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LeBong | Phono Mono Gear Johannes LeBong,
Karlsruhe (Germany) March
28. 2007
johannes@lebong.de
Memories of the sound of the past :: Putting an old record on the turntable and cueing in the needle full of anticipation … these are exciting moments, before the real enjoyment starts. One time you listen to an exciting solo of a genius jazz guitarist – perhaps "Wes Montgomery" – on another occasion a fine classic interpretation, or the Beatles record "A Hard Days Night" triggers a romantic flashback. (Label: Parlophone PMC 1230 - Mono - 1964) On the Mono LP the Fab Four sound more as a whole while the Stereo recording seems to be divided into isolated left and right events. This is typical for many important products of this time, they often sound more naturally in the original mono edition than they do in stereo. Classic enthusiasts are acquainted with great mono-recordings anyway. There is no "better", not even with modern technical means. It was by the way the Beatles and a little bit later the Rolling Stones, who turned me on to buy an electric guitar and play in a beat band when I was young. When the Beatles released the single "Love Me Do", I owned a rather simple Lenco record player with crystal pickup, which fed into a japanese "Monarch"-amplifier connected to Isophon speakers. It was the time of 45 rpm singles. 33 LPs were larger than my tiny turntable platter and mostly wowed, because they did not lay flat. This simple equipment did not stay long with me, since school days I switched gear constantly and worked my way up in consumer hierarchy. I always had the ambition to hear better. However nowadays my possibilities are definitely better and more divers. All about mono recordings :: This paper will only cover playback technology for long play records beginning from 1948. This was approximately the time the first vinyl discs featuring "microgroove" started to be sold. Shellac discs with so called "standard groove" were obsolete soon after this date. Vinyl has better surface characteristics for better sound, does not break as easy as shellac and enables more rational mass production. In the middle of the sixties hi-fi stereo phonics started to replace mono recordings. These two dates bracket the timeframe and the character of recordings I will cover in this paper. The reproducing techniques talked about in this treatise do not include vintage shellac disks. Our topic gets increasingly complex when viewed in detail. There are many rules and as many exceptions to them. I am interested in finding a way through the thick of information for myself and to sum up the results in writing, perhaps as a starting point for other music enthusiasts. I will invite friends and aficionados to bring along their mono recordings and listen to them on my equipment as we have done in the "greencone project". This was fun for everyone. Ancient cutting techniques :: What is the difference between old mono recordings with microgroove and modern hi-fi stereo recordings? Mono recordings were cut with wider grooves and thicker styli. In addition they featured technically different and divers pre equalizations. On every record treble is enhanced and bass is diminished. This is called equalization curve and was not standardized at that time (see also my paper about the 6SL7-Phono preamp 6SL7-riaa ) For playback of these old mono recordings with their wide groove one needs special styli and – according to the age of the recording – special preamps, ideally with adjustable playback frequency characteristics. The goal of this preamp is to relinearize the sound mirroring the employed equalization curve. The way sound is cut into a record and encoded in the groove differs basically between mono and stereo recordings. Why mono styli? ::
Stereo recordings require totally different styli and generators than mono recordings. Instead of electrically translating only the lateral oscillations of the stylus, stereo pickups also use the vertical stylus oscillation to (re-) generate the tonal signal. Mono - image of the record groove - Stereo
Regarding today’s stereo recordings, sound information for left and right channels are cut into the groove walls under an angle of 45 degrees. The inner groove wall encodes the left, the outer groove wall the right channel three dimensional. This method enabled by the way mono compatible stereo recordings. That is the theory. Real world problems are different. Vinyl recordings all look the same at first, but cannot be played using the same technique. If you do not use the right stylus for microgroove, but a stylus with a slim hi-fi cut, this needle will ride on the bottom of the groove and generate hiss and noise. Needles cut too thin or too sharp will bounce through the groove, generate signal degradation and phase deviations; in addition diamond and recording suffer from abnormal wear. We will have to consider another mechanical tracking effect. The highest frequency a stylus will be able to track in a groove depends on the stylus thickness and shape. So we can verify that a thicker spherical stylus works integrating and as a mechanical noise filter. Such a stylus gives less distortion or 'rattle factor' compared to an electronically ex post filtering solution. There is no need to further argue to make clear, why modern stereo styli are totally inadequate to reproduce the potential of monophonic recordings exhaustingly. The best compromise are spherical styli with 25µ on a generator mechanically fit for lateral as well as vertical movement. Using such a needle we can also playback modern stereo recordings without damaging the record. The stylus in this case does not dive as deep into the groove. You use a stereo compatible mono stylus. The other way round is not possible.
Examples given: At Decca classical monos were prefixed LXT, stereos SXL. RCA (Victor) prefixed monos LM, LPM, LOP, RD, stereo variants were prefixed LSC or LSP. At Atlantic (US Jazz Label) stereos were called SD, monos did not have a prefix to the catalog number. Contemporary mono reissues and editions from the seventies are always almost mastered with a stereo cutter. Exempt: Classic Records Reissues. There are modern editions mastered on old Westerex cutters. Anyone can still buy mono pickups today :: First to mention are the classic studio pickups from Ortofon. e.g. model CG25. The motor in the CA & CG models is little changed from Ortofon's first cartridge design from 1948, when the company was called Fonofilm. The CG25 is a classic "old style mono pickup" and therefore destructive to hi-fi stereo recordings, you inadvertently place under it. You have to be sure about the technical characteristics of your records and always be aware that the stylus does not move vertically. This cartridge only tracks lateral movement in the grooves. SPU cartridges are loved because of their full rounded sound characteristics. This is true for the complete family of these upper class pickups.
The "broadcast cartridge" Denon DL 102 also has many fans among mono enthusiasts, that is why I mention it here. Although I cannot understand why. It is a relative of the legendary Denon DL 103 studio cartridge (my favorite for stereo), but built for another special use: the 102 was formerly used in AM* broadcast stations to play stereo recordings monophonically. In fact it is no cartridge for mono microgrooves, because a 17µm stylus does not belong in a 60µm groove. No doubt about that! In addition the higher frequency characteristics of this cartridge must not be flat, since the high frequency will be cut off in the broadcast studio. Denon especially specifies a range from 50 to 10.000 Hz. Perhaps this cartridge has so many fans, because its generator is built rather simple? I can only speculate on that.
* Audio frequency Bandwidth for AM Radio in Germany is truncated to 4500 Hz. That is the reason why AM Radio is missing tonal heights. Music and spoken words sound dull. It is possible to hear basal tonality of common musical instruments, characteristic tonality results from higher "formants". Now to my "secret favorite" the Ortofon OM D 25 M. It is - as far as I am concerned – neither accepted by dealers, nor by high enders. It seems to lead the life of Cinderella, maybe it costs so little and does not look spectacular. Compared to contemporary high end cartridges, such cartridges are not able to excel. Compared to cutting and reproducing technologies of the fifties and sixties, these systems are well above average with a lot of headroom. I recommend a close look at the technical data of these systems in my comparison table below. Crucial is the right stylus. With this system the stylus assembly is perfect and is even changeable when worn out.
I my system this Ortofon mono cartridge is allowed to show off. My mono recordings sound cleaner with the Ortofon D25 M than with a Denon DL-103 wired to mono. Why this is the case I have tried to explain above. practical advisory: Ortofon specifies different data for tracking force. I use in my table the data from my systems paper slip. I run the cartridge with a tracking force of 4.5 grams.
It may be useful to have a look at and compare technical data (according to manufacturer).
I will not be liable for actuality, errata or incompleteness of the data. I will gladly accept friendly corrections, should you find any errors on my side. No doubt you may find some more mono cartridges on the market than the ones listed here. I will only mention those I am personally interested in. As an aside I read about costly "Lyra-Mono-Pickups" with special cuts – but am rather a friend of spherical cuts for mono. You will easily see you have to compromise on some aspects. I left aside any vintage cartridges; collectors items, which are usable as long as the stylus and the stylus assembly are in good shape.
If you do not mind the "cool" design and if you have an SME compatible tone arm, you may use an Ortofon Concorde Pro S. Although it was designed as a robust DJ cartridge, it has a unique selling proposition, making it attractive for mono specialists: an extra thick spherical stylus with 26µm diameter.
Wiring a cartridge for mono :: The pins on the back of the cartridge are bridged to mono. This means the pins R+L as well as Ground R+L are respectively connected with wires.
This connection is also recommended by manufacturers. I have fond it in old application notes from Shure and Pickering. Please do not solder directly on the pins of the cartridge!
The use a Y-cable at the input of the phono preamp would be the second best solution.
By the way: It does not matter if the wiring is done by connecting the coils in
series or parallel, as long as you phase them properly.
Both the series and the parallel connections eliminate the vertical signal by
adding together the signals from the left and right coils.
There is no difference signal remaining at all. Cancellation is perfect as long
as the coils are in perfect symmetry, your cartridge is perfectly aligned as to
azimuth, etc. From there on the best way is to keep the signal in consequently mono. I do hear mono with only one speaker. This way localization and presence of an orchestra or singer are much more realistic than via stereo equipment. You will hear this best with a solo instrument.
Examples for adjustable tube based mono preamplifiers :: As I tried to explain there is no short supply in Cartridges. There is more need for useful adjustable preamps for mono recordings. Newer consumer hi-fi preamps correct the signal according to RIAA norm specifications only. Preamps offering older "house norms" for vintage labels are scarce – none using tubes. That is my motivation to pick up the topic. There is a prehistory to my ECC83 mono-vario-preamp pictured above, which is written down in my weblog from 15. April 2006. Meanwhile my self made device is on evolution level three. Basic considerations towards the design of a variable phono preamp: On the internet you can find many documents and tables which detail in depth the time constants of pre equalization for many record labels. To be precise, subsequently one should use in ones listening setup a studio preamp with dozens of scales and switches to dial in the appropriate correction values. Presupposed you are 100% certain how the mono record you quickly want to hear was cut. More often you will be uncertain of that – in addition I'd find this approach unnerving. Isn’t there a more comfortable way? And how was this done in former times? It's so easy – a preamp by ELAC :: Fortunately I one day discovered a draft by ELAC from the fifties which convinced me instantly with its elegant concept of use. This I had looked for for a long time. In addition you only need one tube – this is great! The Elac apparatus had been built for turntables with the then new magnet cartridges. And exactly for the recordings in stores at that time. Oops that are exactly those we are talking about in this paper. I may be allowed to cite from "Funkschau" 1955 (number 14): "Friends of the best sound reproduction – ELAC coined the word "Klangfreunde" (sound friends) for this interest group – start to switch to magnetic cartridges at home as well as abroad. … Corresponding amplifier circuits are recommended by cartridge manufacturers, who partly even offer readymade preamplifiers." Such a circuit did function then as well as today. Why not? Good enough or not, we would see. Considering todays component quality and current supplies should enable us to get it a lot more high end – here we go…
Parts list
Reproduction by friendly permission of ELAC Electroacustic GmbH, Kiel "Finger exercises" – experiences with the reconstruction :: First I reconstructed the ELAC circuit from the original circuit diagram. It indeed works as expected. As current supply I used a regulated power supply delivering 150V DC stemming from another project. After completion the amplifier played at first with a mono bridged Denon DL 103. This functions well if you bear in mind the relatively high exit impedance of the ELAC circuit. Low impedance inputs and long connects can not be driven by this circuit. Load impedance should be 500k minimum. In my existing chain it fits. You fix the right sound quickly by ear. That's exactly what I expected. This intuitive tuning of filters I could imagine as exercise for accuracy fetishists, who see the tolerances of RIAA curves as the measure for all things. They might be surprised, how insensitive their ear is to small changes in frequency changes. You do not get musical sound from playing with figures. If you take a closer look at the curves from the ELAC preamp you will find the RIAA curve among them. Anyway it is fun to fiddle with the knobs. If you detune the filters on purpose, you are able to produce a kind of nostalgic retro or steam radio sound. On the other hand sound optimum is only a twist away.
Photo: LeBong | Monotube Adjustable mono preamplifier with triode ECC83 Iterative evolution :: As a follow up I tried to optimize the circuit for my purposes. From today's view the biasing for the first tube in the original circuit seems to be a little bit iffy and also the DC feedback of the tubes poses some riddles. I do definitely not need a volume knob in a phono preamp and the lineout stage by Elac is not compatible with today's line inputs. So I started to transplant the basic idea into a new frame and left behind the original blueprint.
Adjustable mono preamp (classic) Johannes LeBong 2007 Main element of the circuit are the two potentiometers for continuously bass and treble adjustment. This RC filter network is a classic "cow tail equalizer" (Kuhschwanz-Entzerrer) as german engineers like to say. (Better: "Baxandall-Filter"). The circuit is named after P.J. Baxandall, who published the idea 1952 in Wireless World as a newfangled tone control for a tube amp. Experimental prototype ::To design the circuit more minimalistic and more funny, I use in the following variant the so called "LED-biasing" method. At the foot of the tube you find a LED instead of a cathode resistor with bypass capacitor.
On my PC I am able to perfectly simulate the filtering circuit and study the outcome of small changes. That way it was easy to visually reconstruct the frequency curves documented in literature. Here I see some future points for fine tuning together with accumulated hearing experience. This will take some time. My last circuit diagram is more a draft, where the set points for the tubes are not yet precisely defined.
Photo: Adjustable mono preamplifier with triode ECC83 and LED biasing LED biasing :: Morgan Jones, one of the best known authors about tube amplifier technology of our time, had talked about his studies of LED biasing. Not all diodes are equally suited for LED biasing. Selection criteria are not only noise or nonlinearities, but also the relationship between threshold voltage and color of the LED in question. If you want to keep things easy then briefly you best take LEDs which emit a dark red colored light. Thankfully Morgan sent me some of his diodes from England, which he got the best results with. By the way a little excursion about the history of mono cartridges :: This is an interesting area for vintage-audio-freaks. I will insert the picture of a rare collectors item. It is a Cellule Clement. Due to my contacts to the french DIY-colleagues from "Mélaudia" in Paris I had more than one opportunity to hear such cartridges on a "Système de qualité". Pierre Clément at first built cutting lathes for records. 1949 he constructed his first pickup cartridge. Around 1952-1953 he delivered his excellent goods en gros to RTF (Radio Télédiffusion Française). Later he did refuse to build stereo pickup cartridges – disappointing the french audiophiles. The sound of an intact Pierre Clément is unsurpassed in the opinion of many connoisseurs.
Supplement :: Here I show a classical preamp circuit for magnetic cartridges from the fifties by the former german company PE (Perpetuum Ebner) residing in St. Georgen in the Black Forest. It is also usable as a shellac preamp and also needs only one EF40 tube.
(sorry, but I give up on the technical detailed description on the pros and cons of this circuit diagram; too many technical vocabulary!) "Die Eisendrossel L mit 0,4H bewirkt in Verbindung mit den Querkondensatoren ein steiles Abschneiden der Rauschfrequenzen. Je weiter der Schalter H nach rechts gedreht wird, um so früher beginnt der Höhenabfall. Dieser erfolgt stets steiler als bei Verwendung eines einfachen RC-Glieds, so dass wertvolle Höhen, die für die Brillanz der Musik wichtig sind, auch beim Abschneiden des Nadelrauschens weitgehend erhalten bleiben. Mit B wird der Grad der Bassanhebung eingestellt. Sind beide Kontakte geschlossen, so bleibt die auf der Platte vorhandene Absenkung bestehen. Öffnet man Kontakt II, dann erhält man mittlere Anhebung, bei geöffneten Kontakten I und II dagegen starke Anhebung. Die Entzerrerverluste werden in der folgenden Röhre mehr als ausgeglichen, so dass am Ausgang eine Tonspannung von 0,7V zur Verfügung steht. Der verhältnismäßig sparsam bemessene Kathodenkondensator von 4 µF unterdrückt die Verstärkung tiefster Frequenzen und damit etwaige restliche Rumpelgeräusche des Laufwerks." (Quelle: Funkschau) A petite inconvenience when reconstructing this circuit is the coil in the input stage. You have to wind and tune it yourself, since it is not available readymade on the market. I have not yet built this preamp, so I can not estimate how it will coexist with modern MM-Cartridges. :: :: ::
March 2010:
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