Double Bass Advice needed.

[PhilCo35]

Hello one and all, I need advice with a EUB I have made from a damaged Ministar Fretless Bass Guitar (It had been run over by a Fork truck and I decided to readapt it into a EUB).

I have extended the scale to 42” using the same type of wood (American Rock Maple) as the original instrument. The extension is about 8” and it is also wider, the back is reinforced as well. I have made a new curved bridge in 3mm thick brass. The original magnetic pickup has been retained in its original position. Two small steel blocks have been added to the middle poles of the pickup to raise the height of the poles to allow for the increased height of the middle strings. When playing Pizzicato I am impressed with the sound which is deep, rich and mellow with excellent sustain.

As I would also like to play Arco; two Piezo strips of 2. 1/2” were placed between the bridge and the solid maple body. These strips were connected to a Blend Pot along with the magnetic pickup (The other two pots are Volume and Tone) the idea being that when the pot is advanced to its limit in one direction one pickup will be active and the other silent, and visa-versa. Hopefully there may have been a blend of the two depending on the position of the pot’s knob. But this isn’t the case, the Piezo’s are silent.

I am electronically illiterate. My only claim is to have soldered up wiring to the connections to the various Pots’. I am now thinking that as Piezos require a surface that will vibrate to become active, and due to the solid nature of the wood/brass bridge connection, the lack of vibrating surfaces will not allow the Piezo to function. Am I correct or could there be another cause? A friend suggested that as the two types of pickups have different impedances, that this could be a problem.

Is there a solution to my Piezo problem, or is there another type of discrete pickup that could be installed to allow for Arco playing? I also have an EBS Black Label Microbass II Outboard Preamp to assist with whatever course I take.

Thanks, Phil

PS I have pics' but don't know how to post them...

 

[DoubleMIDI]

For arco playing you need a curved bridge AND a curved fingerboard!

Piezos need an impedance buffer in front of any pots.
(Have a look at Francis Deck 's solutions in the internet, the simple quick'n'dirty is fine the HPFpre is even better.) Otherwise use the EQ on your amp.

Piezos under the bridge feet are a good solution for arco playing.

 

[ricobasso]

... I am now thinking that as Piezos require a surface that will vibrate to become active, and due to the solid nature of the wood/brass bridge connection, the lack of vibrating surfaces will not allow the Piezo to function. Am I correct or could there be another cause? A friend suggested that as the two types of pickups have different impedances, that this could be a problem..

Yes you are correct. A piezo element has to bend to produce an electric output. I had the same problem when I built my EUB and I was using a small hollow acoustic body! I had trapped a piezo film between the the belly and the foot of a conventional double bass type bridge. It did not bend enough and I got very little output. What type of piezo are you using?

I know there are some designs which will work trapped under the foot of a bridge but I had a spare Ashworth pickup (no longer made) which I use in the wing of the bridge. It is described as "differential". It has one dot in the middle of one side and two dots at the ends of the other side. The bridge wing must be carved so that these dots are the contact points with the piezo. This ensures that the piezo bends in its most sensitive mode/direction by the bridge vibrations.

I get a very "double-bassy" sound which I am very pleased with. If I do get any change, I invariably find that the pickup has become misaligned away from its sweet spot in transit.

Re impedances, your friend is also correct. If you want to mix pickups you will need to use a two input mixer. The piezo will need a 10M Ohm input impedance and the magnetic pickup 1M Ohm.

Hope this helps, Good luck.

 

[DoubleMIDI]

Piezo crystals have a different sensitivity to bending/compressing in different directions.
The piezo disks are cut for high sensitivity on bending, the bridge wing piezo for high sensitivity on compression (I think).
You might want to stack two of them fo higher voltage (but then the impedance will rise) or construct your pickup so that a difference in compression will be tranformed into a difference in bending (not easy to contruct in practice).
Magnetic pickups often work well with impedances around 300KOhms, piezos depending on construction with impedances between 1 MegOhms (thicker piezo crystals) and 22 MegOhms (Piezo film). Single piezo disks are usually in between (depending on size and thickness). Stacked piezo disks may come close to 10 MegOhms.
The preamp/impedance buffer needs to be really close to the pickup if the preamps input impedance is larger than about 3 MegOhms, otherwise the cable might get microphonic (tapping on the cable will get audible over the amp) and sensitive to electrical noise. You won't like this on a long cable on the floor.

 

[PhilCo35]

Thanks for your responses and suggestions, the EBS DI Box I'm using has a 10M Ohm buffer so that shouldn't be an issue. Stacking the Piezos could be worth a try as will creating a slight bend.

I've taken it to my friend who will advise me further. In the meantime I'll try to get some pic's posted.

Thanks
Phil