This will increase your finds...the Equinox (and Explorer, eTrac, CTX) uses a 'balanced' DD coil. Its balanced on a razor's edge. The balance is super touchy, when a metal target upsets the coil balance this is what triggers a signal to your headphones.
Inside the coil there are two windings of copper wire. A transmit winding and a receive winding. The transmit winding creates a magnetic field at the transmit frequencies, this field penetrates the ground. The receive winding sits quietly listening but there's a trick to this. Normally if you put a receive winding near a transmit winding that's blasting out a signal the receive winding would pick up that transmitted signal. But via a trick of electronics, if you physically overlap the transmit and receive windings precisely (balance them), the transmitted signal is canceled out on the receive winding. When you look at both the transmit and receive windings on an oscilloscope during balancing you can watch the transmit signal on the receive winding get smaller and smaller until when the two windings are overlapped just right, the transmit signal vanishes from the receive winding, the coil is now in balance. But the balance is super touchy, if you just barely nudge the overlap of the two windings, just a smidgeon, the coil is out of balance and the transmit signal re-appears on the receive winding.
So we have established that the coil's balance is super touchy, that the transmit winding generates a magnetic field and the receive winding is positioned inside this magnetic field sitting quietly in perfect balance. When a metal target is waved through this magnet field, the transmit signal induces a signal into the metal target which bounces back and is picked up by the receive winding, the coils balance has been upset, the target's signals is now on the receive winding, the detector processes this and gives you a tone.
Understanding just how sensitive this balance is, brings us back to the Threshold tone. The absolute deepest (or smallest) target a detector can detect, that just barely upsets the coils balance, is a super weak signal that may only warble the threshold tone. It will be too weak many times for the detector to determine a TID, or even give a tone, but it will warble the threshold tone.
Way back maybe my 1st year using an original Explorer XS I was hunting with Dave Z a Whites DFX master. He called me over to check a signal. I was using a 15 WOT coil and wasn't hearing anything, but I was listening for a real signal with a tone and ID. Dave proceeded to dig several IH cents from a depth I thought was mind boggling. He had his arm in the hole to his elbow. Since my Explorer routinely crushed his DFX on depth I was baffled. So I asked him, Dave you had no idea what that was right, you knew something was down there because something was barely upsetting your machine, so you took a shot. He looked at me and said, "You don't miss much do you".
So at the very edge of what your detector is capable of even detecting, its the threshold tone. :icon_
That sux... I really love the threshold for not only it's blanking ability on ferrous / discriminated targets, but for it's ability to make those slight whispers indicating a deep conductive / accepted target. As usual, thanks for the info Hugh, regardless... LOL
