THÖRESS

"Behind The Curtain"

THÖRESS P.A.A. was founded after I graduated in mathematics (subsidiary subject physics) at the RWTH University of Aachen. My laboratory is located in the centre of Aachen in walking distance to the famous Cathedral constituted by Charles the Great in the late seventh century. The splendid core of the early medieval edifice, known as Carolingian Octagon, attracts tourists from all over the world regularly. Founded by the Romans, Aachen is a city with a very long and turbulent history located close to the border triangle where the territories of Germany, Belgium and the Netherlands meet, about 50 miles away from Cologne and Düsseldorf.

My hometown is blessed with an opera house, a modestly sized concert hall (both of which offer favourable acoustics) and a division of the Cologne Conservatory of Music, which regularly offers all kinds of concert events whith free entrance to the public. The orchestra and the choir of the Bach Society of Aachen gratify music lovers with passionate performances of J. S. Bach-s sacred music in local churches regularly. Whereas the Society of Modern Music of Aachen presents appealing concert events of modern Jazz, Avant-garde and Experimental Music held in a dedicated modestly sized auditorium, again endowed with enjoyable acoustics. Concert events in Düsseldorf and Cologne are within reach as well. So the region of Aachen is by all means a favourable area for a music lover to strike root!

MUSIC... has always played an important role in my life. I do appreciate all kinds of musical styles... Classical Music, Jazz, Rock, Blues, Metal, Fusion, Funk, Disco, Pop...! It never occurred to me that one particular music genre is more valuable than any of the others. There is only good and bad music. And I firmly stick to this attitude until the present day. Accordingly, it is my firm conviction that a truly world-class audio system should be designed to be NEUTRAL and UNIVERSAL in the sense that every kind of program can be reproduced in an equally appealing and convincing way. Which particularly includes the requirement that the system is capable of reproducing the timbre of ACOUSTICAL instruments with utmost accuracy, of course. Otherwise the system will narrow the musical horizon of the user, which from my point of view is an intolerable constraint, if not a sin...!

In my childhood I was so fortunate to own one of those large 1960s vacuum tube AM+FM radiograms. Made by Philips. It was an outstanding if not avant-garde piece of audio gear which I only fully appreciated decades later on...featuring a 800 ohm (!!!) oval paper cone full-rage transducer fired by a unique single-ended push-pull (!!!) output-transformer-less (!!!) vacuum tube power stage (launched by Philips along with the EL86 tube in the late 1950s, the famous so called EISENLOSE ENDSTUFE, iron-less power amplifier). I truly adored this old machine as it day-inday-out delivered fantastic sound quality flawlessly. So no wonder that I had my beloved tube receiver still in use even when I reached my teens and solid state audio gear had become prevalent.

Since my early teenage years I have also attended all kinds of live concerts regularly and such became acquainted with numerous concert halls, opera houses, churches and jazz clubs within reachable distance to my home. In each auditorium of which I over the time had figured out specific sweet spots where I was keen to take seat on concert events. Fortunately, expensive seats were rarely blessed with extraordinary listening quality, which kept my concert affairs in the affordable range. After school classes, I used to travel to Düsseldorf frequently where I would hunt for hot vinyl program in various record stores until closing time, so as to move on to TONHALLE, SCHUMANN SAAL or some church in order to attend a thrilling concert event. On other occasions, I travelled to Cologne, to go to the opera house or the SENDESAAL DES WDR to listen to the world famous WDR Big Band or to take part in some avantgarde music happening. Well before these auditories opened their doors it was my habit to drop by the fabulous Saturn Media Store (which at that time was the biggest shop of this kind in Europe) which I usually left with a bag full of of vinyl records under my arm (and an empty purse in my pocket).

On one fine day my valued music teacher, a passionate music lover himself, assembled a herd of committed teenager scholars to go visiting the famous STUDIO FÜR ELEKTRONISCHE MUSIK at the Cologne Conservatory of Music (founded in the early 1950s by iconic avant-garde composer Karl-Heinz Stockhausen). I remember vividly that after a few tracks of freshly brewed electronic music had been presented to the gang, played back through a pair of vintage monitor loudspeakers, I found myself stepping forward and, after giving expression to my delight about the tracks we had auditioned, utter ruthless words of criticism about the deficiencies of the studio loudspeakers. Probably to the amazement (amusement... embarrassment...?!) of my class mates, our dear teacher and the superintendent of the famous studio.

In view of such a lifestyle it was only a mater of time until High Fidelity Audio would become a major point of concern in my life. After decades of involvement with live and recorded music, I have gained a crystal clear imagination about the sonic profileof a truly MUSICAL system. And all my design choices have always been ruled by this ideal, whereas aspects of marketing and commerce have been predominantly neglected.

DESIGN PRINCIPLES... The THÖRESS story is about AMPLIFIERS AND LOUDSPEAKERS. A fact which has been frequently overlooked in the past. Perhaps the rather unique amplifier components had stolen attention from the loudspeaker creations...? Or maybe the radical and idiosyncratic solutions of my speaker design (regarding technical principles and appearance) did consternate...? In any case, I strongly believe that amplifier and loudspeaker design are so closely related to each other that they need to get treated simultaneously in order to reach the very top of musical reproduction. Decades of search and struggle for the right design principles have led me away from stereotyped mainstream design recipes to rather unique solutions such as undamped transmission line woofer loading... cabinets made of low-density poplar plywood... constant directionality horns... underhung motor transducers... crossovers optimised for maximally flat group delay... Design choices foremost meant to optimise transient response, the cardinal point of loudspeaker design (and audio science in general). Music by its nature is nothing other than a (highly organised) universe of acoustical transients, after all. Meanwhile, THÖRESS loudspeakers gain increasing reputation aside from the amplifier products which I admittedly note with satisfaction and pride.

In case of amplification, I strongly believe in the supremacy of MINIMALISTIC,  SINGLE-ENDED,  ZERO-FEEDBACK,  VACUUM TUBE TOPOLOGY while strictly avoiding balanced schematics, push-pull operation in particular. My Parametric Phono Equalizer (Phono Enhancer), for example, offers purely active MC amplification (no built-in step-up transformers!) with excellent signal-to-noise performance by solely employing two single triodes per channel operated (in singleended mode) with HIGH IDLE CURRENT. Even the thought of forcing the phono signal through a sequence of integrated operational amplifier circuits (OPAmps) whereas each chip consists of a network of at least 30 transistors run at PIDDLY IDLE CURRENTS linked by loops of negative feedback makes me shudder.

BALANCED VERSUS SINGLE-ENDED TOPOLOGIES... Balanced topology (roughly speaking) not only requires twice as many (active and passive) parts as comparable single-ended structures and as such contradict my minimalist design approach at the root (not to speak of ICs as they are highly common in balanced technology, which typically relies on OpAmp basedschematics). Balanced structures exhibit a very particular distortion behaviour in sharp contradistinction to single-ended circuitry known as

HARMONIC CANCELLATION... EVEN overtones (in principle harmless and pleasant impurities added to the wanted signal due to non-linear amplifier distortion) CANCEL AWAY, whereas UN-EVEN overtones (ugly haze) DOUBLE! Consequently, the sound impurities induced in the wanted signal by a sequence of balanced amplifier stages entirely consist of UNeven overtones initially produced by the input stage, superimposed with UN-even overtones produced in the following amplifier stages, plus a chain of UN-even overtones OF UN-even overtones "inherited" from preceding stages... a devilish avalanche of ugly haze takes place, whereas ALL harmless EVEN overtones cancel away completely. Completely. Hereby EACH balanced gain stage adds twice, yes TWICE(!!!) the amount of UN-even overtones to the wanted signal than a comparable single-ended structure (built around the same active elements under comparable operational conditions).

IN-HARMONICITY  AND  NEAGTIVE FEED-BACK... Classic electronic engineering science concludes that this is not a point of concern because these hazardous distortion products can be reduced to almost nothing by furnishing excessive over-all gain (by adding gain stages, and such even more harmonics to the signal) and applying a large amount of NEGATIVE FEEDBACK. Yet still admits that they cannot be eliminated completely.

Curiously, the following simple common-sense considerations combine to suggest that the common theory of non-linear distortion and negative feedback is only a very rough approximation of reality - at its best...

The notion of distortion reduction via negative feedback essentially relies on the assumption that the frequencies of the overtone impurities added to a pure sine wave test signal by a distorting amplifier device (sine waves do not carry overtones themselves and as such can be considered as the ""indivisible atoms"" of arbitrary periodic signals, so to speak) are UNFLAWED fractions of the respective test signal frequency. Accordingly, non-linear distortion products of electronic amplifiers are often referred to as HARMONICS, suggesting a similarity of conditions between amplifiers and vibrating strings. But can this crucial assumption be in accordance with the real world conditions...?

CERTAINLY NOT! 

Let me illustrate my doubts with the following allegory...It is well known that the overtones of REAL vibrating strings are IN-HARMONIC in the frequency domain! This is to say that the frequency of each individual overtone of a real world string deviates to a certain (very small) degree from the unflawed frequency of an IDEAL (textbook) vibrating string, as predicted by the basic physical theory. Inspite of its tiny magnitude in-harmonicity of real world strings turned out to be a measurable quantity whereas the grade of in-harmonicity was found to be dependent on material, length, diameter and tension of the string element in question. Even a formula has been put forward which explicitly describes the dependency between in-harmonicity and the concrete composition and parametersv of the living object. Furthermore, in-harmonicity is a commonly accepted notion in the field of instrument making and is solidly incorporated in the design and the tuning practice of all kinds of string instruments. Interestingly, strings exhibit the lowest grade of in-harmonicity if they are long, thin and exposed to the highest possible tension. Obviously conflicting requirements which have to be accounted for and brought into balance in string instrument design in view of reducing unwanted in-harmonicity. So it is no wonder that grand pianos incorporate a heavy steel frame to take the enormous tension arising from hundreds of long metal strings stretched out nearly to their breaking point. And that harpsichord players use beautifully sounding brass strings (instead of steel parts) on those key positions where they can withstand the required tension. In the light of these findings, it would be quite a miracle if real world amplifying elements yield UNflawed perfectly harmonic overtones as distortion impurities...

The overtones arising in electronic amplifiers due to non-linear distortion MUST BE IN-HARMONIC in the frequency domain ... !!!

Furthermore, it is easy to imagine that the various amplification elements such as vacuum tubes, FETs, MOSFETs and bipolar transistors exhibit different grades of inharmonicity under different operational conditions. Curiously, I have not come across a single scientific paper in which even the notion of in-harmonicity has been brought up. Not to speak of a comprehensive investigation of the phenomenon itself. Although it can be safely assumed that the measuring tools of the digital age are sufficiently sophisticated and sensitive to permit a revealing analysis of the conditions.

Astonishingly, the concept of in-harmonicity seems to be totally unknown in the field of audio electronics, until the present day … !!!

But what happens to distortion products in the form of IN-HARMONIC overtones in the presence of negative feedback...???

Their amount is reduced, that is certain, as it is clearly shown by common distortion measurement. Measuring procedures which are by far too coarse to ever observe or even quantize the tiny frequency shift given by the in-harmonicity effect (due to thevery limited frequency resolution of the band pass filters used for extracting the sequence of harmonics from the test signal). On the other hand, it is equally certain that the reduction of non-linearity overtones via negative feedback as demonstrated by the standard THEORY with the aid of the so called Fourier Series Transformation of periodic functions COLLAPSES if one declines the unflawed frequency assumption and admits in-harmonicity in the frequency domain! This naturally gives rise to intriguing yet uncomfortable and awkward questions like...

What are the technical and PSYCHOLOGICAL impacts of in-harmonicity in audio...??? Do vacuum tubes exhibit lower in-harmonicity than solid state devices...??? Does (high current) class-A operation of amplifying elements lead to lower inharmonicity than (low current) class-AB operation...??? Does negative feedback reduce harmonic distortion by introducing or increasing some other UNKNOWN form of signal degradation...??? Is it possible to extend the existing theory of linear distortion and negative feedback in a meaningful way by including the concept of in-harmonicity in electronic science...???

DAMPING  FACTOR  AND  NEGATIVE FEED-BACK... In case of power amplification there is another blind spot of negative feedback more humble in nature than in-harmonicity worth a closer consideration. It is related to the so called damping factor DF, the ratio of the load impedance of the driven loudspeaker Rload versus the output resistance of the power amplifier Rout, DF=Rload/Rout. The lower Rout of a power amplifier the higher its ability to obstruct unrestrained cone movements of the bass transducer at its (in-cabinet) resonance frequency, favouring an impeccable transient response. In fact the amplifier gains control over the cone movement of the low frequency transducer by shorting the voltage (often referred as back-EMF (EMF electro-motive force) induced in the voice coil as it moves around in the magnetic flux pervaded air gap of the transducer motor (set into motion and exited by the signal current imposed on the voice coil by the power amplifier)) due to the induction principle. It is important to note that Rout and Rload are connected in series and hence form a two-stage R+R voltage divider! Thus a (small, idealy negligible) portion of the signal voltage induced by the signal current flow between the amplifier and the loudspeaker is developed across Rout (and such gets stuck inside the amplifier, so to speak) and in this sense does not reach the loudspeaker load. As a matter of consequence a low output resistance also ensures that the voltage developed across the loudspeaker load does not drop or even break down at those frequencies where eventual impedance dips of the loudspeaker load occur. So a reasonably low internal resistance/high damping factor is in-arguably a desirable feature in the context of power amplification.

Nevertheless, it is of crucial importance how exactly a low output resistance is achieved in terms of circuit design!

The most basic and natural approach to design a power amplifier circuit with low output resistance is by choosing amplifying elements (vacuum tubes or solid state devices) with low internal resistance and let them operate in class-A mode with high idle current. As high idle current as it is practical. An approach which obviously leads to a costly device with modest to low power output and miserable power efficiency. Due to forceful amplifying elements, an elaborate and costly power supply (large transformer, filter capacitors with high capacitance) and big heat sinks in case solid state devices. In this scenario a (fairly) low output resistance of the amplifier is gained in a tangible way by extensive current flow, without bringing negative feedback into play. A design choice which matches most naturally with single-ended circuit topology where class-AB or class-B operation is nonsensical and class-A operation obligatory anyway. Consequently it is EXCLUSIVELY employed in ALL of my amplifiers, including the Dual Function Amplifier and particularly the EHT integrated and EHT mono blocks.

By contrast, the wast majority of power amplifiers, no matter if vacuum tube or solid state based devices, make use of push-pull class-AB or class-B operation and offer (unnecessarily) high and often massive power output capabilities at fairly low idle currents in their power stages and accordingly low power consumption. Yet still they come along with high, sometimes insanely high damping factors.

How is this possible...???

Well, by taking advantage of the omni-present recipe in conventional audio science: EXCESSIVE NEGATIVE FEED-BACK...!!! A regulation mechanism where the output voltage across the loudspeaker load is monitored, fed back to the inwards of the amplifier, therein compared to the faithful signal and then, dependent on the amount of feedback applied, more or less pitilessly corrected. If the voltage developed across the loudspeaker load tends to break down due to an impedance dip of the loudspeaker because the output stage is too weak to push a sufficient amount of current into the load (as a current-strong class-A output stage would do) the feedback-correction jumps in and restores the voltage drop in question instantly. Instantly...??? Well not instantly, let us better say in ALMOST no time. The gift of negative feedback is provided to the non-discerning audio engineer at the expenses of a nasty kind of distortion in the TIME domain (distortion is not always about linearity and harmonics)...!  Still a tempting gift, as it allows to easily construct amplifiers with high power output, almost no power consumption, negligible heat dissipation yet almost infinite damping factor (output resistance near zero) and ultra low total harmonic distortion figures...amazing spec sheets guaranteed...and all thisat fairly low expenses! Quite an amazing bunch of advantages all of which are gained simultaneously by the magic of negative feedback. Well, there is a concealed “little” disadvantage..........................impaired sonics!!!

Let us namely recall that there is a voltage disturbance back-fired from the voice coil of the loudspeaker into the amplifier, the motional back-EMF! While this signal is shorted and such “”absorbed”” to a large extent by an amplifier with reasonably low output resistance (high damping factor) given by substantial IDLE CURRENT flow in the absence of negative feedback, in the feedback scenario the back-EMF disturbance is allowed to creep in the wanted signal via the feedback loop as the correction mechanism SHAMS A VIRTUALY low output resistance of the amplifier. Such negative feedback, supposedly a universal remedy in audio, turns  into a nasty source of signal distortion modulated by the back-EMF of the loudspeaker! This unpleasant side effect of negative feedback does not show up in any of the common measurement procedures (carried out with humble dummy load resistors rather than a load with adventurous impedance and back-firing voice coils) so it can be stubbornly ignored by conventional audio science and fabulous specs can be conveniently certified to amplifiers employing heavy amounts of negative feed-back. And audio companies can proudly offer “”high end”” amplifiers with 1000+++ watts of power and absurdly low damping factors (Rout below 0.001 ohm !!!). Such insanely low output resistances appear especially ridiculous if one considers that each meter of cable inbetween the amplifier and the loudspeaker adds at least 0.1ohms (0.1=0.001x100) to the EFFECTUAL output resistance acting upon the loudspeaker load. This story readily explains why so many conventionally constructed power and headphone driving amplifiers regardless of claim and price do mostly appeal to people who believe in fabulous specs, audio myths and fairy tales developed in the marketing departments of the audio industry, while they do sound unconvincing and odd to the ear of the non-biased advanced music lover.

With such considerations in my mind, I have over the years evaluated all circuit topology one can think of as a practitioner, by ear and measurement, and clearly found that balanced technology and negative feed-back are sound degrading, unfruitfull, als lastly unnecessary concepts. Especially when it comes to power amplification.

Consequently, I decided to devote  myself EXCLUSIVELY to single-ended zerofeedback topology. The sonic coherence and beauty attainable with this kind of circuit architecture is matchless!

From my perspective, the one and only purely technical merit and right to exist of balanced technology is the immunity of the associated signal transfer scheme against electromagnetic interference (EMI) based on the so called COMMON MODENOISE CANCELLATION effect. In order to make use of this effect a dual signal (composed of the wanted signal and a 180-degrees phase shifted clone) needs to get transmitted from the sender to the receiver component (accordingly cable lines for balanced signal transfer require 3 leads: hot1, hot2, ground). Hereby the receiver device has to present an active differential amplifier stage (with dual input, inverting and non-inverting, mostly performed by an OpAmp IC) or an audio transformer with centre-tapped primary winding to the incoming balanced signal. Balanced technology might be indispensable in sound studio and public address applications where cable lines likely have to run through environments polluted with electromagnetic smog. Yet it means unnecessary technical overkill in domestic audio installation. Overkill which is accompanied by sonic degradation!

THÖRESS is not about dogmatic tube circuit design. Despite the golden age subtitle associated with my brand, it is not my aim to glorify the bygone age of vacuum tube technology. I dedicate myself to vacuum tubes, particularly TRIODES, because they are outstandingly linear amplification devices which in addition (like field effect transistors, particularly MOSFETs) offer the advantage of POWERLESS MODULATION... solely voltage swing yet no current flow is needed for modulating these devices! A huge advantage which makes vacuum tubes by their very nature perfectly suitable for minimalist zero-feedback circuit design! Nonetheless, my amplifiers and speaker crossover networks are made with meticulous hand construction using CLASSIC/ANCIENT point-to-point wiring techniques, exactly suiting my radical purist design approach.

At a rather early stage of my career it became evident to me that I had to melt together the ancient vacuum technology and the contemporary solid state technology in order to reach ultimate sonics. Accordingly, I freely dispose of the whole palette of ingredients and spices the universe of electronics offers to me. Except for integrated circuits and printed circuit boards (which are employed, beyond the audio circuitry, for remote volume control purpose only). As a matter of consequence, in my circuits vacuum tubes are often complemented by solid state devices like light emission diodes, JFETs, MOSFETs or bipolar transistors as side elements in order to create optimal high current operation conditions for the tubes, aiming at the lowest possible distortion figures and in-harmonicity in complete absence of negative feedback correction. Just as in case of piano strings stretched close to their breaking point, tubes (amplifying elements in general) ought to get operated at the highest practical idle current in order to deliver the most harmonious performance! This approach can be classified as...

CON/temporary  S/olid   ST/ate   I/nterleaved   TU/be   TE/chnology,

CON/S/ST/I/TU/TE.

CONSSTITUTE should by no means get mistaken for the more common vacuum tube transistor hybrid technology, where tube and transistor stages are merely combined rather than interleaved and entangled.

In case of my EHT based amplifier models, I take the CONSSTITUTE approach to the next level by applying it in the context of a unique minimalist tube MOSFET hybrid topology, which I call EHT Topology (E/intakt H/ybrid T/riode, single-ended hybrid triode). The EHT topology is a two-stage scheme...a SINGLE-ENDED (class-A) TRIODE GAIN STAGE followed by a unity gain MOSFET POWER BUFFER also operated in single-ended (class-A) mode with high idle current. It represents the SIMPLIEST and PUREST form of all possible single-ended triode power amplifier configurations and allows to fully exploit and mate the most desirable characteristics of both vacuum tubes and solid state devices determined by their very nature! Inspite of its elegance and extremely high sonic capability, the EHT topology has never been utilized in a serial audio product before, as far as I know. Probably due to its low power efficiency and because its implementation is rather tricky.

TONE CONTROL... Another peculiarity found on crucial components of the THÖRESS product line is worth to get pointed out. Four amplifier models (Phono Enhancer, Dual Function Amplifier, EHT Integrated and EHT Mono Amplifier) offer tone control functions implemented via on-the-fly selectable sound presets, which I call timbre registers. Notably, these timbre registers act in a much more subtle way than common bassand-treble tone control facilities and do not rely on clumsy and sound degrading conventional tone control circuitry! In fact all such registers are realized by interposing solely one additional capacitor (per register and channel) to the respective neutral mode circuit! The timbre registers are useful for restoring tonal imperfections of the listening program, for matching the tonality of the loudspeakers to the listening environment and, in case of the Dual Fuction Amplifier, for equalizing the response of individual headphones by adding bass and/or treble extension to a given component.

TRANSFORMERS... All THÖRESS amplifiers are equipped with proprietary mains transformers, output transformers and filter chokes produced in-house to the highest possible standards, tailored to perfectly suit the respective circuit context. In fact every inductive part ever used to built up a THÖRESS amplifier carries a coil which has been fabricated by myself with the aid of a vintage winding machine made in year 1964, curiously the year of my birth.

APPEARANCE...All components which nowadays form the THÖRESS product line were initially developed to serve my own needs. Hence their appearance and construction style bluntly display my personal taste and attitudes. The small 1D66 loudspeaker model is believed to represent an unobtrusive and timeless piece of style-neutral furniture. Whereas the large 2CD12 loudspeaker follows the example of the self-effacing appearance of  a vintage JBL monitor loudspeaker. As for the amplifiers, I want them to get perceived as apparatus, PURISTIC AUDIO APPARATUS as I call them... precision gear meant to serve committed music lovers to broaden their musical horizon and acquaintance. Components free of redundant frills solely dedicated to soulful and honest reproduction of recorded music, rather than pretentious luxury toys for showing off. The undeniable more-or-less obvious retro touch my products emanate are understood as a TRIBUTE to the tradition of high grade audio components from the vacuum tube era, which I admire and cherish since the day I inherited the fabulous Philips tube radiogram. The machine which has awakened and fueled my unrelenting fascination for music.

Greetings to All Music Lovers World Wide! REINHARD THÖRESS

THÖRESS...

A Tribute to Professional Audio Components from the Golden Age of the Vacuum Tube !