Foreword
The term "Audio" derives from the Latin word audire (= to hear). Consequently, audio technology encompasses everything related to (qualitatively high-quality) sound reproduction. In this book, we aim to broaden this definition and focus on the elements necessary for creating listenable recordings. Microphones and amplifiers must be compatible or adapted to each other. In most cases, this is most easily achieved with specialized microphone preamplifiers that boost the signal level sufficiently for processing by a standard line-in input of an amplifier or sound card. This book offers affordable and sophisticated solutions for microphone preamplifiers. Additions to sound cards address a fundamental contradiction that many audio enthusiasts have likely experienced when experimenting with sound cards. On one hand, the PC with its sound card offers creative possibilities that, not long ago, required a visit to a professional recording studio. For instance, software alone can change the tempo of a piece of music, speeding it up or slowing it down without altering the pitch (and vice versa). Another example is filtering out noise using software. On the other hand, cost pressures and mass production mean that standard sound cards are equipped with only basic amplifiers and converters. The multitude of creative possibilities is thus quickly met with a question mark regarding quality. Additional circuits for input or output amplifiers can provide a solution. While domestic audio systems focus on pleasant sound and listening enjoyment, testing recordings emphasizes accuracy and transparency; in short, mercilessness takes precedence over pleasant sound, allowing even small inaccuracies to be perceived during listening checks. In many cases, the recording equipment needs to be easily transportable, so monitor speakers should not be too cumbersome. This book includes construction proposals for power amplifiers and speakers particularly suited for such monitoring applications. The Gigant 2000 power amplifier, which provides ample power for event halls, discotheques, and theaters, rounds off the offering. This book is a collection of articles from the monthly magazine Elektor. Divided into three sections: "Preamplifiers," "Power Amplifiers," and "Miscellaneous" (including speaker construction proposals), it describes and discusses a wide variety of technical approaches. For those familiar with Elektor, the approach is very concrete: theory is presented only as far as necessary for understanding. Everything required for realization follows promptly: precise circuit diagrams with PCB layouts, parts lists, assembly plans, build instructions, procurement advice for hard-to-find components, and alignment tips. No fundamental concept remains unexplored; each topic is pursued through to the tested construction plan. Especially in the aforementioned area, finished devices are quite expensive due to their comparatively low production volumes. Therefore, DIY is particularly worthwhile here, and many initiatives by young bands, school theaters, or chamber music ensembles are only financially feasible this way.
Chapter 1: Preamplifiers
1.1 Microphone Preamplifier with Tubes
By Dr. Götz Corinth
It is a curious phenomenon that in an era of highly developed semiconductor technology and digital audio recording, tube-equipped audio devices are experiencing a resurgence. A construction guide featuring tubes should, however, leverage the advantages of these components and differ from transistor circuits in more than just the height of the supply voltage. The circuit in Figure 1 is an amplifier for connecting a studio microphone with or without phantom power. The input signal from the microphone is fed to the primary winding of a high-quality input transformer from Pikatron (in 61250 Usingen) with a transformation ratio of 1:20. The secondary winding drives the first amplifier stage with B1a/1b. The series tube E88C (or ECC88), connected as a cascode, combines the high gain of a pentode with the low noise of a triode. The grid bias for the 'upper' system is established by the anode current through a very high grid leak resistor, thus eliminating the often difficult operating point adjustment issues. B2a acts as a post-amplifier and passes the signal to the output stage B2b. This cathode follower provides a very low output impedance. The power supply is provided by a classically designed tube power supply unit. For the tube heaters, a stabilized DC voltage of 6.3V is required, supplied by a three-pin adjustable voltage regulator (0.6A). The anode voltage, which is not necessarily stabilized but well-filtered, is 250V at 15mA. The construction of the tube preamplifier is straightforward, provided the usual considerations for high-gain audio frequency devices are observed, such as correct grounding, low-capacitance connections to the transformer's secondary side and thus to the grid of B1a. The requirements for the load capacity of resistors and capacitors are derived from the voltage and current specifications in the schematic. The calculated values should not be undersized! The gain is set to A = 80 dB with trimmer potentiometer P1, and to A = 40 dB with P3, each at the corresponding end stop of P2. The voltage divider can also be replaced by fixed resistors, specifically 130W for P1, 8640W for P3, and 549W, 9760W, 68100W, and 24100W for P2. At the nodes, fixed 10dB gain stages between 40dB and 80dB can then be selected. The microphone amplifier not only meets the rather lenient evaluation criteria of the HiFi industry but also adheres to the strict rules of commercial electroacoustics during development and final measurement on the prototype (source impedance 200W, load impedance 5kW). The result: See the following measurement value table.
Circuit Diagram (Figure 1)
The circuit diagram shows a tube preamplifier circuit. It features two E88CC/ECC88 tubes (B1, B2), a Pikatron input transformer (Tr1, 1:20 ratio), various resistors (R1-R15), capacitors (C1-C12), and potentiometers (P1, P2, P3) for gain adjustment. Power supply voltages are indicated, including a 250V anode supply and 6.3V heater supply.
Tube Pinout Diagram
Pinout diagram for the E88CC/ECC88 tube, showing pin connections for heaters (1, 6), grids (2, 5), plates (1, 7), and cathode (3, 8). Pin 9 is for the second cathode.
Copyright © 2004 Elektor-Verlag GmbH, 52072 Aachen. All rights reserved. Reproduction or distribution of this publication, in whole or in part, is only permitted with the written consent of the publisher. Information herein is published without regard to patent protection. Mentioned software and hardware names may be registered trademarks belonging to their respective owners. While every care has been taken in compiling texts and illustrations, errors cannot be entirely excluded. The publisher, editors, and authors accept no legal responsibility or liability for any errors or their consequences. Feedback on any errors is appreciated. Cover design: Ton Gulikers, Segment, Beek (NL). Layout and typesetting: Jürgen Treutler, Headline, Aachen. Printing: Veiters, Riga (LV). Printed in Latvia. ISBN 3-89576-152-4. 1st Edition, 2004. Elektor-Verlag GmbH, Aachen. www.elektor.de
