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Permalink Reply by Glenn Reither on November 2, 2010 at 7:49pm

Sorry...don't see any sales hype there. Anyhoo, just passing on something that might be interesting to someone. I'm going to try this sometime as a mid cut.



jim said:

chasing a headless chicken to train it to sing blues! sooo much bs with a mix of sales hipe might help! haa


Glenn Reither said:

Here's another bit of useful passive piezo tone info, using a passive RLC notch filter. I haven't tried this but if you found the right values you could notch out harsh upper-mids, maybe on a toggle switch?

"Hi, just saw your artical on your $8
"Doodle-Bass" and noticed you were wanting to
add an acoustic pickup for it from Radio Shack
parts.

I am currently working on an instrument I call a
Bungee Bass that uses rubber strings (actually
"O" ring material). Anyway, I have used a very
cheap transducer system on several of my
home-made instruments that you may find useful.

Basically, all you need is a Radio Shack 273-073
Piezo Transducer and an output jack. If the
instrument puts out any acoustic sound at all,
it will pick it up, you just need to find the
"sweet spot" to mount it. You (in most cases)
don't even need a preamp or power (battery)

The trouble with this $1.49 transducer as
opposed to a $90. 00 and up "pro" modle is that
is quite "trebley" making you turn the treble
down on the amp. But once you do that, it makes
an acoustic guitar sound just as good as to
"pro" transducers.

A solution for this is a passive filter known as
an RLC Notch Filter. "Passive" means it needs no
external power and "RLC" means a Resistor, Load
(inductor) and Capacitor. "Notch" means it will
notch out a frequency and shunt it to ground.
The circuit does not even need to be mounted on
a PC board ( but shielding it in some cardboard
covered in aluminium foil tape is a good idea as
well as using shielded wire). The drawback for
passive filters is some overall volume loss.

If all the components are "tuned" very tightly
(very low Q) the notch would be very slim. This
is kind of hard to do because of the
availability of the components, fortunatly, we
are better of with a wide notch. So wide, in
fact, that it virtually becomes a low-pass (or
high-pass) filter becuase of the audable range
of human hearing.

I could bore you with a lot of math formula but
the following diagram and parts list will be all
you will ever need.


Black Lead --------------------------------->Output Jack Ground
(T1)
Red Lead ----->R1>----->----------------->Output Jack Tip
|
|
>C1>--->L1>--->Output Jack Ground

Parts List for a Bass Transducer System:

T1 = Radio Shack Piezo Transducer - Cat# 273-073

R1 = 33 ohm resistor

C1 = .0047 uF capacitor

L1 = 156 uH inductor

The L1 - 156 uH inductor it's part of the Radio
Shack Inductor Assortment - Cat# 273-1601 ($2.
49). You can easly identify it as it is Light
Blue, looks sort of like a capacitor and has a
Brown dot on one side, a Blue dot and a Green
dot on top, and a Gold dot on the other side


Parts List for a Guitar/Banjo Transducer System:

T1 = Radio Shack Piezo Transducer - Cat# 273-073

R1 = 33 ohm resistor

C1 = .0022 uF (microfarad) capacitor

L1 = 602 uH (microhenry) inductor

The L1 - 602 uH inductor it's part of the Radio
Shack Inductor Assortment - Cat# 273-1601 ($2.
49). You can easly identify it as it is Light
Green, looks sort of like a little dumbbell and
has "602" on it's top.


Construction Notes:

1. I usually remove (carefully) the black
plastic mounting shell from the transducer.
Then, I'll tape the transducer to different
mounting locations until I find the "sweet
spot". Once I have that, I glue the transucer in
place.

2. It makes no difference if the black wire from
the transducer stops to hook to the ground of
the filter before continuing to the output jack.
The diagram shows it otherwise for clearity.

3. If you need to use a preamp (as I do on the
rubber string bass) then make R1 = 3.3k omhs.
The preamp should "sit" between the RS 273-073
and the filter.

4. You can make a "depth control" using the
following diagram:


Black Lead ------------------------------->Output Jack Ground
(RS 273-073)
Red Lead ----->R1>--->----------------->Output Jack Tip
|
|
>C1>-->H1>-->R2>-->Output Jack Ground

R2 = 50k ohm audio taper potentiometer (linear will do)

The pot has 3 terminals, terminal 1 connects to
H1 and terminals 2 and 3 go to ground.

This control will travel from full filter to
complete elimination of the filter fom the
circuit.

5. If you have an assortment of caps, you can
change the notch to "flavor" the instrument by
experimenting with various values. You can also
try other inductor/cap combinations and even
change the resistors value BUT... the source
(driving) impedance should be LOWER than the
impedance of the filter. Also, the output of the
filter should feed an impedance (amplifier) much
HIGHER than the filter impedance.


All and all, I have found this a very good
sounding transducer especially at the price. If
you put this on a nice acoustic guitar, the
end-pin jack would cost you more than the
transducer system.

Well, I hope this info will prove useful to you
and I'll keep an eye on your page, which has
provided me with alot of ideas on my own
projects.

Thanks Clark Rowden"




Don Thompson said:

Yep, My guess is a much larger value for the tone pot. On the order of 1Meg or more. Piezos simply do not have the same electrical values as a magnetic pickup so what works for one won't work for the other. Don

Roosterman said:

I suspect its just a case of getting the right value parts, but calculating such things is black magic to me.
Id be interested as to what you find in there - could be a leprechaun!

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