Pre Amp Gain and the DFX

[Myownwings]

I read everything on all the forums today and was bored so I decided to read every Jimmy Sierra post. I was somewhat amazed that PAG does not directly effect depth, which I always thought it did; but that AC sensivity has the greatest effect. If this is true then, great, because if I have to set the PAG to '2' I just lose interest because I believe that low of a setting just takes out all the deep stuff. Now couple that with sweep speed, ground filtering, recovery delay, smooth threshold, vdi wrap, and what frequency to use and you got one fine machine to fight you while you try to have fun. Of course, clad at 3 is wonderful....sorry, rough night. So, does anyone regulary use a PAG of '2'?

[del]

i will only run the preamp at 2 if the emi is extremely bad in the area or if my targets in question are very shallow . i can usually run it in 3 as it is very stable with an AC of 74 .

[coinnut]

From the way I understood it... the Pream Gain is kinda the listening part of the machine. In other words, it listens for the signal that is created around the excited object. The Sensitivity is the signal that radiates into the ground and hits a target and excites it. It would not benifit you to overdo the Pre Amp Gain. But leaving it too low may miss the target that has been lit up by the sensitivity signal. I have always run mine on 3. The other functions will help you after that.

[kenfen]

I set mine on 3 as well. I have set it to 4, but it depends on the coil if it's going to stay stable or not. With the 5.3 Eclipse, I can go to 4, but then you are amplifying a lot of noise as well. I set mine as hot as I can, but stability being the key. Have you read Jeff Foster's book on the DFX?

Regards, Ken

[Myownwings]

Have you read Jeff Foster's book on the DFX?

Multiple times...it's in the bathroom so some section gets read everday, for months now. My detector probably needs recalibrated, I've had it 7 years+. Plus the ground I hunt is full of iron and is generally highly mineralized. I just need to get over the fact that if I have to set PAG to 2 then, that's what I have to do. Probably a balanced detector at 2 with ac and dc as high as possible without instability is better than fighting it with PAG of 3 and ac and dc low but still having constant falsing. Of course there is also correlate when I get desperate.

[kenfen]

Myownwings,

Were you ever able to go up to 3 PAG? If yes, I'd think about sending it in for a tune up. Have you tried the Hi Iron program in Jeff's book? I'm sure you have....but just asking. I did that and I was quite happy with the way everything quieted down and I had much more stability.

From what I've learned, yes, you're better off with PAG 2 and have a stable machine versus the other option.

Regards, Ken

[coinnut]

Thinking about it now, I think you may have a coil going or gone bad. You should be able to get a setting of 3 on your DFX. I'm leaning towards the coil. When I had the monster 14 x 8 ? DD coil for my DFX, I could never get it past a setting of 2. I now think that coil was just out of alignment and was not letting the DFX accept a higher number. My smaller 4x6 coil was able to run on a setting of 4, but a couple of years later would only balance on a setting of 3. So it may just be a coil issue. The DFX has been rock steady for me mechanically for many years.

[delta196]

From the way I understood it... the Pream Gain is kinda the listening part of the machine. In other words, it listens for the signal that is created around the excited object. The Sensitivity is the signal that radiates into the ground and hits a target and excites it. It would not benifit you to overdo the Pre Amp Gain. But leaving it too low may miss the target that has been lit up by the sensitivity signal. I have always run mine on 3. The other functions will help you after that.


No no no,,,,

The transmit signal, the one that excites the target is fixed and cannot be changed.
The pre amp gain is the first amplification of the return signal, in trashy ground or highly mineralised ground you do not want your pre amp to high as it will just amplify the trashy signals as well as the good ones, you will then have to turn down your AC to counteract it.
If your ground is clean and targets are scarce increasing the preamp will enhance the quieter signals, you then fine tune with the AC to pick up on these quieter deeper signals.

Hope this helps.

[coinnut]

No no no,,,,

The transmit signal, the one that excites the target is fixed and cannot be changed.
The pre amp gain is the first amplification of the return signal, in trashy ground or highly mineralised ground you do not want your pre amp to high as it will just amplify the trashy signals as well as the good ones, you will then have to turn down your AC to counteract it.
If your ground is clean and targets are scarce increasing the preamp will enhance the quieter signals, you then fine tune with the AC to pick up on these quieter deeper signals.

Hope this helps.


So your transmit signal is fixed and is not your sensitivity setting?? :confused: So what does the sensitivity setting adjust?

[delta196]

So your transmit signal is fixed and is not your sensitivity setting?? :confused: So what does the sensitivity setting adjust?


The pre amp gain is basically a volume control, it increases the volume of the signals from the recieve coil. It increases good and bad targets.

The sensitivity control adjusts how loud the signal must be to be used by the discrimination circuit.

The sequence is basically,,

1. Signal recieved from recieve coil
2. Signal is boosted in volume by the preamp gain setting
3. Certain signals are then passed to the discrimination circuit based on the sensitivity setting.

Hope this helps

[coinnut]

The pre amp gain is basically a volume control, it increases the volume of the signals from the recieve coil. It increases good and bad targets.

The sensitivity control adjusts how loud the signal must be to be used by the discrimination circuit.

The sequence is basically,,

1. Signal recieved from recieve coil
2. Signal is boosted in volume by the preamp gain setting
3. Certain signals are then passed to the discrimination circuit based on the sensitivity setting.

Hope this helps


That helps a lot Delta196. Thank You! I must be misinterpreting some of Jeff Foster's descriptions in Chapter 6. :confused: I need an on site teacher lol

[delta196]

That helps a lot Delta196. Thank You! I must be misinterpreting some of Jeff Foster's descriptions in Chapter 6. :confused: I need an on site teacher lol



This is from Jeff himself, posted on another forum...


The Preamp Gain (PAG) setting can be viewed as being available for adjustment so that it can be turned down – not turned up. Here's what I mean by this.

If there were not a need to deal with mitigating factors, the PAG setting could be fixed internally in the DFX at 4. If this were done, two problems could occur. First, in areas where there is very strong mineralization, &quot:daydream:VERLOAD indications might occur due to just the mineralization. Being able to turn down the PAG allows operation in soil that would otherwise not be possible. The other related problem area is electromagnetic interference (EMI). To better understand this issue, let's look a bit closer at what the PAG thingy does.

The PAG setting controls an amplifier in the DFX that is very much like the pre-amps used in older audiophile systems. These pre-amps are amplifiers that boost the very small voltages, coming out of a phonograph needle or a magnetic tape pick-up, up to a voltage level that the stereo's amplifier, which drives the speakers, can use.

On the DFX, the pre-amp is responsible for boosting the very small currents on the receive coil in the search head up to a level that can be more easily processed by the DFX's computer circuits. In cases where the EMI is very strong, this noise will take up a significant portion of voltage range that the pre-amp is providing to the DFX's computer. One would say that the EMI is using a considerable portion of the dynamic range of the amplifier – only the range of voltages above the highly amplified noise and the max (&quot:daydream:VERLOAD) level is available for the computer to extract information.

In these cases, where the EMI is relatively strong, the A.C. Sensitivity (ACS) can be reduced to a level where the spurious audio and display indications stop occurring. Doing this doesn't address that fact that the DFX's computer is limited as to the range of useful information that is available out of the pre-amplifier. Also, the EMI noise, although it can be scaled down with the ACS below a level where it stops causing false indications with the search coil in the air, this noise is still at the base of the signal coming from the pre-amplifier – buffeting the stronger target signals and adding to their instability.

To address these issues, the PAG setting can be turned down. It doesn't really have a detrimental effect on depth since in these circumstances the limiting factor is the competing EMI signal noise.

Out of time for now,

Enjoy.

[coinnut]

That's a real nice explanation of both settings. Thanks You. The line that threw me from the book was : The PreAmp Gain setting controls the amplification of the electrical signal coming out of the search coil. I always assumed it meant that it was being transmitted out of the search coil into the ground. Now I realize that line means the return signal from the coil to the electronics above. I wish we had an Ask Jeff Foster, ask Carl, ask Andy Sabisch, and ask Eric Foster section lol That would be sweet for us and a headache for them rofl

[Nitro 54]

As a DFX user, there's a lot of good info here :peace: :peace: thumbsup01 thumbsup01

[russellt]

i would run 3 paG. ac 80 dc 70. using m/m. i did find some deep targets with my dfx..i like the V better though