Piezo Hum

[dub]

I have two pickups on my double bass, one is a realist beneath the foot of the bridge, the other is a revolution solo two in the wing slot.

The realist is very quiet on it's own (the piezo unit is covered in copper foil, the strings do not need to be grounded), whereas with the revolution solo on it's own, the strings need to grounded via a shield wire and I need to be touching the strings to prevent a loud hum.

I understand how shielding works but I am puzzled as to why I need to hold the strings to prevent hum if they are already grounded?

I have observed this problem with other instruments too.

It would be great if someone would explain this.

Edited January 25, 2011 by dub

[Ou7shined]

It's a bit of a black art but ultimately it's less to do with earthing and more down to capacitance and how your body becomes part of the circuit - everything has to be "earthed" in order to be connected to "the circuit".

I'm sure String Theory has something to do with it too.

[redstriper]

I had the same problem on my Ashbory with it's piezo, but touching the strings didn't help because they are rubber.
It was fixed by a friend of mine and he described the modification in a post on the Ashbory forum.
I don't know if it's of any help to you, but here's a copy of his post:

[quote]I recently modified an Ashbory bass to fix the hum problem. These basses are mass-produced, and it seems many others suffer from the same problem. It is not an earth loop, as some people suspect (that is something quite different) but is caused by poor screening of the electronics.

When I looked inside the bass, it was clear that Ashbory had gone some way towards proper screening, but had not finished the job. The instrument had conductive paint in the battery compartment (this is good) and thin foil under the plastic back plate (also good).

Unfortunately neither of these were connected to earth. The conductive paint extended under the foil-covered back plate, and was supposed to make contact with it, but didn’t (this is bad). Even if it had done, none of this screening was earthed (also bad). Having areas of screening material ‘floating’ un-earthed near audio circuitry like this is actually worse than having no screening at all.

Anyway, here’s the modification. Follow these instructions at your own risk – it’s not a difficult modification, but I can’t be held responsible if you damage your bass. If in any doubt, consult an instrument repairer or a tame electronics geek.

Parts
5” x 1” strip of self-adhesive foil (or kitchen foil and some glue)
8” length of thin, insulated wire
Small solder tag (not essential, but a very good idea)

Tools
Phillips screwdriver
Soldering iron
Scissors
Long-nosed pliers or 9/16” spanner (only necessary if the jack socket earth tag is difficult to access)

Procedure
1. Remove the back plate (four screws).

2. Remove the battery.

3. Unscrew the battery clip (two screws, but yours may have four, depending on how the person who assembled it was feeling at the time!).

4. Attach the self-adhesive foil as shown (run it from the upper surface of the rear of the body to under the where the battery clip attaches). If you are using kitchen foil you should glue it in place.

5. Solder one end of your new length of wire to the solder tag (if you have one).

6. Re-attach the battery clip, fastening the solder tag with one of the screws at the bridge end of the battery cavity (as shown). If you are not using a solder tag, ensure that you firmly trap a bared end of the wire under the battery clip base plate.

7. Thread the wire through the hole between the battery and circuit board cavities.

8. Next is the only potentially tricky bit. There are three tags on the jack socket, each with a wire connected. One wire (black) goes directly to the battery cavity – don’t use this tag. Another wire (yellow, on the bass I modified) goes to the front of the circuit board – don’t use this ‘inner’ tag either.

The tag you want is the ‘outer-most’ one, and protrudes from the main (threaded) body of the jack socket. This had a green wire attached on the bass I modified, but the wire may be a different colour on yours). The wire from this outer tag also goes to the front of the circuit board.

The pliers or spanner may be useful at this point. I had to loosen the nut of the jack socket in order to rotate the socket into a position where I could solder onto this outer tag. You may not need to do this.

Solder the free end of your new piece of wire to this outer tag, as shown. I used green wire, for consistency. You should now have two wires soldered to this outer tag.

9. Replace the battery, and plug the bass into an amplifier to check that it still works. If you have used the correct tag on the jack socket, the bass will sound pretty much as it did - some hum will probably still be noticeable, but will disappear if you touch the metal of the jack socket. If it hums really loudly (worse than it did originally), you have probably selected the wrong tag on the jack socket.

10. If all is well, replace the back plate. The hum should now be gone (or at least greatly reduced).

11. Celebrate with some liquid refreshment of your choice. I had a cup of tea.[/quote]

Hope this helps, I have some pics of his work too, if anyone's interested.
Steve.

[icastle]

Is this a Revolution Solo or a Revolution Solo II?

One of the things that was improved on the II was that they upgraded parts to make them fully shielded (so it's reasonable to assume that the Solo wasn't...)

[dub]

Thanks for the replies. It is a revolution solo II.
The capacitance makes a bit of sense to me. The most frustrating thing about this is that the problem seems to be at it's worst in the shed I use as a studio.

I've done a bit of research into earthing but I need to find out more. I may need a big copper spike in the ground connected to the earth wires in the studio.

[Owen]

[quote name='redstriper' post='1102367' date='Jan 25 2011, 05:03 PM']I had the same problem on my Ashbory with it's piezo, but touching the strings didn't help because they are rubber.
It was fixed by a friend of mine and he described the modification in a post on the Ashbory forum.
I don't know if it's of any help to you, but here's a copy of his post:



Hope this helps, I have some pics of his work too, if anyone's interested.
Steve.[/quote]

YAY for Funky Pete!