![]() 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. The Morris preamp idea has +20dB gain so that won't work with the above.Piezo transducers are common in a range of different areas. ![]() Long time ago but I think that was how I remember it all. You would need unity gain on the piezo output and even then the signal will likely be louder than the passive magnetic PU. in the case of a passive wired Strat circuit you would use a 250k pot and join the output of that to the output of the passive pot. The output is then mixed into the passive Magnetic circuit using a high value pot. If recall right, you might only need to use a unity gain preamp on the piezo. Yes you will need two separate preamps if you want to use them both at the same time.īe aware that the frequency response of these two types of PU's will be quite different and two independent sections with separate tone circuits might be worth exploring. No Magnetic and Piezo don't mix as the Z of each is VASTLY different.Ī Mag pu has a much much lower Z (impedance) and will just load down the piezo and it will hardly work. Try it if you have bad FB, you might be surprised. I'd guess guitar luthiers would reject the idea as folly. for those that may think it a rather non professional fix up, Violin luthiers are known to use a post to reduce resonance between the face and back, moving it around to find the sweet spot. As it's only wedged in place the mod is usually reversible. ![]() Regarding low freq FB I've found that a central post wedged between bridge and back can help in some instruments, A case of suck it and see if it helps. The early bug type transducers did tend to suffer bass loss without really hi Z, like 10meg. ![]() Past 2meg is just asking for low freq FB issues, especially on large body guitars. I've found that most of the bridge under saddle transducers don't need excessively high Z preamp inputs. You may need to experiment with a few circuits to find a happy solution. As can be seen here the LR Baggs output is almost half of the Morris and quite different curves. The output signal strength as well as the response cuves vary a lot. My Preamp circuit is a copy of an old Morris preamp with a couple of slight mods to fix the excess bandwidth which is a common issue for Piezo pickups.Īctive Piezo's are a brilliant idea but the outputs are often very different. This Screen shot is an A/B comparison of the 2 circuits output curves. I've just been running some tests of the LR Baggs circuit as well as something I built a few weeks back to fix a friends guitar which had a dead preamp module. ![]()
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