Marantz 7 Preamplifier Circuit

Published:2011/5/5 22:09:00 Author:Robert | Keyword: Marantz 7, Preamplifier | From:SeekIC

In electron tube prestage the Marantz 7's position is paramount, and it is believed that the electron tube enthusiasts, who have never heared the name of Matantz 7,may be very few. The main circuit of Marantz 7, which is introduced in late 50 years, is shown in picture 4 below. In the circuit, V1, V2 are used as voltage amplifiers, V3 is connected as a cathode follower to be a signal buffer which acts like the NPN tube to be connected as a emitter follower. This circuit's most significant feature is its overall loop feedback design, which is a major factor making the Marantz 7 famous. But for preventing the high-frequency self-excited problem, it needs to add a 22pF capacitor between V1 and V2 to make up a high-frequency part feedback to reduce the high-frequency magnification times, and also the output port needs to connect to a tri-polar N-N type negative feedback network. This network's high-frequency high impedance is very little (about 20KΩ or less). So this design is suspected of posing a considerable burden to V3. Although its high-frequency's open loop gain is not enough and the negative feedback's improvement for high-frequency distortion is not very well, it's strange the enthusiasts's impression to Marantz 7 is good. In this production, for V1, V2, V3 we first use the 12AX7 made in beijing, but the negative feedback network should change to be connected to the second stage which shows in the dotted portion of picture 4, and also we cancel the 22pF capacitor which used for preventing the seld-excited problem, that can effectively improve the distortion. The 12AX7 is a high U tube whose magnification is large, but its internal impedance is also large. It seems using 12AX7 for V3 is not the only best choice. We have ever used the american 5751, 12AU7 for substitution, and the high-frequency's problem has been improved in some extent, with larger dynamic range and better stability of the sound field. When using 12AU7 as the cathode output, but its distortion is less than using 12AX7 or 5751. It can use 6N10 or ECC82 instead of 12AU7. The Toshiba 5963 also has a certain quality of sound which has the same pin with 12AX7. The light filament can be supplied by 6.3V or 12.6V voltage, but in this machine we use 6.3V power supply. The analytical ability and extension degree of high frequency and low frequency of Marantz 7 is not likely very good. These weakness can be found at some time expecially compared to the most outstanding pre-tube nowadays such as Matisse Refernce, Audio Research Referance 1, Convergent SL-1 and so on. However the most attractive things of Marantz 7 is the unspeakable beauty of the mid-frequency sound, and we think its sound has the sunshine-type bias which can express exhaustively the brightness and power of music. When playing brass music, the brightness of the instruments is full; when playing string music, the sound is also pliable and has texture, and the human voice would have more strong feelings. We evaluate highly for this modified Marantz 7's circuit sound effect (V1, V2 use 5751, V3 use 12AU7 of Amperexr). Its sound is soft and smooth with strong texture, and its high-frequency and low frequency heavily playing performance is still be regarded as first-class, which has a certain extension degree and intensity feeling, but its mid-frequency's performance is even better. Compared to the two-stage 6SN7 negative feedback amplification circuit, the Marantz 7's circuit is better. If V1, V2 are all using Telefunken's ECC803S, V3 is using Telefunken's ECC802C too, we believe this Marantz 7 could make you do not want to use other preamplifiers.



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