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Started by sjturbo, February 25, 2014, 08:58:35 PM

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J M Fahey

To be precise, it's a small capacitor, so it *needs*  a high impedance load to have a flat response.
Otherwise its sound is percieved as "shrill/piercing/spikey"


Quote from: smackoj on May 15, 2014, 07:48:24 PM
question; does the piezo type 'transducer' (passive) have a hi Z?

Yes, very.  It basically looks like a fairly hot (0.5-1Vpk) voltage source in series with a small cap.

I've just measured three disks I have here;

Disk dia, piezo dia, cap.
15mm 9mm 3.23nF (3230pF)
20mm 14mm 7.9nF (7900pF)
20mm 14mm 16.4nF (16,400pF)

A series C with a following shunt R forms a First Order high-pass circuit.  The cut off frequency or hinge point, already -3dB down on mid-band, is where the reactance Xc of this inherent capacitive source equals the input resistance.

To take an example using a smaller wafer, and assume an input Z of 1Megohm and a Cpu = 5nF, the hinge and -3dB point will be;

Xc = 1 / 2 Pi f C

transpose to make f the subject

f = 1 / 2 Pi Xc C

1 / (2 * Pi * (1*10^6) * (5*10^-9)) = 31.8Hz

Just a few more quick sums suggests that you might be looking at 10nF for a bridge-slot piezo.
If you say theory and practice don't agree you haven't applied enough theory.


earlier in this thread Phatt suggested using Roly's adjustable CR circuit to find the right impedence match for a passive pickup acoustic guitar. I have studied this schemo enough to know that the input side confuses me. The 'guitar', 'pickup' and 'cable' don't usually get shown on a schemo. If I want to build this circuit, do I just start the build at the 'input' as I would normally on a pedal build or am I not reading this right?


Yes, the actual circuit starts at the "input", R4/22Meg.

The other stuff at the input are models of the guitar pickup and connecting cable and are "lumped" versions of characteristics that are actually distributed.  A wire or cable in LTSpice is perfect, no resistance, no series inductance, and no stray capacitance between the inner and surrounding shield, so if you want to account for them in the simulation they have to be put in specifically; so they are partly an artifact of the circuit being developed in LTSpice rather than pen and paper.

A couple of caviats; I'll be very interested in how you get on with this and feel free to PM or e-mail me if you have any problems/questions.  Note that I have given no consideration at all to the guitar controls, my intention was to actually build it in as a replacement.  The input is pretty sensitive to stray pickup due to it's high impedance, so the case needs to be metal or internally screened with foil, and the cable between the input and pickup should be short, direct, and a screened lead with as low a stray capacitance as possible.

Start a build thread.
If you say theory and practice don't agree you haven't applied enough theory.


roly; thanks. yeah, I thought the 'guitar' cable etc were just CAD design add-ons but it took a while to get a clear picture with them on the schemo. I have a couple other projects to finish then I will give your CR circuit a go. looking forward to testing it on this particular application.

thanks again,  jack


Well like any new circuit it's still a bit experimental, particularly adding in any guitar controls.  These should have minimum effect when set for full level and full treble.

The "R" control should vary the resistive loading from tens of megohms (R4) down to 100k/R3, and the input "C" should range from a few pF residuals, to 1nF/C4 - if any value is a shot in the dark and depends on the pickup used, it's this one.

Remember, this circuit won't do you any good at all with an active guitar, one with a battery inside, only passive guitars where the pickup can be influenced by outside parameters.

I've been thinking of building this in to my cheap workshop test guitar,  but I'm faced with the options of drilling a third hole in the scratchplate (Vol, Rload, Ctune), or trying to find a suitable double pot for C and R with a co-axial shaft (like a car radio, then find suitable knobs).

A more general idea was as a belt pack so the input lead is nice and short.  I've done a few different projects in a blank remote control case that Jaycar sell that will hold 2x AA's or a 9V battery, glue in a (grounded) foil lining, cardboard tray for internal insulation, and the recessed end is ideal for a tiny panels with stubby controls, finger-settable, but resistant to being accidentally knocked.

If you say theory and practice don't agree you haven't applied enough theory.