DIY Direct-drive turntable

Introduction

This page describes the construction of my direct-drive turntable which I built from commonly available items without any specialised machining. Please see the video above to get an idea of how things are mounted since describing it exactly in written form is a bit tedious and will get unnecessarily confusing.

Why direct-drive?

I wanted to build one because of a fascination with it in part, and because of the torque and speed precision that it offers - speed can be controlled by frequency rather than voltage. I searched on the internet for design ideas and most of them were belt-driven designs. I determined that a synchronous motor can be used for this purpose...but finding a suitable unit was hard. Most available were rated for 230VAC/50Hz and offered RPM ranging from 800 to 1600 RPM. To make it run at the slow turntable speeds would require very low frequency sine waves, that too amplified to 230VAC (and even so the torque would remain questionable at such slow speeds) or using gear mechanisms - something I did not want to undertake. Then I thought about using stepper motors as they can be made to turn as slow as we want via suitable digital means. Again a search revealed mostly discouraging posts about using stepper motors for phonograph turntables. Despite this, I thought it worth trying.

All we need to get it to work is a suitable microcontroller and a stepper motor driver with some programming. This is where the Arduino platform comes in.

Parts required

Drive and control system:

• Stepper motor - NEMA 23, 10 kg-cm, 2-phase (4-wire). The one I used is specified for 1.8 deg/step. A motor with a smaller step angle is preferred as it makes it that much smoother while running.

• DRV8825 stepper motor driver board.

• Arduino UNO (or equivalent) microcontroller board.

Electronics:

• Moving magnet phono pick-up.

• Phono pre-amplifier (parts not included in this article).

• Suitable power supply - one that can supply 12VDC at 2A for the stepper motor/controller and dual +/-12VDC at 1A to power the preamp (if you intend to include it). These are not included in this article.

• 1N4007 diodes - 3 nos.

• 470uF/25V - 1 no.

• Green LED - for the "Ready indicator"

• (Optional) Red LED for power indication (not shown in the schematic)

• 1k/0.25W resistors - number depending on need.

• RCA female (red/white) - 1 pair.

• Flexible/thin wires for cartridge - coloured red, yellow, white, green (or blue) each of suitable length

• Male/female headers - 1 no. 40-pin header strip (break out as per need).

Mechanical:

• Ball bearing 608-2RS (8x22x7 mm) - 1 no. (for the tone arm horizontal pivot)

• Shaft coupler - 1 no. (to mount platter to the motor shaft. Choose as per motor shaft diameter. See video for the kind I used)

• 12-inch dia, 2.5 mm thick round MDF discs - 4 nos. (for the platter)

• 2 inch long bolt and nut 2.5 to 3 mm dia - 1 no. (for the tone arm vertical pivot)

• Plywood (for the box and other items - see video)

• Softboard sheets (for the tone arm, and other miscellaneous use - see video)

• 1/2 inch long Philips head bolts/nuts (to mount the platter to the coupler. See video for how I have done it)

• Small screws

• Rubber-based adhesive, super-glue (for sticking metal parts together)

Miscellaneous:

• Stroboscobe disc. There are PDFs available freely which can be printed and used. This comes in handy to verify the speed.

• Tonearm alignment protractor. Useful to have in order to correctly adjust the cartridge.

Platter construction

Find the exact centre of all the MDF discs and drill a small pilot hole in all of them. Take any one disc and using the coupler as a template mark out as precisely as possible all the mounting holes in it. Ensure that the holes are enough to allow the 1/2 inch bolt shanks to pass through but not the heads - this is the platter base. In the remaining discs mark out holes at exactly the same positions but drill them big enough to accommodate the bolt heads. When placed on the platter base and matched with the holes you will see that when the bolts are put, the bolt heads are nicely concealed and the top surface is completely flat. The nuts go under the coupler (the opposite side) and are fixed concentric to the mounting holes with super glue.

Next, in the centre of all but the base platter make the centre hole just large enough to accommodate a 7mm spindle of about 1 to 1.5 inch height. For those in India, the earthing pin of a standard 6A plug is exactly 7mm in diameter, and of the correct height to comfortably seat a phonograph record.

Stick one such disc to the base platter which we will attach to the motor using the coupler. At this time do not stick the remaining discs (see the Conclusion section as to why). Place the spindle in the centre and stick it with the rubber-based adhesive.

Note: You may choose a different platter diameter to suit your needs.

Box construction

Make a box with internal dimensions as 16" width, 16" depth, 3" height. This particular width and depth is chosen to accommodate the platter and tone arm which I made. You may choose a different width/height based upon the tonearm and platter that you decide to use.

The below image shows the turntable layout (not to scale). 

Note: The video does not show the "Ready" and "Power" lights - those were added much after I had shot the last clip.

Tonearm construction

Please refer to the following link for details of the tonearm design: 

www.analogplanet.com/content/tone-arm-geometry-101 

I chose an effective tonearm length of 11" (10" spindle to pivot plus 1" overhang). Based on the above page, I arrived at a suitable tonearm "skeleton" with which I made the proper tonearm template, and used that to cut out the actual piece from softboard.

The main horizontal pivot is made by attaching a softboard piece to the ball bearing exterior and then fixing the ball bearing by its centre on to its location, determined as per the layout (by measuring a 45 deg line from the platter centre or spindle and marking a point 10" away on it). The vertical pivot is made by attaching two plywood pieces in parallel at the opposite sides of the bearing and sliding in a 2" long bolt through it and the tone arm (see the video for exactly how this is done). 

Cartridge wiring

Since I did not have the required connectors to attach wires to the cartridge, a google search revealed a very simple and cost-effective way of obtaining them from VGA sockets. Please refer to this page: www.vinylengine.com/diy-cartridge-clips.shtml 

The cartridge wires are then terminated on one side to the connectors and on the other to a female header (4 pins). Wires are secured to the tonearm with some paper tape. An additional male header then carries the signal into the box to the preamp. Again, this is done so that the tonearm can be completely disconnected and removed if need be - makes it easier overall to open the box.

Motor mounting

The mounting of the motor is critical. Since I used a stepper motor, it was found that mounting it to the box's upper panel results in a lot of vibration which is very hard to get rid of. The solution is to mount the motor by its base on to the lower panel of the box. Here we have to ensure that the shaft is located exactly at the point where the platter's centre is - because the tonearm length and pivot locations are calculated based on this. Some error, I guess might be acceptable, but I have not tested for this.

Since the motor is mounted to the bottom panel, and the platter is attached to it by a rather large coupler, opening the box from the top panel becomes problematic. To solve this, I simply cut out a square just larger than the coupler so that we can open the top panel only by detaching the platter from the coupler. Reaching the coupler's hex nuts from under the platter is very difficult.

The electronics

Only the motor related parts are described. Since a stepper motor is being used to spin the platter, it needs to be driven by a suitable driver that is capable of microstepping. Microstepping is required because of the way a stepper motor works - in steps. In between it stops, and this makes its rotation very choppy. This means a motor with a smaller step angle will have much smoother rotation than one without. Microstepping helps us to use a motor with a greater step angle perform reasonably smoothly. The DRV8825 driver offers 1/32 stepping which means that it can cause the motor to step at an angle that is 1/32 of its full step. With a 1.8 deg/step motor this evaluates to 0.05625 deg/step. There are a lot of articles on the internet that explain how stepper motors, drivers and microstepping work.

The main code is there in the "phono_stepper_drive.ino" file. The "CustomStepperDriveLib.h" is the module or library that implements code to control the driver board. In most cases this will not need to be modified, only the code in the main "ino" file. If you use the code as it is, the available features are only motor speed selection - start/stop is done by powering on/off the microcontroller supply (referring to the above schematic). The code is set up to work with a 1.8 deg/step motor, and 1/32 microstepping with a DRV8825 board. If you use a different motor (or A4988 driver board), be sure to change the relevant parameters in the code (it is hopefully well-documented). The speed cannot be changed while the system is running, nor is there any capability to finely adjust the speed (I had included it earlier but it was unnecessary, and only added extra complexity). You need to stop -> select the speed -> start. Again, change the main code if you want to change the speed on-the-fly.

After starting, there is a delay of about 25 seconds before the "Ready indicator" turns ON (also done in code). This is because of the stepper motor and platter interaction which makes the speed "ring" for a few seconds before it settles to a steady state. A 25 seconds delay gives enough margin before placing the needle on the record. This " ringing" or motor/load interaction is perhaps why use of stepper motors is discouraged for audio turntables.

Depending on the motor, you will need to change some parameters passed to the library such as the driver initialisation parameters. All the documentation is contained within the code. For any help you can contact me as well.

Speed measurement

In addition to the stroboscope disc I measured the speed using a mobile app and here are the results. Speed is within 1.5% of the target values, speed variation is less than 0.2%, and wow is less than 0.1% at both speeds.

33.33 RPM

• Actual speed: 33.27 (-0.20%)

• Speed variation: -0.06% to +0.05%

• Wow: +/-0.04%

45 RPM

• Actual speed: 44.75 (-0.55%)

• Speed variation: -0.06% to +0.05%

• Wow: +/-0.04%

Conclusion

It is possible to have a direct-drive mechanism using stepper motors provided we keep a few things in mind:

• Use a motor capable of high holding torque (10 kg-cm or better)

• Use a motor with as small a step angle as possible, as well as a driver that provides 1/32 microstepping or better (1/32 is the bare minimum for 1.8 deg/step motors, and 1/16 for 0.9 deg/step motors)

• Determine the best platter mass by adding/removing the MDF discs to ensure fastest speed settling time

Further Thoughts...

There are alternative approaches to achieving the same or better performance without using a microcontroller. 

1. Use a simple pulse generator (such as one based on an XR2206 function generator) to generate the required pulses for the driver.  Of course we would have to increase the pulse rate from a low value to the final one in order to simulate acceleration of the motor, as well as hard-wire the microstepping mode, etc.

2. Use a sine wave split-phase drive to excite the motor directly via a linear amplifier. This approach has the possibility for very smooth motor operation, provided the sine wave frequency is very well controlled. Again a XR2206 based approach may be used to generate the sine wave.

And finally, a demo of the turntable...

That's it. Happy building!!