Tom T

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Permalink Reply by Ron "Oily" Sprague on November 13, 2014 at 5:23pm

Use double sticky tape before settling on your final disc position. Stick it all over the box, both inside and out, until,you find a sweet spot. Same principle as the Barcus Berry Hot Dot (wish I could find my old one...)

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Permalink Reply by Tom T on November 14, 2014 at 12:46am

The double-sided tape trick is a great idea. Beats cleaning silicon off the disc every time I want to try a new position. I've already got a few rolls so I'll be doing some moving this weekend.

Thanks for the tip.

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Permalink Reply by Wichita Sam on November 14, 2014 at 12:49pm

Piezos are pretty "trebly". I suggest that you bias the install toward the bass side of the bridge under the top.. I've done plenty this way and even in bridge/saddle arrangements. A little more silicone ought to help.

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Permalink Reply by Tom T on November 14, 2014 at 1:12pm

Thanks Sam,

I plan to start moving things around this weekend since the weather is expected to be a bust.

With the double-sided tape tip, it should be relatively simple to do a lot of moving.

Tom T

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Permalink Reply by Michael Fred Johnson on November 16, 2014 at 1:07am

I also use 27mm all the time,in pairs,embedded in hot glue,gives good results.

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Permalink Reply by Tom T on November 16, 2014 at 2:19am

Thanks for the photo Michael. It always make things more clear. Is that a piece of PVC pipe around the Piezos?

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Permalink Reply by Michael Fred Johnson on November 17, 2014 at 2:08am

That is waste pipe

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Permalink Reply by jawbone on November 16, 2014 at 8:21am

I'm not sure what Michael does, but I wire mine parallel - that seems to cut the signal down to more what an amp is expecting - seems to take some of the "harshness" out - I don't know what electronic laws come into play ! :-)