This circuit is a from publication 1980, but still can be mounted with BC548 transistors for Q1 and BC549 for Q2, besides the changes we made in the original configuration. The circuit has low power consumption and the input and output signals cables must be shielded. The loop gain is set at trimpot 470k. If powered by source, it must have excellent filtration. The input impedance is high and output has a signal level of a few hundred millivolts, enough to excite the entry of most amplifiers.
