Line Preamplifier Circuit with Tube Acquisition

Overview

The circuit was designed to create a line preamplifier by acquiring the use of double triode tubes. The circuit comprises of three parts, the main preamplifier circuit with tubes, the input selector and delay module, and the power supply module.

Terminology

• ECC82 – a double triode used for audio frequency with features such as amplification factor of 17, transconductance at 2 mA/V, and anode current of 5 mA, and AC/DC indirect heating with serial or parallel supply
• ECC81 – a double triode used for radio frequency with features such as amplification factor of 17, transconductance at 2 mA/V, and anode current of 5 mA, and AC/DC indirect heating with parallel or a serial supply with current limiting device inserted when switching ON
• BD679 – a monolithic NPN Silicon epibase power Darlington transistor with resistors and diode in a TO 126 plastic package and is typically used for AF applications due to its high current gain
• 4081 – a quad dual input AND gate in a 14-pin package generally characterized with small fluctuation, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• LM317 – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V and requires only two external resistors to set the output voltage due to its internal current limiting, thermal shutdown and safe area compensation, making it essentially blow-out proof
• 7812 – 3-terminal 1A positive voltage regulator with short circuit protection, thermal overload protection, output transistor safe operating area protection, output current up to 1A, and output voltages from 5V to 24V

Circuit Explanation

The use of vacuum tubes still exists in today's electronic equipments like the application in preamplifiers. This circuit is divided into 3 parts consisting of the main preamplifier, input selector with delay module, and the power supply. The main preamplifier in figure 1 does not have any innovation in the design. The input stage contains a simple circuit where a relay is being driven by an input selector S2. The signal will be fed onto the following stage which consists of a cascaded shunt regulated push pull (SRPP) amplifier and a cathode repeater in the output. The SRPP circuit utilizes the double triode EC882 in U1a & U1b having a pentode characteristics that is connected in series. This will serve as the input stage of radio frequency, similar to the old TV tubes, which provide high impedance and excellent response in high frequencies. To achieve better quality of audio and response in low frequencies, the circuit utilizes large value polypropylene or MKT capacitors in different points and in the output of the tubes. A good quality volume and balance potentiometer is also used. The cathode repeater found in the circuit output consists of half the circuit part of ECC81 tube U2a. The other half circuit can be optionally used for other channel. This is to avoid cross channel interference since vacuum tubes are not known for good decoupling when used in two circuits. In the stage where the output of preamplifier is connected to the final amplifier, a good quality relay RL6 is used with certain delays to avoid unwanted noise when it opens or closes. The relay will close once the circuit voltage has been stabilized and applied.

The second figure relates to simple circuit of the input selector control relay with a time delay circuit. The selector S2 drives the input of the relay through the JF3 while LEDs D1 to D5 provides various visual indications. The time delay circuit is constructed in the region of IC1 where the first time delay is controlled by RL7 via R6 and C2. This will produce high voltage in the preamplifier path. The filament of the tube is being heated in order to apply high voltage. The operation of RL7 is controlled by Q1 via JF3/9 & JF3/10 with an optical indication on D10. When RL7 closes, it produces high voltage with a time delay that is controlled by R8 and C3. This will cause Q2 to conduct and drive RL6 through the JF3/7 & JF3/8, which will be connected to the output of preamplifier of the final amplifier where LED D11 provides a visual indication. The sound signal at the output can be interrupted by using the switch S3 to force the base of Q2 to O V which will open the contacts of RL6. This action refers to the MUTE function of amplifier.

The power supply contains two transformers for producing high voltage and low voltage. The high voltage powers the RL7 that drives the delay circuit while the low voltage id provided in IC1 and IC2 for regulating the separated control of filament. The ICs used to regulate the voltage of the filament should have sufficient heat sink. The process of regulation starts without any load in the output and eventually with a complete load of filament. The voltages being supplied is being stabilized by the zener diode D8.

Part List

Application

There are a lot of circuits that are actually delivering quality audio with the association of vacuum tubes. The filament in the vacuum tube is heating the cathode so that it will produce electrons, which are charged negatively. They can be designed in a preamplifier that operates with solid state component to function in a variety of applications with input types such as line level input, microphone input, and front panel input. Other circuits that acquire vacuum tubes are built for microphone preamplifier, stereo compressor, equalizer, recording channel, direct box, and headphone amplifier. Nowadays, the tube amplifiers are being used for musical instruments especially for the electric guitar, since they are known in the musical industry for producing unique tones. The feature of producing a vacuum tube preamplifier and amplifier is for musical instruments with tone control circuits, with settings that can be easily stored in a digital memory, with self-explanatory controls, and with programmable controls.

Source:users.otenet.gr/~athsam/tube_line_preamplifier_progect_1_English.htm