In this screenshot you can see the signal at the Base of transistor Q3 and Q4.
This is the signal at the Emettitor of Q3 and Q4.
Trying to null coils I have only 0.6mV at pin 3 of U1
and a little and instable signal at the output of U1, pin 6:
If I move coils in a position where they are not nulled, the signal at the output of U1 is stable but the spikes are present the same.
I don't know why.
Before to start, I have regulated R58 for zero at output having the input connected to virtual ground.
And I have selected optimum values for C7 and C8 to have the maximum signal.
I don't know how set R57 therefore I set it for maximum amplitude.
I don't know how set R26 therefore I set in middle.
Always I have only one tone with ferrous.
Sometimes I have two tones with other metals.
The maximum sensibility is about 15/20cm for 1 Euro coin.
False signal are present rarely.
Sometime is necessary to repeat the movement of the coin near coils to have sound.
As soon as possible, I will show you others images about data of coils.
Please, follow this thread !!!