![]() 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. Electrical tape can be used to help with this.Piezo transducers are common in a range of different areas. Screw the strain relief back on to the jack.īe careful to not let any of the wires, especially around the capacitor, to short or become shorted when sheathed in the strain relief. Electrical tape and surfaces or wires that could contact something of the other polarity.Ĩ. Solder the black wire to the larger outer terminal.ħ. Solder the other end of the capacitor to the inner terminal of the jackĦ. Solder the shorter wire to the capacitor, making sure that it will be able to fit in the strain relief.ĥ. Don't forget to do this unless you really like to de-solderĤ. Feed the wires through the strain relief. ![]() Unscrew and remove the strain relief from the ¼” phone jack.ģ. Cut about 1/2” off the remaining red wire.Ģ. Just solder the red wire directly to the inner terminal.ġ. The pickups will still work fine without the filter. This step is optional but with a little experimenting, you can find your own personal preference. Solder a 10 nF capacitor in series with the pickups as a high-pass filter to help reduce valve noise with minimal effect on the trumpet's tone. Wrap the two bare ends together to form a mechanical connection. Strip the ends of the end of each wire to expose about 1/8th of an inch of bare wire.ģ. Cut the 2 wires to appropriate length, approximately 3/4ths of and inch.Ģ. Wire the pickups in series with a red wire from one pickup connected to the black wire of the other pickup.ġ. ![]() The rest of the items are relatively cheap except for the trumpet. My total cost for this was $3.99 for the phone jacks, and $1.99 for each piezo transducer. A 1/8th” jack could be used to connect to a microphone input on a computer for recording.įor the mute, I used a Stone Lined straight mute because it is inexpensive and has good mounting locations for the pickups. This is set up to be plugged into an electric guitar or bass amp. The phone jack can be replaced with any type of connector you need. More detail on how these work can be found at wikipedia. In short, mechanical vibrations of piezoelectric crystals creates a voltage with respect to the vibrations. ![]() These will be used as pickups to convert the sound to electrical signals. The piezo transducers are often marked as buzzers or elements. 2 – Piezo Transducers - Radio Shack 273-0073ġ – 1/4” Phone Jack - Radio Shack 274-0340 ![]()
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