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ANALOG AUDIO PART 1 of 3: The Idler Wheel Turntable


by John Kim

Part 1/3:  Setting the stage: getting an idler wheel turntable working

Finished customized idler wheel turntable playing Isis (track 2) from Bob Dylan's album Desire

This project documents my path in choosing a way to make digital recordings of vinyl records. My goal is to have a system for capturing in digital format some of the characteristics of records that people enjoy.  Part 1 of the project is the first step in this process -- "putting together" a turntable to play the records on.  The path I choose is largely idiosyncratic, and only partly narrowed by careful consideration of options.  It is just as much the result of cost considerations, the availability of parts, and information I happened to have found.  But at some point, one has to make choices in order to move forward. 

A very brief, very incomplete (and somewhat accurate) history of turntables

Sixty years ago, idler drive turntables dominated the turntable market.  The idler drive mechanism is simple: a motor turns a 2-3” idler wheel whose outer edge is made of rubber; the rubber of the idler wheel makes direct contact with a turntable platter; when the idler wheel turns, so does the platter.  A picture and some information about one classic manufacturer of idler drive turntables – Lenco - is available here:

In the Lenco L75, the turntable platter sits on a spindle and spins freely in the recessed metal top plate (figure 1):

Figure 1: recessed metal top plate

Figure 1: recessed metal top plate

The platter is turned by contact with the idler wheel, which in turn is turned as the idler wheel makes contact with the spinning cone-shaped piece of metal connected to the motor (figure 2):

Figure 2: idler wheel and motor

Figure 2: idler wheel and motor

Idler drive turntables went out of fashion partly because of the noise resulting from the direct contact of the idler wheel with the platter. When you play a record, a stylus (needle) fits into the groove of the turning record.  The vibrations caused by dragging the stylus across the physical "bumps" in the record grooves are picked up by the cartridge, which translates vibrations into continuously changing voltages.  After several stages of amplification, that voltage flows through the wire coil of a speaker.  Voltage fluctuations through the coil physically move the coil because it is set in a magnetic field, and because the coil is also attached to the cone of a speaker, movement of the coil results in movement of the cone.  Movement of the cone pushes air.  The result is the music you hear.  

But ANY vibration that the stylus and cartridge pick up - not just the music encoded in the record grooves - are amplified as well.  If there are scratches on the record, then when you play a record you will hear the scratches.  If there is static on the record, then when you play a record you will hear crackles and pops.  And if vibrations of the turntable motor are transmitted to the turntable platter via contact with an idler wheel, you will hear that as well.

A change in turntable design largely solved this problem:  replace the idler wheel that couples the motor and the platter with a rubber belt.  In belt drive turntables, the motor turns the platter by turning a rubber belt, which is fitted around the platter (or a subplatter that the platter sits on top of).  When the belt turns, the (sub)platter turns.  But the belt does not transmit motor vibration to the platter, and so belt drive turntables reduced rumble quite considerably compared with idler drive turntables.  

By the early 1970's, belt drive turntables dominated the market, which they did until digital technology eclipsed analog audio, initially with the advent and widespread popularity of CDs in the 1980's.  

Setting the stage for this project

After my first attempt at building a hi-fi stereo tube amp, I broke out my old Dual belt drive turntable and played Abbey Road.  It was lacking in bass and generally sounded thin; it was definitely a worse listening experience than listening to a CD of Abbey Road on the same stereo system.  There are many reasons the record may have sounded bad-- a bad phono cartridge, poorly adjusted tonearm counterweight and tracking force, etc.  Instead of looking into why the record didn't sound good, I decided to learn more about turntable design and its impact on how records sound.  Among the things I found was that among vinyl record enthusiasts, there was renewed interest in the old idler drive turntables from the 1950's and 1960's, e.g., heavy platter Lencos, the Garrard 301 and 401, Rek-O-Kut turntables.  These old idler drive turntables purportedly had better bass than belt drive turntables had, and could handle complex musical passages better than belt drive turntables could.  To overly simplify the reason given for the supposed superiority of idler drive turntables in these areas, their turning platters maintained constant speed better than belt drive turntables did because the idler wheel pushed against the metal turntable platter to turn it, and so they didn't slow down (much) due to the drag of the stylus across heavy bass or complex dynamic musical passages encoded in the record groove.  On the other hand, the drag of the record against the stylus does affect the speed with which a turntable platter turns in belt drive turntables: rubber belts isolated the motor from the platter, but at the cost of not being able to push the platter at a constant speed if the motor was turning at a constant speed, or so the story went. 

So maybe there is some advantage idler drive turntables have over belt drive turntables, at least among low- to mid-cost turntables. But what of the problem with rumble in idler drive turntables?  A solution is to attach the idler drive turntable to a massive plinth in order to dampen the effect of motor vibration.  

Back to this project- building the plinth for a Lenco L75 idler drive turntable

This is my jumping off point.  The Lenco L75 is my choice of idler drive turntable for this project based on successful projects in the DIY community and its relatively modest cost; you can’t find a Garrard 301 for less than $1000.  In addition to issues with rumble, Lenco L75 turntables also have a relatively flimsy metal top plate.  Thoughtful design of the massive plinth can incorporate a fix to this shortcoming by having the plinth provide additional support for the metal top plate.  (I roughly followed the Lenco L75 plinth design, available at Lenco Heaven:  The Lenco Heaven forum also discusses other ways to modify the Lenco L75 for better sound, some of which I will also use.  For additional information, search the internet for Jean Nantais.)  Here's a picture of my heavy plinth in mid-construction (figure 3):

Figure 3: Three layers of mdf and plywood for plinth

What you see are three layers of the plinth, which will be stacked and glued together.  The piece on the left is actually three pieces: a thin piece of dense plywood glued to a thin piece of MDF, which is in turn glued to a thicker piece of dense plywood.  This plywood-MDF-plywood composite will be the top layer of the plinth.  It has holes cut to fit the Lenco L75 turntable and two additional tonearms. (Notice that the wood is cut to fit the turntable motor and tonearms, but much of the remaining wood in the center of the plinth will provide additional support for the metal top plate of the Lenco L75.)  In the middle of the picture is a thick piece of MDF glued to a thick piece of plywood; this will be the middle layer of the plinth.  On the right is another thick piece of MDF on top of a thick piece of plywood; these will be cut and glued (much like the middle layer), and will be the bottom layer of the plinth.  (I follow this layering of MDF and dense plywood on the recommendation of Nantais, who in the mid-2000's found this combination of plinth materials to offer a good balance of dynamics and neutrality for the Lenco L75.)  The top of the plinth is 24" x 24", and it will be 1.25" + 1.5" + 1.5" = 4.25" deep when the plinth is fully put together (figure 4):

Figure 4: plinth top angle

Modifying the Lenco L75

I made several modifications to a used Lenco L75 that I acquired: 

1.  I removed the power switch, which had the dual role of powering the motor and engaging the idler wheel.  I removed the mechanism that engages the idler wheel, opting instead to make the part of the plinth that sits beneath this mechanism solid in order to strengthen the support the plinth provides the metal top plate.  I will add a simple power switch after the turntable is attached to the plinth.
2.  I removed the spring that engages the idler wheel to the motor and moved it to the other side of the idler wheel mechanism to replace the mechanism that disengages the idler wheel.  To engage the idler wheel with the motor, I installed fishing line to the idler wheel to which weights will be hung in order to engage the idler wheel.  
3.  instead of trying to rehabilitate the stock Lenco L75 tonearm, I replaced it with another tonearm – the Ortofon AS-212 -- that has the same effective length as the Lenco tonearm (and so is nearly a drop-in replacement).

(Replacing the spring-based idler wheel engaging mechanism in (2) and replacing the stock Lenco L75 tonearm in (3) are discussed on the Lenco Heaven forum.)  

The modified top plate looks like this (figure 5):

Figure 5: modified chassis w Ortofon AS-212 tonearm

Tonearm boards for extra tonearms

The plinth has holes cut in it to fit two tonearm boards so tonearms that are different in length than the stock Lenco L75 tonearm can be used with the turntable.  I used hardwood for the tonearm boards, which will sit inside the cutouts in the plinth.  For now, I will use an Audio Technica ATP-12T tonearm on the left side of the plinth, with a Denon 103R low output moving coil stereo cartridge, mounted on a maple wood tonearm board (figure 6):

Figure 6: Audio Technica ATP-12T tonearm

On the back of the plinth, I will use a Rek-O-Kut 160 tonearm, with a Rek-O-Kut headshell and a Denon 102 high output moving coil mono cartridge, mounted on a cherry wood tonearm board (figure 7):

Figure 7: Rek-O-Kut 160 tonearm

Other hardwoods that I may try in the future include (from left to right) poplar, birch, oak, walnut and mahogany (figure 8):

Figure 8: hardwood tonearm boards

I used this modular design so I could use different tonearms by attaching them to appropriately cut hardwood tonearm boards as needed.  This also makes it easy to change tonearms by simply switching out the tonearm boards they are mounted on.

The main platter bearing

Figure 9: platter bearing spindle assembly

For good playback, the turntable platter must spin freely.  A rule of thumb from the Lenco Heaven forum is that when the idler wheel is disengaged from the platter, the platter should continue to spin freely for at least 2 minutes.  When I tested the Lenco L75, the freely spinning platter came to a rest in just over 1 minute.  So I disassembled the main platter bearing that the spindle sits in to clean it and to make some recommended modifications (figure 9).

The spindle is coupled to the platter by friction from the weight of the 9 lb. platter resting on the (tapered) spindle.  The spindle and the platter are effectively one unit, so the spindle must spin freely inside the main platter bearing in order for the platter to spin freely.  To accomplish this, people on the Lenco Heaven forum have recommended various modifications to the main platter bearing.  I adopted three of these:

1. I replaced the old nylon  thrust plate at the bottom of the main platter bearing with another soft , durable material with a friction coefficient similar to nylon; I obtained a piece of Delrin from a Lenco Heaven member for this purpose.  (Thank you, wer.)
2. I replaced the metal ball that sits between the thrust plate and the spindle with a ceramic ball.
3. I used a synthetic lubricant in the well of the main platter bearing instead of oil.

Figure 10: disassembled main platter bearing

In order to make these changes, I disassembled the main platter bearing (figure 10). 

After cleaning the pieces of the main platter bearing with isopropyl alcohol and implementing changes (1) – (3), I reassembled the main platter bearing, reattached it to the top plate, and soaked the wool washer that sits over the spindle between the top plate and the platter with the synthetic lubricant.

Miscellaneous changes and the final product

In order to increase the inertia of the spinning platter, I bought a second Lenco L75 platter and fit it on top of the original platter using a piece of metal machined to sit on top of the spindle, effectively extending the spindle by about an inch.  The result is a dual platter that weighs about 18 lb.  

The rubber mat that sits on top of the platter was dried out and cracking, so I replaced it with a glass mat.  Inertia from a heavy spinning platter and the idler wheel mechanism is not enough to ensure that there is constant speed between the stylus and the record: the top platter may slip on the bottom platter; the mat may slip on the top platter; the record may slip on the mat; etc.  

I am relying on friction from the weight of the glass mat to prevent slippage with the top platter underneath it, and the weight of the glass mat plus the top platter to prevent slippage with the bottom platter underneath them.  But the record is light and so may slip on the mat, especially a glass mat.  To help prevent this, I bought a 1 lb. record “clamp” that fits over the spindle  and simply rests on the record, holding the record down on the mat.  Here’s what the turntable and plinth look like from the top (figure 11):

Figure 11: turntable and plinth - top view

Finally, I put the entire plinth-plus-turntable on 4 rubber balls sitting in rubber cups to help decouple the turntable/plinth from the surface that it rests on.  (I used the IsoBall/iso-Cup product from Herbie’s Audio Lab.)  Here’s what the final setup looks like from the front (figure 12):

Figure 12: turntable and plinth - front view

There are some final touches that I will leave for another time, i.e., after I finally decide where this turntable and heavy plinth will sit. My current thinking is that it will sit inside a wood cabinet, in which case only the top of the plinth needs to be finished.  But I may have it sit on a tabletop, in which case I will have to find a way to finish the top and the sides.   

The turntable is working now.  There is some rumble due to the fact that I haven’t yet glued all the pieces of the plinth together yet, and have yet to put a thin layer of neoprene between the bottom of the turntable and the plinth (and between the armboard and the plinth).  But it sounds pretty good so far, playing through a solid-state Carver preamplifier and its built-in solid-state phono preamp.

Now it is time to turn to the next stage in the audio chain.  In Part 2 of this project, I will build a tube-based phono preamplifier, and a tube-based line preamplifier with a selector switch.

Arlene Suda