This circuit was found in a 80’s American documentation. The adjustment trimpot is from 47k to 100k. The diodes are general-purpose types use and the supply voltage depends on the relay.
Suitable for recovering tape recorded media from a common tape player mechanism, this circuit is...
This circuit is indicated to operation with piezoeletric transdicers or other with high impedance...
Of course, the current is extremely small, but a voltage of 300 V can be obtained from 9 V of a...