A pre-main amplifier that improves transmission characteristics while enhancing functions by newly devised tone circuits and filter circuits.
The output terminal of the power amplifier is equipped with a 3-diffused mesa type PNP/NPN80W class power transistor with uniform characteristics, a pure complimentary service OCL circuit directly connected to all stages of differential-amplification, and a large power transformer made of highly permeable permalloy silicon steel core [Orient Core]. In particular, the pre-stage differential amplifier circuit is separated from the large-current drive of the final stage by selecting the transistor and providing an independent power supply circuit, thereby improving stability.
The equalizer amplifier uses an emitter-emitter negative feedback circuit in which three stages of NPN transistors are directly connected. The three transistors reduce the number of attached parts, and the simple circuit configuration ensures stability regardless of temperature or time. This circuit is operated at a high voltage of 70 v, and the voltage gain when no negative feedback is applied is very large. This allows the RIAA negative feedback and DC negative feedback to be applied deeply, further improving various characteristics. The PMA-255 uses a non-inductive resistor with a constant error of less than 1% for the negative feedback element, and along with the deep negative feedback, it achieves excellent RIAA deviation. In addition, the PMA is designed to maintain and improve the S / N ratio while widening the bandwidth by selecting the type with less noise generation from the type name selected in the design.
On the PMA 255, the tone amplifier, filter and buffer circuits in the control amplifier section do not normally have gain. To obtain the voltage required to drive the power amplifier, a two stage booster amplifier with a gain of 14 dB is installed behind the volume. This booster amplifier ensures the loudness and tone control characteristics of the front and back circuits.
The tone amplifier employs a modified BAX negative feedback type devised by DENON. It is a two stage direct-coupled circuit that combines low-noise FET amplification and emitter follower of PNP transistor. This method is easy to use for the negative feedback circuit that determines the control characteristics and has low waviness. Both Bass and Treble of the PMA 255 are rotary switches that can be adjusted in 2 dB steps. The central position of the knob automatically disables the capacitor of the feedback circuit.
The filter circuit adopts a new replacement active type that uses transistors for both low and high frequencies.
The buffer circuit is provided so as not to be affected by external wiring when the Pre-out terminal is used.
The power transformer is placed in the center and the heat sink of the output stage is arranged on the left and right sides. This improves the convection effect of heat radiation and improves the balance and separation of the left and right channels. In addition, the power transformer is provided with a strict 6-layer magnetic shield, and the cabinet part for the amplifier input terminal is changed from steel plate to aluminum, reducing magnetic induction across the transformer, steel plate and input terminal, and improving the S / N ratio.
In order to keep the resistance from the power transistor to the speaker output as low as possible, we carefully select wires, stabilizing wires, muting relay contacts and speaker terminals that have low specific resistance.
A protection circuit with a fast response speed is provided to protect the power transistor from destruction due to short circuit of speaker circuit, etc.
The printed circuit board is highly self-extinguishing, and the power cord is made of a synthetic rubber sheath cord that is resistant to heat and aging.