Transducer Pickup Tone Control Problem

[rbkxiiowe]

Hi all.

I'm trying to fit a transducer pickup (the kind salvaged from a piezo beeper or xmas card). I'm using this circuit for the volume and tone control - www.guitarelectronics.com/product/WD1H11_00/Guitar_Wiring_Diagram_1_Humbucker1_Volume1_Tone.html - but the tone control doesn't do anything! I'm using a 47n capacitor and a 500k pot. I assume it's something to do with it being the piezo pickup instead of a magnetic one? The different impedances? Any ideas?

Cheers.

[ChrisK]

but the tone control doesn't do anything

Try a larger cap value or check your wiring. I would think that these values should have some effect.

I'd check the wiring (and ground connections) first since it's free and the easiest thing to do.

Of course, it depends on the piezo sensor and where it's mounted. If it's really only detecting and producing a sine wave, they're no harmonics to remove with a tone control.

I mean if this sensor is the size of Rhode Island, you may need a larger cap.

[JohnH]

Try what Chris says first.

Are you just wiring the piezo direct to the volume and tone controls? Piezos usually are better with an active preamp, at least as a buffer.

My thought on the passive set up is that the tone control, even at max cut, is just adding a capacitor in parallel with the piezo. The Piezo is iteself, electrically very much like a capacitor and so your tone cap is loading all frequencies evenly, ie no treble cut.

If you are getting a basically decent signal from your piezo and want to stay passive, I think that doing tone control downstream, in an EQ box or at the amp or PA will work better. You might just get some more tone control action with your current set up with a resistor in series with the piezo, somewhere in the range 2k to 5k at a guess.

When I added a piezo, it got quite involved before I could get a sound that I liked - see this thread:
guitarnuts2.proboards45.com/index.cgi?board=schem

John

[ChrisK]

JohnH is correct in that he had to do a lot of prestidigitation to get the sound that he wanted.

This is primarily due to the piezo sounders used as specific frequency output devices. Piezo beepers are used as self-resonant circuits and have a very peaked response.


While a piezo is capacitive in nature, it is modeled as an AC voltage generator in series with a capacitor. Aside from the uneven frequency response output voltage as indicated above, it forms a capacitive voltage divider with the external capacitance.

Unfortunately, the frequency response of a capacitive divider is not the same as an inductive pickup driving a RC tone circuit.

[rbkxiiowe]

Thanks for the reply.

JohnH - comme ca?



(will a 4.7K tone pot do the trick?)

ChrisK - didn't understand a word of that at 11pm last night ..... makes a bit more sense today (only been wielding a soldering iron in anger for about a year but getting there)

[newey]

RBX-

I don't know about the value of the pot, but your diagram doesn't show a capacitor on it. You initially indicated a 47nf cap, but I don't see one in your drawing.

Is there a 47nf cap wired to the tone pot, and how is it wired? You may just have omitted the cap from your diagram but I bring it up because your original post referred to the tone control not working.

[rbkxiiowe]

newey -
JohnH said:

You might just get some more tone control action with your current set up with a resistor in series with the piezo, somewhere in the range 2k to 5k at a guess.

That's what I was trying to do in my diagram?? Like I said, this stuff scared me until about a year ago, so I'm not the sharpest tool yet!

[JohnH]

yes, but what I meant was, take the circuit that you had, with tone pot tone cap volume pot, and instead of the wiring the piezo directly to it, wire the piezo with a resistor in series with it , between piezo and the controls.

But are you getting any reasonable tone so far? Unless you are close to getting a good sound, this will probably not fix it, because usually an active buffer is needed. What it will hopefully do is to allow the tione pot to do more though

John

[rbkxiiowe]

OK, I've just tried putting a 3.3k resistor between the piezo and the volume and it did nothing except perhaps made the output quieter. Then I tried another 47n in parallel with the first capacitor and all it did was seem to make it quieter as I turned it down until there was no output at full ccw. So I tried a 220n in parallel with the first cap and it seemed basically the same.

I'll try this www.till.com/articles/GuitarPreamp/ between the piezo and the volume and see what happens.

Oh yeah, ChrisK - the piezo is 27mm wide. I'm English but I think Rhode Island's a bit bigger than that (based purely on my knowledge of a certain cartoon).

[ChrisK]

FYI,

While JohnH does exhibit some symptoms of OCD (Obsessive Compulsive Disorder aka CRD - Compulsive Rewiring Dementia), it is somewhat likely that he did what he did for a very good reason.

guitarnuts2.proboards45.com/index.cgi?board=schem&action=display&thread=3718

LR Baggs have the following info;
www.lrbaggs.com/html/products/pickups_xbridge.shtml

www.lrbaggs.com/manuals/x-bridge_us_standard_manual.pdf

They show no passive piezo tone controls.

Fishman also shows none.

Fender does have a piezo tone control in the American Power Tele. It has an active preamp. www.fender.com/support/diagrams/pdf_temp1/telecaster/010_5700_02C/SD010_5700_02CPg2.pdf

My Godin has piezo tone controls (a graphic equalizer). It has an active preamp.


Ergo, one must use an active buffer to effect a piezo tone control in-body.

or

To exercise the daemon of duality*, an inductive component might be used in conjunction with a pot to work with the capacitive nature of the pickup.

* the DOD. An inductive pickup is used with a capacitive tone control circuit.