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【Forum】Dynaco Stereo 70 Series 2 [PDF Manual and Electrical Schematic] ~ FusoElektronique

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exclamation-circle Dynaco Stereo 70 Series 2 [PDF Manual and Electrical Schematic]

01 Feb 2017 14:19 - 24 Apr 2017 09:03 #1 by Trony
Trony created the topic: Dynaco Stereo 70 Series 2 [PDF Manual and Electrical Schematic]
Photo by Rene Rivo

INTRODUCTION

Tens ofthousandsoforiginal Dynacotube amplifiersarestill in use around the world. In fact, more quality music systemshave included Dynacotube preamps and power amplifiers than any other combination. The original Dynaco Stereo 70 is a true classic still regarded with awe by audio enthusiasts worldwide. The ST70 II marks the introduction of the second generation of vacuum tube products bearing the Dynaco name. This "new 70" fully reflects both the illustrious history of Dynaco's involvement with vacuum tube designs as well as the latest advances intube circuit technology. Through its long history, the Stereo 70 has proven a robust, reliable design of exceptional value. Dynaco has sought to preserve the considerable virtues of the original 70 while making fullest use of much better and more modern parts plus advanced circuit techniques. When comparing "old 70" with "new 70," one cannot help but notice the external similarity (i.e., the metalworkisofidentical dimension but with an entirely new connector array). Internally, however, the two units differ radically from each otherin both general appearance and execution of design. (Gone, for example, is nearly two-thirds of all internal connecting wire.)

TUBE VS. TRANSISTOR
Tube amplifiers are THE choice among many knowledgeable audiophiles, and they are preferred in severe service applications where even short-term failure must be avoided. Tube designs are inherently able to withstanduser and signal abuse that would instantly damage solid-state electronics. Field replacement of socketed tubes is much more convenient than solid-state parts. Generally speaking, tube amplifiers will give warning of impending breakdown. Sonically, tube amplifiers do not clip as harshly as their solid state counterparts. Upon clipping, solid-state amplifiers abruptly limit the signal resulting in the generation of high level distortion components (simulating a Square wave if driven hard enough). By contrast, tube circuits "soft clip" (resulting in a more pleasant compression rather than harsh clip). Ideally, neither is desireable. But a tube amplifier of modest power specification can sound as full and rich as a very large, solid-state amplifier on dynamically rich music passages. It is precisely these characteristics which are responsible for the "tube sound" so prized by audio enthusiasts. Tube amplifiers are regarded by many as "more musical" eventhough their measured specifications often appear far inferior to a comparably powered solid-state unit.

Photo by Rene Rivo

DESIGN PHILOSOPHY
Your ST70 Series II amplifier draws upon the proven design philosophies of the original 70. The low open-loop gain philosophy is maintained, thereby minimizing Transient Intermodulation Distortion (TIM). TIM distortion is regarded in many circles as the type of distortion to which the ear is most sensitive. The Pentodes Triode gain circuitry is virtually the same as original, thus keeping the signal path as short as practical without unnecessary distortion inducing complexity. This is the true elegance of the Stereo 70. The output tubes remain the fine EL34/6CA7's. The excellent output transformers are also unchanged from the original ST70. Three output impedances are still available (i.e., 4, 8, and 16 ohm) -- all properly matched with equal power ratings.
Dynaco has made extensive use of the most modern parts in the ST70 II. Precision metal-film resistors, poly-composition capacitors and miniaturized high-capacity power supply electrolytics (unavailable at the time of the original design). Newer parts are also more conservatively rated than some of the original parts. In compliance with general practices, a heavy-duty three-pin grounded power cord is provided. Signal ground is the same as electrical ground and chassis. All signal connectors are gold-plated. The speaker connections are 5-way binding posts ready to accomodate any connection method you choose, from bare wire ends to 3/4" spaced banana connector pairs.

Photo by Rene Rivo


The original ST70's power supply has long been the design area most vulnerable to legitimate criticism. Dynaco has addressed this problem as follows:
• Use of an entirely new and larger dual primary power transformer with improved line regulation (due to added iron in its core). Can now be wired for 220 volt / 50 Hertz operation.
• The original 5AR4 tube has been replaced by solid-state rectifiers, thus removing the 5AR4's inherent losses and reduced regulation, while eliminating a failure-prone tube in the process. (Note: Original high quality "large-globe"5AR4's are out-of-production and unavailable.)
• The original power supply filter capacitance has more than TRIPLED. This has been desireable for a long time but has only recently become practical (i.e., much greater capacitance can now be packaged in casings of diminuative size.)
• The troublesome selenium rectifier used in the bias circuitry has also been replaced by a solidstate rectifier with additional supply capacitance.
• The filterinductor has been reduced in valueto complement the increase infiltercapacitance with a resulting reduction of DC resistance. This provides more voltage for use by the outputs and results in higher power output.

Photo by Rene Rivo


All of the above changes serve to improve "stiffness" of the power supply by lowering the effective power supply impedance and improving regulation while increasing available voltage for maximunsafe power output. It is Dynaco's position that full electronic regulation of the plate power Supply does not result in cost effective sonic improvement.
The original ST70 design made use of roll-offs within the active circuitry to tailor the frequency response of the amplifier -- reducing open-loop gain at the frequency extremes. This open-loop gain reduction resulted in the characteristic distortion increase at the frequency extremes in the original design. Extensive testing and analysis has revealed that roll-offs may not be the most appropriate form of response limiting. Dynaco has installed a input frequency limiting filter network to limit bandwidth to 20 to 20,000Hz (long regarded as the human hearing range). This filter allows the removal of the tailoring components within the active circuitry and permits the active gain circuitry to be flatter across the frequency spectrum. The audible benefit is a great improvement in distortion components at the frequency extremes while still maintaining adequate bandwidth. This network requires the drive impedance from the preamplifier to be less than 2K ohms for proper top end response. This is acceptable for most modern day preamplifiers. Even tube types frequently achieve less than 2K using cathode follower output drivers. This also minimizes the effect of cable capacitance between the preamplifier and the power amp. It is possible to change C12 and C22 to permit higher impedance drives if absolutely necessary.
A further improvementin distortion, particularly in the higher frequencies, was achieved by moving the feedbackpoint from the 16 ohm tap of the output transformer to the actual loaded 8 ohm tap.

Photo by Rene Rivo


Since the most often loaded tap is the 8 ohm tap, the feedback from this point eliminates the distortion caused by imperfect coupling of the signal through an unloaded (16 ohm) tap. Gain resistors have been altered to compensate gain back to normal. Transient response is improved.
The original ST70's signal tube 7199 is out-of-production and unavailable. The ST70 II uses, instead, the 6GH8A. Since this tube has been extensively used in high-frequency applications, availability is excellent. It works exceptionally well in this application, yielding better consistency and lower distortion. (TECHNICAL NOTE: Though most tube manuals do not list a noise spec for the 6GH8A, extensive testing by Dynaco of large numbers of 6GH8A tubes has revealed noise floors that are, on average, lower than modern production 7199's. The 6GH8A is NOT a drop-in replacement for the 7199 tube aspinout for the two tubes is different. Do not attempt to interchange them.)
The printed circuit board material of the original ST70 was exposed economical phenolic which deteriorated with age creating unpredictable electrical changes resulting insonic differences. The Series II unit uses military-grade, double sided epoxyfiberglass PC material and shields the recessed PCB with a metal cover plate. The board is also populated by the finest parts. Sensitivity to internal wiring placement is eliminated since wiring to the output tubes is accomplished via the Printed Circuit Panel. In fact, nearly two-thirds of the trouble-prone original point-to-point wiring has been eliminated.


Music and Electronic Devices Designer. Drum and Bass Addict. Creative Mind as Lifestyle. Cat in past Life. Soccer, Ice Hockey and Snowboard Lover. Marxist.
Sono Il Re sul Carro Danzante. Attraversando La Città Getto Via I Coriandoli Della Vostra Tristezza Regalando Passione Ed Estasi Nei Vostri Occhi.
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Last Edit: 24 Apr 2017 09:03 by Trony.

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【Forum】Dynaco Stereo 70 Series 2 [PDF Manual and Electrical Schematic] ~ FusoElektronique Page 1 | Dynaco Stereo 70 Series 2 [PDF Manual and Electrical Schematic]. Published by Trony on Wednesday, 01 February 2017 in Audio Designer (Design's Factory)
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