Replies to This Discussion

Permalink Reply by Doc Oakroot on July 8, 2009 at 2:18am

The output is pretty similar to a EM pickup, but I don't know the actual voltage.

The impedance is 1-2 MOhm (that's MEGA-Ohm - he may not believe that the first time you tell him).

Permalink Reply by Kevin Lawton on July 8, 2009 at 3:01am

Yeah, a piezo is really high impedance - about a couple of megohms. If your guy finds that difficult to believe, thn ask him to think of the old days of 'crystal' microphones and 'ceramic' record-player cartridges as they all used piezos too and were also very high impedance. A piezo does not need to be loaded at all, so the input impedance of the valva amp is best be about two megohms and as little capacitance as can be managed.

People who are used to working with valves should know how to do really low capacitance wiring as valve circuits tend to be very voltage-operated, thus high impedance, and generally pretty sensitive to capacitive loading. Using a two megohm resistor to isolate the piezo from the grid of the input valve is the usual way to do this. If a volume control is wanted at this point, then a two megohm log pot would do great.

The output voltage of an unloaded piezo can get pretty high - almost into the order of a volt or so - but it all depends on the mechanical arrangement you have between the strings and the piezo. Building the piezo into the bridge, so that the sound vibrations have to go right through it, will give more output than just sticking it on the the sound-board.

Diode clamping is often used to limit the input voltage to a valve circuit to prevent saturation but, if you do this in a guitar amp, then you risk getting pretty harsh clipping when you overdrive it - which sounds horrible. In a guitar amp, you aim to allow the valves to go into saturation for a nice warm, creamy kind of distortion - so you want to avoid any kind of diode clipping at all.

Do bear in mind that high-impedance circuits are far more prone to picking up inteference and hum than others, so you will need to make sure everyting in both the CBG and the amp is well shielded.