This invention concerns stringed instruments with built-in preamplifiers. The instrument-preamplifier combination is designed for amplification through a hi-fidelity stereo system, and more particularly, through an automobile stereo system.
Stringed instruments, including guitars, incorporating preamplifiers are known. Guitar players frequently seek to duplicate the sound of vacuum tube based amplifiers, which sound is recognized as important to the classic rock guitar sound. Preamplifiers that substantially reproduce the classic vacuum tube sound are known. What is needed is a preamplifier that substantially reproduces the classic vacuum tube sound where that preamplifier is incorporated into the stringed musical instrument.
Signals from stringed musical instruments frequently are amplified by amplifiers. Known amplifiers for stringed musical instruments generally are bulky, lack easy mobility and are powered by alternating current. What is needed is a stringed musical instrument incorporating a preamplifier that allows the user to substantially reproduce the classic vacuum tube amplifier sound, by incorporating a preamplifier in the instrument, where the preamplifier also is engineered to be amplified by a hi-fidelity stereo system, such as a direct current powered automobile stereo system.
Additionally, known preamplifier (“preamp”) designs are not as efficient and economical as they could be, and generally require vacuum tubes to generate the classic vacuum tube amplifier sound. Known vacuum tube based musical instrument amplifiers consists of one or more preamp stages, followed by a power output stage. The preamp stages are typically single-ended gain stages, while the power stage is commonly a class AB push-pull amplifier. Each type of stage contributes a distinct non-linear distortion characteristic to the amplifier, which when blended in suitable proportions can be used to add to the harmonic complexity of the amplified sound in a musically pleasing way. Harmonic accentuating characteristics typically are further enhanced by frequency response shaping elements, both incidental (such as inter-stage coupling circuits and parasitic elements), and intentional (such as variable tone controls, and fixed filter circuits).
The vacuum tube amplifier frequency response characteristics usually can be approximated accurately, but the non-linear characteristics are more difficult to reproduce with reasonable accuracy. A common technique for approximating the non-linear characteristics of a push-pull tube power output stage is shown in
According to theory, a properly balanced push-pull output stage produces mainly odd-order harmonic distortion components due to its symmetry, whereas a single-ended amplifier stage, such as used in the preamp stages, will produce both odd and even order harmonic distortion due to its inherent asymmetry.
To attempt to approximate both the odd and even harmonics produced by a typical tube amplifier, the approach depicted in
Whether symmetrical or asymmetrical in nature, simple diode clipping occurs rather abruptly as the signal voltage reaches the conduction threshold of the diodes in the clipping network. This is the opposite of the soft-compression characteristic of a single-ended vacuum tube gain stage; hence the overall approximation by simple diode clipping is poor for all but the heavily distorted case.
What is needed is a preamp that substantially reproduces the vacuum tube amplifier sound, using solid state electronics, which preamp is incorporated into a stringed musical instrument, and where the preamp is designed to operate through a direct current hi-fidelity stereo system, such as an automobile stereo system.
This invention is an instrument-preamp combination designed for amplification through a hi-fidelity stereo system, and more particularly, through an automobile stereo system, where the preamp simulates the sound of analog tube amplifiers. The instrument-preamp combination (“Guitar”) comprises a known electric guitar and an analog vintage valve (vacuum tube) simulator (pedal) preamp built into the electric guitar. The preamp is designed with output parameters compatible with an automobile hi-fidelity stereo system that includes an amplifier and at least one speaker. A cable connects the preamp output to a hi-fidelity stereo system.
A novel preamp design improves approximation of amplification characteristics, both small and large signal, of typical musical instrument amplifiers based on vacuum tube technology, using only solid-state active devices in the preamp to achieve the desired sound.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims ad accompanying drawings, in which:
In vacuum tube amplifiers, the soft-compression characteristic of a single-ended tube gain stage is caused mainly by transconductance modulation effects, whereby the amplified signal modulates the gain of the stage dynamically through modulation of the plate current.
As shown in
The capacitor C113 is connected to junction 17, which is connected to the diode D114 and the current source I1. The diode D114, and the current source I1 are also connected to junction 18, which is grounded. The current source typically will be in the order of 10-20 microamps to achieve a typical diode slope resistance of a few kilohms.
In the preamp stage shown in
By suitable choice of circuit parameters and excitation levels, it is possible to achieve harmonic distortion characteristics (albeit scaled by absolute signal voltage level) very similar to those produced by the single-ended gain stage of
As shown in
The preamp output parameters preferably are tailored for 1 Volt (peak) and less than 5,000 ohms output impedance for compatibility with known automobile stereo systems, though the parameters may be varied for compatibility with different automobile stereo systems. The preamp output also can be modified for compatibility with numerous instrument amplifiers if the user prefers to amplify and play the preamp signal through systems other than an automobile stereo system. The preamp also is equipped with a switch which disconnects the preamplifier from the guitar pickup output so the guitar can be played through a valve or other amplifier customarily used to amplify known guitars.
Note that since the signal current flowing in the R1 branch 52 (clipping-diode branch) is determined by the input voltage impressed across the impedance of the resistor modified diode branch 51, the two branches, 51 & 52, can be thought of as representing two independent cascaded signal processing stages, while sharing a single operational amplifier as a common gain element. In this manner, the circuit provides an overall non-linear gain characteristic that is analogous to cascading a single-ended vacuum tube preamp with a push-pull vacuum tube output stage. The diode branch 50 (or the resistor for modified diode branch 51), and the R1 branch 52 (clipping diode branch) simulate two distinct valve amplifier stages. Without the R1 branch 52 (clipping diode branch), the preamp circuit simulates a single triode preamp stage. The two branches, the resistor modified diode branch 51 and the R1 branch 52 (clipping diode branch) are combined in a novel manner to achieve two cascaded processing stages using a single gain element, the first operational amplifier 11.
The preferred embodiment shown in
As discussed above, the first operational amplifier 11 serves as the single gain element for the two cascaded stages, the resistor modified diode branch 51 and the R1 branch 52 (clipping diode branch). The second operational amplifier 20 stage is a second order, low pass filter, having cutoff frequency fo and Q parameters chosen to simulate the sonic characteristics of a typical guitar speaker cabinet. This extends the simulator of the preamp to include the speaker of a valve amplifier system.