Hi Willy,
When testing the two coils, are you using an air test? If so, the mono will appear to be more sensitive simply because of the width of the detection zone. When you switch to mono, the detection zone is now the width of the transmit winding so the response will be there for a longer period of time. This allows the electronics to act on the target longer. The result is, what appears to be a louder signal.
Now, I am not sure just how ML is processing the audio but if it is using the filtering I suspect, this additional width also allows for a louder signal because of the technique of filtering involved. One should realize that this part about the filtering used is strictly opinion.
The real test is a buried nugget test. When doing this test, one has to be more careful of sweep speed also when using a DD coil. Once again, the reason is the filtering process involved can influence the target response, but also the total target signal as it passes under a DD coil. The narrow detection zone becomes important in what happens to target signals, and this is a problem that is further compounded by target size. On either side of the main detection zone, the target will generate a signal of opposite response. This negative response signal has to die out before the positive signal can be heard. If a person goes too fast, this may not occur.
On a DD coil, the main target response zone is quite narrow so the target response will be brief. Too fast of a sweep and the target may not have time to cause the proper reaction in the circuitry. The old saying "low and slow" becomes more important when using a DD.
One other thing that was mentioned to me was the ML DD coils are "tuned' to minimize the ground signal rather than nulled for minimum response. In this case, it is possible that smaller gold might not respond as well as larger gold, simply because of this DD coil alignment. This alignment is such that the signal from the ground response is reduced. This occurs because the signal from the overlap is providing a slight negative response which is used to minimize the ground signal. Now, a target has to overcome this negative response before it can generate a positive response. This condition isn't obvious on target signals that are loud or big targets.
When I first began building DD coils, my test target was a small nugget and my detector was my Goldquest clone. What I found was I could get the best response when I didn't align the coils for a null, but rather left the overlap zone wider. This extra detection width wasn't a problem with the lower powered units, but as I found out, did become more of a problem on the higher powered units. DD coils would almost eliminate the ground signals on a lower powered unit, but still cause a strong signal on a higher powered one, and the alignment became much more important.
Hope this helps.
Reg
