The Long Tail Pair phase inverter or Schmit is formed by a pair of identical and matched PNP transistors in common emitter configuration (Q 2 and Q 3). This stage highlighted in red color is formed by Q 1, Q 2, Q 3, Q 4, Q 5, R 1, R 2, R 3, and C 1: 1.2.1 JRC4558 Long Tailed Pair Differential Amplifier. The input stage is formed by a Long Tailed Pair (LTP) differential amplifier with a current mirror. Q 1 is a plain current PNP constant current source.Q 13 and Q 7 are configured as a classic current mirror, meaning that the current through Q 13 (I 13) will be mirrored in Q 7, making I 13 = I 7.The external current mirror load of this current source is connected to the collector so that almost the same current flows through it and the emitter resistor. The circuit operates as a constant current source: As a result, the output current I 9 is almost constant even if the load resistance and/or voltage changes. The Zener diode stabilized voltage V Z drives an emitter follower Q 14 loaded by a constant emitter resistor R 9 sensing the load current. As long as the Zener current (Iz) is between certain levels (Iz min and Iz max) called holding current, the voltage across the Zener diode (V Z) will be constantly working in the Voltage Regulation Area: The Zener diode D 2, when reverse biased has a constant voltage drop (Vz). So, the JFET current source sets the constant current I 15 through Zener diode D 2 making I 15 = I z Once the drain-source voltage reaches a certain minimum value, enters saturation where I DS current is approximately constant. Eventually, a stable equilibrium will be attained through feedback, where V GS is just right for the current flowing through the JFET. The gate will become negatively biased with regards to the source, negative values of V GS will start to shut Q 15 off. As the current increases, a voltage drop will develop across the Zener, until the diode will be reversed biased. With zero V GS the transistor enters in saturation region allowing a large current to flow through the JFET, so the current will increase. Then the voltage drop across the Zener diode D2 and V GS will be zero. When the +Vcc voltage source is turned on, no current is yet flowing through the JFET Q 15.The start-up of this part of the circuit is simple: The JFET Q 15 is a self-biased current source, it will provide a constant current source independent from the voltage supply. This stage highlighted in green color is formed by Q 1, Q 7, Q 13, Q 14, Q 15, R 1, R 9 and D 2: A stable current source is important in order to improve linearity and noise rejection. The Active Current Supply it is a utility stage which provides a constant flow of current to the different blocks. Find below the simplified circuit provided by JRC, broken down into blocks: Current Supply, Input Stage, Voltage Amplifier Stage (VAS), Output Stage (OPS) and Feedback Path. 'Jimmy' Lin an RCA researcher) following the classic 741 type opamp structure with some modifications. The JRC4558 internal schematic is based on the Lin Topology (named after Hung C. Table of Contents:ġ.2.1 Long Tailed Pair Differential Amplifier. Despite its questionable audio performance, it seems to be well suited to duty in overdrive circuits.
As such, it was used in huge volumes of Japanese audio equipment. This IC was undoubtedly chosen in the beginning by Japanese design engineers because it was one of the cheapest dual opamps on the market with acceptable audio performance. Many designers included this part in some of the most successful effects like the Orange Squeezer, DOD YJM 30, Boss OD1, Tube Screamer or Peavey equipment. The 4558 dual amp is linked with the history of the guitar pedals development. The first RC4558 monolithic dual opamp was developed by Raytheon Semiconductors in 1974. This chip has industry standard pinout, meaning that there are several different suppliers manufacturing pin compatible 4558 devices and also it can replace other standard dual op-amps like the TL072, NE5532 or the OPA2134. The high voltage gain (100 dB typ.), good input impedance (5 MΩ typ.) and versatile power supply (± 4 to 18 Volts) make it perfect to fit in pedal circuit designs. The JRC4558 integrated circuit by Japan Radio Company is a dual operational amplifier internally compensated and constructed with bipolar transistors on a single silicon chip.
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