The circuits and descriptions below assume flat response to the fundamental frequency, but this can be changed by changing the value of the input resistor. If the fundamental is boosted, many players will find the sound to have excessive bass, so it's not always wise to ensure that you have flat response down to the fundamental frequency. While it's generally assumed that a pickup should respond to the lowest fundamental (82Hz for guitar, 41Hz for 'traditional' bass), a characteristic of most plucked or struck strings is that the second harmonic is usually dominant (depending on the striking/ plucking position and/ or style). This is increased when feedback is applied. Even the TL071/2 has an input impedance of 1TΩ, with typical bipolar opamps only providing around 300MΩ or so open loop. The OPA2134 (or the single OPA134) has an input impedance of 10TΩ - and that is not a misprint. 9V alkaline batteries have a typical capacity of around 580mA/h, so with a load of ~10mA it should provide over 50 hours operation (including the LED). Be aware that the OPA2134 draws roughly double the supply current of a TL072, so battery life will be reduced. A single 9V battery is not advised for a TL072 because it is not designed for operation at less than 10V. While the OPA2134 is a fairly expensive opamp, they have much lower noise than the common-or-garden TL072. The three circuits are shown using a 9V battery and an OPA2134 (or NJM2068) opamp. Some players tune for C 1 (32Hz), but that's still easily accommodated with the circuits shown below. The circuit is simplified considerably for musical instruments, because response below 40Hz isn't necessary (the lowest note on a traditionally tuned double bass is E 1 - 41Hz). The circuits shown in the article/ project are intended more for a specialised bench amplifier, and aren't really suitable for musical instrument pickups. The general principles are discussed in some detail, but it can't be considered a 'true' project for several reasons.įor starters, the final circuit (shown in Figure 10 of the referenced article) requires the use of a 1GΩ resistor, and these are not easy to get and are expensive. The basic principles are discussed in the article High Impedance Input Stages / Project 161. I've also experimented with piezo transducers. However, there is an exception to the 'high impedance' rule that's covered later (charge amplifier)įor reasons that escape me, I've not described a piezo preamp before this, even though I have used piezo transducers in a number of projects developed for customers. While bipolar input opamps can be used, there will be a significant noise penalty. Almost without exception, this means a FET input, either JFET or FET input opamp. In most cases, the input impedance needs to be somewhere between 'high' (1MΩ or so) to exceptionally high (greater than 100MΩ). They can be used as pickups for various musical instruments, such as acoustic guitars, violins, cellos, double-bass (aka upright bass), ukuleles and mandolins (etc.), and also as accelerometers. a 25V part is perfectly fine.ĥ) This particular preamp (an FET source follower) can handle quite a big signal without distorting, so it shouldn't be bothered by the fact that your pickup is "hot".Piezo transducers are common in a range of different areas. Doesn't matter if it's considerably more than 9V, i.e. It needs to be rated for well over 9 volts. The original value of 4.7uF is a bit stingy, and a 47uF cap is still very cheap. (But be kind to your amp and speakers - plug in everything, keep the amp off, power up the preamp, wait a few seconds, then turn on the amp.)Ĥ) I increased the preamps power supply filter cap to 47uF. This will reduce thumps and bangs if you plug your preamp into the input of an amplifier that's already turned on. If you still have enough signal level with the added cap, you don't need to build a preamp at all!ģ) If adding a 4.7 nF cap kills signal too much, and you do build the preamp, it's important to add an additional resistor from output to ground (R4 in the schematic attached to this post). Lacking this, I can't tell you exactly how much the output will be reduced by adding the 100pF cap.Ģ) Since your pickup is already "hot", try connecting a 4.7 nF (same as 4700 pF) capacitor between pickup output and ground it will reduce the output signal, but it will also make the pickup compatible with a 1-megohm amplifier input impedance. Unfortunately, the manufacturer of your Copperhead does not seem to provide the one specification we need: the actual capacitance of the pickup. Here's a cleaned up circuit, stripped of the stuff you don't need.ġ) If you add a 100pF cap across the input, it will reduce the output voltage from your piezo pickup.
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