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JRC Internal Circuit. The Active Current Supply it is a utility stage which provides a constant flow of current to the different blocks. A stable current source is important in order to improve linearity and noise rejection.
With zero VGS the transistor enters in saturation region allowing a large current to flow through the JFET, so the current will increase. As the current increases, a voltage drop will develop across the Zener, until the diode will be reversed biased. The gate will become negatively biased with regards to the source, negative values of VGS will start to shut Q15 off. Once the drain-source voltage reaches a certain minimum value, enters saturation where IDS current is approximately constant.
As long as the Zener current Iz is between certain levels Izmin and Izmax called holding current, the voltage across the Zener diode VZ will be constantly working in the Voltage Regulation Area: The Zener diode stabilized voltage VZ drives an emitter follower Q14 loaded by a constant emitter resistor R9 sensing the load current.
The circuit operates as a constant current source: 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.
This topology is generally the best choice for an operational amplifier, it provides high input impedance, good voltage gain, common noise rejection as well as extra inputs for a summing feedback. To acquire real benefits from using an LTP input, the currents flowing through the differential transistors must be equalized. A very effective way to do it is using a current mirror.
The thermal coupling between both transistors is ideal since they share the same package. There most important reasons to use a current mirror are to enhance the gain and the signal integrity: 1. Signal integrity is improved because the current mirror gives equal quiescent current for each side of the LTP.
The JRC differential amplifier has a single-ended output, where the gain is the half if we compare it with a differential output circuit. The high gain voltage amplifier stage is the core of the power amplifier. Its task is to amplify the low amplitude input signal to a suitable level.
This VAS circuit works in class-A mode since they basically require only a small amount of current, and therefore power losses over the active device can be retained reasonably small.
Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base-emitter voltage is the sum of both base-emitter voltages.
This block is highlighted in blue color and is formed by Q11, Q12, R6, R7, and R8: The function of the Output Stage is to provide enough current gain so that voltage potential provided by VAS can exist over the low load output impedance. This stage is efficiently driven by the VAS.
Variations in the bias with temperature, or between parts with the same type number are common, so crossover distortion and quiescent current may be subject to significant variation. The output range of the JRC is about 1. Counteract is to bias the transistors so that their idling voltage never drops below the forward voltage.
Without a controlled bias voltage, the quiescent collector currents of the output power amplifiers may be excessive, causing thermal failure. The classic Vbe Multiplier topology consists on one transistor and two resistors.
The JRC uses a two transistor approach, a variation from the classic one: The Vbe Multiplier acts as a variable resistor which is mounted to the same silicon device as the output transistors, so they are thermally coupled.
Temperature changes of the device affect the gain of the servo transistor; this consequently changes the voltage drop over the Vbe Multiplier circuit. To improve the temperature coefficient between the Vbe multiplier matching and the output stage. The circuit Vbe Multiplier is equipped with an additional transistor Q8.
It has an important part in error correcting as well as in bandwidth and gain limiting. The capacitor C2 provides frequency selective negative feedback. This technique is called Miller Compensation or Dominant Pole Compensation because it introduces a dominant pole which masks the effects of other poles into the open loop frequency response.
The JRC Myth. Some guitar players believe that the JRCD original chip from the 80s has superior sounding characteristics, especially when it is placed in certain pedals like the Tube Screamer. In this case, the op-amp is claimed to be the holy grail if you want to get the original vintage sound ; many enthusiasts search and open mass produced electronics from the 70s and 80s looking for the original series chips.
They appear in every piece of Nippon electronics from this period. They continued to produce op-amps -labeled as NJMD- but the factory was equipped with new production equipment.
The specs are the same as the old one, but the suspicion still lingers - since they changed facilities, are the reissue chips really as good as the old ones?..
In the beginning, the JRCD was used for one single reason: it was cheap. So it was placed in tons of Japanese electronic equipment, a junky cheap old stereo or a clock radio could have several hidden inside it. Some people claim that they can really appreciate the difference between two chips from different manufacturers or even between two identical JRC from the same manufacturer.
However, in a guitar pedal there are a lot of factors that can modify the sound even more than the opamp can do: the components placement, values tolerance, circuit layout, soldering joints, temperature, power supply, etc There is also plenty of alternatives to the JRC chip, different users may have different opinions about their performance, but anyway all pin compatible: NJM by New Japan Radio Company.
RC by Texas Instruments. It has a nice, complex distorted sound. OPA by Analog Devices. Rounder sound, makes it ideal for blues players OP by Analog Devices. LT by Linear Technology.
MC by Texas Instruments. With very nice highs.
Op-Amp - 4558, Dual, 8-Pin DIP
JRC Internal Circuit. The Active Current Supply it is a utility stage which provides a constant flow of current to the different blocks. A stable current source is important in order to improve linearity and noise rejection. With zero VGS the transistor enters in saturation region allowing a large current to flow through the JFET, so the current will increase. As the current increases, a voltage drop will develop across the Zener, until the diode will be reversed biased. The gate will become negatively biased with regards to the source, negative values of VGS will start to shut Q15 off.
4558D View Datasheet(PDF) - Japan Radio Corporation
4558D Datasheet, PDF, Circuit Diagram, Application Notes