PEARL, INC. - Miscellaneous Downloads


 
 


 
    . . . . . .This page is something of a precursor to the construction of our site. Here we've made available a number of PDF files that will eventually be available from the site's main navigation pages. In the meantime, and so that those viewing posts archived on various list servers will find live links in the future, we'll keep this page as permanent resource.
 
 


 

 
 
  001 - PEARL CryoValve Tubes
One fairly comprehensive page of process description followed by several pages of customer comments -- alright, raves -- on the sound of these tubes. The latest version includes a HiFi News and Record Review interview with Ed Meitner of EMM Labs wherein mention is made of the benefits that accrue to cryo-treated vacuum tubes.

1.4
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  005 - PEARL RB300 3CX Data Sheet
The RB300 3CX is a 300W anode dissipation rated, metal-ceramic, audio triode intended for low-velocity (i.e. quiet) fan cooling. Although not presently available due to difficulties with single-source supply the data sheet is nonetheless an interesting download.

625
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010 - QUAD ESL-57 Rebuild & Modification
245 pages of information on the QUAD ESL-57 that includes:

  • A 2009, Ken Kessler article on the ESL-57,
  • PEARL's complete rebuild of the original QUAD service information,
  • a compendium of Sheldon Stokes excellent work on repair and modification downloaded from here,
  • several pages from Stewart Penketh on panel module repair along with info on setting up the '57s in pairs and quads,
  • Peter Walker and D. T. N. Williamson's 1955, 3 part, Wireless World series of articles on ESL design,
  • the original UK and subsequent US versions of Walker and Williamson's patent on ESL technology in general and the ESL-57 in particular. All the patents have been rebuilt from the scrappy looking patent office originals into rather more presentable, readable, form . . .

12
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  015 - Altec Lansing 260A Product Literature
This is a rebuilt version of the original literature for the Altec/Peerless, push-pull, 813-based 260W, 183lb power amp that while commonly mistaken for a boat anchor is in fact an astonishing performer once slightly modified.

576
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  020 - Altec Lansing "Voice of the Theater" Product Literature
High quality , full color scans of a 1960s vintage, 5 page, original Altec brochure. A nice little nostalgia trip . . .

3.4
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  025 - JFET Input, UL Cascode, Passive RIAA Stage
While I don't suggest this is a "world beater," it has handily trashed any number of commercial offerings. It's presented here as a "freebie" and that's as far as the offering goes. If tech support is desired, you'll have "pay the piper" for the time it'll take to sort through any inquiries.

884
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  030 - W. Marshall Leach Jr. on the subject of noise
Downloaded from here and then collated into one large, navigable PDF.

4.8
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  045 - Fostex NF-1, Near-field Monitor
This is a very interesting speaker. Fostex have done some excellent research and FEA work to arrive at a cone shape not previously seen, one that opens a whole new line of inquiry for this developer.

4.1
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  048 - Ironless Loudspeakers - Guy Lemarquand et al.
We note the drawbacks of classical loudspeaker motors: the inductance varies with the coil s position, there is a reluctant effect and Eddy currents appear because of the iron in the motor. We then present ironless structures of loudspeaker motors to eliminate these drawbacks. These structures are studied with the use of Coulomb s model of permanent magnets, which affords analytical calculations. Thus the design can be optimized to create a uniform, high level induction in the space where the coil moves.

20
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  049 - Magnet-only Loudspeaker Magnetic Circuits: A Solution for Significantly Lower Current Distortion - Benoit Merit & Antonin Novak
This paper presents a study case which leads to a significant decrease of current distortion caused by the magnetic circuit of the loudspeaker: the solution consists in removing all of the nonlinear soft-iron pieces that are used in traditional loudspeaker motors. Thus, in the loudspeaker motors we study, the static magnetic field is only created thanks to a combination of permanent magnets. This combination leads to the realization of the so-called magnet-only loudspeakers.

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  050 - Cone and Diaphragm Materials
Published by Pioneer Corp. about 1982, this paper is an excellent and thoroughgoing technical review of then-recent developments of materials for loudspeaker diaphragms in Japan. Detailed are developments of beryllium and boronized titanium diaphragms for high frequency applications and carbon-fiber reinforced olefin and polymer-graphite materials for use, in the former case in mid- to low frequency applications and in the latter, applications across the audio band.
Much information useful in the assessment of materials for diaphragm applicability is given.

1.9
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  055 - Direct Radiator Loudspeaker Enclosures - Harry Olson; Audio Engineering, Nov. 1951 !
[...] The third factor, namely, the exterior configuration of the cabinet, influences the response of the loudspeaker system due to diffraction effects produced by the various surface contours of the cabinet. The diffraction effects are usually overlooked and the anomalies in response are unjustly attributed to the loudspeaker mechanism. Therefore, in order to point up the effects of diffraction, it appeared desirable to obtain the performance of a direct-radiator loudspeaker mechanism in such fundamental shapes as the sphere, hemisphere, cylinder, cube, rectangular parallelepiped, cone, double cone, pyramid, and double pyramid.

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  060 - Misleading Measurements - Raymond Cooke; KEF Electronics, UK
It is not widely appreciated that acoustical measurements are prone to inaccuracies and that interpretation calls for a degree of experience and insight which is probably unequaled outside of medical diagnosis.
Testing out of Doors: There are in fact very few really large anechoic chambers of first class quality in existence and of these, still fewer are available for loudspeaker measurements. For this reason measurements out of doors are often resorted to in an attempt to obtain precise results.

1.0
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  061 - Shaped Tone Burst Testing - Sigfreid Linkwitz; Hewlett-Packard, 1980
A properly shaped tone burst is used to evaluate the dynamic behavior of a loudspeaker within narrow frequency bands. The raised-cosine envelope of a five-cycle burst reduces the low-frequency content of the test signal and confines the spectrum to a one-third-octave width.

4.0
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  062 - Time-frequency Display of Electroacoustic Data Using Cycle-octave Wavelet Transforms - Don Keele, Jr; DBK Assoc.
A cycle-octave time-frequency display is created by plotting the magnitude of the wavelet transform, using a Morlet complex Gaussian wavelet, on a log-frequency scale versus time in number of cycles of the wavelet's center frequency.

7.0
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  063 - Detection of Audible Resonances in Loudspeakers - Ivo Mateljan; Faculty of Electrical Engineering, R. Boskovica bb, 21000 Split (ARTA), Heinrich Weber, Ante Doric. The paper discusses the problems of the detection and analysis of loudspeaker resonances, audible and not. Basic psychoacoustic researches by Fryer and Toole and Olive have shown that the threshold of resonance detection can be classified by resonance level and Q-factor. In this present work a third criteria is introduced, the energy of the resonance. By analyzing the influence of resonances on the frequency response and group delay, it is shown that it is almost impossible to detect resonances that are near the threshold of audibility.
Perhaps most importantly, the author introduces the concept of Transfer Function Identification (TFI) by which means the resonant frequencies, Q-factors and levels with respect to the output level of the loudspeaker are clearly displayed; an important and to date completely overlooked development.
The PDF contains not only the original paper but many of its cited references and many of their references as well as links to several others in book form present here and not included due to size. 713 pages.

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  064 - Resonance Identification in Loudspeaker Driver Units: A Comparison of Techniques - S. J. Loutridis
A number of techniques with the scope of identifying loudspeaker cone resonances have been examined. The cumulative spectral decay waterfall plot (CSD), the wavelet transform and the empirical mode decomposition (EMD) scheme were compared on the basis of time frequency resolution and damping estimation. The commonly used waterfall plot is only acceptable at the upper range of the acoustic spectrum. The wavelet transform is especially well suited for the analysis of transient signals from loudspeakers and is a significant improvement over the waterfall method. The newly developed EMD scheme has the highest potential in separating the modal components. By application of the EMD both instantaneous amplitude and frequency can be accurately determined.

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  065 - A Multi-physical Loudspeaker Model Including Breakup Membrane Modes; Parts 1 & 2 - Yamaha Corp, Japan & KU Lueven, Mechanical Engineering, Belgium
In this paper, a coupled electro-vibro-acoustic model of a loudspeaker is proposed. To model the loudspeaker membrane, rather than using an experimental approach, mode shapes, resonance frequencies and modal damping are obtained from Finite Element Method (FEM) and then used to perform numerical simulations. The predicted vibration amplitude on the membrane is properly scaled considering the effects of the back electromotive force caused by the electro-magnetic system.

15.8
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  069 - Power Distortion - Kurt Steffenson
Written by Kurt Steffenson of JoeList and TriodeFest fame and edited by yours truly this is a post to the JoeList made about 2000. Kurt engages the matter of "current driving" bass units in a most intriguing article.

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  075 - The High-End Mythology of the Toroidal Power Transformer
If you look at the AC power transformers used in most high-end audio equipment these days, you will find that a very large majority are toroid transformers. These donut-shaped transformers seem to have taken the high-end industry by storm. Their major advantage is that they do not radiate much of a magnetic field--a very useful property. Dealing with stray magnetic fields from EI-frame transformers (non-toroid) inside a high-end component is not a trivial undertaking. While toroidal transformers have one significant advantage regarding radiated magnetic fields, toroids have a number of "problems" that severely limit their performance in high-quality audio equipment. We'll try to help you understand what these problems are and show you how another kind of transformer, the EI-frame transformer can be a superior performer if designed, manufactured and installed properly.

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  083 - Kiseki Gems - Jimmy Hughes Samples Five Kiseki MC Cartridges, one of which costs £3,500 - New HiFi Sound
To some observers of the current [1987] hi-fi scene it must seem now as though that most exotic of all audio products - the expensive moving-coil pickup cartridge - is very much an endangered species. CD is increasing in popularity with each passing day, while a number of smaller record companies have ceased issuing titles on LP altogether.

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  084 - The Wear and Care of Vinyl Records and Styli - Harold D. Weiler; Shure Bros, USA - 1954
This booklet is dedicated to the thousands of record users throughout the world, whose cooperation made it possible. Their replies to a survey conducted by the author showed the necessity of providing an easily understandable book on the wear and care of records and styli, and also served as a basis for our tests. These tests proved conclusively that the average record user is damaging his records and is not obtaining the high quality of reproduction which can be expected from modern recordings.

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  085 - Tonearm Geometry and Setup - Martin D. Kessler & B. V. Prisha
Optimum geometry of tonearms has been the subject of several articles over the last three decades, the earliest complete mathematical study being that of H. G. Baerwald in 1941. His analytical study of tracking-error distortion showed that optimum geometry of a tonearm of given effective length will have a corresponding offset angle and overhang.

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  086 - More Than One Vertical Tracking Angle - Jon Risch & Bruce Maier; Discwasher Corp.
The authors go to some lengths to illuminate the differences between vertical tracking angle (VTA) and stylus rake angle (SRA); the all important differences amoung the shapes of various forms of styli and other frequently overlooked considerations when properly aligning a phono cartridge for lowest distortion playback. Conical styli are shown once again to be the very worst of all worlds . . .

1.8
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  087 - Historical Development of Analogue Disk Recording - Takeaki Anazawa; sourced from: "National Museum of Nature and Science Technology Systemization Report; Vol. 21 - March, 2014". The history of analog recordings dates back to 1877 when American inventor Thomas Edison came up with a new phonograph that enabled users to record sound onto a recording cylinder and replay that audio. In 1887, just 10 years later, German inventor Emile Berliner created the gramophone.

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  088 - Basic Disk Mastering - Larry Boden A primary need of anyone interested in a close study of the disc recording procedure is a firm and complete grasp of the physical nature of record grooves. Perhaps the best way to do this is to thoroughly explain how monophonic grooves work first, and then move on to stereo.

9.5
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  089 - Physical Processes of Cutting Gramophone Records - Collected by Fritz Nygaard
Sound recording , i.e. the recoding and storing of sound signals aimed at later reproduction, plays an extraordinarily large part in today's communications systems and has a great importance in the history of civilization.

3.8
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  090 - Amperex Electron Tubes and Semiconductors; 1956 thru 1963 - Amperex Electronic Corp, Hicksville. NY
Four catalogues: Amperex Tubes for HiFi, 1956; Amperex Electron Tubes and Semiconductors, 1958; Amperex Electron Tubes, 1962 and Amperex Electron Tubes, 1963.

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  091 - General Electric 5-Star Tubes; 1952 - Tube Dep't, General Electric, Schenectady 5, NY
G-E Five-Star tubes are specially designed, manufactured and tested; they are not "selected" tubes.
The Five-Star high-reliability line includes tubes capable of performing virtually every electronic function required of receiving types in critical applications where operational dependability of tubes is essential to protect life, customer's investment or product reputation.

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  092 - RCA 5691, 5692 & 5693 Special Red Tubes; 1948 - Tube Division, RCA Corp, Harrison NJ
The present "Special Red Tubes" include a high-mu twin triode, 5691; a medium-mu twin triode, 5692; and a sharp-cutoff pentode, 5693. They are for industrial applications where 10,000 hour life, rigid construction, uniformity, and stability are paramount. The electrical characteristics of the 5691,5692, and 5693 are very similar to those of the 6SL7-GT, 6SN7-GT, and 6SJ7, respectively.
Also included is copy of the, "Performance Evaluation of 'Special Red' Tubes" from the RCA Review, Sept. 1953.

12
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  093 - RCA Nuvistor Tubes for Industrial and Military Applications; 1963 - Electronic Components and Devices, RCA Corp, Harrison, NJ
Developed by RCA, the nuvistor design utilizes a lightweight, cantilever supported, cylindrical electrode structure housed in an all-ceramic-and-metal envelope.
This unique design represents a combination of materials, processes, and fabrication techniques that is a major breakthrough in electron tube performance, reliability, and size. This combination of strong structural assembly with all-brazed connections and seals, all-ceramic-and-metal construction, and high-temperature processing provides very small tubes which give dependable performance under extreme environmental conditions such as thermal or mechanical shock, continuous vibration and high temperature.

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094 - Sylvania Gold Brand Electronic Tubes; ca 1955

Noteworthy qualities of Sylvania Gold Brand tubes are:
  • 10,000 hour life, exceptional stability, maximum uniformity and extreme physical ruggedness.

These qualities are designed and built into Gold Brand tubes through:

  • specialized processes, materials, and methods borne of continuous research programs in metallurgy, chemistry and basic tube technology. The features built into Gold Brand tubes and their contributions to the elimination of common failure mechanisms are described on pgs. 4 and 5,
  • uncompromising quality standards that are rigidly maintained at each development stage. The extent of this control program is best illuminated by the fact that one in three plant employees is engaged in quality control,
  • automated production of tube parts and sub-assemblies for maximum uniformity.

More info is seen here.

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  095 - Sylvania FRAMELOK GRID; 1958 - Sylvania Electronic Products, NY, NY.
Sylvania's radically new frame grid concept christened the FRAMELOK GRID mechanically locks in alignment with other tube elements whilst its mass dissipates large amounts of heat thus minimizing a cause of thermal misalignment and spurious screen current. In fact, the current drawn by a FRAMELOK screen grid is approximately half that of a conventional screen grid.
This reduction in screen grid current alters the tube so that a FRAMELOK GRID tube is not a universal replacement for similar conventional tubes in presently marketed equipment. However, as with most new component innovations, the FRAMELOK GRID will help improve future equipment designs.
Also included is copy of, "A Frame Grid Audio Pentode for Stereo Output" from the "IRE Transactions on Audio", July-August, 1959.

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  096 - Tung-Sol 5881 - A New Beam Power Tube - Tung-Sol Lamp Works, Inc., Newark 4 , NJ.
Power output tubes get get rough treatment. In the endeavor to obtain maximum output, amplifier designers frequently operate the tubes at, and sometimes beyond, established ratings. This has been especially true in the case of the 6L6 and its glass equivalents, the 6L6G(A). As a result, failures are sometimes too common, particularly in continuous- duty service. Some of these tubes stand up remarkably well, but different production runs of the same tube type often exhibit considerable variability in marginal operating environments where some special characteristic is being exploited or where it is necessary to rely upon the stability of a certain parameter under extreme conditions. [...]
Also included is an interesting study of secondary emission from nickel and carbon anodes in the 813 power pentode done by PEARL, Inc about 1995.

2.0
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  099 - Strategies to Repair or Replace Old Electrolytic Capacitors
Unfortunately, the selection of high voltage electrolytic capacitors today is both smaller and different from the past, so the chances these days are you won't find an exact replacement for your original equipment electrolytic.
For low voltage applications, like cathode bypass capacitors, most vintage types have an axial configuration, which is less common today but still available. The more modern radial configuration can also be used if their leads are long enough and they don't violate your notions of aesthetics . . .

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  100 - General Description of and Application Guidelines for Aluminum Electrolytic Capacitors - Nichicon Corp., Japan An aluminum electrolytic capacitor consists of cathode aluminum foil, capacitor paper (electrolytic paper), electrolyte, and an aluminum oxide layer, which acts as the dielectric, formed on the anode foil surface. A very thin oxide layer formed by electrolytic oxidation (formation) offers superior dielectric constant and has rectifying properties. When in contact with electrolyte, the oxide layer possesses an excellent forward direction insulation property. Together with magnified effective surface area attained by etching the foil, a high capacitance yet small sized capacitor becomes available.

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  103 - Electrolytic Capacitors from Inception to the Present - Jens Both
The 120-year history of electrolytic capacitors is the story of a simple idea, the use of an anodically formed oxide on aluminum as a capacitor dielectric, which grew into a large range of products which address electrical and electronic applications.
Although we usually associate electrolytic capacitor with the small cylinders on our computer motherboard or somewhat larger cylinders in power supplies, electrolytic capacitors have grown to a large family that includes both liquid and solid electrolyte technologies of varying anode metal oxide dielectric which cover the voltage range from about 2 to 630 V ratings.
The history of electrolytic capacitors starts in the early days of electric power (1890s), around the time that the three-phase universal system was first being established. [ . . . ]

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  105 - Measurement of Noise Voltage in Chemical Batteries - Chadwick K. Boggs, Alan D. Doak, F. L. Walls - NIST
Ultra low noise voltage sources are often required in measurement systems and other applications. Common voltage regulators have performed inadequately in some applications. As an alternative, battery cells have been used. Of the various types, Hg cells have been credited with the best performance. However, actual values for the voltage noise in batteries have not, to our knowledge, been reported. In this paper a measurement system capable of measuring voltage noise below ·20dB nV/rtHz [about 20 dB below the equivalent input noise of the quietest JFETs and BJTs of all time ! ] is discussed and its ability to characterize experimentally high performance voltage references is explored. The results of such measurements on common batteries are presented, and potential applications are considered.

References included:

Thermal Agitation of Electric Charge in Conductors - H. Nyquist; Physical Review, July, 1928
"Dr. D. B. Johnson has reported the discovery and measurement of an electromotive force in conductors which is related in a simple manner to the temperature of the conductor and which is attributed by him to the thermal agitation of the carriers of electricity in the conductors. The work to be reported in the present paper was undertaken after Johnson's results were available to the writer and consists of a theoretical deduction of the electromotive force in question from thermodynamics and statistical mechanics."

Thermal Agitation of Electricity in Conductors - Dr. J. B. Johnson; Physical Review, July, 1928
"In two short notes a phenomenon has been described which is the result of spontaneous motion of the electricity in a conducting body. The electric charges in a conductor are found to be in a state of thermal agitation, in thermodynamic equilibrium with the heat motion of the atoms of the conductor. The manifestation of the phenomenon is a fluctuation of potential difference between the terminals of the conductor which can be measured by suitable instruments."

Design Considerations in State-of-the-Art Signal Processing and Phase Noise Measurement Systems - F. L. Walls, S. R. Stein, James E. Grey and David J. Glaze - Frequency and Time Standards Section, National Bureau of Standards, Boulder, CO, USA.
"Introduction - The recent rapid improvement of oscillator phase noise has resulted in significantly more stringent requirements for signal handling equipment. However, information concerning the phase noise performance of the two most important types of circuits - amplifiers and mixers - is often difficult to find. Some general principles are presented which allow one to estimate the phase noise performance of an amplifier. Also, techniques are described which permit one to obtain the best possible results from the traditional double balanced mixer. A measurement set-up which has 15 to 25dB improvement in its noise floor is shown in detail to illustrate proper mixer drive and termination. Although traditional circuits can with extreme care achieve [phase noise of] -175 dB or slightly better, this is not sufficient for all present requirements. One technique to obtain an additional improvement of 10 to 40 dB in measurement system noise is to reduce the mixer and amplifier contributions to the noise floor by the use of correlation techniques. A circuit to accomplish this is discussed along with some preliminary results."

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  110 - Animated FEA of a Woofer Basket - Done by Ulrik Skol when he was running iCapture.dk, this well illustrates the sorts of deflections commonly seen in such structures; exaggerated for clarity.

 

 

 

  115 - CompCore 1.5 - Design Analysis Software for Magnetics Engineers - National Magnetics Corp. 1991, now MK Magnetics, Inc..
CompCore is a magnetic design analysis program written [in DOS] for the IBM PC and IBM PC compatible computers. It evaluates varied design criteria and applies it to tape wound cores accessed from an extensive National Magnetics Corp. database. The program provides input screens featuring project and name identification, input parameters, expected magnetic performance and completed transformer estimate.

Manual
6.2 MB
DOS App
2.1 MB

   
  120 - Vibration and Radiation Behaviour of Loudspeaker Cones - F. J. M. Frankort (Reprinted with permission from the Philips Technical Review. Vol. 36. no. I. pp. 1-15; 1976 - Included in the references).
Many treatments of loudspeakers begin with the comment that the loudspeaker is the weakest link in the audio reproduction chain. The complex vibrational behavior of the conical loudspeaker diaphragm which noticeably affects the frequency response of the loudspeaker has been a frequent challenge to mathematical analysis. Only recently, however, with the availability of the computer to carry out extensive numerical computations, has it been possible to obtain any detailed picture of the behavior of the loudspeaker cone. The picture gives a satisfactory explanation of the irregularities in the behavior of the cone as a radiator which had long been known from acoustic measurements. At the same time it has now become possible to indicate the dimensions and material properties that will produce the desired frequency response.

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  125 - Frequency Dependent Mechanical Properties of Violin Varnishes and Their Impact on Vibro-mechanical Tonewood Properties - Various Swiss Federal Laboratories, 2001. Violin varnishes influence the vibrational properties of tonewood. However, the frequency dependence of the varnish influence and mechanical properties of typical varnishes has received little attention. The viscoelastic properties of various violin varnish materials over the audible frequency range were characterized by dynamic mechanical analysis. The properties of the studied varnishes showed comparable frequency dependencies. For all varnishes, E [Young's modulus] increased and tan delta [loss factor] decreased with increasing frequency.
The PDF contains not only the original paper but most of its cited references and many of their references as well as several other relevant references. 251 pages.

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  130 - Interface Micro-discharges - Pierre Johannet The present invention relates to a manufacturing process of an electrical conductor or circuit compensated for radio interferences such as micro-discharges, and an electrical conductor or circuit obtained by this process.
In the field of processing of electrical signals then their storage or their transformation into sensory phenomena directly perceived by human physiological receptors, numerous works have been carried out up to the present in order to maintain, indeed improve, the signal to noise ratio after each transformation, due to the processing, with the object of improving the reproduction and therefore the perception of these sensory phenomena.
Such concern is not moreover specific to the single field of sensory phenomena, such as the reproduction of sounds, but appears also in the much wider field of electronic signals creation, their transmission, their storage and their use by electronic or electrical transducers specially adapted to this purpose.

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  135 - Chladni Patterns in Vibrated Plates - Dep't of Physics; U of Toronto, Canada
Chladni patterns are a classic undergraduate demonstration. You can visualize the nodal lines of a vibrating elastic plate by sprinkling sand on it: the sand is thrown off the moving regions and piles up at the nodes. Normally, the plate is set vibrating by bowing it like a violin. It helps to put your fingers on the edge to select the mode you want, much like fingering the strings of a violin. This takes some practice.

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  140 - The Catgut Acoustical Society Journal - Published semi-annually and complete here from May, 1964 to May, 2004, the final edition.
The 40-year existence of the Catgut Acoustical Society represents a tiny but important segment of man's age-old fascination with and need for understanding sound, not only for survival but also for communication and enjoyment. Primitive man created stringed, wind, and percussion instruments that have become further sophisticated in various ways in different societies over many millennia.
Research related to CAS objectives can be traced back to the work of Michael Praetorius in 1619, and to numerous scientists and mathematicians of the 18th, 19th, and early 20th centuries (please see my paper entitled "The History of Violin Research"). This important work set the stage for the eventual founding of the CAS in 1963.
From the final edition, "The Catgut Acoustical Society Story" is here.

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.zip File

 

Excerpted from the PEARL Archive
 
  374 - Use of an Ideal Matching Transformer for Impedance Matching
Excerpted from "Applied Electronics" published by the MIT Press

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  1150 - Inherent Feedback in Triodes - H. Stockman; S. D.
The triode is imagined to be replaced by an infinite- impedance pentode with a fictitious EMF in the grid circuit to represent the back action of the anode on the field at the cathode It is shown how this transformation makes it possible to obtain practical triode circuit formulę from conventional feedback theory. [ . . . ]

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  1385 - Critical Damping: Missing Link in Speaker Operation Parts 1 & 2 - D. J. Tomcik; Chief Engineer, Electro-Voice, Inc.
In audio reproduction, a subject of considerable importance to the high-fidelity enthusiast is amplifier damping factor and its effects on speaker operation. Misconceptions have arisen concerning this subject, and vague and incomplete answers have too often been given to the many questions involved.
Are the high damping factors found in present high-fidelity amplifiers byproducts of high-feedback circuits and, as such, unimportant in the operation of the system? Or is the ultimate, as some loudly proclaim, to have the highest possible damping factor built into the amplifier? Why does a particular speaker sound better with amplifier A than with amplifier B, although both show identical frequency response and power capabilities under bench checks? Why does that $2.00 speaker with the 6-ounce magnet (inefficiency and distortion included) seem in some cases to have more bass than the high-fidelity unit with the 5-pound magnet? Why is it that one enthusiast found reproduction more pleasing when he used a little current feedback from the output circuit yet another didn't when using the same circuitry? [ . . . ]

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  1386 - Loudspeaker Damping, Parts 1 & 2 - Albert Preisman
Part 1. A discussion of theoretical considerations of loudspeaker characteristics, together with a practical method of determining the constants of the unit as a preliminary step in obtaining satisfactory performance.
"One of the considerations in the design and application of loudspeakers is the adequate damping of their motion. Thus, owing to the masses and compliances involved, the sudden application or removal of current in the voice coil tends to produce a transient oscillation of a damped sinusoidal nature. In particular, the sudden cessation of current in the voice coil may find the loudspeaker continuing to vibrate in the manner described, so that the sound "hangs over". Anyone who has experienced this unpleasant effect will seek ways and means to eliminate it."

Part 2. A discussion of theoretical considerations of loudspeaker characteristics, together with a practical method of determining the constants of the unit as a preliminary step in. obtaining satisfactory performance.
"We come now to the question of damping of the loudspeaker mechanism by the electrical circuit. In Fig 3 is shown the electrical equivalent of a loudspeaker illustrated in Fig. 2, with the addition of an electrical source of internal resistance RG feeding it. This normally represents the Rp of the output tube or tubes as viewed from the secondary terminals of the output transformer."

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  1387 - Acoustic Damping for Loudspeakers - Benjamin Bauer; Shure Brothers, Inc. Chicago, Ill
The fundamental resonance of loudspeakers is recognized by many as a source of annoyance. Usually this resonance can be damped electrically by suitable selection of the amplifier impedance.
What is less well known is that damping can also be achieved by acoustical means incorporated into the loudspeaker or the enclosure.
This paper deals with the theory and methods for providing acoustic damping. It is concluded that transient response of loudspeakers and enclosures can be effectively controlled by acoustic damping. Furthermore, the response-frequency characteristic of the loudspeaker system need not be adversely affected, and it actually may be improved. Loudspeakers with acoustic damping may operate from high-impedance amplifiers without "hang-over." Performance characteristics become largely independent of the amplifier impedance.
Acoustic damping may be designed in a straightforward manner by ascertaining the acoustical constants and using standard experimental techniques of equivalent circuit analysis. We conclude, therefore, that acoustic damping for loudspeakers merits far more serious consideration than it has had heretofore.

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  1389 - Loudspeaker Enclosure Design - Damping with Acoustic Resistance - Parts 1 & 2 - E. J. Jordan
A seminal piece of work that as far as I know has been completely ignored. Written in 1956 by E. J. Jordan , then in the employ of Goodmans Industries Ltd. UK, the paper outlines the manifold advantages of bass loading using what he called an "Acoustic Resistance Unit," an ARU, to control low frequency damping.
Completely unaware of Jordan's work, I independently developed this same method about 1980 and have used it with great success ever since. I called it "Distributed Acoustic Impedances" or DAMPS and in the estimation of many, many listeners across some 3 decades it is the best bass loading method yet developed, far surpassing the ubiquitous Thiele-Small alignments for "reflexive," or, resonant, enclosures. Additionally it's group delay numbers border on these produce by first order filters such ESLs and are below researched audible thresholds.

4.3
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  1816 - Twinning Push-pull Audio Output Transformers - Bill Perkins, PEARL, Inc, Calgary, AB
This is a very brief, schematic description of a method for connecting pairs of single coil push-pull output transformers that materially improves the AC and DC balance of the composite pair so formed. AC balance is very important to low-value, high frequency distortion and feedback loop stability and is typically very poor in conventionally wound push-pull output transformers. The measurements shown for a Dynaco ST 70 output transformer illuminate the problem very well . . .

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