. . . . . .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.
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. Althoughnot presently availabledue to difficulties with single-source supply
the data sheet is nonetheless an interesting download.
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
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 . . .
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.
- 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.
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.
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.
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.
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
Much information useful in the assessment of materials for diaphragm applicability is given.
[...] 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.
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.
- 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.
- 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.
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.
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
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.
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.
To some observers of the current  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.
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.
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
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 . . .
- 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.
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.
- Physical Processes of Cutting Gramophone Records - Collected by Fritz Nygaard
, 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.
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.
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,
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.
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.
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
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.
- 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.
- 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 . . .
- 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.
- 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. [ . . . ]
- Measurement of Noise Voltage in Chemical Batteries - Chadwick K. Boggs, Alan D. Doak, F. L. Walls
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
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
- 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,
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.
- 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.
- 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
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.
- 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.
- 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.
- 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
From the final edition, "The Catgut Acoustical Society Story" is here.
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. [ . . . ]
- Critical Damping: Missing Link in Speaker Operation Parts 1 & 2- D. J. Tomcik; Chief Engineer,
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? [ . . . ]
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
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."
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.
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.
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 . . .