This is the purist approach solution to the tinny contact mic sound problem if you want the lowest electrical noise, or the lowest distortion at higher levels. Piezo mics aren't usually considered hi-fi, so distortion isn't usually a problem of the amplifier - at high levels the mic will not be particularly linear. However, they are remarkably low noise. Although they are high impedance, they don't need an outrageously high load impedance - not like an electret mic, where you need gigaohms to avoid losing bass. The calculation in the introduction showed a load of 330k was enough, so 1meg is plenty for a contact mic. The first thing that sprang to mind was a TL071 - the overall schematic is shown below.

Matching levels of the 1kHz test tone to correct for the slight difference in gain shows the NE5534 offers a noise floor 10-20dB lower than the 2N3819, in return for extra complexity. At the back of my mind was the wonder whether a piezo mic would not have more self-noise anyway, so I removed the test tone and put in the mic. This is a reasonably quiet location. The hum is because this circuit is on a breadboard with the mic laid on the shelf, and some of the hum is mechanical, from the transformer of the power supply powering the unit, which is on the same shelf unit. About 3/4 of the way in some piece of machinery is started by my neighbours - I did not hear this at the time but it was conducted through the house into the shelf with the mic. You wouldn't normally have to put up with this level of hum - building the circuit into a metal box and using screened cable to the piezo device would fix that. However, the piezo mic has not raised the noise level significantly. -------------------------

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The bad news, however, is that the manufacturers of FETs don't control their parameters well, and have somehow conned us into living with the problem. The gate-source voltage needed to bias the transistor into the linear region can vary between 0.25V and 8V, which leaves a good 7.75V down to a hopeless 0.4V for the transistor and load if used with a typical NiCad 8.4V PP3 You'll have to get more FETs than you need and throw out the dogs. It's easy enough to test, and this parameter is a given for a particular device - it doesn't age of change greatly with temperature. Design manuals get all sniffy about that sort of thing because selecting FETs obviously adds to the cost if you are mass producing something. That's not the case here, and there's just no way to cope with a manufacturing tolerance which can throw more than 90% of the battery voltage away in variations in manufacture without screening the bad 'uns. Ideally you'd run the FET from a higher power supply voltgae, like two batteries in series and perhaps double the values of R2 and R3, but it would be a shame to have to use two batteres just because the manufacturers couldn't be bothered to grade by Vgs. You can tell if you have a good 'un by measuring the voltage at the drain and source of the FET in circuit. Ideally you would like Vs to be about 2.5V and Vd to be about 6V (assuming a 8.4V Nicad PP3) In practice you can live with Vs at 1 to 3.5V which will correspond to a Vd of 7.4 to 4.9. This will run the FET at 0.25mA to 0.9mA I prototyped this and tested it out with all the spare FETs I had in my junkbox, on a supply voltage of 8.5V device Vs Vd usable 2N3819 #1 2.07 6.3 OK 2N3819 #2 2.03 6.2 OK 2N3819 #3 1.4 6.8 OK J309 #1 2.1 6.1 OK J309 #2 2.27 5.98 OK BF244B 2.39 5.86 OK BF244 4 4.2 BAD

Mandolin info

http://www.youtube.com/watch?gl=GB&v=wZRIz3juVG8 http://www.tonetechluthiersupplies.co.uk/ http://liutaiomottola.com/construction/NeckCarve.htm http://www.liutaiomottola.com/construction/NeckBlank.htm http://www.eguitarplans.com/making_a_guitar_neck.pdf http://www.touchstonetonewoods.co.uk/ http://www.luthierssupplies.co.uk/ http://www.mandolinluthier.com/HospTriage.htm It seems that "most" of the Mando's I see are made with a 1" neck at the nut. I have alot of trouble chording mandos with the narrow neck . Therefore I have built mine with a 1-1/4" to 1-1/2" Neck at the nut. From what I have seen, 1" would be a narrower neck than most out there. I think 1 1/16" might be more common. I build my Traveler mandolins with about 1 3/32" at the nut. Seems to be just right for most folks, even if a hair wider than the norm. Look at Weber, Gibson, Collings and Breedlove specs and you'll see most are 1 3/32 wide. My first build was 1-1/2" and while every guitar player who has tried it likes it , mandolin players do not. I am currently working on 1-1/4" for my next mando. A friend has an early teen(1912?) Gibson, and it's that neck that I copied for my own mandolin, which is 1-1/4" at the nut. What year did they narrow it up? Maybe his is older than he thinks?

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