iRPM: iPhone app for measuring turntable speed

Dulcimer · « **on:** 20 Apr 2013, 04:29 pm »

I’m the new and delighted owner of a Garrard 401 turntable and SME 3009 tone arm that had been in storage for twenty years in a friend’s basement.

The turntable platter is cut for a 50Hz strobe, so I can’t use it. But there is a free iPhone app that measures rotational speed. Find it here:
https://itunes.apple.com/us/app/irpm/id406885844?mt=8

According to the iRPM app, my Garrard's rotational speed varies between 33.3 and 33.4, which I think is good for turntable that sat unused in a basement for 20 years.

I’m curious to hear from others who have used iRPM. How accurate is it? Should I try to get a perfectly steady 33.3 RPM or is it OK to have a variation of .1 RPM?

kgturner · « **Reply #1 on:** 21 Apr 2013, 01:53 pm »

Well, it's a free app so worth a try. I downloaded it to my iPhone 5 and noticed my SDS settings were low. I haven't done any listening, but wonder if the weight of the phone might have any effect on the turntable speed.

Kevin T

EDIT: Weighed my iPhone 5 in it's case. It weighs 128 grams. I guess my worries about weight are unfounded.

JCarney · « **Reply #2 on:** 21 Apr 2013, 02:35 pm »

Dulcimer, thanks for sharing! Way cool. Tried it on both tables. One is like yours, between 33.2 and 33.4, the other, spot on.

Thanks again,
JCarney

kgturner · « **Reply #3 on:** 22 Apr 2013, 01:20 pm »

Just a followup. I placed my iPhone 5 on my center weight so that it is centered on the platter and actually calibrated the app before starting. My 33.3 speed was dead on with my original SDS setting. My 45 speed
was about .15 Hz off from my original SDS setting. I had previously set my SDS with a KAB Speed Strobe so this app seems to pretty accurate in my estimation.

Kevin T

Dulcimer · « **Reply #4 on:** 22 Apr 2013, 03:16 pm »

Thanks for the update. I'm glad to hear the iRPM app is accurate, making it a useful tool.

I agree that the iPhone's weight isn't an issue but it's probably good to center if on the platter or to place an equal weight diametrically opposite the iPhone.

nascar03 · « **Reply #5 on:** 26 Apr 2013, 05:49 am »

dulcimer...thanx for the iRPM find.
What a great app! It checks perfect with my strobe discs.
My Oracle is right on, flashing 33.3.
thanx.

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jarcher · « **Reply #6 on:** 2 May 2013, 08:11 pm »

A fun / useful / free app. Placement is best as suggested in the photos. If you put the iphone / touch lengthwise / parallel to the spindle, or to far on the rim of the platter, it tends to eventually spin off the table.

VPI traveler seems to be good within .1 rpm (@33.3 RPM). An older Kenwood beltdrive was slightly fast (but consistent), and used this app to fine tune a Pioneer Direct Drive table that despite the strobe / platter markings was slightly off. An older Conrad Johnson Sonographe turntable was quite a bit off - average 32 RPM! - so I now know that one probably needs a new belt (might also explain the occasional pitch drop).

Larry LShore · « **Reply #7 on:** 15 Jul 2014, 01:19 pm »

Cool app and it's hard to argue with the price, but as with most things, you get what you pay for.

I've been using a dedicated turntable tachometer for several months now. It has 3 decimal places of resolution (2 orders of magnitude better than the iapp) so there's no need to guess if the speed is close to 33.3 or 33.4. The really nice feature about this tach, is it can be connected directly to the companion power supply unit and automatically adjusts the output frequency of the PSU to keep the platter locked on speed within ±0.005 RPM.