Abstract: In a coin operated machine, the coins are tested by utilizing a symmetric, dual feedback oscillator, each having a tank circuit with a reactance, one of the reactances being a reference reactance and the other one a sensing reactance being passed by a coin to be tested. Separate outputs are derived from symmetrical points in the oscillator and are compared in some form either through phase detection or by detecting the sum of the oscillating signals or by detecting the common mode and/or the differential mode of the envelopes or a combination of several of these tests whereby common mode and differential mode detection of the envelopes together is preferred. The reference reactance includes preferably a coin exactly of the type to be tested. Multi-coin variance and digital processing is additionally disclosed.