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Amplifiers with Positive and Negative Feedback (Audio 1961/04)

Details: Category: Audio USA; Hits: 706

Amplifiers with Positive and Negative Feedback
CHARLES P. BOEGLI (Product Planning Manager, Bendix Corporation, Cincinnati, Ohio)
Audio, April 1961, Vol 45, No. 4

Contrary to a widely held belief, this author discovered that the cathode-coupled phase inverter (“long-tailed pair”) introduces a significant amount of distortion. By including this stage in the negative feedback loop he achieved an unusually low-distortion amplifier.

Several years ago, the writer had two articles¹ published on the design and construction of audio amplifiers utilizing over-all negative feedback with internal positive feedback. A number of readers constructed these amplifiers and satisfaction was the general result.

Those who are interested in the details of these amplifiers should refer to the original articles. Several difficulties were encountered with the circuits, primary among which were:

The output transformer was not designed for the manner in which it was operated.
The output transformer secondary was at a small d.c. potential above ground.
The inverter (the first stage of the amplifier) was not included in the negative feedback loop, so that the distortion introduced by this stage appeared undiminished in the output.

Both amplifiers used ordinary output transformers with the secondaries connected in unusual fashion. The speaker lines were connected to the 0- and 16-ohm taps of the secondary and the 4-ohm tap was grounded (for a.c.), so that a balanced output was being drawn from a transformer intended for unbalanced operation. The output transformer was carefully specified, and those who were foolhardy enough to construct their amplifiers with other transformers usually paid the penalty of instability or oscillation. For some time, the reason why one transformer worked well while another did not, remained a mystery, but it was thought that unbalanced capacitances between each end of the winding and ground might be responsible.

One hundred per cent negative feedback was obtained by connecting the ends of the secondary directly to the cathodes of the driver tubes. Internal positive feedback was brought from each driver plate to the grid of the other driver. Bias for the drivers was obtained by inserting a bypassed resistor between the center tap (that is, the 4-ohm tap) of the output transformer secondary and ground, so that the entire secondary was at a d.c. potential equal to the bias on the driver cathodes. If a speaker line became shorted to the chassis of the amplifier, the bias was disturbed, and oscillation usually occurred. Nevertheless, speaker lines are usually not grounded, and this did not prove to be a very great shortcoming.

Push-Pull in HiFi (Audio 1959/4)

Details: Category: Audio USA; Hits: 1114

Push-Pull in HiFi
Author: MANNIE HOROWITZ
AUDIO, APRIL, 1959, VOL. 43, No. 4 (Successor to RADIO, Est. 1917).

The push-pull amplifier has become standardized as the optimum circuit arrangement for providing adequate power output with a minimum of distortion - so long as the tubes are used under proper conditions. The author makes the performance of this type of amplifier thoroughly understandable.

The push-pull power output stage can be studied from many angles. A theoretical discussion on composite tube characteristics is interesting and informative. A survey of the practical applications of different push-pull or driver circuits is an important asset to any audiofan's library.

In this article, several refinements in push-pull circuits will be discussed. These refinements are frequently designed into the amplifier on an intuitive basis rather than a scientific one. The importance of a scientific analysis rather than instinctive motivation can be well appreciated by the serious hi-fi enthusiast.

Graphical Analysis

A typical self-biased triode push-pull output amplifier is drawn in Fig. 1. Everything discussed about this triode refers to the pentode as well – but to an even larger degree due to the greater curvature of the tube characteristics.

How can you extend the life of your battery tubes

Details: Category: Radioamator i Krótkofalowiec; Hits: 1295

How can you extend the life of your battery tubes
J. Pastor, Radioamator i Krótkofalowiec, Rok XI, march 1961

Having often difficulties with the purchase of electron tubes for a "Pionier" type battery receiver, I was wondering how to extend the "life" of these tubes. It occurred to me that if the filament voltage of those already worn-out tubes working under normal conditions was increased by about 0.4V, their work should improve. So, losing nothing, due to the used tubes, I used 1.8V instead of 1.4 filament voltage, and I kept the anode voltage unchanged. These tubes started to work like almost new again in all wavelength ranges.
After 360 hours, I increased the filament voltage again to about 2.4V and the work improved again for about 320 hours, so the total work of the tubes was extended by about 680 hours, which is one sixth of the warranty standard. I used dry cells to power the tubes, connecting two in series with each other - one new cell with a voltage of 1.4V and the other - used cell, with a voltage of 0.8V. I connected a 5Ω wire resistor to these cells in series.

Modernize your Williamson Amplifier

Details: Category: Audiocraft; Hits: 3342

David Hafler
Modernize your Williamson Amplifier
Audiocraft, Volume I, Number 3, January 1956

The Williamson amplifier circuit was first publicized in England in 1947, and in this country in 1949. It has achieved wide acceptance and popularity, and has been the basis for several modifications of the original design. The most basic change was the ultra-linear version of operation, which I developed and subsequently described^*. This arrangement corrected 2 of the basic defficiencies in the original design - it increases the power capability of the amplifier to 25 or 30 watts, and it improved the margin of feedback stability.
Now, as always happens, progress in amplifier design has continued: it is possible to make further improvements in the Williamson design (both triode and ultralinear versions). These improvements again correct for limitations with respect to power output and stability.

Increasing Power Output

Present thinking on requirements for audio power is vastly different from that of a few years ago. Then, most people said, "Ten watts is enough for me". Now, however, modern program material has been increased the power requirements substantially for realistic, undistorted reproduction. In addition, source material frequency responce has been extended, and this also introduces the need for re-evaluation of amplifier power requirements. Increased frequency response means that the amplifier has to handle power at greater extremes of frequency. At these extremes, the impedance characteristics of the loudspeaker change from the nominal values. This means that the amplifier is mismatched at frequency extremes, and a mismatch decreases the maximum-power capabilities of any amplifier.

All About Electric Guitar - Part II

Details: Category: Radioamator i Krótkofalowiec; Hits: 2335

Eng. Konrad Widelski
All About Electric Guitar - Part II
Radioamator i Krótkofalowiec Polski, Year 16, October 1966, No. 10

The first part of the article (no. 9/66) provides a description of a simple amplifier based on two electron tubes. This amplifier, properly made, will fully satisfy the guitarist's needs. Nevertheless, its power in some cases (in larger rooms, especially for a bass guitar) may turn out to be insufficient. Therefore, for more advanced radio amateurs, we present a design description of the amplifier in a push-pull circuit with an output power of 12W. Of course, the construction of such an amplifier should be started only by radio amateurs who already have some positive achievements in the field of amplifiers, because building this device on one's own is not easy.

PUSH-PULL AMPLIFIER

The amplifier was assembled from a minimal number of elements, quite easily available on the market. Only the output transformer should be made by yourself, as it is untypical and cannot be bought.
The amplifier has very good - considering amateur requirements - parameters. They have been achieved by simple means, namely the use of a negative feedback system at several points and direct feedback between the stages.
Figure 9 shows a schematic diagram of the amplifier. As you can see, the number of tubes and elements has been significantly reduced to a minimum. One of the ECC83 electron tube triodes works in the first stage. The anode of this vacuum tube is directly connected to the control grid of the next amplifier stage. This stage, equipped with the second triode of the ECC83 electron tube, works in a phase inverting system.

Fig. 9. Schematic diagram of the 10W amplifier

The phase reversal system used (the so-called "cathodine") is reliable in its simplicity. The output stage uses a pair of EL84 type tubes. Of course, in line with the requirements of modern Hi-Fi technology, this stage uses negative feedback in screen grids (the so-called "ultralinear system"). These grids are not connected - as is usually the case - directly to the high voltage source, but to special taps on the primary winding of the output transformer. The use of such feedback complicates the production of the output transformer somewhat, but it is very cost-effective as it reduces the nonlinear distortion caused by the power stage by about two times. In addition, the entire amplifier is subject to deep feedback, which runs from the secondary winding of the output transformer to the cathode of the pre-amplifier (the left triode system in the diagram). The implementation of the feedback covering the entire amplifier is possible, among others, thanks to the direct coupling of its first two stages.
The following parts and components are needed to make the amplifier:

All About Electric Guitar - Part I.

Details: Category: Radioamator i Krótkofalowiec; Hits: 2516

Eng. Konrad Widelski
All About Electric Guitar - Part I
Radioamator i Krótkofalowiec Polski, Year 17, September 1966, No. 9

Due to the unflagging interest in electric musical instruments, and especially in such a very popular electric guitar - we are publishing the first part of an article written on this subject. The entire study, consisting of three parts, should give interested persons an answer to their doubts.

An electric guitar differs from a normal (mechanical) guitar in that it requires a suitable amplification device for its use. However, before we take a closer look at this apparatus, we will devote some space to the guitar itself. Its principle of operation is by no means complicated. Figure 1 shows a schematic diagram of the so-called magnetoelectric transducer, which is an essential element of the instrument.

Fig. 1. Construction of a magnetoelectric transducer

Such a transducer consists of a permanent magnet and two spools with a winding made of thin insulated wire, mounted near its poles. The whole thing is placed directly under the steel strings of the instrument. During play, the string set in motion changes its distance from the front part of the magnet. This, in turn, causes changes in the magnetic flux in the system and the induction of electromotive forces in the winding. The electrical voltages produced by the transducer most closely correspond to the vibrations of the string, and thus to the sounds it produces. These voltages should then be suitably amplified and reproduced through the loudspeaker.
The mechanical vibrations of the air generated by the loudspeakers perceive listeners as sound impressions. A block diagram of this type of electroacoustic set is shown in Figure 2.

Fig. 2. Block diagram of the electroacoustic set

You can also use an existing standard mechanical guitar as an electric guitar. For this purpose, a magnetoelectric transducer should be mounted to it. Such transducers are factory-produced and sold in music stores for about PLN 100.
The transducer/pickup can be easily attached to your guitar, following the instructions given in the pickup's factory manual.
Making the transducer yourself, although it is also possible, should probably not be an option, because it is a task (especially in relation to the mechanical part) that is too difficult to do at home.

Stereo on a Shoestring

Details: Category: Electronics Illustrated; Hits: 3218

Ernest Wayland
Stereo on a Shoestring
Electronics Illustrated 1958/10

The most exciting news for the hi-fi music listener since the introduction of the LP record is the development of the stereo disc. Stereo records now available take you from the blaring brass band of the bullfight ring to the whirling three-quarter time of the Strauss walts... and the stereo bandwagon is just beginning to roll/ By the time you read this, the record bins will be chock full of music for every taste - the designed for "two-ear" listening.
The big question is - How much? What do the stereo records and a playback machine costs? The good news is that you don't need the wealth of Midas to set yourself up with three dimensinal hi-fi. The present list price of the stereophonic records averages only about a dollar higher than a standard LP.
How about the playback equipment? Go about it the right way, and you can match the stereo records in cost, with a complete setup for approximately fifteen dollars.

The two stereo channels are fed separately to each of the hearing aid type headphones. The two knobs seen at lower left are optional volume controls

Modern High-End Valve Amplifiers based on toroidal output transformers

Details: Category: Books; Hits: 2917

Ir. Menno van der Veen
Modern High-End Valve Amplifiers based on toroidal output transformers
Elector Electronics, Dolchestr, England, 1999

CONTENTS

About the author (xiii)
Introduction (xv)

Why Valve Amplifiers?
Output Teansformer Specifications
The Output Transformer, Valves and Loudspeaker
The Output Transformer in the Complex Domain
Frequency-Domain Calculations for Toroidal Output Transformers
Theory of Overall Negative Feedback
Output Transformer Low-Frequency Tuning
Special Output Coupling Techniques
Single-ended Toroidal Output Transformers
Building a Push-Pull Valve Amplifier: the Phase Splitter
Building a Push-Pull Valve Amplifier: from 10 to 100 Watts
Read more: Modern High-End Valve Amplifiers based on toroidal output transformers

Build your own audio valve amplifiers

Details: Category: Books; Hits: 9432

Rainer zur Linde,
Build your own audio valve amplifiers
Circuits for hi-fi and musical instruments
Elector Electronics (Publishing) P.O. Box 1414 Dorchester England DT2 8YH,
First Published in the United Kingdom 1995, This Edition September, 1997

Contents

Introduction

To most people working in electronics, the thermionic valve or electron tube is history. Indeed, few of us would be content nowadays without the ease of use, the technical quality, the low weight and the small dimensions of transistorized or integrated electronic equipment. None the less, for many hi-fi enthusiasts and musicians, a valve amplifier still forms the nucleus of their audio equipment. The combination of modern high-quality peripheral equipment (signal sources, loudspeakers) and a classical valve amplifier to come into their own as far as tonal quality is concerned. In modern hi-fi valve amplifiers much attention is paid to obtaining a signal transfer that is as nearly linear as possible. This obviates the need of complex tone control, which is the past were necessary to mask the deficiencies of signal sources then available. For instance, a CD player does not produce stylus noise which, when the audio was young, was combatted by chopping off the higher frequencies so as to obtain an acceptable sound quality (which modern listeners would not tolerate).

Whether it is nostalgia, interest in the technical parameters, the appeal of a gleaming amplifier chassis with softly glowing valves, respect for the technical know-how of earlier generation, or perhaps the firm conviction that the sound of a valve amplifier can not be bettered, it is a fact that this technical tradition, which deserves a place of honour in any science museum, is still in demand. It is particularly gratifying that many of the younger generation admire valve amplifiers. Perhaps this is due to the popularity of the electric guitar.

The field of the thermionic valve is extensive, shown by the enormous amount of literature it attracted in the 1960s.

This book is intended for a broad cross-secion of the public: apart from projects for preamplifiers, power amplifiers, and two amplifiers for musical instruments, aimed at the practical audio/hi-fi enthusiast, it offers much information on the operation od electron tubes, while the first chapter gives a short history of the thermionic valve.

Part 1. GENERAL
- Chapter 1 - History of the thermionic valve (1)
- Chapter 2 - Operation of the electron tube (39)
  Read more: Build your own audio valve amplifiers

Marantz 40/20-watt Power Amplifier - Marantz Audio Consolette

Details: Category: Audio USA; Hits: 3003

Marantz 40/20-watt Power Amplifier - Marantz Audio Consolette
(Equipment Report)
AUDIO, AUGUST, 1956, VOL. 40, No. 8 (Successor to RADIO, Est. 1917).

If the average audiofan were to start building a preamplifier-control unit exactly fo suit his fondest dreams as to performance, absence of hum and noise, flexibility of control, and over-all appearance, it is quite likely that he would come quite close to dublicating the Marantz Audio Consolette - if he had the necessary ezperience, ability, and perseverance. And that is just about what Saul Marantz did, and over many months he worked out the design. The rezult was sufficiently "commercial" to warrant putting the unit on the marked. The performance curves in Fig. 1 show why.

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