Experimental Hybrid Preamp Design
By Kurtus Richter
Here is a collection of diagrams for a very versatile hybrid guitar preamp, meaning a preamp involving both solid-state and tube-type circuits in one project. The reasons for hybridization are as follows. (1) With purely analog circuitry, tube-amps sound better than solid-state amps, and which also applies to preamps. (2) Hybrid preamps allow for the mixing of a nearly pure input signal from the solid-state circuitry with a distorted signal from a tube circuit in which the tubes are deliberately overdriven. (3) Tubes have a slower slew-rate than solid-state devices; by orders of magnitude. In fact, a guitarist who plays very fast scales can exceed the slew rate of the tubes in a tube-type amp, resulting in indistinct attacks of individually picked notes, heard as a muddy sound, but solid-state devises have slew rates that cannot be exceeded. Consequently, for analog circuits (i.e., no digital processing involving modern sampling technology), one remedy is to use two kinds of amps, or two preamps; one solid-state for attack definition, and one tube-type for the tone. Hence my design for a hybrid preamp.
The block diagram above illustrates how different circuits in my Hybrid Preamp can be connected together, though other arrangements are possible. I have deliberately placed input and output jacks on each of the solid-state subcircuits, and recommend they be retained, in order to maximize versatility. This gives the user the ability to connect the different circuits as desired and insert external devices, such as stomp-boxes, between any two such circuits, though the tube circuit is self-contained, since it is a distortion circuit inspired by the Garnet Herzog unit made famous in the ‘60s and ‘70s by guitarist Randy Bachman, who used it on hits American Woman and Takin’ Care of Business.
But first I show the input preamp, which has highly user-adjustable parameters, and can also be considered a distribution amp, where one output can be a direct signal sent to the Mixer, another can go to the solid-state 5-Band Tone Control or the Booster, or both in series, and the third to the Tubes.
The table in the diagram lists the low corner frequency that can be set by switches S1 and S2, according to the user’s personal taste. For instance, for deep lows, use the 20 Hz setting, but for no lows at all use the 240 Hz or even the 400 Hz setting. The gain of the preamp can also be adjusted from 2 to 200, depending on the setting of switch S3 and the position of the Gain control. High gain, in conjunction with a sufficient setting of the input Level control allows for enough signal voltage to be sent to all three preamp outputs so there should be no trouble adequately driving subsequent circuits. Below is a 5-band active tone-control circuit, which is of my own design.
Ordinarily, 3-band tone controls work fine for the electric guitar. Yet, I designed the above 5-band control for users who want greater adjustment of the sound of the solid-state circuitry. There is also volume control on the output of the Tone Control circuit, and a Booster circuit has been provided, with its own simply tone-control, to help the signal from the 5-Band Tone Control compete with the signal from the Tube Circuit.
The 5-Band Tone Control’s frequency bands divide the audio spectrum by 5, labeled here as Bass, Lo Mid, Mid, HI Mid, and Treble. The approximate frequencies are:
Bass < 250 Hz, Lo-Mid 250 – 500 Hz, Mid 500 Hz – 2.5 kHz, Hi-Mid 2.5 kHz – 10 kHz, and Treble > 10 kHz.
But, of course, the user must set the positions of pots by ear to compensate for personal taste and how the human ear hears different frequencies.
Next up is the Booster circuit.
The booster has four controls; Input Level, Tone, Gain, and Volume. The Input Level control is needed to keep the Booster from being overdriven if it is to be used cleanly, but will also allow it to be overdriven if desired. The Tone control here only affects the highs, the Gain control adjusts the amount of boost, and the Volume control sets the circuit’s output voltage. And here are some active filters; one low-pass with a high cutoff, and one high-pass with a low cutoff. In series they form a 2-stage audio band-pass filter, to mitigate unwanted subsonics and ultrasonics generated in the other circuits. They can be used separately or in series in any order.
And we have a 4-input mixer, to mix-down as many as four different signals. And while the project is designed to provide three channels, one direct, one amplified, and one for the tube circuits, there is a fourth input to the mixer in case it’s needed.
The added mixer input is provided in the event an external splitter is used for some reason between any two stages and the user has an extra signal to be mixed with all the others. Below I show the power-supply and the pinout diagrams for the ICs used in the solid-state circuits. This supply is only for the ICs. The tubes have their own supply.
Note: A builder of this project may wish to make two or three of these supplies, each providing voltage to only two or three ICs, to keep from overtaxing a lone supply.
As said, the Tube Circuit is my version of a Herzog distortion unit. It is in fact a single-ended low-wattage guitar-amp in which the output tube is used as a preamp tube, for the purpose of overdriving it to get its distortion sound. The tube-type sub-circuits are given as follows. They must be treated collectively as one channel.
For a power-amp build to go with this preamp, consider the following resources.
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
Audio Amplifier Boards & Modules Amp Kits | Mojotone Amp Maker: Guitar amp kits
Amazon.com: 1 pc LM3886 Amplifier Board Power Amplifier Audio Amplifier OPA445 high
Voltage Version : Electronics
As an example, however, I copied a power-amp design below, or the builder can keep the power-amp separate from the preamp, according to personal preference.
Should anyone build this project, know that I have not built it in order to test it, so it must be considered EXPERIMENTAL CIRCUITRY all the way. Also, the builder must assume all responsibility for the outcome. I cannot be responsible for someone else building this project.
Comments welcome here, and/or send me an email. hkurtrichter@gmail.com
By Kurtus Richter
Here is a collection of diagrams for a very versatile hybrid guitar preamp, meaning a preamp involving both solid-state and tube-type circuits in one project. The reasons for hybridization are as follows. (1) With purely analog circuitry, tube-amps sound better than solid-state amps, and which also applies to preamps. (2) Hybrid preamps allow for the mixing of a nearly pure input signal from the solid-state circuitry with a distorted signal from a tube circuit in which the tubes are deliberately overdriven. (3) Tubes have a slower slew-rate than solid-state devices; by orders of magnitude. In fact, a guitarist who plays very fast scales can exceed the slew rate of the tubes in a tube-type amp, resulting in indistinct attacks of individually picked notes, heard as a muddy sound, but solid-state devises have slew rates that cannot be exceeded. Consequently, for analog circuits (i.e., no digital processing involving modern sampling technology), one remedy is to use two kinds of amps, or two preamps; one solid-state for attack definition, and one tube-type for the tone. Hence my design for a hybrid preamp.
The block diagram above illustrates how different circuits in my Hybrid Preamp can be connected together, though other arrangements are possible. I have deliberately placed input and output jacks on each of the solid-state subcircuits, and recommend they be retained, in order to maximize versatility. This gives the user the ability to connect the different circuits as desired and insert external devices, such as stomp-boxes, between any two such circuits, though the tube circuit is self-contained, since it is a distortion circuit inspired by the Garnet Herzog unit made famous in the ‘60s and ‘70s by guitarist Randy Bachman, who used it on hits American Woman and Takin’ Care of Business.
But first I show the input preamp, which has highly user-adjustable parameters, and can also be considered a distribution amp, where one output can be a direct signal sent to the Mixer, another can go to the solid-state 5-Band Tone Control or the Booster, or both in series, and the third to the Tubes.
The table in the diagram lists the low corner frequency that can be set by switches S1 and S2, according to the user’s personal taste. For instance, for deep lows, use the 20 Hz setting, but for no lows at all use the 240 Hz or even the 400 Hz setting. The gain of the preamp can also be adjusted from 2 to 200, depending on the setting of switch S3 and the position of the Gain control. High gain, in conjunction with a sufficient setting of the input Level control allows for enough signal voltage to be sent to all three preamp outputs so there should be no trouble adequately driving subsequent circuits. Below is a 5-band active tone-control circuit, which is of my own design.
Ordinarily, 3-band tone controls work fine for the electric guitar. Yet, I designed the above 5-band control for users who want greater adjustment of the sound of the solid-state circuitry. There is also volume control on the output of the Tone Control circuit, and a Booster circuit has been provided, with its own simply tone-control, to help the signal from the 5-Band Tone Control compete with the signal from the Tube Circuit.
The 5-Band Tone Control’s frequency bands divide the audio spectrum by 5, labeled here as Bass, Lo Mid, Mid, HI Mid, and Treble. The approximate frequencies are:
Bass < 250 Hz, Lo-Mid 250 – 500 Hz, Mid 500 Hz – 2.5 kHz, Hi-Mid 2.5 kHz – 10 kHz, and Treble > 10 kHz.
But, of course, the user must set the positions of pots by ear to compensate for personal taste and how the human ear hears different frequencies.
Next up is the Booster circuit.
The booster has four controls; Input Level, Tone, Gain, and Volume. The Input Level control is needed to keep the Booster from being overdriven if it is to be used cleanly, but will also allow it to be overdriven if desired. The Tone control here only affects the highs, the Gain control adjusts the amount of boost, and the Volume control sets the circuit’s output voltage. And here are some active filters; one low-pass with a high cutoff, and one high-pass with a low cutoff. In series they form a 2-stage audio band-pass filter, to mitigate unwanted subsonics and ultrasonics generated in the other circuits. They can be used separately or in series in any order.
And we have a 4-input mixer, to mix-down as many as four different signals. And while the project is designed to provide three channels, one direct, one amplified, and one for the tube circuits, there is a fourth input to the mixer in case it’s needed.
The added mixer input is provided in the event an external splitter is used for some reason between any two stages and the user has an extra signal to be mixed with all the others. Below I show the power-supply and the pinout diagrams for the ICs used in the solid-state circuits. This supply is only for the ICs. The tubes have their own supply.
Note: A builder of this project may wish to make two or three of these supplies, each providing voltage to only two or three ICs, to keep from overtaxing a lone supply.
As said, the Tube Circuit is my version of a Herzog distortion unit. It is in fact a single-ended low-wattage guitar-amp in which the output tube is used as a preamp tube, for the purpose of overdriving it to get its distortion sound. The tube-type sub-circuits are given as follows. They must be treated collectively as one channel.
For a power-amp build to go with this preamp, consider the following resources.
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
Audio Amplifier Boards & Modules Amp Kits | Mojotone Amp Maker: Guitar amp kits
Amazon.com: 1 pc LM3886 Amplifier Board Power Amplifier Audio Amplifier OPA445 high
Voltage Version : Electronics
As an example, however, I copied a power-amp design below, or the builder can keep the power-amp separate from the preamp, according to personal preference.
Should anyone build this project, know that I have not built it in order to test it, so it must be considered EXPERIMENTAL CIRCUITRY all the way. Also, the builder must assume all responsibility for the outcome. I cannot be responsible for someone else building this project.
Comments welcome here, and/or send me an email. hkurtrichter@gmail.com