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INTEGRATED STEREO AMPLIFIER
Power output stage employing MOS FETs in a triple push-pull configuration MC input to output directly coupled in a straight DC configuration Complementary-symmetry push-pull design based on Accuphase's original circuitry
All-stage push-pull circuitry. All signal paths directly couple MOS FETs in a triple push-pull configuration can drive ever
Though an integrated amplifier, the E-303X incorporates much of the sophisticated technology and quality circuit components originally developed for use in Accuphase's separate amplifiers. Its resulting high-level performance permits the E-303X to handle even purely digital audio sources, such as Pulse Code Modulation (PCM) recordings and Compact Discs.
The power output stage of the E-303X uses high-speed MOS FETs in a triple push-pull configuration to deliver one of the highest outputs of any integrated amplifier: 150 watts per channel (into 8 ohms, from 20–20,000 Hz, with no more than 0.01% THD). This means the E-303X has more than enough power to drive even low impedance loads. For example, output into 2 ohms is an enormous 250 watts per channel.
The circuitry is based on Accuphase's original complementary-symmetrical push-pull design and generous employment of the cascode push-pull configuration realizes ideal high-frequency characteristics. This design raises overall performance to the highest level possible. Furthermore, all amplification stages from MC (Moving Coil Cartridge) input to output are directly coupled in a straight DC (capacitorless) configuration to provide amplification with a minimum of coloration.
The E-303X also has an input impedance selector to assure optimum performance with all sorts of MC cartridges. Other features include tone controls, automatic loudness compensation for listening at low-volume levels, abundant facilities for tape decks, and a complete range of input terminals to accommodate even large, multicomponent audio systems. Inside and out, the E-303X is fully equipped to meet the increasingly complex needs of today's high quality audio components.
Triple push-pull MOS FET power output stage delivers 150 watts of clean power per channel.
The power output stage employs a triple push-pull configuration using six MOS FETs having a generous PD (maximum power dissipation) of 600 watts. MOS FETs have already been used in a great number of Accuphase power amplifiers where they have gained a solid reputation for their sonic purity. For your reference, the main advantages of MOS FETs are described later.
The circuit diagram for the power amp section is shown in Figure 1. As you can see, the design is clean and allows easy operation. The preliminary stage is a differential push-pull circuit consisting of transistors Q2 and Q3. Next comes a cascode push-pull circuit with transistors Q6–Q9, and the MOS FETs Q12–Q17 are driven by the Q10 and Q11 pre-drivers. In this circuit design, the most important point affecting performance is the cascode push-pull circuits respectively composed of Q6 and Q8, and Q7 and Q9.
The cascode configuration is most often used in the radio frequency amplifier circuits of tuners because this type of circuit is not plagued by the Miller effect even when handling high frequencies. Along with the cascode's high gain, this makes a significant contribution to improving performance in all power amp stages.
Fig. 2 Circuit Diagram of Equalizer Amplifier
2 Large power supply and powerful output stage drive even 2-ohm speakers with ease.
The nominal impedance of a speaker is nothing more than a representative value obtained at a specific frequency. The actual impedance fluctuates a lot across the speaker's frequency range. Consequently, the actual impedance of a speaker system having a nominal impedance of 4 ohms may drop to as low as 3 ohms or even 2 ohms for some frequency ranges. Impedance can also drop momentarily when certain kinds of transient signals are reproduced. Because of this, only an amplifier capable of handling these low impedances can drive a speaker system accurately.
A directly coupled solid-state amplifier generates increasingly large currents in proportion to decreases of the load impedance, which can result in destruction of output elements. Because amplifier output in general can handle currents only up to a certain strength, some sort of protection circuitry must be provided to prevent currents above a certain level from reaching the outputs (and speaker). As a result, the maximum output level for low impedance loads is severely restricted with these amplifiers.
On the other hand, the power amp section of the E-303X has a high power output stage and a generous power supply section to supply very high outputs safely when necessary. As a result, the E-303X can be connected to low impedance speakers. Into 2 ohms, for example, the E-303X can output an awesome 250 watts per channel.
Birectly coupled circuitry with DC servo preserves MC signal purity from input to output.
The E-303X employs a virtually ideal configuration in which the MC head amp, equalizer amp, high-level amp and power amp are all directly coupled. This construction guarantees that the original signal is amplified and output in a virtually unaltered state to provide an extremely high level of fidelity. To eliminate DC drift completely, a problem that exists in all directly coupled amps, a powerful DC servo is used to stabilize each unit amp.
Fig. 3 Circuit Diagram of Head Amplifier
All-stage push-pull EQ amp with differential amplifier input, cascode pre-drive, and Darlington pair output.
Disc sound quality largely depends on the equalizer (EQ) amp, because the EQ amp determines RIAA characteristics. The circuit diagram of the E-303X's EQ amp is shown in Figure 2. The input stage is a differential amplifier with an FET buffer, the pre-drive stage is a cascode circuit composed of Q9–Q12, and final stage is a Darlington pr output composed of Q13–Q16. Each stage employs push-pull circuitry.
The pre-drive stage cascode connection consists of two pairs of transistors, Q9 and Q11, and Q10 and Q12, respectively. As stated in Section 1 describing the power amp, these cascode connections have excellent high-frequency characteristics to offer stable, low-distortion performance over a wide frequency range.
The Darlington pair output is also composed of two pairs of transistors, Q13 and Q15, and Q14 and Q16, respectively. Because the input impedance of this stage can be raised to a value multiplied by the hFE (static forward current transfer ratio) of Q13 and Q14, pre-drive stage performance is not easily affected by the load. This makes possible the creation of an amp having low distortion and excellent stability.
In this manner, Accuphase has put together a series of outstanding individual circuits in a completely push-pull configuration to raise amplifier performance to the highest level possible before adding negative feedback and to obtain the pure quality sound.
Power supplies are provided by constant voltage power supplies on each printed circuit board to reinforce operation further.
in a straight DC servo configuration. Accuphase 2-ohm load, delivering 250W/ch.
Differential amplifier input + Darlington pair output all-stage push-pull head amp guarantees optimum performance with all kinds of MC cartridges.
The E-303X has a complete MC head amp that even has its own amplifier with flat characteristics placed before the equalizer amplifier. The circuit diagram is shown in Figure 3. Because there are no capacitors, the delicate signals received from the MC cartridge can be sent directly to the input circuit. To prevent the generation of noise, carefully selected low-noise circuit elements are emimpedance
Because MC signals are so weak, even noise from the constant voltage circuit of the MC head amp's power supply can seriously degrade the S/N ratio of the signal path. In the E-303X. all elements used in the constant voltage power supply have been carefully selected to guarantee a completely low noise design.
The gain of 30 dB, a high value for a head amp, neans that even low output MC cartridges can be produced at a sufficient volume level. To be compatible with the wide range of MC cartridge sound quality, the E-303X has a 3-stage impedance selector (10, 30, and 100 ohms). For cartridges having an impedance of around 3 ohms, a load impedance of 10 or 30 ohms is best. Cartridges having an impedannce of more than 10 ohms are suited to 30- or 100-ohm setting.
Logic controlled relavs keeps the signal path as straight and short as possible.
The roundabout signal paths necessitated by switching connections for the input source, tape monitor, and other mode selections can degrade high-frequency sound quality and lead to an unstable operation. In the E-303X, the input termi-Relays located near this board then route the orders from a logic circuit.
The relays are highly dependable crossbaraled in airtight containers to guarantee out-
Switch type tone controls are employed to permit adjusting response in discrete steps of 1.5 dB Fine control like this means you can obtain precisely the response curve desired in almost any to either 200 Hz or 500 Hz, the treble turnover frequency to either 2 kHz or 7 kHz
3-step loudness compensator provides well-balanced lowvolume sound.
At low-volume level, very low and very high frequencies are more difficult to hear than the frequencies in between. Consequently, some degree of compensation, or boost, in these ranges is required to preserve a feeling of flat response at compensation must vary automatically according to the volume setting to maintain the feeling of flat, balanced response
To do this, the E-303X has a 3-step loudness compensation system that lets you adjust response according to the acoustics of the listening room, speaker system characteristics, and personal preferences.
Direct readout peak power meters
The power meters use a logarithmic peak scale to and watts. The soft-blue transparent illumination
adds an extra touch of elegance to the front panel and your listening room.
Tape monitor and dubbing switches
once for convenience and versatility. The tape monitor switch permits monitoring the signal being recorded, and dubbing switch permits copying (dubbing) a tape from one deck to the other regardless of the input selector's position.
1 Other functions and facilities
The E-303X is also equipped with many other useful features
A subsonic filter can be activated to eliminate subsonic noise generated by record warps.
An attenuator allows lowering the volume level without disturbing the volume control.
A speaker switch lets you switch between two speaker systems or use both at once.
A stereo/mono mode switch permits switching to monophonic reproduction to check for correct phase response between the left and right speaker
These and many more controls mean the E-303X is fully prepared to handle any listening or recording need. It even has two extra pairs of line input terminals (in addition to the TUNER terminals) to allow connection of a Compact Disc player, video cassette recorder, or other components without having to disconnect any other
"POWER MOS FETs"
VOLTAGE CONTROLLED MOS FETS PERMIT SUPERIOR DRIVER STAGE DESIGN
MOS FET power transistors have a high input-impedance
MOS FETS PRODUCE HIGH GAIN
MOS FETS PRODUCE HIGH GAIN The high gain attainable from only one stage of com-plementary push-pull Power MOS FETs is equivalent to the gain obtained with two or three bipolar transistor amplifier stages. The reduced number of stages for MOS FET amplifiers simplifies signal path circuitry and helps to create a superior power amplifier with higher stability and improved phageabriding.
SUPERIOR HIGH FREQUENCY PERFORMANCE
Irrequency characteristics within the tw/ (Wegative Peeddack) loop in audio amplification circuits where large amounts of negative feedback are required. This helps to prevent TIM (Transient Intermodulation Distortion) and obtain a more accurate, faithful reproduction of music. The wideband characteristics of MOS FETs make this possible, and helps to prevent TIM more effectively.
LINEARITY
Compared with Junction-type FETs, MOS FETs have a
BUILT-IN PROTECTION AGAINST OVERHEATING
BUILT-IN PROTECTION AGAINST OVERHEATING MOS FETs have a Negative Temperature Coefficient in the high current area, a characteristic which basically differs from bipolar transistors. This helps to protect itself from damage in case of trouble. For example, if an abnormal current flow occurs resulting from some circuit breakdown, a sudden rise in pellet temperature will cause this negative temperature coefficient of the MOS FET to decrease current flow, reduce heat and protect itself from damage. A similar breakdown may cause thermal runaway with bipolar transis-tors which would require protective countermeasures and evention.
Nevertheless Accurbase has adopted MOS FETs be-
SUPERIOR HIGH SPEED CHARACTERISTICS ENSURE
mpietely. This carrier storage effect and notching distortion are not countered with MOS FETs because of their superior
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