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Diy piezo guitar body12/1/2023 ![]() ![]() The feedback network of the second op-amp, together with C2 forms a low-pass filter against radio frequencies and other high frequency noise. I use the higher cutoff settings when using the preamp for my cymbal reverb. This gives a variable cutoff between 47hz and 4.8khz, which is a pretty good and usable range for removing bass and even mids from the pickup. The formula for the filter frequency is f = 1 / (2π * C * R)Ĭ being C4 and R being R4+RV2 in this case. Next, signal goes through high-pass filter C4, R4, RV2 and the second op-amp, which acts as a buffer with a little additional gain (same formula, a fixed factor of around 10dB). Calculating the gain works like this: G = 1 + (RV1/R2) The first op-amp provides the buffering and gain, through R2 and RV1, the gain being a factor of 1.0 (0 dB) when the pot is at zero resistance, to a factor of 101.0 (+40dB) when the pot is at 10k. Those can be pretty high and in case you drop it or hit it too hard, the diodes will lead the surplus current to + and – (called VCC and VEE in the schematic). The two diodes before the first op-amp will protect the chip from voltage spikes coming from the piezo element. If you are not familiar with star grounding, read about it here and here. Eventually, both ground and shield connect together, but in order to prevent ground loops, make sure you adhere to the star grounding scheme as much as you can. The microphone cable shield goes to pin 3. Connect the positive piezo lead to pin 1 and connect the ground lead to pin 2. In order to get a good noise performance, use shielded microphone cables. The signal enters at “from piezo” on the left. Let me quickly walk you through the schematic: That way, the hum and noise induced in the system can be greatly reduced. The shield is connected to ground along with the “black” or ground lead of the disc. Next to the circuit itself, it’s important to both keep the piezo leads short and to use shielded cable. Hitting or touching the cymbal at such a high gain setting will lead to brutal distortion, so mind your levels □ The high gain settings will pickup extremely small vibrations and are useful for picking up resonance that is caused through the air, like in the example of the cymbal being excited by another instrument. In fact it goes from a mere +10dB gain up to a huge +50dB gain. In addition to the Megalithia version, I added two potentiometers for variable gain. Note that if you have never used buffered piezos, you will be amazed at how much bass comes out of these things. ![]() The -3dB point, or cutoff, is around 47hz, which is basically “off” and can be adjusted up to around 4800hz. Since I will be using this mostly for experimental reverbs, I opted to add a hi-pass filter so I could filter out the bass as much as I wanted to, without having to use up filter channels. Like the Megalithia one, it has extra protection diodes that will keep the piezo from doing harm to the ICs in case you accidentally hit it or drop it. Thanks a lot □ It has two channels and runs off a single 9v battery. It’s based off the Megalithia op-amp piezo buffer and has design changes which the awesome folks at GroupDIY have patiently helped with. Most preamp recipies out there use FETs, but I opted for a little more complex, but in the end very rewarding op-amp ( operational amplifier) solution. In addition, you get all sorts of hum, buzz, noise and more dirt, all associated with low signal level.Īctually piezos can put out a lot of signal and hardly need any amplification, but they desperately need buffering and that can be done through various methods. If you don’t match this, you get a nasal, honky, quaky, thin sound, because the impedance mismatch will impose a hefty hi-pass filter on your signal. Perfect output impedance of devices hooked up to a line mixer is around 600Ω. Typical piezo impedance is 1MΩ, which is way too high. Since piezos have a very high impedance, they need to be buffered in order to bring them to an impedance compatible with line level. Unfortunately, just connecting them to your line mixer is going to give you exactly that. Piezos are little mics in disc form that are dirt cheap to buy (about 1€) and have a terrible reputation for sounding thin and noisy. I showed the DIY ilski PB-1 (“PiezoBanana-1”) preamp, which I will explain thoroughly in this post. In my post about cymbals reverb with the help of piezo mics, I explained the technique of using a piezo transducer (also called contact mic) in order to get beautiful, musical stereo reverbs from drum cymbals. ![]()
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