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This post comes a little late, since I’ve just spent two weeks in Morocco on a Country Walkers’ walking tour. Morocco is very interesting, with a rich culture and really friendly people. The weekend before flying to Morocco, I attended a “Tube Tasting” at Oswald’s Mill in eastern Pennsylvania. Here is the mill itself:

It is a 18th century grain mill built right into the house, and has been refurbished over the last ten years by Jonathan Weiss. Jonathan’s “Tube Tastings” have been an invitation-only event since 2002, where people into vintage, exotic, and exceptional home-built equipment can set-up and compare their projects. Jonathan is also a world-class cook, and cooked all the food for the gathering - and the food was fantastic!

The main listening room takes up nearly all of the third floor. The open beams, various vintage paraphernalia (verging on steam-punk), and the 3rd-to-4th floor opening (see the large windows on the picture to the left) gave plenty of room for the speakers to breath. The window openings in the two-foot thick stone walls made perfect turntable mounts.

The impressions given in this article can only give a glimpse of the totality of the tasting. The official 2008 Tasting page is not yet up (although 2003 through 2007 tastings can be seen here), but there is a pretty complete review of the 2008 Tasting at the 6 Moons site, with many tasty pictures. If I leave anyone’s equipment out of the following text, it is due to my inability to listen to everything plus my fading memory of the event three weeks ago.

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In the course of writing a series of posts about a proposed dipole speaker, I had a few thoughts about vibration control, a topic near and dear to speaker and turntable designers everywhere.

First, the “mainstream” approach of using every known method to get emitted vibrations as low as possible. Thanks to a post by jzagaza at diyAudio, we have some very interesting links to Fujitsu’s advanced audio research project at:

http://www.timedomain.co.jp/tech/tech_e.html

http://www.timedomain.co.jp/tech/theory/td_theoryA4_eng.pdf

And a rare Stereophile review that actually describes the real-world results of this project - compared to something completely different, an Avante-Garde all-horn system!

http://stereophile.com/standloudspeakers/107fuj/

By the way, the Fujitsu Ten Eclipse TD712z speaker is about as close to an ideal monopole - in terms of extremely low diffraction and fully decoupled vibration control - as any speaker I’ve ever seen. This seems to represent the limiting case for the “mainstream” method of rigorous isolation and mechanical sinking of vibration from the driver. The physically small size of the single-driver speaker also limits unwanted emissions from the cabinet, something that is quite troublesome in big audiophile (and dipole!) speakers.

And for the other view - instead trying to completely get rid of vibration, over at Mother of Tone, they’re aiming for “consonant” vibration, using selected woods and varnishes:

http://www.mother-of-tone.com/mother.htm

http://www.mother-of-tone.com/lacquer.htm

This isn’t as ridiculous as it sounds. Thom Mackris and I auditioned identical compressed-carbon (not carbon-fibre) record mats with, and without, a woodworking varnish in a direct A-B test. The difference was equivalant to the difference between a $500 and $5000 tone-arm, or a $200 and a $2000 phono cartridge. More musical, as you’d expect, but also much more lifelike and a remarkable absence of “mechanical” or metallic coloration. Instead of adding a “musical” coloration, it removed quite a bit of harsh and grainy “mechanical” coloration, and revealed much more of the musical dynamics, tone colors, and timbre. The impression of space was also quite a lot better, remarkably so, and it didn’t sound like a coloration to me, more like the removal of a number of resonances.

Are the “absolute” and “consonant” approaches in conflict? Not as much as it appears. Yes, it’s good to get coloration as low as possible, but there’s always going to be a residue. That residue might as well be musically consonant, so it can subjectively disappear. The Mother of Tone approach using solid woods and selected lacquers (not C37, by the way, but conventional wood lacquers) seems like a direct and cost-effective way to build agreeable-sounding speaker systems using commonly available materials.

I’m sorry for the lack of posting recently, but I’ve been working a lot as well as having bit of a writer’s block. I’ll soon be posting the first of what I hope will be an extended series on amplifier design topics. Here are a few things I’ve collected over the last few weeks:

The Lossless Audio Blog has switched from blogger to WordPress (the same software used here). Their new URL is http://www.losslessaudioblog.com/

Speaking of lossless compression, the Philadelphia Orchestra is offering performance recordings for sale online in both MP3 and FLAC format. The prices for most works are $4.99 for MP3 and $5.99 for FLAC, although Beethoven’s Symphony #9 goes for $9.99/$11.99. These prices are for the entire piece, so range from $0.37/minute for Britten’s Sea Interludes from Peter Grimes to $.11/minute for Schubert’s Ninth Symphony for the FLAC version (these were two I bought). Not really cheap but not bad for 16-bit 44.1KHz lossless files.

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This the second half of Mark Kelly’s article on developing a three-phase turntable motor driver:Ā

With confirmation that we needed a three phase drive and the full 30 watts it came down to how we go about it. The three phase oscillator is easy enough - I had constructed several such for the experiments on my Garrard so I used the modified Dippy oscillator which gave the best performance. To improve reliability I decided to put this on a PCB and to use a commercially available Vactrol analog optoisolator. It ended up looking like this (fig. 1):

The oscillator board is mounted in a separate insulated sub-enclosure and is wired directly to the power supply. This avoids the oscillator bounce at start up but more importantly it keeps the RC components of the oscillator at a fairly steady temperature, reducing frequency drift. This is further improved by using ordinary metal film resistors with a typical tempco of + 100ppm/Ā°C and matching them with polyphenylene sulphide film caps which have a tempco of - 100 ppmĀ°/C. Once the oscillator warms up it holds frequency within 0.01 Hz which is about as much as can be expected with analogue techniques. The output voltage is set at 1.0V.

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Mark Kelly, an engineer in Australia, who has a particular interest in audio electronics and turntables, has graciously agreed to submit an article on his experiences building a motor drive system for a vintage Empire turntable motor. Further installments are forthcoming. Now Mark Kelly:

Some time ago I was contacted by a vinyl lover [GW] resident in Japan, asking whether I could design and build a drive for his precious 1960s Empire turntable. GW said the motor ran on mains power with phasing capacitors and required 30 watts. Our first thoughts were that it was some form of synchronous AC motor, although I could not understand why any manufacturer would use such a high power synch motor. Anyway, I designed a high power quadrature drive, but fortunately I decided to ask GW to send the motor to me so that I could optimise the drive for it, which he duly did (in less than five days).

When the motor arrived I was very surprised - it was one of the famous Papst “flywheel” motors with external rotor (fig 1). As I checked it over it became obvious that it was neither two phase nor a synch motor. The motor had three leads, as do many two phase synch motors, but the resistances of the windings as measured between the three leads were equal. This is typical of three phase motors, a two phase motor will typically show twice the resistance across the outside of the pair of windings as between each winding and the common centre. I couldn’t detect any rotor magnetisation, so the motor either had the best shielding ever or it was not synchronous.

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Morgan Jones, the British writer on high-quality audio and author of Valve Amplifiers was reviewing my comments on unipivot tonearms (see the introductory part of “Mackris on cartridge set-up“) and sent me the following comments regarding his experience with these arms. He has plenty - he has been building these since he was 15 years old! There are significant ideas here that I haven’t seen described before. Here is Morgan Jones:

All “uni-pivot” arms plus many conventional arms have the center of mass of the counterweight below the pivot point, and will have the effect of making the effective VTA very sensitive to arm angle. The tracking force has an even greater effect on the angle of the stylus to the groove as the stylus swings up and down.

A unipivot only needs to have its center of gravity fractionally below the pivot point, yet most arms have it way below the pivot point in a misguided belief that it’s more stable. It isn’t. The center of gravity should be fractionally below the pivot point so as to minimise the length of the pendulum and maximise its resonant frequency. A higher resonant frequency allows damping to be applied more effectively. Further, most unipivots apply their damping poorly. You need least damping for normal up and down motion following warps, a little more to follow record eccentricity (which is slower) and most to damp torsional movement of the arm. The other important point ignored by most unipivots is that the pivot height ought to be at the same height as the stylus tip. Again, it’s a misguided belief about stability. Naturally I’m biased, but here’s the arm I made that uses all of these design features (see pictures in this posting - J.A.). It looks as though the center of gravity is very low but that’s because the pivot height is at stylus/record height, not 3/4″ above as is common.

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Direct-Drive Turntables

We have covered the basics of DC and AC motor design, and will now concentrate on a specific implementation: the direct drive turntable. Among the many headaches for a turntable designer are the various methods for slowing down a fast motor to the desired platter speed, whether through belts, idler wheels, or gears. Once self-commutating brushless DC motors became economical, due to the advent of semiconductors, the idea of driving the platter directly from a motor became feasible. The first direct drive turntables became available in the late 1960s, and by the mid-1970s, had become a fad. As with all fads, there were some good implementations and a lot of mediocre ones.

Direct-drive turntables became popular with DJs (both radio and later dance-hall DJs), because they could start quickly, kept accurate speed, were rugged, and were very reliable. One of the classic DJ turntables, the Technics SL-1200, has been in continuous production in one form or other from 1973 until today, practically a record for hifi gear!. Hi-Fi purists tended to avoid direct drive turntables, and stuck to their belt-drive versions. Direct-drive turntables are rarely seen today in an audiophile’s system, but as will be shown later, some excellent versions do exist.

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Peter Bartok, as it turns out, was the son of the famous composer, and after coming to America, became the engineer for Folkways Records. The AudioXpress article not only has a wide-ranging discussion of the early days of magnetic tape and the LP record, but also has two circuits from Peter: a mag-tape preamp dating from the late Fifties, and the all-tube cutterhead amplifier that he used for mastering records.

There’s also a sidebar on the equalization choices made in the early days before RIAA equalization became the standard - a straight 6dB/octave slope over the whole audio spectrum gives too much HF pre-emphasis (leading to distortion), and no EQ at all results in too much HF noise and excessive groove excursions in the bass.

The RIAA curve we use today is a half-way house between the extremes. In the early Fifties, record companies had many different proprietary curves of their own, and audio magazines simply advised listeners to adjust the tone controls until the got the sound they wanted. Hifi enthusiasts and the equipment manufacturers demanded the record industry agree on a common standard, and the RIAA curve came out of that. The 45/45 Westrex stereophonic LP standard made a few years later was a similar industry-wide agreement.

This article is the kind of thing that is all too rare in audio writing; a serious technical article about one of the real pioneers in audio, with an extensive discussion of the technology of the day. Thank you, Ed Dell and Paul Stamler.

Thom Mackris recently sent the following observations on cartridge set-up to his friends, and asked me to put them up on the blog. These observations back up something I learned in one of Wally Malewicz’s talks at the Rocky Mountain Audio Fest: that tonearms with under-hung counter-weights will change the effective VTA (Vertical Tracking Angle) quite a bit, depending on the height of the cartridge. All “uni-pivot” arms plus many conventional arms have the center of mass of the counterweight below the pivot point, and will have this effect. The tracking force has an even greater effect on the angle of the stylus to the groove as the stylus swings up and down. Here are Thom’s observations:

Recently, I’ve seen several references to extremely high tracking force recommendations for the Dynavector XV-1s cartridge - forces as high as 2.5 to 2.6 grams which is considerably above the 1.8 to 2.2 grams specified by the manufacturer.

Comments made on various audio forums also referred to Harry Pearson recommending that the XV-1s be set up in this manner. My experiences with the XV-1s is not in agreement with these observations. My results show the low side of the recommended range to work with the two samples I have been running in - even during the early stages of break-in when the cartridge’s suspension is the least compliant.

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Thom Mackris, of Galibier Designs, pointed me to a news item on MSNBC’s web site - a review of the Denon DL-103 cartridge. The reviewer was most incredulous that you could still buy a 44-year-old electronic item, but we (at least us here in America) should be glad that Denon is now officially importing it again! It gets a “Highly Recommended”. When the “Main-Stream Media” starts covering the arcane esoterica of vinyl playback, the chances are good LPs are here to stay!

What Makes a Good Turntable Motor?

The last posting reviewed how motors work - now let’s look at the details of what makes a good turntable (or tape recorder) motor. It turns out there is no one obviously superior type - it will basically come down to either a good synchronous motor or a speed-controlled DC motor.

Constant Speed

An obvious criteria for a good turntable motor is constant speed. Ideally the speed should be constant over the long term (so you don’t have to keep readjusting it) and over the short-term with line voltage changes or other perturbations. The AC synchronous motor is an good choice. In countries with reasonably-sized power grids, the power-line frequency is kept quite constant. If the motor is supplied by a built-in AC supply, these can be made quite stable, or even locked to a crystal oscillator. The AC induction motor is relatively stable, but in all but the cheapest designs can be replaced by the superior synchronous motor. It will no longer be considered. A high-quality DC motor running “open-loop” (i.e. no speed locking) is relatively stable, but is susceptible to long term drift in speed due to wear and to temperature changes (which causes changes in the bearing oil viscosity and hence load torque). However, it is easy to change the speed by changing the voltage. Various speed locking techniques can be applied to the DC motor to “servo” it to a constant frequency reference. This is done, for example in the Papst motors used in many SOTA turntables and in all “direct-drive” turntables. We will discuss speed locking later. If implemented competently, this can make a very stable motor.Ā read more »

Motors, Explained

A motor is basically a device to turn electrical energy into rotational energy, the vast majority using electromagnetism to do this. There are two parts to motor: the stator - the part that is fixed (i.e. at mechanical ground), and the rotor - the part that is connected to the rotating shaft. The rotor is also called the armature. Coils may be wound on either the rotor or stator or both, depending on the motor design. Most motors are built in a cylindrical form, but some use a flat “pancake” form factor.

All rotary motors work by generating two radial magnetic fields, one in the rotor and one in the stator. By having one of these fields rotate, the attractive and repulsive forces of magnetism cause a torque on the other field. In some cases one of the fields is fixed, generated by either a DC current or by a permanent magnet. In other types, both fields are rotating relative to their rotors and stators.

We will be looking at motors applicable to turntables: small, constant speed, and with a smooth output torque. There are three main categories of motors that are usable: DC, AC Synchronous, and AC Induction. After reviewing each of these types, it will become clear that there are many fundamental similarities between these types.

DC Motors

A DC motor has one fixed field, generated by either a DC electromagnet or a permanent magnet and the other, rotating, field generated by a series of coils distributed evenly in a circle. For this discussion, it will be assumed that the fixed field is in the stator and the rotating filed is in the rotor. However, many DC motors are arranged the other way around (fixed field in rotor, rotating field in stator). These are both equivalent and differ only in implementation details. read more »