In our previous posts, we have introduced 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.

Today, we will discuss the idler drive, and go through its typical troubles.

Idler Drive

The belt drive has been dominant for such a long time, it's easy to forget it wasn't always like this. From 1940s all the way to the late 1960s, belts on turntables were an exotic rarity. Idlers ruled the industry, and were found on everything from affordable briefcase-type players to the most expensive studio drives. Why did the idler drive disappear so quickly and so completely?

But first, a few words on how an idler turntable drive works.

Just like with a belt drive, we need a way to convert a fast and uneven rotation of an affordable motor to a slow and smooth movement of the platter. A suitable RPM reduction is achieved by a rubber-tyred idler that sits between the small diameter motor shaft and a large diameter platter. It also couples the motor and the platter quite rigidly. In effect, the inertia of the motor's rotor is increased by the (comparatively huge) inertia of the platter.

 

A. Motor

B. Motor Shaft

C. Idler Wheel

D. Platter's Inner Rim

E. Platter

F. Main Bearing

G. Idler Bearing

Compared to the belt drive, idler drive represents a very different approach to torque ripple compensation: instead of filtering the ripple, larger rotating mass is smoothing it out. A relatively inelastic link also solves the problem of energy replenishment with load variation. Remember, the platter is virtually an extension of the motor. Great, isn't it?

Not so fast. Hard-coupling the motor to the platter transmits all sort of noises right into it. Electrical hum, grating bearings, squeaking motor supports—all there to be picked up by the stylus. This makes the idler drive a lot more demanding of motor quality. And, of course, it requires a solid heavy platter to work.

Alas, most consumer idler drives used the cheapest fan-type AC motors and lightweight stamped platters. This resulted in truly terrible wow and flutter performance plus an even worse dose of annoying rumble. No wonder the consumers switched to much quieter and steadier belt-driven turntables as soon as they appeared.

Are all idler drives that bad? Let's look at their characteristic problems and decide.

 

 

We started our belt drive problem discussion with the resonances. The great thing with idlers is, they don't have any. The combination of hardness and damping of the tyre elastomer means it doesn't have a perceptible own frequency. But many other belt drive problems do have corresponding idler drive ones.

The rubber compound used for idler tyres is usually harder and more resilient than a typical belt formulation. The tyre might appear good even after 50 or more years of duty. Of course, it is not. No elastomer on Earth can keep its original properties after half a century.

Old worn idlers slip just as well as dirty pulleys. Old idlers warp. They develop cracks that are not visible until the pressure is applied. Old idlers lose their grip and need a lot more force applied to them to drive a platter reliably.

Just like with a belt drive, axis misalignment spells trouble. Here's a side view of a typical idler that's slightly off. What happens when the vertical axes of the idler A and the driven platter B are not exactly parallel?

In our illustration, the idler A would try to "drive" down on the platter rim. This would put a vertical load on the idler bearing (and it was never designed for it). It would also create a tipping force on the platter, trying to turn it over away from the viewer. And, of course, there is a telltale scraping or squeaking sound.

Why does it happen? In most cases, the reason is a worn idler bearing. Also, the turntable might have been dropped or mishandled. In this case, the idler bracket or the shaft might be bent out of shape, introducing a permanent angle α.

Interestingly, an idler can be designed to create much less of a side load than a belt. But unless you're looking at a EMT, chances are the idler you see pushes on the platter with quite a force.

This creates exactly the same problems for the main bearing that we have described in our previous post: oil starvation, dry spots, poor stick-and-slap performance.

On the positive side, designing low diametric force idlers is easy. There's no need for extra moving parts, just a careful consideration of a drive geometry. Another approach is a Lenco-type drive (covered below) that doesn't create any side force at all.

 

Bad design isn't exclusive to belt drives. Before the belts became dominant, inept engineers used ill-positioned idlers to tip the turntable main bearings.

Fortunately, the idler drive is a bit more resilient in respect of this force. Yes, the bearing is stressed exactly the same way as with a belt drive, but no wander or scraping is introduced. The drive will run fine until the bearing self-destructs.

 

Here lies one of the key reasons for the idler drive's fall from grace: its typical middling measured performance.

Remember, commercial wow and flutter meters only concern themselves with speed variations that fall between zero and 250-350 Hz ("perceptual weighting"). More rapid fluctuations get filtered out.

Belt Conveyor Idler

Let's take a typical belt drive, with a 13-15mm motor pulley driving a 6" (150-160 mm diameter) subplatter at 33 1/3 RPM. This means the motor rotates at about 300-400 RPM. The belt's defects (variation in thickness, elasticity etc) will change the effective diameter of both pulleys as they rotate. If you think about it for a second,

• The smaller one matters a lot more. Its effective diameter variation has proportionally greater impact;
• The smaller pulley RPM times 2 is the lowest frequency limit of the fluctuations that belt imperfections can cause

In our case, it means the speed varies 600-800 times a second. No wow and flutter meter will detect this—but our ears certainly will!

What about an idler? With a typical 2"-3" idler driving an inner 11" rim, the speed fluctuations caused by the idler not being exactly circular will fall between 120 and 200 Hz.

Bam! The wow and flutter meter arrow moves to the right, the magazine reviewers give the thumb down, the public stops buying "inferior" idler-driven turntables.

 

 

For completeness' sake, here's a diagram of a warped platter too. This mostly happens with stamped platters, but some Zamak and aluminium cast ones can also suffer.

I'll spare you the calculations here; if you see anything like this, the platter is beyond hope. Get another one (or a different turntable).

Similar Drive Types

In the late 1950s, the Swiss Lenco company came up with an ingenious variation on an idler drive. They placed the motor shaft horizontally, and drove the bottom of the platter through a vertical idler.

 

 

A. Motor

B. Motor Shaft

C. Idler Wheel

D. Platter Bottom Surface

E. Platter

This arrangement virtually eliminates the side and tipping forces on the main bearing. It also puts the motor vibration into a vertical plane: mono cartridges, designed for horizontally modulated mono LPs, didn't pick it up at all!

Unfortunately, instead of improving performance, Lenco used the inherent advantages of this drive type to cut the costs right to the bone. Yet, even with the dire manufacturing quality and the crude Soviet-style tonearms, Lenco L75/L78 turntables remained competitive all the way to the late 1970s. They were sold in hundreds of thousands, if not millions.

A Lenco-type drive has its own unique take on idler scrubbing.

Normally, the axis of the idler should be perpendicular to the tangent B of its path A. When it isn't and the angle α is present, the idler tries to "run away" from its path A.

Not only this degrades the wow&flutter performance, but also brings about all sorts of strange intermittent problems like idler arm resonance, clicking sounds, platter rocking etc.