Why we built the RPM Clamp
A small speed error on our cutting turntable made a batch of records play sharp. This is how we found it, and why we built a clamp to measure it.
A local artist asked us to cut 50 records for their launch event. The grooves came out wide and loud, and every record sounded great on our own systems.
Then the artist played them at Slow Boat, our vinyl listening cafe, and noticed the pitch was higher than it should be.
What went wrong
We had recently recalibrated our cutting turntable. During a cut, the cutterhead rests on the platter, and after the recalibration its weight pressed down harder than before. That extra load slowed the platter by 0.3 RPM. A record cut slightly slow plays back slightly fast, so every copy came out sharp.
Why the usual tools don't work
The speed has to be checked the way the lathe is used: with the cutterhead down on a blank disc. The usual tools don't work there. Our laser tachometer isn't made for this turntable. A phone app needs the phone lying on the spinning platter, right next to the cutterhead, where it could fly off. The other way is to cut a test, play it back and compare it with the original. It works, but it is slow: a blank and a playback pass every time, the playback deck adds its own speed error, and matching by ear only gets you close.
We needed something that sits on the spindle, stays put, and shows the speed while we cut.
Building the RPM Clamp
That became the RPM Clamp. We told our friend Mike from SpinnerTownBlanks about the idea, built a prototype, and now we make it in our studio.
The clamp shows the speed on its own display while the platter turns. It is made for setup, servicing and record cutting, where even a small speed error changes pitch and timing.
- An integrated 3-digit LED display that reads up to 100 RPM.
- Made for standard turntable spindles.
- Rechargeable, and charges over USB-C.
The companion app
The free companion app connects over Bluetooth when you want more: an extended RPM range, live stability information and a history of your measurements, kept on your device. No account or internet connection is needed.
RPM Clamp by RCRDLAB is free on the Mac App Store. The iPhone and Android versions are coming.
Kevin Chu
RCRDLAB, Singapore