[DIY] AC Power Supply for Michell Gyrodec

DIY design for an outboard AC PSU for Michell Gyrodec turntables or AC motors

How to trust “precise” mains frequency and live happy

Product: DIY outboard AC motor PSU (for 24V motors)
Manufacturer: not for sale, free TNT-Audio DIY design
Approx. cost: from 60 to 80€/$, depending on components
Authors: Design by Ruggero Sanna - Introduction and test by Lucio Cadeddu - TNT Italy
Published: February, 2022

Foreword

Nowadays most turntables rely on DC motors, while in the past synchronous AC motors were all the rage. On this kind of motor, the speed accuracy was mainly based on the precision of the mains frequency (50 or 60 Hz, depending on the country). Frequency accuracy can't be improved, or indeed altered, as it depends directly on the electric power company.

If this frequency accuracy isn't good enough you can use a PSU (Power Supply Unit) that generates a precise 50Hz/60Hz sinusoid, based on a precise quartz oscillator. More or less, it is an amplifier that puts out a pure 50Hz tone. This SHOULD guarantee a better speed accuracy for AC motors. And this was exactly the idea behind the Linn Valhalla, introduced in May 1982 by Linn, to feed their Sondek LP12 turntable. Years later, precisely in 1995, Naim designed the Armageddon, a beefy 430VA PSU intended to give a precise voltage output to the Linn Sondek motor, without the use of a quartz-controlled device. The Valhalla, indeed, wasn't very reliable, to say the least. On the other hand, the plus of a quartz-controlled PSU is that it can generate another (higher) frequency to make the turntable spin at 45rpm. This is impossible to do with a PSU like the Armageddon, for example.

As said, many turntables used (or still use) a synchronous AC motor. For example, Michell used Papst 24V AC motors for some of their turntables. When the PSU goes bananas, you can decide to purchase a new standard one (normally expensive), a better one (more expensive), a second-hand unit (for how long will it last?) or try the DIY route.

Our case study is the original Michell Gyrodec 24V AC PSU, which is a very simple device. It can be found on the second-hand market for, say, 100-150€/$, but you can save some money and get a completely new (and better) unit by deciding to build our dedicated DIY PSU. This will work with any other turntable with similar features (24V AC motor).

I was discussing this idea with a friend with some very good DIY skills (Ruggero Sanna), and we came up with a DIY design that won't break the bank and can be built with minimal skills.

Of course, our goal wasn't to make the best outboard PSU, just a simple one that can be easily built and that works fundamentally better than the stock Michell unit. You can modify/upgrade our design as you desire. For example, you can use a beefier toroidal transformer (ours is 50VA, much larger than the original Toroid International TI-66636 of the Michell PSU), a higher spec'd cap or a different mains filter. You can even decide not to use a mains filter. Naim, for one, claims mains filters are harmful if used in turntable PSU's like the Armageddon. Just try and decide for yourself.

In use, our DIY PSU works flawlessly, as it is completely silent, with no hum, noise or vibration of any sort. Moreover, speed accuracy is slightly better, hence the sound gets a small improvement as well. When speed accuracy improves, you get a firmer and better focused 3D soundstage and the bass range becomes deeper and punchier. Small improvements, of course, but still audible on a good system.

Let's the designer speak

The first thing to remark is that there are many different Michell Gyrodec versions out there. This PSU will work with 24V AC motors only, at 33rpm. 45rpm is not possible, but for sure you can change the position of the belt on the motor pulley. For DC motors you should follow a different route.

The first step is the list of the components you need for building a 230V unit. For 110V units please vary values accordingly. Of course, the output of the transformer must be 24V in any case. These are just mere suggestions, you can decide to use different/better components, provided the scheme values are preserved.

Scheme

As you can see, it is a very simple circuit, just a toroid (T1) and a cap (C1). There is a couple of security devices such as the PTC that protects the motor from overload and the mains fuse (F1), just to be 100% protected from mains faults. We have used the original Michell cable: you just need to cut the old cable and connect it to the new PSU, preserving the color code, as shown in the scheme.

Have a look at the pic below in case of doubt. If your skills aren't up to the task, ask a friend or a technician do the soldering job and the connections for you.

Usual security disclaimer here Since electricity kills, do not attempt to build this device if you haven't the minimal skills required.

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