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As I mentioned in an earlier post, the waters get a lot deeper here, compared to the much simpler low-to-mid crossovers. Why? Two reasons:

1) 1 kHz is about 14 inches long (sound travels 345 m/Sec). This means polar patterns are going to be much more important than at lower frequencies, where drivers are usually closer together than one-half wavelength. When drivers are one wavelength - or more - apart, you start getting lobing in the polar pattern, just like a TV or FM antenna. In addition, the phase angle between the drivers becomes important, since this steers the lobe up or down - and changes with polar pattern vs frequency are very audible as a “phasey”, electronic-sounding, or artificial coloration.

2) Remember the Fletcher-Munson curve? That applies to audibility of frequency response variations, and distortion. A 2dB variation in frequency response at 200 Hz is less audible than a 0.5dB variation at 2kHz - in fact, normal listening rooms have 5 to 10dB variations in the 100 to 500 Hz region, and we take these for granted. At the higher frequencies, room modes are so closely spaced they no longer affect the direct-sound response, so we are mainly hearing the speaker itself.

Even more important than frequency response variations, audibility of distortion follows the Fletcher-Munson curve as well. This results in IM and THD distortion in the critical 1 to 5kHz region being far more audible than lower frequencies. Audibility of noise, for example, reaches its peak in the 5 to 8 kHz region, which is why DBX and Dolby compandors use frequency-selective curves, as well as FM broadcast de-emphasis curves, NAB tape recording equalization, and the RIAA recording and playback curve. Audibility of noise and distortion in the upper-mid frequencies has been known going back to the Twenties, thanks to Bell Labs research on telephone transmission.

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Several updates have been made to the homepage: Internet Explorer users can now finally see the header picture and a list of recent comments have been added to the right-hand column.

Lynn reports that editing a post while using the Safari browser under Mac OS-X doesn’t work. This is apparently due to a problem with Java scripts, and WordPress hopes to fix it. In the meantime, the workaround is to use a different browser, such as Firefox or Opera, at least for editing.

I have set the threshold for comment moderation so that if there is more than one imbedded URL, the comment will be held for moderation. This is a compromise that will hopefully keep the comment spam down.

- John Atwood

Lynn pointed me to an interesting new on-line magazine: DIYMAG. Bas Horneman in the Netherlands publishes it. It is in pdf format so you can print it out and take it to the bathroom (if you want). It’s got a nice mix of articles that are reminiscent of Sound Practices and Valve, plus it has a good sense of humor. There are three issues out so far. This definitely fills a gap now that there are so few print DIY magazines.

In the most recent issue is a long article on turntable motors by Mark Kelly in Australia. He covers some of the same stuff I am covering, but goes into much greater detail in some areas, especially on motor control circuits. Consider it a strong adjunct to my series of postings. Mark really knows his stuff.

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.� read more »

I am half-way through reading a great book: The Timeless Way of Building by Christopher Alexander (Oxford University Press, 1979, ISBN 0-19-502402-8). Christopher Alexander is a mathematician and architect who came up with a technique of describing and designing buildings and towns based on he calls a “Pattern Language”. This is a set of rules for design that are analogous to how DNA creates living things. In following various Pattern Languages, people build their living spaces in similar but not identical ways.

What does Alexander’s work have to do with sound and music? It is the criteria he develops for making good quality designs. He calls it “the quality without a name”. He is careful to not to name it, because he feels that existing words in English will restrict its meaning. However, he dances around the meaning using words like “alive”, “whole”, “comfortable”, “free”, “exact”, “egoless”, and “eternal”. He relates the quality without a name to the concept of being “alive” - people being natural and spontaneous. It struck me that this is exactly the quality needed to make good music, both in the performance and in the playback. Music that is made by uninvolved musicians just following notes on the page has a dead quality. I call it “sleep-walking music”. Playback systems that are carelessly designed or designed to certain abstract ideologies often deaden the music being played through them.

The concept of a “pattern language” for building design also exists for electronic design. Virtually no piece of electronics is designed in a vacuum (so to speak) - its topologies and constraints come from previous designs, and the ones that are well proven or perform well seem to embody the quality without a name.

Anyway, this is a fascinating book, and is well-recommended.

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 »

Yes, OK, I know I’m supposed to get started on the high-frequency crossover next - and this post ain’t it. Well, it’s a big subject, involving vector math, a discussion of how a loudspeaker is both a transducer and an antenna (and you don’t want a Yagi), remembering that 1kHz is about 14 inches long, and a lot of stuff about phase synchronizing the two drivers with each other. And so on.

What I’ve really been doing while John A. is keeping the fires lit on the blogspot is getting acquainted with my two new computers. My old MLSSA speaker-measuring system gave me a good scare a couple of months ago when the hard drive failed - fortunately, my Tektronix pal and PC genius Gary Pimm had a spare antique IDE drive with DOS and another copy of MLSSA, so a UPS delivery from rainy Portland and an afternoon with the PC and MLSSA was up and running again, this time on the replacement hard drive (thanx, Gary!!!)

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I’m in the process of building the motor for my next turntable project…

… just kidding!� This is actually a turbo alternator from page 9 of Electric Machinery, (by Fitzgerald and Kingsley, McGraw-Hill, 1952), a great resource on motor theory and design.

Let’s get Quantitative

We will now look at how to quantitatively calculate the dynamic effects of the motor/platter/cartridge interactions. In order to do this, an analytical model of the system needs to be constructed. Since mechanical systems are analogous to electrical systems, the same tools used to study electrical networks can be applied to the mechanical model.

An excellent book to help do the mechanical analysis is Physical Networks by Richard S. Sanford (Prentice-Hall, Inc. 1965). It is out of print, but used copies can be found at places like Powells.com. In this book, electrical, motional, rotational, and hydraulic models are all treated the same, and analytic tools familiars to an electrical engineer, such as nodal and loop analysis, matrix analysis of networks, Laplace transforms, closed-loop feedback systems and stability, are applied to these models. Complex electrical networks have been studied in much more detail than mechanical networks, so the tools of an electrical engineer are helpful here.

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Steve Schoenherr, a professor of history at the University of San Diego, has a really good webpage showing the history of sound and video recording technology at http://history.sandiego.edu/gen/recording/notes.html.� This covers not just the technology but the musical culture, too. For example, under “New Popular Music” at the year 1926, see how Bing Crosby “began to use the new microphones developed by Bell Labs that encouraged the “crooner” sound when held close to the singer’s mouth”.� This site provides good background to the ideas expressed at the ClariSonus site.