Universal Feedback Amplifier Circuit (Audio January 1960)

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Category: Audio USA

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Universal Feedback Amplifier Circuit
ARNOLD J. KAUDER (Principal Engineer, Bendix Aviation Corporation, North Hollywood, California)
AUDIO, January, 1960, VOL. 44, No. 1
A simple amplifier of exceptional performance which should be adequate for practically any installation is the basis for this article, but its greatest value lies in the "universal" instructions for adjusting any feedback amplifier.
(Note: The original notation of units used at the time of writing the article has been preserved.)

   The amplifier to be described has performed well with five different output transformers, which has led the writer to use the designation "universal." The amplifier has in each case been found completely stable with (a) no load, (b) 8-ohm resistive load, (c) 8-ohm loudspeaker load, and (d) a 0.1 μF capacitor load added to any of the load conditions of (a), (b), or (c) above. The feedback factor employed has been 20 db ± 1 db.

   Few of the "Williamson Type" and other amplifiers seen by the author have been capable of meeting such a stability test. Breathing of the loudspeaker cone, due to very-low-frequency oscillations, and supersonic oscillations readily seen on an oscilloscope are all too common. Either type of oscillation can produce negative charges on the grid sides of the output-tube coupling capacitors, with distortion and limited power output resulting. Marginally stable amplifiers have also been observed which do not normally oscillate, but are highly regenerative at extreme frequencies and do oscillate when audio signals with steep leading edges on the waveforms are fed to the input terminals.

   A brief history of the development of the circuit is believed to be of interest and is as follows:

Development

   The author was a "high fidelity" fan many years ago and is still not ashamed of the performance of a class-A push-pull 2A3 triode amplifier (power output of 7 watts) still on hand. After a lapse of 10 years, a reviewed interest in high fidelity led to a study of feedback and the present day amplifiers which have achieved recognition in the literature. The writer found to his annoyonce that it was not possible to duplicate a published amplifier circuit and employ a different output transformer and a more compact layout - unless extensive redesign of the coupling and feedback circuits was carried out.

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

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Category: Audio USA

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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:

1. The output transformer was not designed for the manner in which it was operated.
2. The output transformer secondary was at a small d.c. potential above ground.
3. 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.

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Push-Pull in HiFi (Audio 1959/4)

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Category: Audio USA

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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.

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How can you extend the life of your battery tubes

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Category: Radioamator i Krótkofalowiec

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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

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Category: Audiocraft

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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.

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All About Electric Guitar - Part II

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Category: Radioamator i Krótkofalowiec

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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:

Read more: All About Electric Guitar - Part II

All About Electric Guitar - Part I.

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Category: Radioamator i Krótkofalowiec

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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.

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Stereo on a Shoestring

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Category: Electronics Illustrated

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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

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Modern High-End Valve Amplifiers based on toroidal output transformers

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Category: Books

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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)

1. Why Valve Amplifiers?
2. Output Teansformer Specifications
3. The Output Transformer, Valves and Loudspeaker
4. The Output Transformer in the Complex Domain
5. Frequency-Domain Calculations for Toroidal Output Transformers
6. Theory of Overall Negative Feedback
7. Output Transformer Low-Frequency Tuning
8. Special Output Coupling Techniques
9. Single-ended Toroidal Output Transformers
10. Building a Push-Pull Valve Amplifier: the Phase Splitter
11. Building a Push-Pull Valve Amplifier: from 10 to 100 Watts

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