1 Classic Mission 774 turntable tonearm

Transcription

1 1 Classic Mission 774 turntable tonearm In the heyday of the record industry in the last century, the Mission tonearm was aiming for the audio gourmet. We are talking about the late 70's in the last century. The first 774 arm is as far as I know the foundation of the name and fame of the English brand 'Mission'. This arm is designed by an American engineer; John Bicht. Besides in America, John Bicht has lived a period in Great Britain, and in that time he was looking for a turntable to fit his high quality standards. Because at the time, the marketed tonearms did not meet John Bicht's standards, John designed a tonearm himself. After acquainting with the English HiFi world, his tonearm was taken into production by Mission with the name of 774. This tonearm raise like a new star in the HiFi world with its clever design, the purposeful construction and material selection, supported by an endless list of positive listening and test results. Mission suddenly gained attention in the audio market. In my opinion, the quality index of Mission has never been as high in the audio market again. Figure 1: The Mission 774 tonearm mounted on a Denon record deck (picture is from TMRAUDIO) The success story of the Mission 774 tonearm was taking place around 1980, when John Bicht returned to America and picked up new and other designs, partly in the audio industry. 1.1 High compliant super tracking cartridges At the time of the introduction of the original (first version) 774 tonearm around 1980, record players were equipped with mostly MM cartridges that were meant to be used with a low tracking force. With the soft mounted generators in these cartridges, the cartridges showed very good tracking with low tracking forces of e.g. 1 gram, with the supposed advantage of low record wear. These cartridges are generally characterized by the audio industry as highly compliant. HENK SCHENK 1

2 Figure 2: The Mission 774 with Shure cartridge (picture AudioAsylum.com) I qualify these types of cartridges as lightweights. Particularly because high compliance requires a low effective mass of the turntable tonearm to bring the basic arm resonance to the desired range around 10 Hz. This requires low-effective tonearm mass as well as a low self-weight of the cartridge because these two masses added together play a role in obtaining the tonearm resonance. The Mission 774 (the original version designed by John Bicht!) is the ultimate tonearm in this battle field because the arm exhibits a very low effective mass of around 6 grams. 1.2 Compliance of the pickup cartridge Cartridge compliance as a feature reflects the cantilever support flexibility and this flexibility allows the stylus to follow the groove when playing a record disc, together with other cartridge construction characteristics. The compliance has as a consequence (and purpose) that the cantilever changes position by forces that occur at the stylus. Not all changes in the position of the cantilever are intended and this can lead to a proverbial pitfall. 2 Compliance consequences, theory Having said that the cantilever changes position by force on the stylus tip, I now want to observe several forces that occur at the stylus when playing a record. 2.1 Forces on the cantilever position One of the forces is the adjusted tracking force required for the playback process. This force makes the cantilever slightly change its tracking angle, or "flex", when the cartridge drops on the record. HENK SCHENK 2

3 Furthermore, there are skating forces and frictional forces occurring by "dragging the stylus" when playing a record and these vary with the (degree of) groove modulation. The frequencies of the played notes from the records also affect this frictional force and vary continuously. The skating force varies dynamically, while the anti-skating set in the turntable tonearm is almost always as good as static. Thus variations in lateral forces will occur which will move the cantilever horizontally (laterally). The varying frictional force attempts to draw the needle at a greater distance from the fixed tonearm bearing point. Increasing the distance can only occur when the VTA angle decreases. As a result, the vertical cantilever angle varies continuously. (This last effect is slightly affected by the position of the vertical bearing of the tonearm, which I do not further elaborate upon). Likewise undesirable, but of visible influence are forces that arise from record warps. Because the tonearm with its effective mass has difficulty following fast up and down movements of a cartridge, it will move after the cartridge following the slopes of warps. As such the tonearm trails warps a cartridge encounters and the resulting displacements relative to the record played generate changes in suspension position. Even if these changes are not visible to the naked eye, they will be present and may have consequences for the information traced and played on records. In addition to riding warps, a similar phenomenon is present due to eccentricity of records where the centre hole is "not perfectly in the middle". However, the impact of this effect I regard relatively small. 3 Compliance consequences in practice and my preference After the hype of super tracking cartridges from Shure, ADC and Stanton using a very high compliance in the 70's, I was positively surprised by the bomb-proof dynamics and unconditional stability in the musical display of the first MC carts that I got across. MC cartridges have a different build construction and usually (not necessarily but this is relevant in this article) a much lower compliance than the superlight MM carts! I linked these perceived results to the lower impact of dynamic forces on the stylus that occurs in a low compliant cartridge. The dynamic forces on the stylus and the associated constant suspension and cantilever angle changes bring about a kind of wow and flutter and also a change of generator efficiency, thereby affecting the dynamics in the music played. The wow and flutter phenomena undermine the stability in the music played. The audio-dynamic side effects are more complicated and probably also of a lesser order. In practice, the influence of the dynamic forces appears to be increasing with the compliance of a turntable cartridge. In fact, the cartridge and tonearm masses are not low enough to obtain a favourable resonance with featherweight MM s. And this unfavourably low resonance frequency induces the added perceived adverse effects. (The example of the extremely short and light tangential tonearms in B&O record players with the super small and light inserted cartridges is the exception that confirms this proverbial rule). In short: 30 years ago, I shifted my preference to low compliance cartridges, and I have maintained that stance until today. HENK SCHENK 3

4 Figure 3: Vertical displacements of the stylus illustrated by the yellow tangent line 3.1 Consequences of my then new preference The consequence of my switch to low-compliant MC cartridges was that these preferred cartridges were not, by definition, compatible with the low effective mass of the Mission 774. The primary tonearm\cartridge resonance of 14 to 15 Hz as a consequently was too high in comparison of the supposed optimum. As a side note; I'm less concerned with audible effects of a highish resonance frequency than of a lowish frequency. The Mission 774 also sounded very good with the MC carts, but there is more than the primary arm resonance that affects the sound. Let s take a look at these other effects. Low compliant cartridges require relatively high tracking forces. From the associated and thus also relatively high forces in the cartridges generator assembly, relatively high reactance forces will arise and radiate or transfer towards the tonearm. A MC cartridge "radiates" more mechanical, "vibrational", energy into the tonearm than a flyweight MM cartridge. And through the tonearm into the turntable and possibly the turntable base. The insights of John Bicht in the 70's were specifically aiming to the minimization of vibrations, resulting from vibrational energy of the cartridge, in the Mission 774 tonearm. Controversially, the low effective mass aiming at the "featherweight" carts, is compromising this objective of suppressing vibrations. In summary, avoiding vibrations in the 774 tonearm is counteracted by the quest for a low mass. HENK SCHENK 4

5 4 Adaptation to the compromise in the Mission 774 In the 80's of the last century I performed many experiments with cartridges, turntables and tonearms. I have been able to review and listen to a range of factory products in my time as a reviewer for the Dutch magazine Audio & Techniek. MC carts got my preference and similarly turntables and tonearms in which the effects of vibrations were minimized. My choice for MC cartridges and search for elimination of vibrations have led me to adapt the Mission 774 tonearm. The effective mass of a tonearm when applying low compliant MC cartridges can easily be upped to 16 or 20 grams, at least 10 grams higher than the standard Mission 774. Mechanical vibrations can be reduced by the extra mass due to the resulting mass-damping and energy-absorption obtained. 4.1 Alternative tonearm wands and additional damping Initially, I wrapped the Mission 774 (removable) arm tube with a 1 mm thick silicone rubber mat. The effective mass is increased 6 grams as a result and the vibrations are audibly suppressed. I still have this arm tube available today and it is retained next to an untouched original sample. The original sample I rarely use, but I save this sample so it can serve as a kind of reference or starting point. Figure 4: The original 774 arm wand wrapped with silicone mat Secondly, I used a titanium tube to build another alternative arm wand. Titanium is lighter and stronger than the aluminium / magnesium alloy of the original arm tube. But the purchased titanium tube has a larger wall thickness relative to the original tonearm wand and is therefore heavier and much stronger. The effective mass is increased to about 20 grams. This titanium tube is less perturbed by vibrations when compared with the previous variants, and the resonances found appear relatively high in the frequency spectrum. I applied additional damping to the titanium tube as well. Because the weight of this titanium arm tube including use of a relatively heavy Kiseki cartridge is high, I have chosen a felt layer for damping. With felt as damping the added mass by damping remains low. HENK SCHENK 5

6 Figure 5: Titanium arm wand with felt damping Thirdly, I used a double aluminium tube as an alternative arm wand. The base aluminium tube is similar to the original tube and slipping over it I glued a second aluminium tube. This produced a stronger arm tube with a mass that is very useful for the MC cartridges, just slightly lighter than the Titanium version. Figure 6: The larger diameter double tube tonearm wand with some additional features HENK SCHENK 6

7 5 30 years later, Anno 2015 Since 2015, I was looking for a favourable combination of a tonearm and turntable with a van den Hul Colibri lightweight cartridge. My Mission 774 is quite flexible because I can choose between 4 arm wands with different features. For the Colibri, with a specified optimum tracking force of 1.4 grams, I chose the light original tube with added silicone damping. The combination seemed to track well and gave a primary resonance in the good regions (12 Hz). However, due to unfavourable listening experiences I switched to the use of the slightly heavier double aluminium tube. The basic resonance lowered somewhat and a slightly softer sound was the result of the differences in tonearm tube and wiring. But listening experiences still were not as expected and called for further analysis. The solution was finally found by increasing the tracking force to 2.15 gr, a value that is usually applicable to the more stiff MCs. 5.1 Results after the changed setting for the Colibri With a tracking force far out of specification, the Colibri started to sing beautifully. However, now the tonearm wand started to sing as well. Close (and even not so close) to the turntable, besides audible "needle talk", the arm wand could be heard to produce a specific tonearm talk. Like my familiar and stiff MC cartridges, this Colibri releases quite a lot of mechanical, vibrational energy which finds its way through the arm wand and through the bearing into the chassis of the turntable. This is a clearly audible phenomenon, presumably resulting from a set of characteristics specific to this combination of cartridge and tonearm. To sum up: 1. High tracking force is accompanied by a lot of vibrational energy that is being generated. I now am using the Colibri with a high tracking force, so it is generating a lot of vibration energy. 2. The Colibri weighs a mere 5.5 grams and in this low mass, much energy cannot be absorbed. As a result much of the energy is transferred into the tonearm. 3. The Colibri cartridge body consists of 1 piece of cast Nylon / Polymer plastic. There are no transitions of different materials that, as a mechanical barrier, could block the transfer of vibrations to the tonearm tube. 4. The double aluminium arm tube I used is glued to mechanically form an undivided structure, also without obvious mechanical barriers. There was no damping around the tube. (Finally, I put a rubber damping around a part of the tube and found that there was a difference in the sound and in the tonearm talk of the tube.) 5.2 Results and deduction The aluminium arm wand built from two concentric layered tubes does not seem to be able to completely control the energy of the Colibri, witnessing the screeching sounds that clearly audible emanates from this wand. However, the sound produced by the Colibri via the speakers is very good and convincing enough to recognize a famous masterpiece in this van den Hul cartridge sample. But the noise emanating from the tonearm made me believe improvements were possible. Thus, as a first option my incentive was to go for the titanium secret weapon. From experience I know of the additional "control" this heavy weapon can deliver. However, breaking up my beloved combination with the Kiseki kept me back. And besides, I was curious to test an alternative idea. Modern materials like carbon fibre and new insights about strength and size of tonearm wands brought the idea forward to make something new, in this case; The Mission 774 on steroids. HENK SCHENK 7

8 5.3 Motivation My thoughts are based on the following ideas; 1. Carbon fibre is nowadays readily available in various sizes and shapes. 2. The carbon filaments are soaked in a resin matrix with properties as rigidity and mechanical inertia and stability. From this matrix resin I expect a beneficial effect when coping with mechanical energy and vibration. 3. Using light carbon material, larger diameter tubing can be used compared to aluminium or other metals, without getting too high a mass. 4. Stacking concentric tubes can provide additional damping in addition to extra strength. This makes it possible to avoid the assumed conundrum that high strength leads to strong resonance. 5. The S.A.T. tonearm uses carbon fibre as main construction material to an extreme extent and I have looked at this design from the first appearance with awe and admiration. I am, I fear, a follower of the SAT sect. I did not need more arguments to transform the thought into shape and function. 6 Building a Carbon Fibre Mission arm wand The current industrial revolution has put Carbon Fibre tubes readily available on the market and is shipping the products in house with just a mouse click. By careful judging from product descriptions, I chose tubes with as many longitudinal fibres as possible to optimize the strength in the length direction. This way I hope for a high first resonant frequency and efficient energy transfer from the cartridge position towards the other end of the arm, at the tonearm bearing and the counterweight's damper. Because I had a construction in mind of a tapered tonearm with more material (thickness) close to the arm bearing, I purchased several diameters of tubes. Figure 7: Carbon fibre tubes of 1 meter length HENK SCHENK 8

9 Figure 8: Carbon fibre tubes with increasing diameters The different diameters allow to stack the tubes by sliding larger versions over one another and thereby increasing the total diameter and wall thickness. Using different lengths result in a tapered shape. The carbon material is easy to work with, the tube lengths to be used are shown in Figure 9. Figure 9: Carbon fibre tubes in incremental diameters en diminishing lengths I assembled these tubes into the desired tonearm tube. The intermediate result is shown in Figure 10, onto which the head shell and wiring must be added still. HENK SCHENK 9

10 Figure 10: The Carbon fibre arm wand taking shape The wiring is inserted as shown in Figure 11. Figure 11: The Carbon fibre arm wand with silver/teflon wires I find it particularly hard to create a head shell (or rather; head block) with the correct size of the cartridge mounting holes. The shape follows function and my construction is certainly not spectacular in visual design. I made the cartridge mountings flexible to be able to compensate for deviations from the ideal cartridge geometry. I do find these deviations rather frequently in various cartridges, so from experience I know mounting flexibility is required. Hence the short slots as mounting holes. Acquiring a smooth and pleasing finish (onlay) is a task for which patience is needed, but I am satisfied with the results. As head block material I have chosen nylon plastic to match the Colibri housing. Nylon is easy to work on as well, assuming you can stay out of the way of its low melting point. HENK SCHENK 10

11 Figure 12: The Carbon fibre arm wand with the Nylon head block Swiftly I continued to mount the cartridge and installed the wand on the Mission 774 bearing house. When balancing, the arm was found to be a fraction heavier than the double aluminium tube and approximately a fraction lighter than the titanium arm tube. Well suited to the Colibri based on my previous experience. Figure 13: The Colibri cart mounted onto the carbon fibre arm wand HENK SCHENK 11

12 Figure 14: The carbon fibre arm wand in Situ Figure 15: The carbon fibre tonearm in helicopter view Construction complete. 7 The resulting sound My first hours of listening to the result yielded remarkable impressions. In fact, the first half hour was most revealing. After two record sides I was beginning to believe my initial favourable impressions. I slowly started to accept the positive and, in basically all aspects, unmatched good results. I had had faith in my idea of building this arm wand, but did not expect it to be this convincingly positive. That this design was my own "idea" is of course relative and not entirely correct; I certainly do not claim ownership of the design approaches and ideas in trend setting examples like SAT and others using carbon fibre in the arm wand. I just designed an implementation based upon the original and cunning design of John Bicht. And with the result I am extremely content (euphemistic). HENK SCHENK 12

13 Figure 16: After this record side it was clear to me; this combination is tantalising 7.1 Ease What I always find positive is that replicated music is easy on the ear. Ease in the sense that there are no hints of irritations or negative artefacts like noise, cracks and signs of strain in forceful outbursts in the music. This carbon arm tube offers this form of ease more than the armbands I used earlier. 7.2 Voices display detail en sound natural Voices appear in the sound stage in a surprising, exemplary natural way and are more beautiful and detailed than usual. Particularly multiple voices singing in harmony are easier to separate and stay separated and natural all the way through dynamically demanding passages. In any case, voices come out of nothing and appear in the sound image with ease that keeps capturing. 7.3 A wealth of details Using the new arm wand music is reproduced with a sea of detail that unfolds with an ease that reminds of the concept reality. From ordinary, standard records an endless stream of details is now unmistakably presented in all forms of music. Details that create an impression of how music is organized in space, for HENK SCHENK 13

14 example. Also, and clearly recognizable, when the perceived spaciousness was created with the use of pan pots and studio gear effects, this is revealed very clearly. 7.4 Dynamics With this tonearm, the dynamics sound very natural to my ears and grab my attention again and again. In particular, I appreciate the absence of an unnatural "edge" or forced and compressed sounds with strong cuts in the recorded music. Also, the place of a voice or instrument remains stable in space, regardless of all dynamic changes in the display. 7.5 Spaciousness Perhaps carried by the exceptionally detailed display, all individual sounds appear in the sound stage clearly and effortlessly in a large open space. Spaciousness may be created in the recording studio with artificial means, and that is often recognizable, but it is stunningly impressive and constantly present. This is one of the most important features of 'good sound reproduction for me, and this carbon tonearm offers this to an extraordinary extent. 7.6 Lows clean and tight with exceptional delineation The bass is sounding less coloured than before. Previously the lows were regarded as very good, but now it is clear some colouring was still present. My Thorens record deck has its recognizable own 'colour' in the range between about 30 and 40 Hz and this is clearly audible. I know this as a fact, but have not been able to solve this so far. In these very low frequencies, this colour is not irritating to the ear and in this case it certainly does not give an idea of a large peak and exaggeration. With this new arm wand this effect is audible, even better recognizable, but it still is presented as a pleasant effect adding some audible subbass. Apart from this effect, which I know well in this deck, the tonearm itself does not give any doubts about colouring, which is as good as absent. The bass sounds are very beautiful and appear unveiled and can be placed in the spatial image of the music. I'm a (sub) bass admirer and with this arm I get more than usual fun in this low bass area. 7.7 Etc.. 8 Other results The time I needed to work out the concept for the new tonearm, to order parts, to build it and to get it deployed on the turntable with cartridge, required a lot of patience, of which I do not have an unlimited supply. Although I'm generally known as reasonably patient, the anticipation and looking forward to a new and supposedly better component causes the need to skip all non-essentials. Listening is the goal, as quickly as it can be arranged. But daily realities are unmistakable factors that cannot be ignored. But for the rest of the time; build and setup the new tonearm with the Colibri cartridge listen! Paragraph 7 contains the impressions of my listening experience. I rarely am tempted so loose myself much to the superlatives recorded in there. But in this case I find everything in the produced sound so correct that I did dwell somewhat into the strong and positive phrasing written in paragraph 7. At the same time there I had a lingering uncertainty regarding the VTA, SRA and azimuth in my mind, especially because my Colibri is deviating from the standard cartridge geometry. By adjusting these key HENK SCHENK 14

15 aspects to the naked eye, I get insufficient certainty about the accuracy of the results. The last national holiday offered me the time to make an effort to get SRA and azimuth insightful with a USB microscope. Figure 17: The beauty of a delicate stylus and cantilever With the microscope I determined the Stylus Rake Angle by approximation. The nowadays norm of 92- degrees still feels somewhat uncomfortable because I have been aiming for 90 degrees for decades. After the first impressive listening results, I was curious about the SRA: 90 or 92 degrees? Or something else?? HENK SCHENK 15

16 Figure 18: Stylus Rake Angle (SRA) of close to 92 degrees With the result of almost 92 degrees I was pleasantly surprised. The photographic shot in Figure 18 is made with full tracking force but static on a non-rotating platform. Based on the knowledge that during the playback of a record the dragging forces will decrease the needle angle by a few tenths of a degree, my sensible setting will be between 91 and 92 degrees. Right now, I do not feel the need to try and further improve the SRA setting, backed by my very positive listening experience. Again with the help of the microscope, I have approximately determined the azimuth of the needle position. The azimuth setting I had performed by naked eye was referenced to the coil armature which is very clearly visible in the open construction of the Colibri cartridge. I now wanted to know whether Coils and stylus were in line with one another. It was difficult for me to get a clear picture of the stylus from the front and the results can only be interpreted with some feeling. Nevertheless, I place the resulting picture (figure 19) here and hope that this is showing the stylus clear enough. HENK SCHENK 16

17 Figure 19: Stylus in front view seems to be close enough to 90 degrees I think for now this is OK as well and see no need to adjust some more. So I can rest assured my listening experience is backed by a reasonably accurate cartridge adjustment. The end result is this cartridge is skewed a few degrees above the record platter due to its deviating geometry. I'm reassuringly convinced that the cartridge is set up correctly and the enlarged images in figures 18 and 19 do not contradict my earlier findings. 9 And now for Listening to some more music.. Henk Schenk HENK SCHENK 17