Audio Dynamics Dynakit Stereo 35 Power Amplifier 14635013 Manuel D’Utilisation

DESCRIPTION

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.

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