Getting ready to build the MPP

[KRinAZ]

This is the output section that I'm using with my GB detector, it could be modified to use with a GB version of the MPP.

GND would become -5v and +16v would become +5v. The ground symbol is TX coil common.

The output is variable pitch and level.

Hmmm, many thanks for this, I'll take some time & look over...

[KRinAZ]

Regarding the absolute value circuit, it is good to have it with GB because beyond the hole response is inverted, and in case you have indication only for positive going responses, you can't have any response of a long TC target directly underneath the coil. You'll find that all PI machines with GB have absolute value circuitry for that reason. Minipulse is not originally designed as a GB machine, and therefore no absolute value circuit. There are a few ways to add it in.
The way I introduce GB to minipulse is by introducing a GB sample that is in ratiometric relationship to the main sample, so that ground responses are eliminated. It most certainly introduces a hole, and it most certainly reverses phase for TCs larger than a hole.

If I understand correctly the threshold you speak about, you have a quiet(ish) constant tone which increases when targets are nearby, and it is a nice mellow sinus. I also played with such indication, only I call these modulators. Any balanced modulator will perform this way, and by means of introducing a slight mismatch to the balance (e.g. an offset) you get a quiet background tone. Most balanced modulators have very limited balance, and they attenuate a LO by only about ~60dB, which is quiet but audible. I believe this is the best possible indication method, as there is no real threshold that obliterates tiny targets, and it must work preferably well for seeking gold.
Since I found a few creeks in my backyard with gold, and a friend of mine found a real nugget in one of them, building a rig that can find them started making sense. I myself panned a small amount of gold dust, and it was a great joy. I'd starve if I had to earn money by digging gold, but it is a joy.

OK, on the absolute value circuit - an observation - in my testing of the 8 turn 8x10 BW coil, one experiment I tried was 2 coils (since I now have two 8x10 BW's) - the 7 turn as the TX and the 8 turn as the RX - and I ran into the phenomenon where - while air testing so no ground effect - there was no target response as a target approached the coil, but a response when the target retreated from the coil - was quite strange & if I understand correctly the absolute value circuit would correct this, the same way it corrects the response hole created by the GEB circuit & sampling. Yes?

On the threshold tone, yes you've got it right, this would be a form of amplitude modulation, perhaps more simple than the balanced modulation you outlined though - I believe the implementation is the target response produces a varying voltage which biases the transistor that drives the audio output - much like the output of a standard audio amplifier (that's my understanding anyway). The Geotech IDX project is based on the Whites Classic IDX (it's in the Whites Classic series I referred to) & I believe uses this approach for threshold, there's a schematic in that forum (don't remember the post number) but some of the components listed in the schematic are incorrect (the RX preamp for instance has the wrong component) & I believe the tone is originated by a 400Hz square wave oscillator. In my opinion 440Hz would be a better choice as it's an in tune musical note (it's an A4) and will sound more musical.

On the gold in your back yard, how cool! - hope you make a good find - maybe there's a nugget with your name on it back there...

[KRinAZ]

OK - now I understand.
The MPP has a VCO-type audio output, where the amplitude of the target signal controls the audio frequency. This arguably provides the most sensitive method of detection. However, if you would prefer a non-VCO audio, where there is a change in loudness instead of frequency, then I suggest you take a look at Fig. 16 in the Hammerhead Article.

Many thx for this article! - I'll take a look at the Hammerhead, as I personally very much prefer the amplitude modulation (Whites style) threshold tone.
For what it's worth this (non-VCO AM) is the threshold tone method used on the most sensitive gold specific detectors - the Whites GM series and the Fisher Gold Bug II (I think most would agree for either IB VLF or PI technology no other detectors match the sensitivity to small gold of these two).

[Qiaozhi]

Since I'm primarily a prospector searching for gold, I personally usually don't use disc modes if present on a detector - only all metal mode - so I'm actually referring to the Whites threshold sound in all metal mode (Classic series) or just the Goldmaster ("black box" & GMT/GMZ models) threshold (no disc mode so all metal all the time, where the threshold tone is always present (unless threshold control is turned down too low) as a faint hum & gets progressively louder with RX signal strength, but the freq is constant at around 1khz to 1.5khz (I believe, will try to verify) & sounds like a nice sine wave signal, hope that helps & let me know if you'd like more info on it.

OK - now I understand.
The MPP has a VCO-type audio output, where the amplitude of the target signal controls the audio frequency. This arguably provides the most sensitive method of detection. However, if you would prefer a non-VCO audio, where there is a change in loudness instead of frequency, then I suggest you take a look at Fig. 16 in the Hammerhead Article.

[mickstv]

This is the output section that I'm using with my GB detector, it could be modified to use with a GB version of the MPP.

GND would become -5v and +16v would become +5v. The ground symbol is TX coil common.

The output is variable pitch and level.

[Davor]

Regarding the absolute value circuit, it is good to have it with GB because beyond the hole response is inverted, and in case you have indication only for positive going responses, you can't have any response of a long TC target directly underneath the coil. You'll find that all PI machines with GB have absolute value circuitry for that reason. Minipulse is not originally designed as a GB machine, and therefore no absolute value circuit. There are a few ways to add it in.
The way I introduce GB to minipulse is by introducing a GB sample that is in ratiometric relationship to the main sample, so that ground responses are eliminated. It most certainly introduces a hole, and it most certainly reverses phase for TCs larger than a hole.

If I understand correctly the threshold you speak about, you have a quiet(ish) constant tone which increases when targets are nearby, and it is a nice mellow sinus. I also played with such indication, only I call these modulators. Any balanced modulator will perform this way, and by means of introducing a slight mismatch to the balance (e.g. an offset) you get a quiet background tone. Most balanced modulators have very limited balance, and they attenuate a LO by only about ~60dB, which is quiet but audible. I believe this is the best possible indication method, as there is no real threshold that obliterates tiny targets, and it must work preferably well for seeking gold.
Since I found a few creeks in my backyard with gold, and a friend of mine found a real nugget in one of them, building a rig that can find them started making sense. I myself panned a small amount of gold dust, and it was a great joy. I'd starve if I had to earn money by digging gold, but it is a joy.

[KRinAZ]

Since I'm primarily a prospector searching for gold, I personally usually don't use disc modes if present on a detector - only all metal mode - so I'm actually referring to the Whites threshold sound in all metal mode (Classic series) or just the Goldmaster ("black box" & GMT/GMZ models) threshold (no disc mode so all metal all the time, where the threshold tone is always present (unless threshold control is turned down too low) as a faint hum & gets progressively louder with RX signal strength, but the freq is constant at around 1khz to 1.5khz (I believe, will try to verify) & sounds like a nice sine wave signal, hope that helps & let me know if you'd like more info on it.

Update - the fixed freq may actually be appx 400Hz, & may vary some between models - as I get my hands on various models I will start measuring the threshold tone & if I remember correctly the Whites TDI [PI] series also have a very similar threshold sound & behavior.

[KRinAZ]

I'm not sure what you mean by Whites threshold, so if you are up to it, you may try to demonstrate it in a video?My MPP stuck at all components on board, but zero conditions for me to breathe life in it. I intend to make it work with minimum changes, and when I'm happy with it proceed with a GEB circuit. I have it ready, and I have an absolute value circuit ready, but I'm not pushing it yet since I did not test it myself. A GB circuit should work for small gold even without the absolute value circuit though.

Great idea on the video, I'm in between Whites detector's at the moment - I buy them when I come across a good deal & sell them bundled with some training, so there's a flow of Whites going thru my hands, next time I have one (hopefully soon) I will see if I can produce a short video on the threshold sound. & I don't recall reading any posts on your absolute value circuit, could you elaborate on that or refer to post #'s? Many thx for that & the update on the GEB circuit

[KRinAZ]

I assume you're referring to a silent-search mode? In other words, it only beeps when a target is found, otherwise there's silence (apart from the occasional chatter).

Since I'm primarily a prospector searching for gold, I personally usually don't use disc modes if present on a detector - only all metal mode - so I'm actually referring to the Whites threshold sound in all metal mode (Classic series) or just the Goldmaster ("black box" & GMT/GMZ models) threshold (no disc mode so all metal all the time, where the threshold tone is always present (unless threshold control is turned down too low) as a faint hum & gets progressively louder with RX signal strength, but the freq is constant at around 1khz to 1.5khz (I believe, will try to verify) & sounds like a nice sine wave signal, hope that helps & let me know if you'd like more info on it.

[Qiaozhi]

Thought I'd see what popular opinion is on the Threshold sound for the MPP - personally I have spent many hundreds of hours swinging Whites Goldmasters & Classics & prefer their Threshold sound - never fatigues me & I can distinguish faint tiny targets with it. I have also put in enough time on Garrets, Tesoro's etc. (& they are similar but not the same) to know I like Whites Threshold best. What do others think? Would this be good for the MPP project? or perhaps make a mod to convert MPP to Whites style Threshold for those who want it? Almost all my experience is in the VLF world but I would think it could be the same/similar sound for PI...

I assume you're referring to a silent-search mode? In other words, it only beeps when a target is found, otherwise there's silence (apart from the occasional chatter).

[Davor]

I'm not sure what you mean by Whites threshold, so if you are up to it, you may try to demonstrate it in a video?

I read thru these posts recently but was unsure of the state of GEB for the MPP - I saw where Davor & others did a lot of great work on, & SilverDollar created nice schematics, do we have a plan & procedure to add GEB now?

My MPP stuck at all components on board, but zero conditions for me to breathe life in it. I intend to make it work with minimum changes, and when I'm happy with it proceed with a GEB circuit. I have it ready, and I have an absolute value circuit ready, but I'm not pushing it yet since I did not test it myself. A GB circuit should work for small gold even without the absolute value circuit though.

[KRinAZ]

Threshold on MPP

Thought I'd see what popular opinion is on the Threshold sound for the MPP - personally I have spent many hundreds of hours swinging Whites Goldmasters & Classics & prefer their Threshold sound - never fatigues me & I can distinguish faint tiny targets with it. I have also put in enough time on Garrets, Tesoro's etc. (& they are similar but not the same) to know I like Whites Threshold best. What do others think? Would this be good for the MPP project? or perhaps make a mod to convert MPP to Whites style Threshold for those who want it? Almost all my experience is in the VLF world but I would think it could be the same/similar sound for PI...

[KRinAZ]

MPP GEB

I read thru these posts recently but was unsure of the state of GEB for the MPP - I saw where Davor & others did a lot of great work on, & SilverDollar created nice schematics, do we have a plan & procedure to add GEB now?

[KRinAZ]

It depends on what type of target you were using for the depth tests. Sampling earlier allows you to detect targets with very short time constants, such as small gold nuggets or fine jewellry; and you will find a difference in depth for these objects. Unfortunately, sampling earlier also increases ground response.

Yes, I've found this to be very much true! I can perform air tests but without GEB it is unusable on the ground here in the heavily iron mineralized Arizona dirt. OK, one other change I made (don't think it made a difference after all the other mods I did though) is swap the NE5532 for a LM4562 - & since I'm mostly interest in seeking small gold nuggets I mostly test with small lead pieces that are all odd shapes like natural gold nuggets. Also the most prevalent metal trash in AZ is ammo shells so I test them also. For testing: SAT set for fastest recovery without affecting air depth tests, sensitivity at most sensitive, delay at 7uS (the minimum stable delay for my setup) - .44 & .61 gram lead at 2.75"/7cm, .96 gram lead at 5.75"/14.6cm, 1.97 & 2.51 gram lead at 6"/15.24cm, aluminum pull tab 11.5"/29.21cm, .22 caliber brass shell at 6.5"/16.52cm, aluminum .44 caliber shell at 12"/30.48cm, 7.62x39 steel & brass NATO shell at 11"/27.94cm, large iron objects (the edge of my utility trailer) unmeasured exactly but at least 40"/100cm. These all far exceed the Whites Classic ID VLF detector I compared with so I would consider depth & sensitivity a success, just need GEB to complete this project & start using in the field...

[KRinAZ]

----------------------------------------------By an '8 turn version' do you mean that you are going to have 8 wires in each of the slots or about 40 turns on the coil? That should put the coil well in excess of 450uh and I don't think it would decay in under 10us, just guessing. If you take a look at my post #31 in the CHANCE PI COIL thread the attached pictures show an 8" round coil with 39.4 turns of wire yielding a coil with 458uh of inductance. This coil will not allow my CHANCE PI to operate down at even 10us so a larger 8X10 elliptical coil would have even more inductance with the same number of turns. By the way the increase in inductance after putting a shell or armor on the coil I believe comes from slightly crushing the open coil geometry with the wraps and is almost unavoidable. I have a couple of coils that were used in development of my 3:1 oval or RACETRACK style and after much handling un-armoured, they exhibited an increase of inductance I attribute to crushing. The change was from about 310uh to 325uh or about 5%. I am in the process of armoring with fiberglass a 4" X 12.5" 3:1 'Racetrack coil' and a picture of it ready for fiberglass is in post #232 of the CHANCE PI COIL thread. It now measures 328uh.Best of luck,Dan

Yes, by 8 turns I mean the number of windings needed to advance from position 7 to position 8 of the basketweave winding pattern. Since I also wanted to try changing the proportions of the various elipses that make up the template, I went ahead and made the 8 turn 8x10 with the coil spread out a bit & flatter. This modification did not work out well and confirms what is probably obvious (but it never hurts to experiment) - that it is best to keep the windings as tight & compact as possible & design the template accordingly. I will be making a new version of the template with this in mind. So - this 8 turn experiment with wide spacing made a coil with 338uH (quite low for 8 turns), 4.0 passive ohms, critical damping appx 1K(I need a better assortment of 2W resistors to get more accurate), minimum delay about 12uS. From my testing results so far it seems that the 7 basketweave turns for this size coil is the best balance between inductance (RX sensitivity) & speed (sub 10us sampling). I will next post the air test results of my MPP & the 7 turn BW (basketweave) mono coil...