Patent Publication Number: US-2009224826-A1

Title: Output circuit of vacuum-tube amplifier

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an output circuit of a vacuum-tube amplifier, and more particularly, to an output circuit of a vacuum-tube amplifier, which maintains a voltage between a ground terminal and an output terminal of the vacuum-tube amplifier as 0V to output an amplified signal without using a coupling condenser or a transformer and maintains a voltage of a cathode of a vacuum tube of an output stage uniform by a variable self-bias circuit to obtain linearity of output power in proportion to an increase in an input signal without being affected by the cathode when the input signal is increased, increase the output power and improve distortion. 
     2. Background of the Related Art 
     In spite of advantages of a semiconductor device, lovers of music still prefer an audio amplifier using a vacuum tube because the audio amplifier using a vacuum tube has sound quality higher than that of a semiconductor amplifier. 
       FIG. 1  is a circuit diagram of an output circuit of a conventional vacuum-tube amplifier. Referring to  FIG. 1 , the output circuit of the vacuum-tube amplifier includes an amplification unit and an output buffer unit. The amplification unit receives an input signal through a grid G of a vacuum tube  1 , receives a voltage V 1  through a plate P of the vacuum tube  1 , amplifies the input signal, and outputs the amplified signal through a cathode K of the vacuum tube  1 . A coupling condenser C 1  and a bias resistor R 1  are connected to the cathode K of the vacuum tube  1  such that the amplified signal is output to an output node N 1  through the coupling condenser C 1 . 
     In case of a multi-stage amplifier, multiple vacuum tubes having the same configuration as the vacuum tube  1  are connected in parallel, a bias resistor and a coupling condenser are connected to the cathode of each of the multiple vacuum tubes, and output nodes are commonly connected in parallel.  FIG. 1  illustrates only one vacuum tube. 
     The output buffer unit receives the signal of the output node N 1  of the amplification unit through a grid G 1  of a vacuum tube  2  and outputs the amplified signal to a speaker  4  through a plate P 1  of the vacuum tube  2 . A resistor R 2  and an AC bypass condenser C 2  are connected in parallel with a cathode K 1  of the vacuum tube  2  to construct a self-bias circuit  5 . 
     To output a signal amplified by the vacuum tube  1  in the amplification unit, the coupling condenser C 1  or a transformer for cutting off a DC voltage must be used at the cathode K of the vacuum tube  1 . A general method sets the resistance of the resistor R 1  such that a voltage, obtained by halving the plate voltage V 1  of the vacuum tube  1 , is set to the cathode K of the vacuum tube  1 . That is, when the amplification unit is directly connected to a following amplifier, the coupling condenser C 1  or a transformer must be used in order to cut off a DC signal and pass only an AC signal because there is a DC components in a signal transmitted to the following amplifier. 
     The output buffer unit connects the resistor R 2  of hundreds to thousands ohm to the cathode K 1  of the vacuum tube  2 . A bias voltage is applied to the vacuum tube  2  using a voltage drop according to the resistance R 2 . That is, a self-bias method is used in the output buffer unit. 
       FIG. 2  illustrates cathode and output power characteristics with respect to an input signal of a self-bias amplification circuit. In the self-bias circuit as illustrated in  FIG. 1 , when the voltage applied to the grid G 1  is increased, current of the cathode K 1  is increased in proportion to the increase in the voltage applied to the grid G 1 . Here, the voltage is increased in proportion to the current by resistance set to the cathode K 1 . A voltage according to a basic current value of the cathode K 1  and the input signal of the grid G 1  are increased in proportion to the input signal voltage in a satisfactory manner in response to the amplification factor of the vacuum tube  2 . When an input signal having a voltage exceeding the voltage according to the basic current value of the cathode K 1  and the input signal voltage of the grid G 1  is applied, however, an increase in the voltage of the cathode K 1  attenuates the input voltage of the grid G 1  by the increase in the voltage of the cathode K 1 . Accordingly, the input signal and output power do not achieve satisfactory linearity, as illustrated in  FIG. 2 . 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide an output circuit of a vacuum-tube amplifier, which employs a zero voltage-maintaining circuit that maintains an output terminal of a front end amplifier as 0V to transmit a signal without using a coupling condenser or a transformer for cutting off a DC voltage at an output node of the front end amplifier. 
     Another object of the present invention is to provide a variable self-bias circuit of a vacuum-tube amplifier, which prevents a voltage set to a cathode of a vacuum tube from being varied irrespective of the power level of an input signal applied to a grid in an output amplification stage of the vacuum-tube amplifier. 
     To accomplish the above objects, according to the present invention, there is provided an output circuit of a vacuum-tube amplifier including: a front end amplification unit for amplifying an input signal using a first vacuum tube; an output amplification unit for power-amplifying the output signal of the front end amplification unit using a second vacuum tube and outputting the amplified signal to the outside; a zero voltage-maintaining circuit for controlling a voltage applied to a plate of the first vacuum tube in response to the voltage of a cathode of the first vacuum tube to maintain the DC voltage of the cathode of the first vacuum tube as 0V; and a variable self-bias circuit for maintaining a bias voltage of a cathode of the second vacuum tube uniform irrespective of a variation in the input signal. 
     The front end amplification unit includes: the first vacuum tube having a grid connected to a signal input port and a cathode connected to an output node; a resistor R 11  for applying a negative voltage V 3  to the cathode of the first vacuum tube; a resistor R 12  for applying a positive voltage V 1  to the plate of the first vacuum tube; the zero voltage-maintaining circuit continuously controlling connection/short-circuiting of a resistor R 17  for dividing the positive voltage V 1  with the resistor R 12  in response to a predetermined time constant based on the voltage of the cathode of the first vacuum tube and maintaining the DC voltage of the cathode of the first vacuum tube as 0V according to the control of the positive voltage V 1 ; and a first smoothing condenser connected between the plate of the first vacuum tube and a ground terminal and a second smoothing condenser connected between the ground terminal and the terminal of the negative voltage V 3 . 
     The zero voltage-maintaining circuit includes: the voltage-dividing resistor R 17  connected to the plate of the first vacuum tube; a power amplifier connected to the output terminal (−) of the voltage-dividing resistor R 17 ; and a differential amplifier having a non-inverted input terminal (+) receiving the voltage of the cathode of the first vacuum tube through voltage-dividing resistors R 13  and R 17  and a time constant condenser C 13  and an inverted input terminal receiving the voltage of an output terminal of the differential amplifier through voltage-dividing resistors R 14  and R 15  and a time constant condenser C 14 , the output terminal of the differential amplifier being connected to an output terminal (+) of the power amplifier. 
     The variable self-bias circuit of the output amplification unit includes: a differential amplifier having a non-inverted input terminal (+) receiving the output voltage of the cathode of the second vacuum tube, which is divided by two resistors, and an inverted input terminal (−) receiving a reference voltage set by a zener diode; a power amplifier having an input terminal (+) connected to the an output terminal of the differential amplifier and an output terminal (−) connected to the cathode of the second vacuum tube through a resistor R 23 ; and a smoothing condenser connected between the cathode of the second vacuum tube and the ground terminal to smooth a peak value. 
     While the zero voltage-maintaining circuit is used in a circuit in which the front end amplification unit and the output amplification unit are connected in the present invention, the zero voltage-maintaining circuit can be used in any amplifier circuit using a vacuum tube, which requires an AC coupling condenser or a transformer at an output stage, to output an AC signal without using the coupling condenser or the transformer. 
     The variable self-bias circuit is applied to the output circuit of any power amplifier circuit using a vacuum tube to maintain the bias voltage of an output stage uniform, obtaining stable output power characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of an output circuit of a conventional vacuum-tube amplifier; 
         FIG. 2  illustrates the relationship between an input signal and an output power of a conventional vacuum-tube amplifier; 
         FIG. 3  is a circuit diagram of an output circuit of a vacuum-tube amplifier according to the present invention; 
         FIG. 4  is a circuit diagram of a zero voltage-maintaining circuit of the vacuum-tube amplifier according to the present invention; and 
         FIG. 5  illustrates the relationship between an input signal and an output power of a variable self-bias circuit of the vacuum-tube amplifier according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 3  is a circuit diagram of an output circuit of a vacuum-tube amplifier according to the present invention. Referring to  FIG. 3 , the output circuit of the vacuum-tube amplifier according to the present invention includes a front end amplification unit  10  for amplifying an input signal using a vacuum tube  11  and an output amplification unit  20  for power-amplifying the output signal of the front end amplification unit  10  using a vacuum tube  21  and outputting the amplified signal. 
     The output circuit of the vacuum-tube amplifier further includes a zero voltage-maintaining circuit  12  for controlling a voltage applied to a plate P 11  of the vacuum tube  11  based on the voltage of a cathode K 11  of the vacuum tube  11  of the front end amplification unit  10  to maintain the DC voltage of the cathode K 11  as 0V, and a variable self-bias circuit  22  for maintaining a bias voltage of a cathode K 21  of the vacuum tube  21  of the output amplification unit  20  uniform using a self maintaining circuit irrespective of a variation in the input signal. 
     The front end amplification unit  10  receives the input signal through a grid G 11  of the vacuum tube  11  and applies a voltage V 1 (+) to the plate P 11  of the vacuum tube  11  through a resistor R 12 . A voltage V 3 (−) is applied to the cathode K 11  of the vacuum tube  11  through a resistor R 11 . The zero voltage-maintaining circuit  12  is connected between the cathode K 11  and the plate P 11  of the vacuum tube  11 . 
     The zero voltage-maintaining circuit  12  detects the voltage of the cathode K 11  of the vacuum tube  11 , divides the voltage V 1 (+) applied to the plate P 11  of the vacuum tube  11  and controls the voltage applied to the plate P 11 . The zero voltage-maintaining circuit  12  connects/short-circuits a resistor R 17  for dividing the voltage V 1 (+) applied to the plate P 11  of the vacuum tube  11 . The zero voltage-maintaining circuit  12  maintains the DC voltage of the cathode K 11  of the vacuum tube  11  as 0V by repeating connection/short-circuiting of the voltage-dividing resistor R 17  in response to a predetermined time constant. 
     A first smoothing condenser C 11  is connected between the plate P 11  of the vacuum tube  11  and a ground terminal E and a second smoothing condenser C 12  is connected between the ground terminal E and a voltage V 3 (−) input terminal to smooth a ripple voltage. 
       FIG. 4  is a circuit diagram of the zero voltage-maintaining circuit  12 . Referring to  FIG. 4 , the plate P 11  of the vacuum tube  11  is connected to an output terminal (−) of a power amplifier U 12  of the zero voltage-maintaining circuit  12  through the resistor R 17 . The cathode K 11  of the vacuum tube  11  is connected to a non-inverted input terminal (+) of a differential amplifier U 11  at a reference voltage according to voltage-dividing resistors R 13  and R 16  and a time-constant condenser C 13 . A self output of the differential amplifier U 11  is applied to an inverted input terminal (−) of the differential amplifier U 11  through voltage-dividing resistors R 15  and R 14  and a time-constant condenser C 14 . That is, the zero voltage-maintaining circuit  12  divides the voltage of the plate P 11  of the vacuum tube  11  by the resistors R 12  and R 17  based on the voltage of the cathode K 11  of the vacuum tube  11  and controls the voltage of the plate P 11 . Accordingly, the zero voltage-maintaining circuit  12  maintains the voltage between the cathode K 11  and the ground terminal E as 0V. 
     Referring to  FIG. 3 , the variable self-bias circuit  22  of the vacuum-tube amplifier according to the present invention connects a grid G 21  of the vacuum tube  21  to an output node N 11  of the front end amplification unit  10  to receive the input signal and amplifies the input signal through a plate P 21  of the vacuum tube  21 . The variable self-bias circuit  22  includes a differential amplifier U 21  having a non-inverted input terminal (+) receiving the output voltage of a cathode K 21  of the vacuum tube  21 , which is divided by two resistors R 21  and R 22 , and an inverted input port (−) receiving a reference voltage set by a zener diode ZD 21 , a power amplifier U 22  having an input terminal (+) receiving the output of the differential amplifier U 21  and an output terminal (−) receiving the output of the cathode K 21  of the vacuum tube  21  through a bias resistor R 23 , and a smoothing condenser C 21  connected between the cathode K 21  of the vacuum tube  21  and the ground terminal E to smooth a peak value. 
     The zero voltage-maintaining circuit  12  is included in the front end amplification unit  10  and the variable self-bias circuit  22  is included in the output amplification unit  20 . 
     Though the conventional vacuum-tube amplifier must use a coupling condenser in order to output an amplified AC signal, the vacuum-tube amplifier of the present invention can output an AC signal without using a AC coupling element by maintaining the voltage of the cathode K 11  of the vacuum tube  11  of the front end amplification unit  100  as 0V. 
     The operation of the vacuum-tube amplifier according to the present invention is explained with reference to  FIGS. 3 and 4 . Referring to  FIG. 3 , a voltage V 1  of +250V is applied to the plate P 11  of the vacuum tube  11  through the resistor  12  and a voltage V 3  of −200V is applied to the cathode K 11  of the vacuum tube  11 , for example. If a zero voltage method is not used, the voltage VT of the cathode K 11  of the vacuum tube  11  becomes 50V. However, the zero voltage-maintaining circuit  12  is operated such that the voltage of +250V applied to the plate P 11  is divided by the resistor R 12  and the voltage-dividing resistor R 17  included in the zero voltage-maintaining circuit  12  to apply a voltage of +200V to the cathode K 11 . Accordingly, the voltage of the cathode K 11  of the vacuum tube  11  becomes 0V. It is impossible to continuously maintain 0V due to errors in the vacuum tube and other components. To stably maintain 0V, the present invention constructs the zero voltage-maintaining circuit  12 . 
     In the zero voltage-maintaining circuit  12 , the voltage detected from the cathode K 11  of the vacuum tube  11  is divided by the resistors R 13  and R 16  and applied to the non-inverted input terminal (+) of the differential amplifier U 11 . Here, when the output signal of the differential amplifier U 11  is at a high level, the input terminal (−) of the power amplifier U 12  becomes a low level and the resistor R 17  functions as a voltage-dividing resistor. That is, the voltage V 1  of +250V is divided by the resistors R 12  and R 17  and applied to the plate P 11  of the vacuum tube  11 . 
     When the output signal of the differential amplifier U 11  becomes a high level, the high-level signal is divided by the resistors R 15  and R 14 , and thus a voltage higher than the voltage of the non-inverted input terminal (+) of the differential amplifier U 11  is applied to the inverted input terminal (−) of the differential amplifier U 11 . Here, the divided voltage is delayed by the time-constant condenser C 14  and applied to the inverted input terminal (−) and the output signal of the differential amplifier U 11  is transited from a high level to a low level. 
     When the output signal of the differential amplifier U 11  becomes a low level, the input terminal (+) of the power amplifier U 12  becomes a low level and the output terminal (−) of the power amplifier U 12  becomes a high level so that the resistor R 17  is short-circuited. Accordingly, the voltage V 1  of +250V is applied to the plate P 11  of the vacuum tube  11  through the resistor R 12 . 
     The output signal of the differential amplifier U 11  is transited and connection/short-circuiting of the resistor R 17  is repeated to control the voltage of the plate P 11  and the aforementioned operation is repeated. A time interval of repeating the operation is controlled by the time constant of the differential amplifier U 11  and a ripple voltage applied to the plate P 11  and the cathode K 11  is smoothed by the smoothing condensers C 11  and C 12 . 
     Consequently, the voltage of the plate P 11  of the vacuum tube  11  is controlled by the zero voltage-maintaining circuit  12  to stably maintain the voltage between the cathode K 11  of the vacuum tube  11  and the ground terminal E as 0V. 
     Since the voltage of the cathode K 11  of the vacuum tube  11  of the front end amplification unit  11  is maintained as 0V, only AC signals can be output even when a coupling condenser or a transformer is not used at the output node N 11 . 
     When the front end amplification unit  10  transmits an amplified AC signal through the output node N 11  without using a coupling element, the output amplification unit  20  maintains the bias voltage of the cathode K 21  of the vacuum tube  21  to increase output power and improve distortion. 
     In the self-bias circuit  22  of the vacuum-tube amplifier according to the present invention, the voltage of the cathode K 21  of the vacuum tube  21  is divided by resistors R 21  and R 22  and input to the non-inverted input terminal (+) of the differential amplifier U 21 . Here, the differential amplifier U 21  compares the reference voltage set by the zener diode ZD 21  to the voltage divided by the resistors R 21  and R 22 . When the voltage of the non-inverted input terminal (+) is higher than the reference voltage, the output signal of the differential amplifier U 21  becomes a high level. Here, the voltage of the input terminal (+) of the power amplifier U 22  is at a high level so that the resistor R 23  functions as a load of the cathode K 21  of the vacuum tube  21  according to the operation of the power amplifier U 22 . 
     Since the resistor R 23  serves as a load, the voltage of the non-inverted input terminal (+) of the differential amplifier U 21  is decreased and the output signal of the differential amplifier U 21  becomes a low level. Then, the power amplifier U 22  performs an inverting operation so that the resistor R 23  is short-circuited. The voltage obtained by dividing the voltage of the cathode K 21  of the vacuum tube  21  by the resistors R 21  and R 22  is applied to the non-inverted input terminal (+) of the differential amplifier U 21 , and thus the output signal of the differential amplifier U 21  becomes a high level. Accordingly, the resistor R 22   23  is operated as a load of the cathode K 21  of the vacuum tube  21  according to the power amplifier U 22 . 
     When the aforementioned operation is continuously repeated, the voltage of the cathode K 21  of the vacuum tube  21  is fixed by a voltage value set by the resistors R 21  and R 22 . That is, the resistor R 23  functions as a load and short-circuits repeatedly according to the operations of the differential amplifier R 21  and the power amplifier U 22  and a peak value is smoothed by the smoothing condenser C 21 . Accordingly, the bias voltage of the cathode K 21  of the vacuum tube  21  is fixed and does not affect the output power in response to the input signal. 
     Therefore, the bias voltage of the cathode K 21  of the vacuum tube  21  is not varied by a value set by the resistors R 21  and R 22  even when a basic current of the vacuum tube  21  is 50 mA through 100 mA, for example. 
     Referring to  FIG. 5 , when the input voltage of the grid G 21  of the vacuum tube  21  is increased, the current of the cathode K 21  of the vacuum tube  21  is increased in proportion to the input voltage of the grid G 21 . Here, when a variable operation of the cathode K 21  is performed by the resistors R 21  and R 22 , the voltage of the cathode K 21  is not varied but fixed in a set range. Thus, the increase in the input signal of the grid G 21  is not affected by the cathode K 21 , the output power W obtains satisfactory linearity in proportion to the increase in the input signal, the output power W is increased by 25% and distortion is improved up to a set limit. 
     As described above, the vacuum-tube amplifier according to the present invention can transmit AC signals without using an AC coupling element by maintaining the voltage of the cathode of the vacuum tube of the front end amplification unit as 0V. Furthermore, the vacuum-tube amplifier according to the present invention automatically maintains the self-bias of the cathode of the vacuum tube of the output amplification unit to a fixed value to obtain output power that is not affected by the cathode and is proportional to an input signal. Moreover, the output power is increased and distortion is improved according to satisfactory linearity of the output power. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.