threshold

[Unregistered]

Hi
Have built hammerhead non vco.
To my liking the threshold seems little high in ref to target signal.
How can i lower threshold audio without affecting target signal audio. Want to punch more target signal throu specially on smaller grain size gold targets.
thankyou
henry

[Unregistered]

Any one with some answer please
tthnks
henry

[Unregistered]

Any Technician out there that can answer problem please

[Carl-NC]

Sorry, if you've adjusted R31 and still can't get the threshold where you want, then you may need to tweak R30 or R32 to get more range out of R31. The easiest way to do this is to just tack in a parallel resistor. It's probably R30 so try adding, say, a parallel 220K resistor and see if the threshold drops. It may take a couple of tries to find what works, you can just touch the resistor to the pads and then solder in a final selection.

- Carl

[Reg]

Hi Henry,

I am not sure just what you are asking but as a guess I suspect you want more gain or more signal from small gold without having to turn the threshold up. If this is the case, then the only way to do this is to increase the gain of one of the stages such as the differential amplifier or the autotune stage.

If you increase the gain on the differential amplifier, you have to do it to both channels. As an example, you can increase the value of both R 26 and R 27 from 100K to something like a 470K. This will add some gain. Likewise, you probably should reduce the value of the two associated capacitors, C 17 and C 18 from a .1 uf to maybe a .027 uf or so. BTW, these capacitors really should be matched for best performance.

Now, you can also increase the gain of the autotune stage also. This can be done a couple of ways. One way is to remove the feedback resistor, install a couple of pins and then add another pot, maybe a 470K or even a 1 meg pot and connect it to the pins. This makes the gain variable. You can add a 100K in series with the pot so the minimum value has the same gain as the original, but as you turn the pot up. the gain is increased.

This is about the only way to get a louder signal from small gold except for shortening the delay. If you can build a coil that operates faster, you can reduce the delay before sampling. This will also increase the signal from small gold. Ideally, you want the delay to be 10 usec or less for the best results on detecting small gold.

Reg