Dreaded HUMmmmmmm.....

Hi Guys,

I thought I was ready to Rock'n'Roll but.....

I have a twofold query.

Firstly. I do not have a solder or experience in electrics etc.. so I got a 'Disc-o-Tone' prewired embedded Piezo pickup Harness from C.B. Gitty. This is a Pot, Jack and pickup embedded in a small circular piece of wood which you attach to the lid with the strong adhesive backing supplied (I'm sure you guys know what I mean with this set up).

Before sealing box, I strung one string to test it. There is a dreadful Hum coming through my little Roland Microcube Amp. When I pinch the exterior of the Jack it disappears or when I pinch the Pot shaft it disappears. How do I eliminate this?

Secondly. What is the point of having a bloody volume pot? During my 'test' I have to turn the volume pot up full AND the 'Master' and 'Volume' on the Amp up nearly full before any should comes through!

Any thoughts?

Do I need to remove the adhesive backing and glue the embedded pickup directly to the lid?

Would this help? I dunno!!

I have it fixed to the lid. snug against the neck (when lid is shut), below the bridge and as close to the 'bass' string as possible.

Yours, slightly annoyed,

Col

Replies

JL June 10, 2014 at 2:10pm

volume issue: you should be able to get a lot of sound out of the piezo by tapping on it with your fingernail while plugged into the amp. that will tell you if it is a placement issue or if it is got too much dampening material surrounding it. a bare disk taped to the inside lid should allow a 10w practice amp to make the spouse cover her ears.
colin mcgrath June 10, 2014 at 1:40pm

I stopped testing with cheap cables. I live in a city, with lots of electronic interference. Those cheap plastic cables can sound awful. I shelled out the cash for a good braided cable. It has cut the electronic hum in order for me to pinpoint a real issue. Just my experience.
robert jones June 10, 2014 at 12:00pm

Wiring is correct.

There is the possibility of a cold joint at the vol pot case ground.

Have you tried the amp plugged into an outlet away from any radio interference?

What about a different amp entirely?

Have not played a piezo through a micro cube so can't say if an impedance mismatch is causing the limited volume problem.

If you can plug into an acoustic amp you can possibly eliminate impedance mismatch as a cause.

You could try alternate locations for the piezo such as the underside of the neck but if you are happy with where it is now carry on.

If you can't sort this out contact Ben (C.B. Gitty), he'll take care of you.
Marcus Darwell June 10, 2014 at 6:49am

Not mad about piezos but have used them. It should be directly under the bridge so bore a shallow hole with a spade bit that's the right OD into your neck where the bridge goes. Looks like yours is quite a way from the bridge.Just use a thin piece of double sided tape to hold it against the lid (or silicone). The hum could be reversed wiring or mebbe a cold joint. Go buy a cheap soldering iron and redo the joints following the wiring schematics you can find on the CBN site. Good luck mate!
- Randy S. Bretz > Marcus Darwell June 10, 2014 at 8:18am
  
  Should of bought a rod piezo bridge off Gitty
  - Colum Burke > Randy S. Bretz June 10, 2014 at 9:20am
    
    very helpful there Randy... Cheers!
Richey Kay June 10, 2014 at 6:26am

Hi mate. Sounds like you have the wires the wrong way round at the jack. Have a look at some wiring diagrams and see if yours matches up.
You don't need to ground the strings with a piezo. That is needed to cut down on hum from magnetic pickups. I think what you have is good old fashioned buzz.
Piezos should be quite sensitive and loud. Yours looks heavily encased. When I use them I just wrap them in a strip of old mouse mat, glue to the lid and it works a charm. Good luck!
Colum Burke June 10, 2014 at 5:41am

here is the wiring.... it looks ok.. I think???
Brian Grise Lucky Voodoo Guitars June 10, 2014 at 4:16am

For the hum problems, Did you ground the bridge/strings?
For the volume problems try moving the piezo around. Might be to tight or loose between the wood.
Not sure, pure speculation, seeing as I am new to both CBG building and piezo placement. Also hard to to say without a photo of the instrument
- Colum Burke > Brian Grise Lucky Voodoo Guitars June 10, 2014 at 4:31am
  
  Hey Brian,
  
  Sorry don't know what you mean by 'grounding' the bridge/strings?
  
  The bridge is a simple bolt which will be held in place when strings are tightened.
  
  The Pot/shaft is tight against the wood/lid. fixed with washer and nut supplied. How can this be 'too tight'? Why on earth would you want it loose?
  
  As for the pick-up, I think it's in the best position I can place it......
  
  Cheers,
  
  C

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