Abstract:
An amplifier for electric guitar including a plurality of foot switch selectable preamplifier channels which can be set to provide differing sonic characteristics, and also including a power amplifier section for driving a loudspeaker that can be switched between at least two different power or performance capabilities. User operable switching dedicated to at least one of the preamplifier channels is provided to select preferred power amplifier parameters in conjunction with differing channel voicings. Thus, differing preferred power amplifier characteristics may be pre-set and assigned to individual preamplifier channels and called up automatically in conjunction with the remote channel selection.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to audio amplifiers primarily for use with an electric guitar.  
       BACKGROUND AND DESCRIPTION OF THE PRIOR ART  
       [0002]     The electric guitar is an instrument with a very wide musical vocabulary. A large part of the electric guitar musical versatility derives from the varying sonic characteristics that may be provided by modern amplification. Unlike the piano, which also can play a wide variety of musical styles, yet always retains the same timbre for each style, the sound of the electric guitar emanates solely from the amplifier and its associated loudspeaker. Moreover, the guitar amplifier is no longer expected to operate exclusively in the linear domain, faithfully reproducing the original “acoustic” sound of the plucked strings.  
         [0003]     Instead, the role of the modern guitar amplifier has expanded to include saturation distortion, an expressive sonic enhancement that is so vital to certain popular musical styles that they could not otherwise be created. In fact, some musical styles have arisen after the fact, in response to, and as a result of a particular amplifier&#39;s characteristics of sound distortion.  
         [0004]     As examples, there is simply no way that searing, sustaining, single-note lead lines could have been played without a major dose of amplifier saturation. Over time, as amplifier over-drive characteristics became more extreme, the musical styles they spawned have become increasingly aggressive as illustrated by the entire genre of “heavy metal” and its various stylistic subsets, many of which are closely, if not nearly exclusively, the domain of certain models and settings of amplifiers and their distortion tonalities.  
         [0005]     The origin of the modern distortion-enhancement amplifier dates to the “dual mode” circuit described in U.S. Pat. No. 4,211,893 of Randall C. Smith which provides a “lead mode” as well as the traditional “rhythm mode”, each being pre-settable and foot-switch selectable. In this design, extra stages of pre-amplification are cascaded such that massive overdrive saturation creates distortion that can go far beyond merely mimicking the sound of an amplifier “turned up too loud”.  
         [0006]     However, the sound of a good vacuum tube amplifier “turned up too loud” must be acknowledged as the historical source of musical distortion for electric guitar. First used by blues players (who set the stage for the rock and rollers that followed), power amplifier distortion became a powerfully expressive tool.  
         [0007]     The fundamental limitation of power amplifier distortion is its inseparable relation to amplifier loudness. Indeed, the distortion occurs only when, and as a result of, the amplifier being “turned up too loud”. One of the benefits of the amplifier of the above noted patent was its ability to separate playing loudness from distortion characteristics and this was accomplished by providing for massive, controllable saturation generated within the preamplifier, then providing user attenuation of the signal amplitude before coupling it to the power section. Though this approach can mimic power amplifier distortion (and more), it is not truly the same thing and differs in many respects.  
         [0008]     First, unlike preamplifiers, power amplifiers are nearly all audio amplifiers are push-pull amplifiers. Second, power amplifiers almost all utilize pentode or beam power tubes, compared to the small triodes of the preamplifier. Third, power amplifiers couple through an output transformer to the loudspeaker. Fourth, there will always remain an abiding opinion that power amplifier distortion is more “legitimate” than preamplifier distortion, due to its historical roots.  
         [0009]     Thus, despite the versatility provided by preamplifier distortion, there remains a strong need for providing authentic power amplifier distortion while remedying the age-old limitations presented by its link to playing loudness.  
       SUMMARY OF THE INVENTION  
       [0010]     The amplifier of the present invention provides at least two user selectable output power levels in conjunction with a plurality of preamplifier channels and further provides switching such that the preferred output capability which best complements each channel&#39;s preamplifier setting can be assigned and automatically synchronized to the channels as they are selected during a performance.  
         [0011]     In a typical embodiment of the present invention, the output amplifier would include four power vacuum tubes arranged in a push-pull parallel configuration. In its full power setting, all four vacuum tubes would be operable for maximum undistorted headroom. This is the power setting that would best complement a preamplifier channel set for a pure, clean style of playing.  
         [0012]     A second preamplifier channel may be set with more gain and then be coupled to the power section alternately configured to operate this time on only a single pair of push-pull output tubes capable of delivering only about half the power of the previous setting. This enables the musician to generate desirable power amplifier distortion at a playing loudness that, while reduced from the maximum possible, actually better matches it in perceived loudness. This is because the amplifier stages, as they are driven increasingly hard into clip, progressively compress the dynamic peaks of the musical material. Thus, a less powerful output section that is driven harder can better match the loudness of a higher power section which is not overdriven and which is fed a more dynamic signal, such as is the case with clean picking or chording.  
         [0013]     Amplifiers with more than two preamplifier channels (or modes) and/or more than two power output capabilities may, of course, utilize the channel-dedicated power amplifier switching of the present invention in a more expanded fashion.  
     
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an illustration of the control panel of a two-channel guitar amplifier showing the control features for a typical (and minimal) embodiment of the present invention;  
         [0015]      FIG. 2  is a simplified schematic diagram illustrating the control circuitry for the amplifier of  FIG. 1 ;  
         [0016]      FIG. 3  is a diagram of a circuit that can be used as the switch  15  of  FIG. 2 ; and  
         [0017]      FIG. 4  is a block diagram of a guitar amplifier circuit in accordance with the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring to  FIG. 1 , at the left side of the panel are two jacks, one for input of the electric guitar signal  1  and another for the foot switch that remotely controls channel selection  2 . To the right of the input jack  1  is a small LED  3  to indicate when the upper channel is active. Arranged in a vertical column are three small toggle switches, the upper switch  4  manually selecting between the channels, the middle switch  5  altering the performance mode of Channel  2  and the lower switch  6  altering the tone control frequencies for Channel  2 .  
         [0019]     The two preamplifier channels of the amplifier of  FIG. 1  have their respective controls arranged horizontally with Channel  1  above Channel  2  as shown by the treble, mid, bass and presence knobs. As the panel drawing shows, the controls are for the most part duplicated between the channels to simplify learning and operating the amplifier. There is one significant difference between the channels, however, and that is the inclusion of the DRIVE control  7  which is optionally functional for Channel  2 . When DRIVE is selected (via the toggle switch  5 ), a high gain saturation circuit is coupled in line with preamplifier Channel  2  in order to provide distortion enhancement that is then controllable by the combination of DRIVE control  7  and GAIN control  8 ,  8 ′ settings. Signal output amplitude for the channels is adjustable by the respective MASTER controls  9 ,  9 ′.  
         [0020]     Next to each channel MASTER control is located a toggle switch  40 ,  40 ′ which provides the musician with the choice of two different wattage capabilities for the power amplifier, such as, for example, 100 watts or 50 watts. These are the Channel dedicated power switches of the present disclosure.  
         [0021]     It can be seen that the control parameters of both of the preamplifier channels may be set independently whether or not DRIVE is selected. When the DRIVE mode option is de-selected, Channels  1  and  2  then have similar gain structures and responses. This provides for two widely divergent settings of the two similar channels that may be further enhanced by the ability to select differing power amplifier capabilities for each channel. For the more traditional player, Channel  1  would be configured for an entirely clean, distortion-free preamplifier coupled to the higher wattage (e.g. 100 watt) maximum headroom power setting.  
         [0022]     Channel  2  could then be set for more preamplifier gain (with or without engaging the DRIVE feature) and coupled to the power amplifier with its output capability reduced to the lower wattage (e.g. 50 watts). This provides the musician with both the capabilities of a higher wattage (e.g. 100 watts) clean amplifier and a lower wattage (e.g. 50 watts) blues amplifier, both of which are highly desirable, well balanced to one another and alternately selectable during a performance within a single amplifier package. Other controls and switches on the front panel are typical and do not directly involve the invention of the present disclosure.  
         [0023]      FIG. 2  is a schematic diagram of a simplified two-channel switching circuit including the channel dedicated power switching of the present invention. Two pairs of power pentodes, P 1 , P 2  and P 3 , P 4  are coupled to the primary winding  10  of an output transformer whose center tap  52  is fed from a high voltage power supply (not shown). The cathodes  11 ,  12  of the first pair of power tubes P 1 , P 2  are coupled to ground  1 . The cathodes  13 ,  14  of the second pair of power tubes P 3 , P 4  are tied together and coupled to a switch  15  such that they may be isolated from ground  1  (as shown) or connected to ground  1  when the switch  15  is closed, as indicated by the dashed line. With switch  15  in the open position as shown, the power section operates on a single pair of power tubes P 1 , P 2  for half power. With the switch  15  closed, full power capability is alternately provided as the second pair of power tubes P 3 , P 4  have their respective cathodes  13 ,  14  coupled to ground  1 .  
         [0024]     As shown in  FIG. 3 , switch  15  of  FIG. 2  may be a coil-operated relay circuit whose action can then be dedicated to either or both of the two preamplifier channels by the adjacent control circuitry. The circuit includes NPN bipolar transistors Q 1  and Q 2  with their emitters coupled to ground  1 . A pull-up resistor  23  couples positive voltage to base  24  of transistor Q 1 . With its base  24  biased high, transistor Q 1  conducts current through the relay coils  35 ,  35 ′ that activate the circuitry of Channel  1 , to the V+supply. Because the collector  25  of transistor Q 1  is coupled through isolation resistor  26  to the base  27  of transistor Q 2 , the collector  28  of transistor Q 2  is simultaneously held in a high state and does not conduct current through the relay coils  36 ,  36 ′ of Channel  2 .  
         [0025]     When either the foot switch  37  or the panel switch  38  is operated to apply ground to the base  24  of transistor Q 1 , the base  24  of transistor Q 1  goes low, opening the collector  25 -emitter  21  path and de-energizing the coils of Channel  1  Simultaneously, the base  27  of transistor Q 2  is biased high through the resistor  26  and relay coils  35 ,  35 ′ of channel  1 , thus turning on transistor Q 2  such that current then flows across the emitter  22 -collector  28  junction, activating the relay coils  36 ,  36 ′ for Channel  2 .  
         [0026]     The full-power/half-power relay coil  29  may be coupled to the channel relay busses  31 ,  32  through blocking diodes  33 ,  34  and the amplifier panel switches  4  and  6  for the respective channels. The blocking diodes maintain isolation between the channel busses  31 ,  32  while providing dedicated coupling of the power selecting relay  29  to operate in conjunction with the selection of neither, either or both channels. As shown in both  FIG. 1  and  FIG. 2 , the amplifier embodiment of the present invention can provide high power (e.g. 100 watts power) when Channel  1  is selected and can provide low power (e.g. 50 watts) when Channel  2  is selected.  
         [0027]     Referring now to  FIG. 4 , there is shown a block diagram of a guitar amplifier in accordance with the present invention. There is shown the input  1  from the guitar, also shown in  FIG. 1 , which is fed to channels  1  and  2 . When the channel  1  relays  35 ,  35 ′ of  FIG. 3  allow the signal to travel to the channel  1  circuitry  50 , channel  2  relays  36 ,  36 ′ inhibit the signal from travel to the channel  2  circuitry  51  and vice versa. The output of operational one of the channels travels to the circuitry of  FIG. 2  and is applied to the control grids of the respective vacuum tubes P 1 , P 2  and P 3 , P 4  in standard manner. In this way, the circuitry of channel  1  or channel  2 , which are preamplifier sections, can be selected along with the power amplifier section which includes vacuum tubes P 1  and P 2  alone or in combination with vacuum tubes P 3  and P 4  to provide the improvements discussed above. The circuitry  50  and  51  could include any standard guitar preamplifier circuitry such as, for example, the circuitry of the above mentioned patent.  
         [0028]     Though the invention has been described with respect to a specific preferred embodiment thereof, many variations and modification will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modification.