Description

INSTRUCTIONS FOR ASSEMBLY AND USE OF THE DYNAKIT STEREO 35 POWER AMPLIFIER

The Stereo 35 is a dual power amplifier of deceptively simple appearance and circuitry. Behind its seemingly conventional design is an extremely sophisticated circuit configuration representing the most recent advances in power amplifier engineering and construction. This high degree of technical refinement, combined with the conserva tive use of premium-grade components, has resulted in a typical Dynakit design offering superb performance at low cost.

Each of the two channels in the Stereo 35 is rated as a 17,5-watt amplifier. Although not evident from this simple rating, the power stated is available on a continuous basis at any frequency from 20 to 2,000 cycles per second. A "music power" rating would make the Stereo 35 a 45-watt amplifier, although even this rating would non disclose that full power is available at low distortion throughout the audio spectrum. Nor would such a rating indicate that the Stereo 35, even when driven to power output levels beyond its rating, is able to handle these overloads gracefully, with minimum detriment to the sound. As a result, the Stereo 35 can, if necessary, duplicate the sound intensity of amplifiers with much higher power ratings when called upon to do so.

The heart of the Stereo 35 is the special Dynaco Z-565 output transformer, a patented design tailored specifically to this circuit. The transformer is free of resonances, has extended frequency response (from 6 to over 60,000 cycles), and is wound by special techniques on a carefully designed core to insure low distortion over a wide fre- quency spectrum. Transformers of this caliber have never previously been used in low-cost equipment of moderate power rating.

The transformer provides an optimum match between the loud speaker load and the output tubes used. These tubes are supplied as a matched group with the kit, and are used in a circuit configuration which gives low inherent dis- tortion and high stability of their d.c. operating point.

The output stage is preceded by a composite voltage amplifier and cathodyne phase inverter (a 7247 tube with direct coupling between sections). The phase inverter used is unique; its operations is independent of the age of con- dition of the tube. The phase inverter stage is therefore able to maintain its capabilities permanently with no need for adjustment or balancing. Gain in this part of the ampli- fier is augmented by a feedback connection from cathode to cathode.

Two negative feedback loops, one mostly resistive and the other purely capacitive, are carried over the amplifier to provide 20 db of negative feedback. This provides low

distortion and noise, a high damping factor, and further benefits. The feedback loop is unconditionally stable under all load arrangements, so that the Stereo 35 is suitable for use with any loudspeaker, including electrostatic types The values of components selected for the Stereo 35 have been carefully determined, to be certain that the operating conditions for each stage have been set at the center of the optimum range. The use of close tolerance parts provides additional assurance that these settings will not shift, and will remain accurate, so that every amplifier will meet its specifications. This is very important to the kit builder, as it gives immunity from the variations to which much electronic equipment is subject. Heavy-duty circuit boards give additional stability and reproducibility of characteristics, assuring a level of performance which takes full advantage of the capabilities of the design. The con- servatively rated power transformer, after testing, is com- pletely sealed in a special encapsulating material which prevents core vibration and also serves to dissipate heat

effectively.

One of the outstanding aspect of the design of the Stereo 35 is the attention which as been paid to the repro- duction of transient signals. Specifications commonly quoted for amplifier refer only to performance with pure sine- waves as the signal content; however, music and speech are actually composed of non-repetitive sounds which are non-sinusoidal in character. It is this irregular, or transient type of waveform which the amplifier will usually be called upon to reproduce. Pulse tests and square wave measure- ments help to evaluate the transient performance of cir- cuits, and these have been applied to derive the correct operating parameters for the Stereo 35. The success of this approach is particularly evident in listening tests where the source material includes percussive sounds-drums, tam- bourines, piano, cymbals, etc. The Stereo 35 reproduces these sounds with a transparency and absence of blurring which is only obtained when an amplifier has extended frequency response, without bounce, flutter or overshoot. In addition, the Stereo 35 recovers almost immediately from overload, so that the tones immediately following are not affected by the overload signals. Thus, even whit low-efficiency speaker systems, where an occasional peak signal might overload the amplifier, reproduction of every- thing except the highest peak of the heaviest passage is effortlessly reproduced.

It is this characteristic subtlety of design approach which distinguishes the Stereo 35 from other equipment with superficially similar specifications. These differences provide a perceptible improvement in clarity and natural- ness of sound; this was the design objective of the Stereo 35, and it has been achieved in a unit of remarkably low cost.