Graham's Blog . . .


Graham's Upside-Down Turntable

Posted by Graham
March 4th, 2018
Graham's Upside Down Turntable
 

What I just did to my 1979 vintage turntable may be seen as crazy to some, but to me it's just plain engineering.

I was originally trained as a mechanical engineer and I could (eventually) see there was a mechanical contradiction about this turntable which in my opinion was either a design oversight, or that it was done intentionally to make it sound a certain way.

There have been a number of third-party tweaks, which either calm or exaggerate its particular sound, some of which I'd tried over a period of many years without satisfaction. Recently I decided to tackle the issues I'd identified head on, and this is a story of my assumptions, what I did, and the outcome.

The motor, a cheaper version of what we know (or probably don't know) as a stepper motor, has a 24 pole rotor with two coils, and is designed to position whatever it is driving to an accuracy of 7.5 degrees. And given suitable control circuitry it will do exactly that.

Inside Stepper Motor

24-pole can-stack construction permanent magnet stepper motor

But here (and since the 1970s) such motors have been cleverly exploited by turntable manufacturers to latch onto the mains frequency as their source of stepping pulses.

Given 50 cycles per second (50Hz), that is 100 pulses a second, and driving both coils (by means of a phasing capacitor) each pulse drives them through a 15 degree angle.

50 c/s therefore gives 6000 pulses per minute. This drives them through 90,000 degrees. Divide by 360 degrees and you'll find they do 250 rpm very accurately. And on 60 c/s (60Hz) mains that's 10,800 degrees equating to 300 rpm.

100 or 120 steps per second when coupled to the light-weight motor board via the motor mount, will obviously attempt to make it vibrate in sympathy. So these are the two contradicting things: motor vibration, and the light-weight motor board, which will (obviously) mechanically amplify the vibrations.

As the fidelity of the signal relies on the coupling between the cartridge and the record (the only thing which should "give" being the stylus), the path between arm base and main bearing should not have any resonance.

Any vibration in that link is going to modulate the signal, and you either get the sum or you get the difference between its vibration frequency and any/all of the frequencies of the music being played. These sum and difference frequencies are not part of the music, but will be heard (or not heard if it results in cancellation).

We could stiffen the motor board with a brace (which it's manufacturer seems to have done since), or make the motor board of such mass that it damps the vibrations (the Linn LP12 which also uses a stepper motor weighs in at 10kg). Stiffening simply ups the frequency. Mass is would seem to be better, and this could be why the Linn LP12 is better (IMO) in the lower registers.

As Linn say "because information lost at the source is lost forever", the answer is right there in the manual (which I never had, so never saw it until the internet came along) "[the motor is] mechanically decoupled from [the] main bearing".

On my turntable the decoupling from the main bearing had originally been done by mounting the motor using a rudimentary "cats cradle" - a wobbly sort of microphone shock mount. In later times this had been replaced by a large self adhesive foam pad. Neither in my opinion had solved the bass problem.

Taking a look at a particular hand start turntable it can be seen that it uses a separate pod mounted motor, the reasoning is it decouples the motor from the motor board which carries the main bearing and arm.

This type of turntable also uses the stepping type motor, but a much lower powered unit, hence the need to run it up to speed by hand. The thinking behind it being that lower power equates to even lower vibration through whatever the turntable is sat on.

Having one on loan from Analogue Works I'd noticed it dug deeper into the bass and the overall sound was far clearer and more together. This was the pivotal moment when I decided to make my turntable's motor pod-mounted.

The difficulty only having layman's tools was how to make it and of what? Thus began a thought process about the easiest way to make a motor pod, without leaving home and office to hunt things down.

My first thought was to use a stub of thick wall plastic pipe but not having any of a suitable size and the minimum order length being 2 metres, I decided that was too costly and wasteful.

The thought of plastic pipe led me to recall the use of uPVC pressure piping in the fabrication of printed circuit processing machinery, which I'd been involved with in my distant past, and with that came the thought of using a straight socket.

A George Fischer 2 inch equal socket at £6.11 each (RS Components 212-3133) looked like it would serve the purpose. I also bought a backing flange, George Fischer (RS Components 279-1843) at £8.96 to fashion a base for the motor pod.

Motor pod materials

Materials used in the motor pod

The socket was cut down to size using a regular hacksaw, and finished using a belt sander. The backing flange was drilled to accept the power cable, and the socket face drilled and tapped for the motor mounting screws.

Using a slightly different motor (see sidebar) the socket was cut away to clear its terminals. The assembly was blutack glued together, and some sorbothane feet stuck on for grip with, and decoupling from the mounting base (it was to be stood on a shelf).

Motor pod complete

The completed motor pod

Next thing was to seriously bastardize the motor board. An 80mm hole saw was taken to it where sufficient "meat" was available, left of the main bearing, centred at the same distance from the main bearing as the original motor shaft position.

The motor pod was then stood in the hole and the belt affixed.

Motor pod driving the platter

Motor pod driving the platter

So, how did it sound? The bass had improved noticeably, was more plentiful and in proportion with the rest of the music. The decoupling had done the trick, and it had proven to me that motor vibration had always been the problem, no matter what tweaks had taken place over the many years to reduce it.

OK, the motor pod really ought to have a bit more mass to it - my rudimentary idea weighed in at just short of half a kilo - but still, the improvement was real.

But Graham, you haven't yet answered why upside down?

Although the sound was much better, it wasn't fully to my liking, and that might have been because the tonearm was getting a bit long in the tooth. I had fitted a Roksan Tabriz sometime in the early 90s which had seen better days.

Quite a few of my subscribers and forum members had told me how good the Jelco SA750 tonearm is, and so I decided to give it a try.

It originally had an Acos Lustre arm and when I fitted the Tabriz I had to elongate the hole to achieve the correct main bearing to arm distance.

Neither position was good for the Jelco, and if the hole had been round I could have glued in a piece of dowel, and then re-drilled at the correct distance.

An elongated hole of two different diameters joined together looked too daunting to try and fill. Even if successful I had visions of the new hole running off centre due to differing material densities, resulting in a right old mess.

My thoughts turned to making a whole new motor board, but that could skew my findings: I wanted to keep as much of the original as possible, using my memories of its sound for comparison.

I knew I could obtain 25mm thick chipboard, which the original was made from, but try buying Formica from a DIY shop in England these days. Things have certainly changed these past 39 years!

The only option to be able to drill a new hole was to flip it over so front became back and bottom became top. Off came the feet - the motor and wiring was already out - and out came the main bearing.

Oh, and the switch? It became redundant because I was using my own home made regenerator, plus I'd cut the 80mm hole straight through the switch wiring when making the opening for the motor pod! I was aware of what I was doing, believe it or not.

A little easing of the main bearing hole was required and in went a brand new main bearing, and then it was time to mark out the new arm mounting hole position.

I'll not go into detail regarding that in this post but here are some photos...

Marking out the tonearm hole using the Jelco rule

Marking out the tonearm hole using the Jelco rule

Cutting the tonearm hole

Cutting the tonearm hole

Arm mount fitted ready for final build

Arm mount fitted ready for final build

In went a brand new Ortofon 2M Bronze and the start of some serious listening. A very big improvement overall but still with some edginess. OK, new arm, new cartridge, new bearing - it all needs some running in - but I couldn't resist some tweaking.

Completed turntable

Completed turntable

Experimenting with different mats including not using one at all, I decided to try some heavy rubber sound-proofing sheet left-overs. At 2mm thick it is of similar weight to Lead! At time of writing it's giving me the best tonal balance and minimum distortion.

2mm thick 'lead-weight' mat

2mm thick 'lead-weight' mat

I also ordered some Buckram cloth which is quite stiff. The reason being that a guy called Dave Rusby (Lynwood Electronics) once demonstrated a mat made of it and I was very impressed with the resulting sound. More on that another time.


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Comments

Tarek Mahmoud

Hi Graham,

I hope that we will see your ideas put in production. Waiting for the 1st Graham Slee turntable.

Regards,

Tarek

John Whitaker

Lots of work to your turntable. Thank goodness I own a SME 15A turntable & purchased prior to the new owners of SME hiked their prices across the board by 20% to 25% ! My cartridge the Transfiguration Alexia.

The plinth in your final photo looks a right "Pigs Ear" you have to change it. What about the Boo Plinth that upgrades Linn LP12 models done by Brian & Trevor in Manchester...

...Well done though

Barry Brennan

The aesthetic of this table is as good a barometer as any of your solid engineering credentials, Graham - it is truly terrible looking!

Sidebar

The Allied-Premotec 110V motor (part # 9904 111 31813) used in this turntable, in fact all this make's models which run off the mains, is rather expensive.

At time of writing it is £77.50 plus VAT from Farnell Element14, and that's trade! That's more expensive than a true stepper motor.

Although advertised as specially made for turntables there is no escaping this technologies main use as a stepper motor - often used for program timing in early automatic washing machines!

For use on 230V (the UK line voltage) it has to be used with a dropper resistor, and 6.8 kilohms at 5 watts is recommended. But you'll most likely find a value of 10 kilohms used, and this is to minimise the motor vibration (so they knew about it all along!). The 10 kilohm dropper resistor lowers the voltage to around 90V.

At the same time as doing this experimental work I was developing a quartz regenerator motor drive for use with such motors, and wanted to test it at the highest power it could be expected to do. To that end I found an Airpax (part # 9904-111-31104) 3.5 watt motor.

Running it from the regenerator set for its rated 220V, I had it housed it in the original motor position, and the only difference was that the platter reached speed in half the time. By the way, this motor is priced at £34.40 plus VAT.

I'd been doing some other experiments with the regenerator and I hadn't realised I still had it set for 110V when I reconnected it.

The motor ran absolutely silent. This in marked contrast to the original motors which always made a "cogging" sound, even though not noticeable unless you had your ear right up to the turntable.

It also (at this reduced voltage) got up to speed in about the same time this turntable usually does. I was really impressed.