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Integrated Circuit Based High End Audio Line Preamplifier
The circuit is designed to create a high end preamplifier that would produce a better result of stable and quality audio line.Preamplifier (preamp) – a device normally used together with the sound equipment to be of assistance in improving the overall quality of sound by detecting and strengthening weak signals for subsequent and more powerful amplification stages NE5534 – a high performance low noise operational amplifier having a combined excellent AC and DC characteristics and features such as capability of external compensation, capability of offset nulling, wide range of voltage supply, low harmonic distortion, with output voltage swing at peak to peak, high slew rate, 100 dB common mode rejection ratio, high gain of DC Voltage, unity gain bandwidth at 10 MHz, and noise voltage of equivalent input OPA604 – a FET-input operational amplifier designed for enhanced AC performance, data acquisition, transducer amplifier, active filters, spectral analysis equipment, PCM DAC I/V converter, and professional audio equipment by providing superior performance in high quality audio and other applications requiring excellent dynamic performance due to its low noise, very low distortion, and wide bandwidth. 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 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
The main part of the circuit is shown in figure 1 with the acquisition of quality relays in the audio line stage. The relays should be of a closed type with an inductor connected in parallel with a diode within the package. Otherwise, an external diode can be connected. In all parts of the circuit, a capacitor is always present in the path for the DC coupling which aids the operation of trimmers TR1. The buffer stage comprises of two single op-amps IC1 and IC2, receives the acoustic signal from the input although no gain is being produced. All op-amps are allotted with a trimmer to ensure precise voltage of around 200 uV that is being adjusted in the output stage while suppressing the excessive voltage. A jumper switch is attached to every trimmer that functions as a selector if the supply would pass across the positive or negative line. The way of choosing the line is decided by the type of op-amp used, wherein the polarity of voltage line is dependent. The modification of the value suppressed by the trimmer comes after the preamplifier functions a few times. The connection of power amplifier in the preamplifier output will be possible if no potentiometer with lowest resistance is connected. The location of op-amp IC5 and IC6 should be in the positive line together with their respective trimmers.
A good quality volume potentiometer follows the output stage that is composed of IC3 and IC4, which drives all types of amplifiers due to the gain level of the signal. However, the gain can be modified by changing the values of R14 to R18 which provides protection to the op-amp and are placed in the feedback region. Long cables can also be suitable in the output of the circuit. The relay RL6 located in the output closes upon the passage of time delay from C13 and IC7 & IC8 to provide unwanted noise in the power supply. The IC5 and IC6 function as buffer to drive the record output.
One of the factors of producing a quality sound is the choice of op-amps. The cheaper and classic NE5534 is a good choice but can be replaced with more expensive ones like the OPA604, OPA627, or OPA637. OPA637 rise above the rest but does not provide stability with gain under X4. Since OPA604 has good value and characteristics, it could be a better choice especially when combined with FET because of low noise, big bandwidth and low distortion, which is capable of driving a load of 600 ohms.
An input selector with delay output is used in the circuit which consists of relay S1, tape monitor S2, mute S3, and the delay output connection. The relay is supplied with 12 V which simultaneously turns ON the LED D1 to D4 for the input indication. The output of IC7A becomes high when positive voltage is supplied by the preamplifier through R28 while charging C13. This will cause IC7B go high while Q1 closes by the RL6 deflector. The delay time can be adjusted with reference to the value of C13. The mute operation by S3 is normally closed but will be triggered as the switch opens which interrupts the supply to RL6 whose contacts are opened. This will prevent the flow of signal to the output of preamplifier which will turn ON the LED D7 to indicate the mute operation. The contacts of RL6 closes with the closure of S3 thereby restoring the supply. The 12 V in the S3 is the supply coming from the contacts of S1. Each time the rotary switch S1 moves, it ensures that RL6 will obtain minimal time supply. The output of preamplifier is always interrupted every time the input is being chosen while preventing noise to be produced.
An adequate power supply, which eliminates and filters EMI via JF1 plug, is essential for a high end preamplifier. The power supply in the audio part and relay part has dissimilar ground line. A double voltage stabilization can be selected to build a powerful filter that would ensure reject noise and ripple while simultaneously creating minimal thermic and mechanical fatigue from the duration of usage. Each regulator consists of multiturn trimmers in the stabilizer in regulating the voltage output. It is prerequisite to provide heatsinks to all the regulators while C31 and C32 ensure the fast discharging at the closure of main supply.
Some high-end preamplifier can be used for enterprise and sever storage markets that is used in the TDA5360 family since it is highly integrated and low noise solution for the server and enterprise storage markets, which is focused on video streaming and high-speed data access applications. Other high-end preamplifiers are used for line stage preamplifier with very high performance, for producing high value audio and home theater components, for combining neutrality with a tube flair capable of presenting music, for making surround sound processors, for high performance audio/video products, and for making tuners.