The Sound of the Wood™
Speed Box Mk. III
by David Plummer
http://www.audiospecialtyco.com/
9/27/06
1. Introduction
Synchronous AC motors are often used to power turntables. Examples are the various AR models (XA,XB, ES-1, etc.), Linn Sondak, and the Thorens TD-1xx series. An AC motor is one which is designed to be powered by alternating current – such as that provided by common household power receptacles world-wide. The drive element of a synchronous AC motor rotates at a speed which is determined by the frequency at which the supplied alternating current fluctuates. In the U.S., that frequency is nominally 60 cycles per second (Hz). The more pure the supplied current in terms of frequency, the steadier the rotation of the motor and hence the turntable platter driven by that motor.
In order to change the rotation speed of an AC motor, the frequency of the driving AC must be changed. Normal synchronous AC motors have the ability to respond correctly to a rather narrow change of frequency – on the order of +/- 2 to 5%. Most turntables employing synchronous AC motors have no provision for adjusting the frequency of the supplied AC and hence have no way to allow fine tuning of the speed at which the platter rotates.
The Speed Box Mk. III has been designed to allow adjustability of AC frequency, to hold the set frequency constant, provide a reduction of distortion in the resulting AC power, and reduce power line noise through isolation. The general specifications of the Speed Box Mk. III are:
· 60 Hz output +/- 2.5 Hz adjustability
· +/- less than .07 Hz periodic fluctuation in the measured output frequency
· Total harmonic distortion + noise less than 2% at 60 Hz
· 6-8 watts of output power
2. Wall Current and Power Filters
The standard 120 VAC found in homes in the U.S. is often not very pure. That is, the harmonic distortion of such power often is on the order of 5% or more, depending on the environment and operation of other appliances within the home. Noise is always present, and often at radio frequencies. The 60 Hz. frequency of the current can vary. And the voltage of the AC current can fluctuate widely – even when there technically is not a “brown-out” of the power grid.
Almost power line conditioners are designed to reduce noise in the supplied AC – accomplishing this by using inductors and capacitors and/or line isolation by transformer. Some supplies use a transformer with multiple taps to allow stepping up or down the voltage in response to under or over voltage situations. Note that conditioners using auto-formers are not providing true line isolation in most cases.
Very, very few power conditioners can reduce harmonic distortion or affect the frequency of the AC. The Speed Box is such a device. It also delivers all of the advantages of the common power line conditioners including a degree of output voltage regulation inherent in its design topology.
3. AC Distortion and Turntable Speed Stability
All electro-mechanical devices designed to rotate at a constant speed are subject to periodic error (fluctuation) in their rotational speeds. This can be observed in turntables by using a strobe light to measure the rotation of the platter, or even by hearing a wavering in the pitch of a sustained note. Note that serious telescopes employ motor drives to cancel out the rotation of the earth so that images may be held constant in the telescope’s field of view. Scopes without such drives must be repositioned frequently to keep the observed object in view. The better grade of telescope drives use periodic error correction to greatly reduce the tracking error brought about by rotational speed fluctuations in the driving motor.
It is generally not appreciated, even by turntable designers, that the harmonic distortion content of the AC supplied to synchronous motors affects the periodic error of the drive system. The reason for this is subtle but clear. It is a tenet of physics that any periodically fluctuating signal can be constructed from a series of sine waves. For a signal without distortion, only one single sine wave is needed to construct the signal. A distorted signal must be constructed by using multiple sine waves. In fact, harmonic distortion is seen on a spectrum analyzer as a family of signals, related mathematically to the original signal. Since a synchronous motor “locks” on to the frequency of the driving AC, the presence of multiple sine waves causes the motor to “hunt” in a periodic fashion. This produces a change in rotational speed of the platter. Hence, the lower the distortion in the supplied AC, the lower the periodic error – all other factors being held constant.
Interestingly, some add-on turntable speed controllers output a saw tooth signal rather than a sinusoidal signal. Such a signal is rich in harmonic distortion – thus guaranteeing a higher degree of periodic error in any turntable connected to such a device.
It is worth noting that while a heavy turntable platter can reduce the magnitude of the rotational errors, it does so by spreading the errors out in time – not by eliminating the error. So a loud, fast wavering note becomes a softer, more slowly wavering note. But no turntable platter without infinite mass can eliminate the wavering!
4. Speed Box Mk. III Design and Use Overview
The topology of the Speed Box Mk. III is very straightforward. A high quality signal generator emits a sine wave AC voltage whose frequency is determined by a voltage controlled oscillator. A potentiometer is used to adjust the voltage and hence sets the frequency of the output signal. The generated sine wave signal is directed to a small, high quality amplifier which is capable of driving complex loads. The amplifier output feeds a step-up transformer which converts the amplifier output to wall AC voltage levels. DC for the power amplifier is provided by purpose-designed power supply. Input AC is directed to the power supply via a modular, switched and fused IEC connector on the back of the unit. Note that the fuse tray accepts 5x20mm, .5 amp slow-blow devices. Normally, a spare fuse is supplied inside of the tray at the front.
The Speed Box has three controls: a neon-light power switch on the rear of the unit, integral with the IEC connector, a front panel mounted miniature toggle switch, and a rotary knob. A blue LED is also present on the front, above the rotary knob. When the blue LED is illuminated, the rotary knob varies the frequency of the output AC within the specified range. When the LED is dark, the output frequency is set to 60 Hz. The miniature toggle switch is used to select between the two modes of operation.
Care should be taken to not overtax the output capabilities of the Speed Box. The power rating for the turntable being used should be a maximum of 6 watts. Note that the frequency adjustment will have no direct effect on the rotational speed of turntables using DC drive motors.