For all the public perception that today's musicians embrace or pioneer the latest in tastes and fashions, the electric guitarists among them are surprisingly conservative and overwhelmingly traditional when it comes to their musical instruments. To the unaware, this would seem particularly odd because electric guitarists are commonly viewed as the most radical breed of rebel in a modern band of stylistic outlaws. But where else in the 1990's could be found such traditionalist xenophobes that the two perennial best selling guitars both originated in the 1950's and the only type of electronic amplification technology that rates serious consideration is based on the vacuum tube!
Nowhere else does the tube vs. transistor rivalry still so overwhelmingly come down in favor of the antique, and for several compelling reasons. First, the traditionalist argument again: The original sounds of electric guitar were created through vacuum tube amplifiers and hence, in the view of many, "the sound was thus defined". And as guitar sounds have evolved since, an ever more important element has become the intentional use of massive saturation distortion of overdriven vacuum tubes. While many designers have sought solid state parity to the performance of vacuum tubes in guitar amplifiers, even the best of them have enjoyed only marginal success.
Modern transistor and digital electronics have, of course, totally displaced the vacuum tube in recording studios and in keyboard instruments and several of the modern wonders of this technology are potentially appealing to the guitarist. What the guitar player would most like is an amplifier with self-explanatory controls, capable of producing the entire palette of traditional and modern amplified guitar sounds with perfect authenticity, whose settings could also be easily stored in a digital memory and recalled instantly via a foot-operated controller. Indeed, a few programmable preamplifiers for guitar have been introduced and have enjoyed some success commercially.
But the central element distinguishing a musical instrument from a piece of commercial electronic gear is its longevity. And just as keyboard synthesizers and signal effect processors have notoriously short product lifespans, such has also been the fate of the first programmable preamplifiers for guitar. Some of these even managed to accommodate a tube or two in their signal chain but to no lasting avail. Eventually the newness of programmability wears thin and the amplifier is heard for its essential sound and found lacking.
In the prior art, no one has been able to successfully replicate digitally the traditional analog tone control/volume control network whose particular interplay is absolutely crucial to delivering the essential characteristics of sound quality and controllability that guitarists demand.
As a point of history, the now traditional tone control circuit (illustrated schematically in FIG. 1) was first introduced in the mid 1950's by the Fender Musical Instrument Company of Fullerton, Calif. and is found with only minor alterations in all of the most successful amplifiers since. It is now safe to say that this is the tone/volume control circuit whose particular sound and functionality--though technically flawed by interaction between its various controls--is nonetheless musically "perfect" because it became the standard by which other devices were judged, in part perhaps because of the complex interaction of these components. Any design which deviates from this standard has so far found its lifespan short, particularly among discriminating, trend setting players. Many guitarists feel like they've "come home" when they return to this traditional set up after a period on some other type of tone control.
Yet however simple this circuit is to achieve with analog potentiometers, its replication into a format allowing simple digital control has, in the prior art, largely defied the best efforts of previous design talents.
The prior art has attempted to approximate the tone/volume network with fixed high- and low-pass filters (for treble and bass) working in conjunction with voltage control amplifiers (VCA's), which readily lend themselves to digital control. Unfortunately this approach has intrinsic limitations: VCA's typically operate on +15 and -15 volt supplies and thus cannot handle the high signal voltage produced at the output of a vacuum tube voltage amplifier. So in some prior art the VCA's are located prior to the tube amplifying stages--resulting in unacceptably high noise levels, particularly when massive "Lead Mode" saturation gain is introduced later in the circuit. The alternative solution is to insert an attenuation pad between the tube voltage amplifiers and the VCA's but this approach introduces its own new set of audio characteristics which further confounds the central problem of the entire VCA approach. That central problem is that it fails to faithfully execute the functions of the traditional analog system, in which complex interplay among the elements is a vital ingredient of the overall performance.
Another example of prior art grappling with this problem and at least recognizing the absolute importance of including the traditional tone/volume network has resulted in a handful of units being built with digitally controlled stepper motors twisting actual potentiometers! This design at least has the virtues of faithful sonic performance and extreme ease of operation. Unfortunately, the price and complexity make it more a curiosity than a practical unit. Additionally, even though the traditional circuit is there, programmability is limited to various settings of the same controls; other control values and parameters could not easily be substituted.
There has therefore been a need for a programmable, digitally controlled volume control for vacuum tube amplifiers. The present invention substantially overcomes the limitations of the prior art in that it provides a readily adjustable, simple to use, programmable digital volume control capable of working with the high voltages generated by vacuum tube amplifiers, and operable through a foot controller.
In a key aspect of the invention, all of the circuit elements of the tone control of the current invention remain faithful to the traditional set up with the exception of the variable resistance devices which replace the original analog potentiometers.
Because neither FET nor bipolar transistors are suitable for the very high signal voltages present in the control circuitry (especially with massive saturation distortion), another control device was necessary for replacement of the standard potentiometer.
Light Dependent Resistors (LDR's), which are well known and commonly used in guitar amplifiers to provide low-noise analog signal switching, offered some attractive features. The LDR consists of an LED and a photo-sensitive cadmium-sulfite resistance element facing each other on a light tight package. The "off resistance" of the photo cell is very high, typically 200 megohms or so, while the "on resistance" (depending on the type of device) can be as low as 100 ohms. The virtue of this device as an analog switch in high gain audio amplifiers is that its rapid swing in resistance (from off to on, or back again) precludes the transient "pop" endemic to instantaneous relay switching.
However, to replace the standard potentiometer requires that the device be capable of being used as a stable, repeatable, variable resistor, and the conventional LDR, taken alone, is not because controlling the LDR in its "resistance region" between the conditions of "off" and substantially "on" is very difficult for several reasons. First, cell resistance as a function of LED current (brightness) is extremely nonlinear, falling rapidly from "off" to around 40k ohms just as soon as the LED begins to illuminate and draw current. Second, cell resistance is wildly unstable, differing grossly from one device to another, rising dramatically as temperature is increased, and even changing over time (all other factors remaining stable) as the cell material becomes "light adapted". Thus, while the LDR provides the enticing characteristics of high signal voltage capability plus a potential for resistance variability to one megohm or higher, it presented very difficult problems if it is to be used as stable, repeatable, variable resistor--particularly in the high resistance region required, namely 250k ohms to 1 megohm.
As a result, conventional prior art circuits for use of LDR's were deemed unacceptable for the present application.
In order to achieve high resistance values in the system, the present invention provides an LDR device together with a negative feedback loop around a differential amplifier wherein the feedback loop incorporates a capacitor, such that the resulting RC time constant has the effect of lowering the gain in the time domain such that the action of the differential amplifier driving the LED portion of an LDR compensates for, and synchronizes with, the time lag inherent in the photo sensitive cell's delayed response to changing light conditions. This has the desired effect of creating a substantially steady resistance over a wide range of high resistance values whereas in the prior art the result would be a continually varying cell resistance as the LED portion of the LDR illuminates too brightly at first (overcompensating for the very high "off" resistance of the cell) and then stays illuminated for too long due to the time lag of the cell's delayed response to light, then turns off as resistance swings too low, causing reverse drive from the differential amplifier.
A resistor ladder, comprising a plurality of discrete resistors, provides a reference wherein each of such resistors represents a desirable discrete incremental setting of the equivalent analog control in the traditional circuit. In this manner repeatability and programmability is provided while retaining all of the fundamental circuitry of the traditional tone control circuit.
A general object of the present invention is to provide a digitally operated set of programmable controls which accurately duplicates the traditional analog set of volume and tone controls for a vacuum tube guitar preamplifier.
A specific object of the present invention is to utilize Light Dependent Resistors (LDR's) as the functional variable resistance elements in substitution for the analog potentiometers used in the traditional volume/tone control circuitry.
A further specific object of the present invention is to insure stability and accurate repeatability of setting for the LDR devices under varying conditions of age and temperature as well as from one production batch of devices to another.
Another specific object of the present invention is to achieve high value resistance from an LDR's photo cell by causing the LED in the LDR to blink on and off rapidly as controlled by a servo loop fed by a differential amplifier and referenced to any one of a plurality of fixed resistors, each one of which represents a desirable discrete incremental setting of the equivalent analog control in the traditional circuit.
Yet another specific object of the present invention is that the reference resistors controlling the servo loop/differential pair as described above, will each have a value equal to (or proportional to) the value which the LDR reference cell will become, as controlled by the active cell segment which therefore replicates a similar specific discrete setting of the analog potentiometer which it represents and is therefore equivalent to.
A still further specific object of the present invention is to include in the differential amplifier described above, a negative feedback path such that overall gain in the servo loop/differential pair is reduced to a point in accordance with the lag time of the photo cell's response to--and recovery from, the blinking LED such that stable high resistance results and the "noise" of the rapidly changing cell resistance is thereby reduced or eliminated as stable resistance is achieved.
Yet another object of the present invention is to couple together two such circuits as described above so that the pair of differential amplifiers (each one driving the LED of that pair's LDR) both derive their reference from any single point along a series of fixed reference resistors, where each point represents a desirable discrete incremental setting of the equivalent traditional circuit--and further, that the pair of LDR active cell segments are connected in series to each other in an arrangement analogous to a potentiometer type variable attenuator i.e. a constant overall fixed resistance such that any pair of series and shunt resistance values taken together always adds up to the total fixed resistance value.
These and other objects, advantages and features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of two preferred embodiments, presented in conjunction with the accompanying drawings: