How to Build a Good Turntable

 

Sing me no sad song
  Helen Reddy

By Dominic Vautier
orig 11/2018
update 1/2019


Building your own turntable

I began collecting parts for my turntable some time ago.  I wanted to come up with a unit that played as good as the high priced ones on the market but I also wanted a pretty simple one.  It needed an isolated motor and basic arm functions.  It also had to have standard head-shells since all my carts use standard mounts.  The arm needed queuing, anti-skate, stylus weight and plinth adjustment. 

The plinth adjustment was for VTA (vertical tracking angle) which I understand is not a big consideration but overlooked and sometimes hard to measure without a computer microscope. But mostly I wanted simplicity. I did not want any of the elaborate mechanisms involved in arm return.  Just taking a look under my Technics with all the leavers and switches is a truly frightening experience which I would think requires at least two degrees in mechanical engineering to understand.  My technics is a fine unit but suffers from low rumble and sometimes stops before the last song finishes.  Other than that it is a very tolerable machine. Rumble and early return may be just some of the typical problems of direct drive and automatic return turntables, but there seems to be no way to get away from some rumble or motor noise unless the DC motor is physically detached from the unit itself and the power supply is some distance away.  There may also be resistance created by the automatic return catch lever which happens to me once in awhile.

Parts

This article is intended to help people who want to make a turntable from scratch and donít know where or how to begin.  You have to start by getting the kind of parts you want.  I got a motor, belt, power supply and platter from an old Pro-ject turntable.  Pro-jects have very good motors but I feel their arms are too light and do not use standard headshells. I got a pretty good tonearm from Stanton.  I built everything else.  The total cost was about $250.

Definitions I used here (which are mine and not industry)

Arm length The APPAREANT arm length from tracking pivot to the stylus.  there is about a 26 degree curve in most arms.                    
Arm pivot The point above the deck where the arm pivots up and down.                      
Head-shell Standard straight headshell   
MM Everything described here is in millimeters 
Platter The thing that turns around   
Plinth The (in this case, adjustable) surface of a tone-arm that supports queue lever and lock.  It raises and lowers the stylus angle.
PtoS Tracking pivot distance to spindle.  Here arm pivot=tracking pivot which is different from up and down pivot or arm pivot.               
shell surface Area of the head-shell that mounts the cartridge
Spindle The center hole where the platter spins.                    
Table The surface body that holds the platter and tone-arm.  The motor is not part of the table.
Tracking Pivot The point at witch the tone-arm pivots horizontally to follow record grooves.  The industry refers to this simply as pivot but there are two pivots in a tone-arm.  Both are important.
VTA Vertical tracking angle.  The stylus angle to surface of record.  The optimum is 90 degrees. Use a computer microscope to adjust.

Arm bearing adjustment

A factory tone-arm should have its bearings checked.  I think they are sometimes shipped loose.  If you can feel a wiggle the bearing needs to be tightened.  If you canít feel a wiggle at all it may be OK but you should test it anyway (described later).  If the bearings are in need of adjustment be sure to MARK the location of the center screw FIRST and then release the lock nut one half turn. Observe how many degrees it can be tightened to achieve a very light snug. Then back it off. The rule is 30 to 40 degrees backoff or about 1/8th turn, no more.  After setting, hold center screw in position while tightening lock nut. You donít have to tighten much. Tightening the set screw may change the snugness. Test again for wiggle. After setting the lock nut It should not come loose again.  Mark the final position.

You may have to make your own spanning tool for the locknut.  I took a flat blade and ground it to spec.

Adjusting bearings

For adjusting the table has to be absolutely flat because the tracking pivot axis has to be perpendicular.  There should be no wiggle in the bearings at this point.

To test the arm pivot bearing (the up-down motion), bring the arm to zero weight on the counterbalance and move it to a neutral position just off the platter.  The stylus should be at record play level.  Gently tap it up.  The arm should oscillate at least 5 times before coming to rest again at record play level. If it does this and it has no perceptible wiggle, it is properly adjusted.

To test the tracking pivot bearing (back-and-forth motion), bring arm to slightly less than zero which raises it above the platter by 10 mm or so.  Put on about 1 gram antiskate.  Give the arm a very gentle nudge inward.  It should glide inward half way then return to start of record.  Take off antiskate. Give the arm another gentle nudge.  It should glide to the center stop and slowly glide back.  If you have an auto return this may not work because the arm can get stuck in the auto return mechanism.  But if it glides in its adjusted OK.

  

VTA

Plinth adjustment allows easy vertical tracking angle adjustment.  The mounting surface on a head-shell may not be parallel to the record surface in the play position or the arm pivot may be too high or too low when the stylus contacts the record.  The distance from deck to record surface may also be different.  My arm has an adjustable plinth of from 43 mm to 53 mm above the table and my platter was 28 mm above the table.  This gave me about 18 to 28 mm variance for the distance from the center of the tone-arm tube to the stylus.  The end result of plinth adjustment is to get VTA as close as possible.  Consider the standard head-shell in the diagram.

 

Several conditions play a part in VTA, the angle of the head-shell to the record surface, the distance from head-shell mounting surface to playing surface, and the angle of the stylus to the cartridge mounting brackets. These are a lot of considerations. I do notice that VTAs vary with cartridge size, cartridge mounts and stylus. An easy way to get it right is to mount the cartridge, set it on a record and adjust the plinth.  Use a computer microscope if you have one to check it.

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Here my Shure M97x is close.  Notice the plinth is adjusted at platter height. It needed to come up some more.

There are other adjustments that take more time such as null points.  Here is a good discussion of null point adjustment.


Five Wires

There are five wires to a tone-arm, two signal wires, two signal grounds and a chassis ground.  The chassis ground extends from the cartridge base and headshell base through the arm, shielding the other wires up to the preamp.  The chassis ground is meant to protect against outside electrometric interference and is needed since cartridge signals are so weak. These three grounds are all separate and should never contact each other.  The two signal grounds are totally dedicated to signal.

 


Calculating PtoS and Overhang.

I calculated the PtoS on my table at 222 mm.  I did this by taking the average of the three head-shells I have and measuring the arm length which came in at 234 mm.  This gave me an arm length of 228 mm to 238 mm.  With this I can get an overhang of from 10 to 20 mm.

My Technics has a slightly longer 230 mm null point adjusted arm length, a 213 mm PtoS and therefore a 17 mm overhang but the arm angle is less so I didnít need 17 mm overhang.  I think that the overhang depends on a lot of things but it is usually around 1/13th of PtoS.

The table was eventually cut to 410 by 355 mm.  The spindle hole is 160 mm in from the front and left.  The spindle to motor was 180 mm so when I cut out the motor block it allowed a 4 mm rubber buffer.

Construction

I cut my initial 14 by 17 inch table from 2 pieces of 10-ply and pressured glued them together for a solid 1 Ĺ inch slab.  I then drilled out the pivot hole and cut a larger hole for the motor.  It was only later after I tested that I cut out the motor block.

 

 

I first installed the spindle at 165 mm from the front and 165 mm from the left.  From there I measured the motor location being 180 mm and the arm at 220 mm away from the spindle.  I then measured and installed the motor then began drilling out clearance for the tonearm base.  before drilling I calculated the distance and measured overhang. 

 

 

 

Doublechecking the overhang.

I put two marks at 12 and 20 mm past the spindle.  I then used my AT green and my M97e cartridge to make sure they both fall within the overhang. I really needed to get this part right.

 

 

Here is my motor with regulator and power supply

 

 

The motor was tested and then the motor block was cut out from the deck.  padding was glued to the sides so it would never come directly in contact with the deck.

             

Above is a sample taken from my new turntable.  It shows no rumble.  You can hear the recording here.

 

 

Above is a sample taken from my new Technics of the same passage.  It shows a slight rumble here.

Good luck to all you who want to DIY.  Lots of fun.