Hi Xavier
A metal object should cause a change in amplitude and/or phase of the sine wave at the output of the first amplifier, but the change might be very small, too small to observe on a scope. (Also, the larger the signal present with no target, the harder it is to see a change.) The TGSL uses loads more gain in the following stages to get an audible indication. Your schematic in post no.12 didn't show a sync. det. stage, and you have a much better chance of seeing something worthwhile if there is one included. Without one, the second amplifier is pretty useless as it can't handle 14.5kHz.
Regards,
Gwil
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