Comparing
Konstant FMD: what can closed-loop speed control add?
The Konstant FMD is, according to Transrotor, the best motor control it offers ("the best motor control we offer"). A supply at this level regulates the voltage and frequency fed to the motor. Like supplies of this type, it runs open loop: platter speed is not measured by a sensor. Closed-loop regulation rests on a different principle: it is Isostase's, which replaces the Konstant on compatible turntables.
What Transrotor states
Transrotor's Konstant FMD page presents it as the "ultimate" motor control, recommended for all turntables. It claims even more precise control than the other units, a slow start-up with high torque meant to protect bearing, belt and motors, speeds of 33 and 45 rpm, one to three motors, an elaborate housing and a hand-polished front panel.
An open-loop control
To our knowledge, the FMD, like Transrotor's other power supplies, imposes a voltage and a frequency on the motor without measuring the actual platter speed. Transrotor does not spell this out on its page. This is not a flaw: it is the principle of a control, and a synchronous motor fed with a clean frequency and voltage already follows its setpoint very well. The quality of the power stage, of the frequency source and of the soft start plays a major role.
Open loop, closed loop
| Principle | What is measured | What it corrects |
|---|---|---|
| Fixed supply (open loop) | Nothing: frequency and voltage are imposed | Mains and its noise. Not load, belt or bearing deviations |
| Supply with adjustable reference frequency | Nothing: the reference frequency is set by hand | Nominal speed, through fine adjustment |
| Closed loop | Actual platter speed, through a sensor | The gap between wanted and obtained speed |
A synchronous motor follows the frequency it is given, but the platter is subject to the belt, the bearing and the tonearm load. The control does not see these deviations: only a measurement on the platter shows them, and that is what a closed loop adds.
What Isostase adds
Isostase measures platter speed with an optical sensor. An index disc (59 marks) or an index stuck on the platter edge lets it correct speed once per turn. The disc, where the turntable has room for it, mainly refines the wow and flutter display. Setting the motor phase angle and the FFT analysis of its current are not done in real time: they are separate procedures, run under repeatable conditions, outside playback.
Why only one correction per turn? It is a design choice, not a guarantee. The platter is heavy: its inertia already smooths fast variations, and the correction targets what drifts slowly (load, belt, temperature, bearing). A faster correction would require mastering loop stability and could itself inject wow. In return, a variation faster than one turn is not corrected, only displayed. This reasoning remains to be confirmed by measurements.
Pre-series units are being measured and validated: first measurements are published in Speed, wow and flutter measurements of a Transrotor turntable.
Source: Transrotor, Konstant FMD page (consulted October 2026).
How Isostase does it
Optical speed measurement, turn-by-turn correction, phase angle adjustment, current analysis: see the Isostase page. Product in development, pre-series units being measured and validated.
Related videos: Platter Speed