Verified Split-Supply Preamps for Guitar
Here are six solid-state guitar preamp designs using op-amps, each drawn in the CircuitLAB Editor and verified running that site’s Frequency Domain Simulation. No help is needed to make them viable, though comments and suggestions for changes are welcome. The diagrams represent various preamps with features I would like to see in a guitar preamp. I start off with less complex designs but go on to more complicated ones, so the reader or prospective builder can decide which ones they like best.
While the diagrams have been verified by a simulator, they should still be viewed as experimental, since I have not built them so as to test them. If someone wishes to build one, I suggest breadboard testing before committing to a soldered build.
If interested in investigating CircuitLAB, here’s the link. https://www.circuitlab.com They only allow one free-trial project, but I was so pleased with it that I am making affordable monthly payments to use it. What follows are examples of my most recent efforts. All circuits below are of my own design with exceptions made for some subcircuits, such as the Fender-style 3-band tone-stack; my favorite tone-control circuit. The specified IC is the TL082 dual op-amp, but any other pin-compatible dual op-amp IC will work just as well. Examples: For the cleanest possible sound at any price, use the OPA2134, though the batteries will be drained more quickly. But for deliberately overdriving the whole preamp, to get the JFET distortion, use the LF353, which is cheaper, and drains the batteries less quickly.
The Compensation controls above are my creation, based on tone-control methods used by various designers. You often see a bright switch enabling / disabling a low-valued capacitor across a volume control to mitigate loss of treble as the control is turned down. But after analyzing the frequency responses from different values of such a capacitor, and adjustments that could be made by varying the accompanying resistor, I came up with the Comp circuitry as shown. The Tone control is based on the kind that is found on-board passive guitars, with the exception that the user can select among three different frequency ranges for this control. Should readers wish to keep these subcircuits optional, the on / off switches can be retained to allow the user to enable / disable them. Or omit them from the project altogether. Yet, I included them because I would like to have such extra control over an input signal’s sound since I would expect to be using effects between the guitar and the preamp and might want to change the tonality of the output of an effect or effect-chain coming into the preamp.
Above is a preamp with diode-based fuzz. The diode subcircuit is based on the method used in an old electronics magazine article “Build The Mod Box” by Thomas Henry & Jack Orman, in Electronics Now, May 1997. And though extremely dated, that circuit has been made available online. https://www.muzique.com/news/mod-box-distortion/
Here is much the same preamp but with a more elaborate fuzz circuit. Here, the user will have the choice of either a mellow or bright fuzz tone, depending on the type of diode array chosen. Notice that I send the fuzzed signal through the tone-stack so that it can be shaped with the 3-band tone circuit. Be aware that the diodes will reduce the signal level, so that fact must be taken into account when deciding on how to use the preamp. Switching in the Fuzz will reduce output volume, not boost it. If that is desired, I suggest using two of these circuits, one with and one without Fuzz turn on, and switch between them using my original 4-Channel Effects Loop project in conjunction with my 4-Channel Quiet Switch project (both posts coming soon), while using the other two channels for any other preamps or effects as desired.
What follows is another version of this preamp with an even more elaborate diode array which I call a Diode Matrix. It allows the user to choose between mellow, hard, or bright fuzz tones. Here again, however, the user must decide on how the preamp is used. My 4-Channel Effects Loop and 4-Channel Quiet Switch projects will allow for independent selection of this and the 3 previous preamps. Or else use my coming 1 of 4 Exclusive Selector project to switch between four channels while performing.
For a clean boost, instead of fuzz, use the circuit below. This kind of project would be useful for those who like the idea of overdriving a tube-amp while also having a fairly clean preamp otherwise, or for musicians who simply want a clean sound.
Here too, for the cleanest possible sound, regardless of cost and battery drain, replace the TL084H quad IC with two OPA2134 dual op-amp ICs.
Above, I borrowed a technique from Craig Anderton with respect to the Bass Cut circuit, which will affect both the straight and boosted signals, if used.
Finally, I show a preamp with diode-based fuzz but which puts the diode array in the feedback loop of an op-amp rather than shunting the signal path. This helps keep the diodes from reducing signal level. Note that the array is simple but could be easily replaced by any of the foregoing arrays for even more variability.
Comments with respect to this post are welcome, and private communications with me can be conducted via email. hkurtrichter@gmail.com
EOF
Here are six solid-state guitar preamp designs using op-amps, each drawn in the CircuitLAB Editor and verified running that site’s Frequency Domain Simulation. No help is needed to make them viable, though comments and suggestions for changes are welcome. The diagrams represent various preamps with features I would like to see in a guitar preamp. I start off with less complex designs but go on to more complicated ones, so the reader or prospective builder can decide which ones they like best.
While the diagrams have been verified by a simulator, they should still be viewed as experimental, since I have not built them so as to test them. If someone wishes to build one, I suggest breadboard testing before committing to a soldered build.
If interested in investigating CircuitLAB, here’s the link. https://www.circuitlab.com They only allow one free-trial project, but I was so pleased with it that I am making affordable monthly payments to use it. What follows are examples of my most recent efforts. All circuits below are of my own design with exceptions made for some subcircuits, such as the Fender-style 3-band tone-stack; my favorite tone-control circuit. The specified IC is the TL082 dual op-amp, but any other pin-compatible dual op-amp IC will work just as well. Examples: For the cleanest possible sound at any price, use the OPA2134, though the batteries will be drained more quickly. But for deliberately overdriving the whole preamp, to get the JFET distortion, use the LF353, which is cheaper, and drains the batteries less quickly.
The Compensation controls above are my creation, based on tone-control methods used by various designers. You often see a bright switch enabling / disabling a low-valued capacitor across a volume control to mitigate loss of treble as the control is turned down. But after analyzing the frequency responses from different values of such a capacitor, and adjustments that could be made by varying the accompanying resistor, I came up with the Comp circuitry as shown. The Tone control is based on the kind that is found on-board passive guitars, with the exception that the user can select among three different frequency ranges for this control. Should readers wish to keep these subcircuits optional, the on / off switches can be retained to allow the user to enable / disable them. Or omit them from the project altogether. Yet, I included them because I would like to have such extra control over an input signal’s sound since I would expect to be using effects between the guitar and the preamp and might want to change the tonality of the output of an effect or effect-chain coming into the preamp.
Above is a preamp with diode-based fuzz. The diode subcircuit is based on the method used in an old electronics magazine article “Build The Mod Box” by Thomas Henry & Jack Orman, in Electronics Now, May 1997. And though extremely dated, that circuit has been made available online. https://www.muzique.com/news/mod-box-distortion/
Here is much the same preamp but with a more elaborate fuzz circuit. Here, the user will have the choice of either a mellow or bright fuzz tone, depending on the type of diode array chosen. Notice that I send the fuzzed signal through the tone-stack so that it can be shaped with the 3-band tone circuit. Be aware that the diodes will reduce the signal level, so that fact must be taken into account when deciding on how to use the preamp. Switching in the Fuzz will reduce output volume, not boost it. If that is desired, I suggest using two of these circuits, one with and one without Fuzz turn on, and switch between them using my original 4-Channel Effects Loop project in conjunction with my 4-Channel Quiet Switch project (both posts coming soon), while using the other two channels for any other preamps or effects as desired.
What follows is another version of this preamp with an even more elaborate diode array which I call a Diode Matrix. It allows the user to choose between mellow, hard, or bright fuzz tones. Here again, however, the user must decide on how the preamp is used. My 4-Channel Effects Loop and 4-Channel Quiet Switch projects will allow for independent selection of this and the 3 previous preamps. Or else use my coming 1 of 4 Exclusive Selector project to switch between four channels while performing.
For a clean boost, instead of fuzz, use the circuit below. This kind of project would be useful for those who like the idea of overdriving a tube-amp while also having a fairly clean preamp otherwise, or for musicians who simply want a clean sound.
Here too, for the cleanest possible sound, regardless of cost and battery drain, replace the TL084H quad IC with two OPA2134 dual op-amp ICs.
Above, I borrowed a technique from Craig Anderton with respect to the Bass Cut circuit, which will affect both the straight and boosted signals, if used.
Finally, I show a preamp with diode-based fuzz but which puts the diode array in the feedback loop of an op-amp rather than shunting the signal path. This helps keep the diodes from reducing signal level. Note that the array is simple but could be easily replaced by any of the foregoing arrays for even more variability.
Comments with respect to this post are welcome, and private communications with me can be conducted via email. hkurtrichter@gmail.com
EOF
The batteries for the Split-Supply Preamps above can be replaced by a standard dual-polarity +-9V DC power supply. Here's an example.
This type of supply could power up to four of the preamps (if not more), making them perfect for rack-mount applications.
This type of supply could power up to four of the preamps (if not more), making them perfect for rack-mount applications.