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RPM - App for Turntable Speed

Is your turntable really doing its job?

[Italian version here]

Product: app for measuring turntable speed
Manufacturer: Philip Broder
Approx. price: 10,99€ (a free version is also available)
Reviewer: Piero Canova - TNT Italy
Reviewed: July, 2020

Foreword

For those who were tinkering with turntables in the late sixties or seventies, every turntable worth its name had a stroboscope to measure the rotation speed. There was a flourishing of lights of all possible forms and on the edge of the platter there were dots, squares, bars and other similar shapes to allow the correct setting of rotation speed. Among all my turntables only the Lenco L75 missed out.

With the return of turntables, from the beginning of the last decade, new materials have appeared for platters (I am thinking mainly about acrylic and other polymer compounds), but there is no practical way to measure if your turntable is doing its main job: to rotate precisely at 33,33 RPM and in the most constant and uniform way. The best tool I had was a stroboscopic disk received with a Scheu Premier MK III turntable: it was well built and with an led light blinking at 50Hz to replicate our mains frequency. When I sold the turntable I was right to give it to the new owner with the disk and light, meaning I had to find a replacement.

There are some disks on sale on different sites, but their quality and visibility makes them almost useless.
Fortunately in our hands we have some very powerful computing devices that are our mobile phones and some developers have decided to make a specific app for this; I have tested some of these available both for iOS or Android and the best so far is RPM for iOS. The app is available both for free with limited features or with full performance for 10,99€. I felt this was an investment I could afford so I paid the sum and this is what I have got.

The first thing to consider is how to place your mobile phone centered on the platter: since you have the spindle protruding right where the phone needs to rest. I have built a platter of cardboard in order to flatten the surface. I am completely sure most of you can build something much better than this.

Opening the app you are welcomed by a first screen with, on the upper side, a record with dials indicating the rotation speed.

The white label will become green if the rotation is inside the tolerance you entered into the setup screen. Very useful for me is that the dials, by pressing the small lock button on the top will remain facing you all the time when platter rotates removing the need for your neck to go through some very tiring exercises.

The setup window allows some fine tuning of your measurements; my best advice is:

• if you want to focus on rotation speed go for “freeze after 30 sec” so you have enough time to tune the speed with your pitch control
• A good turntable must be within 0,5% of nominal rotation speed
• if your goal is wow and flutter measurement, go for the minimum amount of samples otherwise the averaging process will obscure the details.
• Use auto calibration since it compensates for the small differences between each gyroscope and it is fairly accurate. You lose the possibility to share the values measured by other users, but you can do this later with a second measurement excluding the auto calibration function
  

OK, but after this long speech what do we get?

Using this app I could set the speed accurately on my turntables that have a pitch control for setting the right speed and don't have a stroboscope built in (Dual 701, Lenco L 75 among others).

I have found that original manufacturer's data for wow and flutter were very optimistic since I haven't recorded any of my turntables bettering 0,08% in spite of all my cleaning, lubricating, tuning etc. As a reference, this is an order of magnitude better than Denon was declaring for their DP 6000 which is so far my best performer.

I have tested other turntables and several times the impression of being “fast” was confirmed by measuring 33,49 RPM.

One more interesting thing

We all believe that our turntables run completely smooth without any vibration perfectly at 33,33 RPM.

The measurement on the right is the real recording made on my Sony TTS 4000. You can see well in the lower part the graph showing the continuous up and down of speed generated by the electronics accelerating or breaking the DD motor to approximate the requested speed. In absolute terms the deviation is small and speed is between 33,31 and 33,35 RPM and the more advanced is the control, the more the curve will flatten, but sadly it will never be completely flat.

Conclusions

For an investment of 10,99€ it seems to me a tool really worth the money since it gives us some better understanding of our beloved turntables and allows some fine tuning. A good example of a digital device being helpful in improving analogue equipment.

© Copyright 2020 Piero Canova - piero@tnt-audio.com - www.tnt-audio.com

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