(1) Field of the Invention
This invention relates to a linear high-voltage amplification stage for use in, for example, high-fidelity vacuum-tube audio power amplifiers having a cathode-follower output stage requiring a large drive voltage swing.
The present invention is an improvement over that disclosed in my prior U.S. Pat. No. 5,859,565 entitled "CATHODE-FOLLOWER HIGH-FIDELITY POWER AMPLIFIER" issued Jan. 12, 1999.
(2) Description of the Prior Art
In the past decade of the art of high-end audio reproduction it has come to be recognized by many audiophiles and amplifier designers that vacuum-tube amplifiers provide superior music reproduction as compared with transistor amplifiers. These tube amplifiers generally have output stage configurations which are either push-pull or single-ended.
Both the single-ended output stage and the push-pull output stage operate in the common-cathode mode which has numerous disadvantages, primarily due to the relatively large magnitude of the tube plate resistance. As explained in more detail below, these disadvantages include the need for a large air gap in the single-ended transformer core to prevent saturation thereby reducing the transformer primary inductance and increasing the distortion due to the resulting steep elliptical loadline traversed by the tube operating point. Further, the high source impedance, inherent in the common-cathode mode, also increases the distortion due to the nonlinear magnetization current of the transformer core. It also results in a high amplifier output impedance at the speaker terminals thereby providing a low amplifier damping factor and poor speaker transient response A further disadvantage of the high source impedance of the output stage is the resulting frequency response nonlinearty when driving typical speakers which have an impedance which varies with frequency. In addition, the high source impedance of the plate resistance coacts with the leakage inductance and winding capacitance of the output transformer to generate phase shift and rolloff at high frequencies. The common-cathode mode also results in a high Miller-effect capacitance at the input of the output stage.
In a push-pull configuration, the high source impedance due to the plate resistance exacerbates the inevitable dynamic and static imbalances of a push-pull output stage accompanying the aging of the output tubes with use over time, and also those imbalances due to asymmetrical drive signals transmitted to the two halves of the push-pull stages.
These disadvantages of the common-cathode output stage are further explained in the detailed description of the prior art set forth below. It is well-known in the prior art that a lower output impedance and reduced distortion in the output stage itself can be obtained by utilizing a cathode-follower mode of operation for the output configuration. However, this has heretofore been impractical because the cathode-follower output stage requires a very large drive signal so that the preceding conventional drive stage would have generated as much or more distortion than would have been reduced by the cathode-follower output mode. Also, the large supply voltage required for the drive stage would have subjected the drive stage tube to an excessively high plate-to-cathode voltage.
My prior U.S. Pat. No. 5,859,565 implements a cathode-follower output stage to obviate the above-noted disadvantages of the common-cathode output stage, in combination with a novel drive stage to provide the required large drive signal. This novel drive stage comprises two series connected tubes which share the large static and dynamic voltages of the drive stage so as to provide with low distortion the large voltage signal required to drive the cathode-follower output stage, and without subjecting the drive stage tubes to excessively high voltages.
More particularly, the novel drive stage of my prior U.S. Pat. No. 5,859,565 comprises at least two vacuum-tube triodes connected in series with the plate of a first drive tube connected to the cathode of the second drive tube. A load impedance is connected from a B+ power supply terminal to the plate of the second drive tube. The plate of the second drive tube constitutes the output node of the drive stage and is connected to the grid of the cathode-follower output stage to drive the latter. The grid of the first drive tube constitutes the input node of the drive stage and is driven by a previous stage. The grid of the second drive tube is driven by a signal responsive voltage divider network driven by the plate of the second tube.
The voltage divider network maintains the grid of the second drive tube at a voltage approximately midway between the voltage of the second drive tube plate and the ground as said plate voltage swings in response to the signal current flowing through the load impedance. As a result, the voltage of the second drive tube cathode and the first drive tube plate is also maintained approximately midway between the voltage of the second drive tube plate and ground. That is, the two series connected drive tubes share approximately equally both the quiescent static voltage and the dynamic output voltage of the drive stage as the latter drives the grid of the cathode-follower output stage with the required large amplitude voltage swings.
As disclosed in my prior patent, the advantages of the prior invention may be best understood by first considering in more detail the problems and disadvantages of the conventional common-cathode output stage configuration of the prior art. FIG. 5 of the drawings of my prior patent shows a conventional single-ended output stage. The output tube designated OUTPUT is generally a power triode having its cathode grounded, its grid driven by a previous drive stage indicated by the voltage symbol designated DRIVE, and its plate connected to one end of the primary winding of an output transformer designated XFMR. The opposite end of the transformer primary winding is connected to a B+ power supply indicated by a voltage symbol designated SUPPLY. The transformer secondary winding is connected to the loudspeaker load designated SPKR.
This prior art single-ended output stage operates in the common-cathode mode which has numerous disadvantages. Several of these are explained with reference to FIG. 6 of the drawings of said prior U.S. Pat. No. 5,859,565 which shows a simplified equivalent circuit of the transformer at low frequencies, and with reference to FIG. 7 of that patent which shows an equivalent circuit at high frequencies. Referring first to FIG. 6, Vlf designates a low-frequency voltage source provided by an output tube (not shown). Rp represents the dynamic plate resistance of the output tube connected in the common-cathode mode. The open-circuit primary inductance of the output transformer is designated Lp. The core loss due to magnetic hysteresis and eddy currents in the transformer is designated by Zm. The loudspeaker impedance, reflected to the primary, is designated Zs. Most of the disadvantages of this common-cathode topology are due to the relatively large magnitude of the plate resistance Rp. In FIG. 7, Vhf designates a high-frequency voltage source provided by the output tube. Rp is again the plate resistance of the tube. Li symbolizes the transformer leakage inductance. Cw designates the transformer winding capacitance. Zs again refers to the reflected speaker impedance. The disadvantages arising from the high plate resistance Rp are as follows:
First, in a single-ended output stage configuration the quiescent plate-to-cathode current flows through the output transformer primary winding and is not balanced by an oppositely flowing current as in a properly biased push-pull output stage. This requires a relatively large air gap in the transformer core in order to prevent saturation of the core. The resulting increased reluctance of the core substantially reduces the transformer primary inductance which presents the output tube with a reactive low-impedance load at low frequencies. This causes the operating point of the tube to follow an elliptical loadline having a steep major axis, thereby increasing the distortion of the signal output at the plate. This distortion is exacerbated by the large plate resistance Rp because it provides a relatively high driving source impedance as seen by the primary inductance Lp. The reactive load is further disadvantageous in that at low frequencies it causes phase shift which reduces the feedback stability margin and prevents the use of substantial amounts of negative feedback, thereby further increasing the distortion of the amplifier. In a push-pull implementation the same problem exists to a lesser extent. That is, the bias currents of the output tubes eventually become unbalanced with use as the tubes age over time. This requires an air gap in the core, albeit smaller than the gap required for a single-ended topology.
Second, the high source impedance due to plate resistance Rp also increases the distortion due to the nonlinear magnetization current of the transformer core, as explained in the papers by Partridge (Ref. 1) and Hodgson (Ref. 2) in the bibliography at the end of this specification. That is, the nonlinear magnetization current results in a nonlinear voltage drop across plate resistance Rp which voltage drop, when subtracted in series from the linear source voltage Vlf, provides a nonlinear voltage to the primary winding of the transformer.
Third, the high source impedance due to the plate resistance Rp results in a high amplifier output impedance at the speaker terminals. This in turn results in a low amplifier damping factor and poor transient response, and also frequency response nonlinearity when driving typical speakers which have an impedance which varies with frequency.
Fourth, the high source impedance of the plate resistance Rp coacts with the leakage inductance Li and winding capacitance Cw to produce a high-frequency rolloff, and also to produce a phase shift at high frequencies, thereby reducing the high-frequency feedback stability margin and further limiting the amount of feedback that may be utilized to reduce distortion of the amplifier.
Fifth, the common-cathode mode results in a high input capacitance due to the Miller effect, thereby increasing the load on the previous drive stage, and also increasing the high-frequency phase shift so as to reduce further the high-frequency feedback stability margin of the amplifier.
Sixth, the high source impedance Rp exacerbates the inevitable dynamic and static imbalances of a push-pull output stage due to said aging of the output tubes with use over time, and also those imbalances due to asymmetrical drive signals transmitted to the two halves of the push-pull stages.
Several of the above-described disadvantages of the common-cathode output stage are explained in more detail at Pages 214-217 and 229-233 of the Radiotron Designer's Handbook (Ref. 3).
It is well-known in the prior art that a lower output impedance and reduced distortion in the output stage can be obtained with a cathode-follower configuration. That is, instead of the output transformer primary winding being connected in series between the power supply B+ terminal and the output tube plate as implemented in the usual common-cathode topology, the transformer winding is connected in series between the output cathode and the ground. Such a cathode-follower output stage is shown by Gilson and Pavlat (Ref. 4) who utilize this configuration in order to reduce the amplifier output impedance and thereby increase the amplifier damping factor for better control over the speaker cone movement. It is also well-known in the prior art that the cathode-follower mode provides low distortion when used in drive stages, as noted at Page 596 of the Radiotron Designer's Handbook (Ref. 3). However, for output stages the latter reference also accurately states:
"Cathode-follower output stages introduce serious problems, and are not suitable for general use, even though their low plate resistance and low distortion appear attractive. The difficulty is in the high input voltage which is beyond the capabilities of a resistance-coupled [penultimate drive] stage operating on the same plate supply voltage."
This page of the Radiotron reference further states that the Gilson et al. (Ref. 4) cathode-follower amplifier is seriously flawed because:
"the total harmonic distortion at 50 c/s is over 1% at 8 watts output, and 1.7% at 20 watts. The high output voltage which must be delivered by the resistance-coupled penultimate stage thus show its effect on the distortion, even though the supply voltage has been increased to a dangerously high value [700 volts]." PA1 "Full cathode-follower operation in either of the three major [output] circuits is not generally used in practice because it is as difficult to obtain (without distortion) the very large drive voltages required as to design the output stage itself with low distortion."
As stated by Crowhurst (Ref. 5):