The Korf Blog

The inside story: our research,
development and opinions

22 November 2022
Turntable Drives. Part IV, Direct Drive Introduction
In our previous posts, we have talked about the turntable belt drive and explored its function as a lowpass filter, rejecting the motor's torque ripple. Then, we went through the key problems associated with the belt drive—and some of the solutions.

Then we shifted our attention to the idler drive, and went through its typical troubles.

We have saved the best for last. This is the first post about direct drive, how it works, what is good about it and what isn't.

Direct Drive History
While the inventors of belt and idler drives are unknown, Matsushita (Panasonic/Technics) claims the invention of direct drive turntable with their 1970 SP-10. This isn't completely preposterous. But, strictly speaking, neither is it true. To understand why, let's first define what a direct drive turntable is.
The most logical definition I found states that in a direct drive turntable, the motor A and the platter B share the same shaft C and rotate at the same angular speed.

This is a very straightforward configuration, and it would be really strange if nobody tried it before 1970. And, of course, during the prewar shellac/78 RPM era, everybody and his dog did, both in turntables and in cutting lathes. After the Paillard buyout in 1963, Thorens mentioned in their brochures a 1928 Paillard patent that supposedly covers a direct drive motor for an electric grammophone. Whether this patent is relevant or not, by 1930s direct drive was quite common.
Many turntable companies, Thorens included, marketed their worm-and-gear drive turntables as "direct drive". Personally, i don't think they are.
Then what did Matsushita invent? And why can't we find any mention of postwar direct drive turntables until Technics came along?

The answer is, those early direct drive motors were unusable with slower (45 or 33 RPM) speeds. They depended on mechanical commutation—carbon brushes supplying current to the rotating copper collector. With a suitable flywheel, it didn't create too much trouble at 78 RPM. But any slower, the jerkiness of mechanical commutation became unbearable.
Matsushita invented the electronically commutated (brushless) direct drive (DD) turntable, steadily rotating at slower speeds. The patent says exactly that. They used semiconductors to direct electrical current to the motor's coils, smoothly and without any noise.

We will come back to electronic commutation later, as it is critical in understanding the benefits and limitations of direct drive. All modern DD turntables are electronically commutated.

How Does Direct Drive Work?
With belt and idler drives, the mechanical diagrams were pretty much self explanatory. These are not very helpful in understanding direct drives. All the cool things are in how the motor itself is designed and driven. Shall we have a closer look at one?

For illustration purposes, we will use a high quality first generation motor: Dual EDS 900. Just like with digital audio a decade later, the caliber of engineers working on early direct drives was often a lot higher than during the technology's supposed "maturity". If you don't believe me, compare the sonic quality of Donald Fagen's fully digital "The Nightfly" from 1982 with anything recorded in the 1990s.

If you are not familiar with the way brushless motors work, I can recommed the following tutorial. Otherwise understanding of what follows might be a bit challenging.
A typical turntable DD motor has a permanent magnet rotor with a good number of poles. Our EDS 900 has 8, other companies used any number from 4 to 64.

I love to use the green film to visualize magnetic fields. It is indispensable when working with anything magnetic.

There's a stator with a few coils. The current flowing through those coils determines the movement of the rotor. EDS 900 has 4 separately driven coils in 2 phases. 2 or 3 phases are typical.

Note the coils have no metal core. Most DD motors are coreless.

For the electronics to "know" the position and speed of the rotor, feedback systems are needed. In our EDS 900, the 2 Hall sensors (brown paint dots on the PCB) give the rotor position.
For speed feedback, Dual uses a soft iron gear rotating inside the circular multipole magnet. The resulting changes in flux are registered by a coil that lies under it.

Other companies used simpler approaches, from a PCB coil to a magnetic head reading a barium strip on the platter. As we will see, the precision of this feedback is critical to the performance of a turntable.

How does this all comes together and rotates the platter at the desired 33 1/3 or 45 RPM? Here's a block diagram of EDS 900. It's fairly typical of the 1970s DD motors.
The frequency generator (in EDS 900's case, a coil under the soft iron gear) generates impulses with a frequency that is proportional to the motor speed.

This frequency is then converted to voltage: the higher the RPM the higher the voltage. The lowpass filter discards the high frequency fluctuations and provides the damping, preventing "hunting" or runaway oscillations of the whole feedback loop.

The resulting voltage is compared to a reference. This is how the speed selection (33/45/78 RPM) is usually done—by changing the reference voltage.

Comparison gives us the error voltage. This voltage is then driving the motor controller itself, telling it to increase or decrease the current through the coils.

If the motor rotates too fast, the reference will be lower than the FG voltage. The comparator will thus "tell" the controller to put less current through the coils, slowing the motor down. If the motor is too slow, the error voltage will increase, "telling" the controller to increase the coil current.
Precision voltage references are quite hard to implement. Instead, later DD motors often used a a quartz stabilized frequency reference inside of a phase locked loop. On paper, this is an amazing advantage: a common inexpensive quartz is stable to 50 ppm (0.005%), while a very good modern voltage reference can be stable to 50 ppm per degree of change in the ambient temperature!

Some turntables, like Micro Seiki's DQX-1000, actually gave the user a choice of two control strategies.

Direct Drive Advantages and Problems
At a first glance, direct drive looks like perfection itself.

There's nothing mechanical to go wrong or create extra noise. The only friction part is the bearing, and it's present in all the turntables anyway. Bearing side loading, the bane of belt and idler drives? Zero. Tipping forces? Absent. Machining imperfections or warps? Not a big deal at all. Expiring elastics? None.

No elasticity in the drivetrain should also mean perfect reaction to load changes. All that torque goes straight into the platter!

Simplicity also means that a direct drive turntable can be made ridiculously inexpensive. Alternatively, the money not spent on idlers and pulleys can go into a monster bearing and a perfectly balanced platter.
Why isn't direct drive dominant, except in the DJ booths? Why do many direct drive turntables underwhelm sonically?

Then why isn't direct drive dominant, except in the DJ booths? Why do many direct drive turntables underwhelm sonically? Why Micro Seiki, having spent the 1970s perfecting the direct drive, went into the 1980s with a belt drive and never looked back?

Direct drive has only two significant problems. One is easy and can be inexpensively solved. And the other... well, the other calls for a separate post or two.
Flux Leakage
Flux leakage is magnetic field escaping the confines of the motor and finding its way into the pickup or wiring. Remember, the typical pickup cartridge is essentially an instrument that reads the magnetic field changes. Even when shielded, the cartridges remain very sensitive—and most moving coil cartridges aren't shielded at all.

Here's a screenshot from a simple DD motor simulation. The black lines represent magnetic flux, escaping above the platter (blue rectangle) where they can be picked up by the cartridge.

Inattention to flux leakage doomed many otherwise well designed and engineered turntables to sonic mediocrity. The saddest and best known example is the magnificient Kenwood/TRIO L-07D.
Stray magnetic fields of most midrange 1970s and 1980s Japanese direct drive turntables can only be described as epic.

Why did it happen? In the 1970s, the two best EMI-fighting tools were not yet perfected: a computer magnetic field simulation and a sensitive realtime broadband magnetometer. The engineers had no way to visualise magnetic fields, or to reliably measure them. Now, of course, both tools can be had for trivial money.
Flux leakage manifests as veiling of sound and loss of perceived resolution. There's not much that the end user can do—homemade shielding solutions seldom make a significant difference.
Speed Stability/Torque Ripple
If you read any advertising copy or magazine reviews for direct drive turntables, you are likely to see the following statements:

  1. The turntable's wow and flutter are below the measurement floor. It plays music with perfect timing and metronomic precision: the speed error is less than a second over the whole LP side!
  2. There's something called cogging that affects our competitor's turntables, but is compeltely eliminated in ours

As is typical in advertising, these statements are obfuscating the very nature of the challenge while being technically true. In our next post, we will try to accurately and impartially describe the complexity of direct drive's speed behaviour.

To whet your appetite, here's a little chart. It shows speeds distribution—how many times a given momentary speed was measured over a given period of time. The speed was taken 5000 times a second with a proprietary contactless Korf Audio system, while a pop music LP was playing. Thin vertical grey line is 33 1/3 RPM.
Brown is a top quality late 1970s Japanese DD turntable. Blue is a similarly priced belt driven turntable of equally high standard. Both were tuned to exactly the same average speed (a little slow).

DD's momentary speed wanders over a much wider range, having at least 2 distinct peaks. The belt driven turntable has the correct speed almost twice as often—despite its much worse wow and flutter specification.

Why is it so?

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