6DJ8 Cascode CR Equalizer Pre Amplifier
By SEKIDO Ryoichi (E-Mail: email@example.com)
Project Completed: July 2006
First Edition: Jul 23 2006
This pre amplifier features CR equalizer and 600 ohm line transformers. No NFB is used, it aims powerful and dynamic sound.
The first stage consists with 6DJ8 cascode circuit followed by 6AQ8. Cascode amp is rarely used for audio circuit however, it is good at high frequency performance and 6AQ8 too. Also much plate current would help strong sound.
The flat amp has 600 ohm output which is generated with the line transformer driven by 12AU7. This transformer is a TANGO NP-216N and has permalloy core. The volume control is located at the secondary. It is a 600 ohm fixed impedance attenuator.
The phono equalizer is a CR filter located between 6DJ8 and 6AQ8. A 6DJ8 is connected as cacode amp because of 1) good performance in high frequency, 2) massive sound of cascode amp using for audio circuit.
The schematics are shown in Fig. 1-1.
The flat amp is a 12AU7 drives 600 ohm line transformer, TANGO NP-216N. This transformer consists with permalloy core and has 20K ohm primary and 600 ohm secondary. The primary DC current is 12mA max, frequency response is 20Hz - 30KHz (-2db).
Unlike usual pre amplifier, a gain control is located at secondary of the line transformer. This amp uses 600 ohm fixed impedance attenuator which has 40 step attenuation with precise resistors. This fabulous attenuator made by TOKYO KO-ON DENPA CO., LTD. is unfortunately discontinuance. The balance control is omitted because the gain control exists left and right separately. Two volume control might be inconvenient however, it is indeed imaginary fears. It must be considered carefully that the flat amp can amplify the input signal without distortions because there is no gain attenuation before the flat amp.
A rec out is also from secondary before the gain control and is disconnected usually. It is connected when the rec out switch turned on. Before the flat amp there is -20 db attenuation switch. A rec out switch and an attenuation switch are linked with LED. (Photo)
A 600 ohm resistor located after the attenuator is needed when the power amp connected to pre out has high input impedance. If the power amp has 600 ohm impedance this resistor is not needed. If this resistor is loaded unsuitable the attenuation will be incorrect.
The flat amp schimatics is shown in Fig. 1-2.
The power supply is stabilized. The heater supply is also stabilized by three-terminal regulator. The schimatics is shown in Fig. 1-4 and 1-5.
This is a real VU meter made by Yamaki Electric Corporation. For practical use, there added an meter driving amp which has a couple of gain can be selected from 0db, +10db and +20db. The amp is connected to 600 ohm line, it must has high input impedance, so an operational amp is used with non-inverted amplification.
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