I found this tucked away with a google search
"The only way to get a passive tone control working with a piezo pickup is to have a resistor in series with the pickup before the signal reaches the tone control. The larger the resistor, the more effective the tone control will be. Unfortunately too large a resistor will also lose output, so a compromise value would be needed. This is the main reason that acoustic preamps are used in the guitar, so that the piezo pickup signal goes straight to the preamp input and the pickup is buffered by that, then the tone controls come after (or within a later stage of) the preamp. Then in that case, they will work as expected, usually with boost and cut of the various ranges."
it was from here
http://charliehall.proboards.com/index.cgi?board=saved&action=print&thread=485
As I have never found a passive tone pot works with a piezo using the same method as for a magnetic pickup (using a capacitor only) I'll have to give it a go.
Has anyone else tried this?
Regards,
David
Replies
jim said:
Glenn Reither said:
"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:
"Piezo pickups need a very high impedance. Usually something like 3 to 5 megohms. Unfortunately typical tone and volume pots do not come in this range so ideally you do need a preamp before any controls.
A standard electric guitar pickup is affected by input resistance and capacitance. Lowering the resistance will reduce the amount of the pickup's resonant peak. Increasing capacitance will change the resonant frequency to a lower frequency.
"A piezo pickup reacts completely differently. Lowering the input resistance will reduce bass. Increasing capacitance will reduce volume. This is why when adding the usual type of guitar tone control to a piezo pickup it acts as a volume control instead of a tone control. Adding a suitable resistor in series with the piezo pickup will help to convert the piezo signal so that a guitar tone control will work more like expected. I would start with a resistor of maybe 100K between the pickup and the tone control. Wattage rating doesn't matter so you can use a resistor type that is physically very small. The tone control should be a fairly high value, maybe 1 meg log. The value of the pot won't affect the bass because the capacitor will stop bass from being lost. The tone capacitor can probably be one of the standard values used in electric guitars such as 0.022uF, 0.047uF or 0.1uF. If the 100K resistor value works then it should do. If it works but not effectively, try a larger value resistor like 220K or 470K until it does. I would then connect that point to a volume pot of 1 meg log or even 2 meg log if you can get one. The pickup will see a total resistance that is the value of the resistor and the volume pot added together. This total will be reduced because of the input impedance of whatever the instrument is connected to. The larger the pot values, the less signal you will lose. The downside is that with a larger value volume pot, you will lose more highs from the signal when the volume pot setting is reduced but you will not lose quite so much bass. Adding a bypass capacitor to the volume pot might help to retain lost highs when the volume setting is reduced. The larger value that your added resistor is, the more highs you could lose with the volume control backed off. I would try something like 100pf as a bypass capacitor if you think you will need one. The value chosen will depend on the capacitance (length and type) of cable you use from the instrument. I would choose the shortest cable that you can comfortably work with. For an external preamp, you can still use a preamp that has a high impedance designed for piezo pickups and you may find one that has extra tone controls to fine tune the sound. Or you can use an FET design with an input resistor to earth or ground of 3.3 or 4.7 megohms. It won't work as well as having the preamp in the instrument and the controls in the usual place within the preamp design itself, but I think it is the only way to do what you want. By juggling with values, you will be able to figure out compromise settings that work as best possible for your instruments."
Roosterman said:
David Lloyd said:
I think the schematic is something like the above - I have yet to work out how the pots work in this schematic (hidden on otherside of PCB and not sure if I can get to them. The resistor measures 22K on my multimeter.Roosterman said:
Id be interested as to what you find in there - could be a leprechaun!
Thanks for the replies gentlemen. After another hour spent playing around with capacitors and resistors (zero result) and drawing a blank on google I have bought one of the "passive preamps" off ebay
http://cgi.ebay.co.uk/Acoustic-Guitar-Piezo-Pickup-Passive-Preamp-S...
Once it arrives from china (if it actually works) I'll take the back off it and see what magical components thet have used.
Roosterman said:
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