# US 5,705,950 A · Hybrid solid state/vacuum tube audio amplifier > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,705,950 A - Document ID: US5705950A - Status: Granted - Kind: A - Application number: 08/601,758 - Filed: 1996-02-15 - Granted: 1998-01-06 - Earliest claimed: 1996-02-15 - Inventors: Brent K. Butler - Web page: https://patentgrep.com/patent/US5705950A ## CPC - H03F 1/327: {to emulate discharge tube amplifier characteristics} - H03F 3/181: Low-frequency amplifiers, e.g. audio preamplifiers - H03F 5/00: Amplifiers with both discharge tubes and semiconductor devices as amplifying elements - H03F: AMPLIFIERS ## Abstract In this hybrid amplifier circuit the vacuum tube is direct coupled or DC coupled to the output of a solid state amplifier in a cathode follower configuration. An automatically operative bypass component, such as a Zener diode, LED, transistor junction or relay, diverts the signal output of the solid state amplifier around the vacuum tube stage when bias conditions or other vacuum tube operating conditions signify that the vacuum tube is still warming up or for some reason inoperative. Thus the hybrid circuit produces amplification at full volume and at low distortion even while the vacuum tube is warming up. The vacuum tube provides a desirable warming coloration to the sound that may be user adjusted by controlling the level of closed loop feedback within the circuit. Lower total harmonic distortion is achieved in a pentode vacuum tube device by direct coupling the screen and control grids. Lower voltage use of power pentodes wired in nonconventional "triode mode" is achieved. ## Claims ### Claim 1 A hybrid amplifier circuit of the type having amplifier input and output ports for inputting and outputting an audio signal, comprising: a solid state amplifier stage for coupling to said amplifier input port and having a first stage output; a vacuum tube stage having an input coupled to said first stage output and having a second stage output for coupling to said amplifier output port; a bypass circuit coupled between the output of said solid state amplifier and said amplifier output port; said bypass circuit being responsive to the operative state of said vacuum tube to automatically divert said audio signal around said vacuum tube stage when the vacuum tube is in a first state of operation and to automatically divert said audio signal through said vacuum tube when the vacuum tube is in a second state of operation. ### Claim 2 (depends on 1) The amplifier circuit of claim 1 wherein said solid state amplifier stage comprises an integrated circuit operational amplifier. ### Claim 3 (depends on 1) The amplifier circuit of claim 1 wherein said solid state amplifier stage comprises a discrete transistorized balanced or differential amplifier. ### Claim 4 (depends on 1) The amplifier circuit of claim 1 wherein the input of said vacuum tube stage is direct coupled to said first stage output. ### Claim 5 (depends on 1) The amplifier circuit of claim 1 wherein the input of said vacuum tube stage is direct current (DC) coupled to said first stage output to couple direct current signals from said solid state amplifier stage to said vacuum tube stage. ### Claim 6 (depends on 1) The amplifier circuit of claim 1 wherein said bypass circuit comprises a semiconductor junction biased for conduction when said vacuum tube is in said first state and biased for nonconduction when said vacuum tube is in said second state. ### Claim 7 (depends on 1) The amplifier circuit of claim 1 wherein said vacuum tube stage is configured as a cathode follower. ### Claim 8 (depends on 1) The amplifier circuit of claim 1 wherein said vacuum tube stage is configured to introduce even harmonic coloration of said audio signal. ### Claim 9 (depends on 1) The amplifier circuit of claim 1 further comprising a housing having a viewing window and wherein the vacuum tube stage includes at least one vacuum tube positioned within said viewing window and said amplifier circuit further comprising illumination means disposed in proximity to said one vacuum tube to illuminate the one vacuum tube and thereby provide a visual indication that the vacuum tube is operational. ### Claim 10 (depends on 1) The amplifier circuit of claim 1 further comprising user adjustable feedback circuit coupled around said vacuum tube stage for controlling the degree of harmonic coloration added to said audio signal by said vacuum tube stage. ### Claim 11 (depends on 1) The circuit of claim 1 wherein said vacuum tube stage comprises a power pentode configured as a direct coupled class A cathode follower. ### Claim 12 (depends on 1) The circuit of claim 1 wherein the vacuum tube stage is operated at a plate to cathode potential at substantially less than 250 volts. ### Claim 13 (depends on 1) The circuit of claim 1 wherein the bypass circuit further includes visually illuminated device for indicating tube warm-up and tube failure conditions. ### Claim 14 A hybrid amplifier circuit of the type having amplifier input and output ports for inputting and outputting an audio signal, comprising: a solid state amplifier stage for coupling to said amplifier input port and having a first stage output; a vacuum tube stage for providing coloration to said audio signal having an input coupled to said first stage output and having a second stage output for coupling to said amplifier output port; a user adjustable feedback circuit including frequency dependent component coupled around said vacuum tube stage for controlling the degree to which said vacuum tube stage is operated under closed loop control and thereby controlling the degree to which the vacuum tube stage provides coloration to said audio signal, stage output to couple direct current signals from said solid state amplifier stage to said vacuum tube stage. ### Claim 15 (depends on 14) The amplifier circuit of claim 14 wherein said solid state amplifier stage comprises an integrated circuit operational amplifier. ### Claim 16 (depends on 14) The amplifier circuit of claim 14 wherein said solid state amplifier stage comprises a discrete transistorized balanced or differential amplifier. ### Claim 17 (depends on 14) The amplifier circuit of claim 14 wherein the input of said vacuum tube state is direct coupled to said first stage output. ### Claim 18 (depends on 14) The amplifier circuit of claim 14 wherein the input of said vacuum tube state is direct current (DC) coupled to said first stage output to couple direct current signals from said solid state amplifier stage to said vacuum tube stage. ### Claim 19 (depends on 14) The amplifier circuit of claim 14 wherein said vacuum tube stage is configured as a cathode follower. ### Claim 20 (depends on 14) The amplifier circuit of claim 14 wherein said vacuum tube stage is configured to introduce even harmonic coloration of said audio signal. ### Claim 21 (depends on 14) The amplifier circuit of claim 14 further comprising a housing having a viewing window and wherein the vacuum tube stage includes at least one vacuum tube positioned within said viewing window and said amplifier circuit further comprising illumination means disposed in proximity to said one vacuum tube to illuminate the one vacuum tube and thereby provide a visual indication that the vacuum tube is operational. ### Claim 22 (depends on 14) The circuit of claim 14 further comprising supplemental feedback circuit that increasingly augments the gain of said vacuum tube stage as said user adjustable feedback circuit is varied to increase the degree to which said vacuum tube stage provides coloration to said audio signal. ### Claim 23 A hybrid amplifier circuit of the type having amplifier input and output ports for inputting and outputting an audio signal, comprising: a solid state amplifier stage for coupling to said amplifier input port and having a first stage output; a vacuum tube stage having an input coupled to said first stage output and having a second stage output for coupling to said amplifier output port; said vacuum tube stage employing a pentode having a cathode, a control grid and a screen grid; said pentode being configured in a cathode follower configuration; said vacuum tube stage further including a means for lowering the total harmonic distortion of the vacuum tube stage comprising a direct connection between said screen grid and said control grid. ### Claim 24 (depends on 23) The circuit of claim 23 wherein said pentode includes a plate and wherein said vacuum tube stage includes a bias circuit in which said plate is open circuited with respect to direct current (DC) potentials. ### Claim 25 (depends on 24) The circuit of claim 24 wherein said plate is not connected to said bias circuit. ### Claim 26 (depends on 24) The circuit of claim 24 wherein said plate and said screen grid and said control grid are all connected together and are separate from said bias circuit, but direct coupled to said amplifier output port. ### Claim 27 (depends on 23) The circuit of claim 23 further comprising a bypass circuit coupled between the output of said solid state amplifier and said amplifier output port; said bypass circuit being responsive to the operative state of said vacuum tube to automatically divert said audio signal around said vacuum tube stage when the vacuum tube is in a first state of operation and to automatically divert said audio signal through said vacuum tube when the vacuum tube is in a second state of operation. ### Claim 28 (depends on 23) The circuit of claim 23 wherein said vacuum tube stage provides harmonic coloration to said audio signal and wherein said hybrid amplifier further comprises a user adjustable feedback circuit coupled around said vacuum tube stage for controlling the degree to which said vacuum tube stage is operated under closed loop control, thereby controlling the degree to which the vacuum tube stage provides coloration to said audio signal.