chance pi build

[WM6]

[satdaveuk]

Hi Dan
Parts list etc
Regards

[baum7154]

Thanks for the quick response Dave. The front end looks like it can be modded to a 2 stage and Ill get a drawing posted of what I think will be a good start. Do you know the minimum sample delay that the unit will run at now? Do you know how long it takes for your coil to come out of flyback? Have you looked at a scope trace of the amp out vs the coil decay showing how long it takes to come out of saturation? What is the sample delay you would like to run at?

Regards,

Dan

[satdaveuk]

Hi Dan
These pics may help
Would like to be able get a bit more out of the design regarding small gold and silver items .
Far as my personal scope measurements am stuck at the moment due to knackered scope but should be sorted soon.
Have got some more scope pictures from the original Barracuda if they may help but they doesnt indicate where measurements were taken like the two I have posted

Regards
Dave

[baum7154]

Hi Dan
These pics may help
Would like to be able get a bit more out of the design regarding small gold and silver items .
Far as my personal scope measurements am stuck at the moment due to knackered scope but should be sorted soon.
Have got some more scope pictures from the original Barracuda if they may help but they doesnt indicate where measurements were taken like the two I have posted

Regards
Dave

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If I am reading the scope shots correctly the delay is set for about 25us and the sample window is about 130us wide. Do you know if that is the minimum sample delay or is that just where 'Delay' was set for the photo? Can't really tell about the coil decay but I'd guess it is somewhere in the 15-20us range.


Dan

[sinclairuser]

the minimum sample delay is 25us on the original barracuda, the lowest useable setting i have tried(with success) is 12us, so your 2 stage approach is very welcomed my friend.
thanks
ally.

[baum7154]

Thanks I will give it my best shot. Gathering info and thinking about the best approach to this.

Regards,

Dan

[baum7154]

the minimum sample delay is 25us on the original barracuda, the lowest useable setting i have tried(with success) is 12us, so your 2 stage approach is very welcomed my friend.
thanks
ally.

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A few questions about your 12us success:

1. Did you have to modify the sample delay circuit of the detector to allow operation below 25us?

2. How fast does your coil decay from flyback?

3. Do you have a scope picture of the preamp output showing how fast it settles? I guess the real question is if the amp is the cause of the inability of the sample delay to go lower than 12us. It certainly sounds like it is and 12us is somewhat impressive for an amp with a gain of 1000. Do you have a picture of the amp output?

Regards,

Dan

[sinclairuser]

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A few questions about your 12us success:

1. Did you have to modify the sample delay circuit of the detector to allow operation below 25us?

2. How fast does your coil decay from flyback?

3. Do you have a scope picture of the preamp output showing how fast it settles? I guess the real question is if the amp is the cause of the inability of the sample delay to go lower than 12us. It certainly sounds like it is and 12us is somewhat impressive for an amp with a gain of 1000. Do you have a picture of the amp output?

Regards,

Dan

question1- no, no modifications to design other than (fast)graphite shielded coil, delay set with trimmer, scope timing read 12us.
2- unknown without further testing, but presumably slightly less than the sample delay.
3- no pics, i'm a hands on stone age kind of guy, i wasn't trying to modify the design, i was simply tweaking and tuning the coil to improove gold response.
the limits in the barracuda design as i understand it are due to the relationship between sample delay and the coil flyback decay time.

[baum7154]

Yes, in order to get the small gold a fast coil in the range of 300uh would be best. This would necessitate a damping resistor change to obtain critical damping.

So it sounds like the detector is capable of even shorter sample delays using the internal trimmer pot. You are right about the relationship between sample time and coil decay time. It won't do to be able to adjust below 10 or 12us if the coil doesn't decay faster than that. I think the original Barracuda coil at about 450uh will have a very difficult/impossible time in getting down to the 10 us range.

I have a schematic drawn up for the Barracuda 2 stage amp with the suggestion to wind a 300uh coil and a change in damping resistor. 300uh is a general number used by many when trying to detect small gold, that said I have a special 335uh coil that decays in about 6us and it works down to about 4.5 grain pieces of 14kt gold (See CHANCE PI COIL thread in the COILS FORUM). So a coil in the neighborhood of 300uh to 325uh should serve well and the 300uh coil would be the easiest 'fast coil' to make.

Thanks,

Dan

[sinclairuser]

a fixed resistor value for that coil would probably be welcomed as well, a lot of barracuda builders have done it with out a scope, by hitting the coil specs of the original, and fitting a 330 resistor, we have scopes but a lot of others dont, and quite a lot of constructive testing and tinkering was done in the barracuda thread by those with more simple bench setups.

[baum7154]

I understand that some don't have access to a scope and have to rely on a prescription damping resistor value for a prescription coil. This frequently results in a compromise in performance. However those with a scope can really improve the performance of the coil by optimizing the damping. In fact setting critical damping provided a massive improvement for me in building CHANCE. I also relied on the 'prescription coil and damping resistor' from the author but was never able to operate CHANCE at the minimum 8us setting until I reduced the coil from 400uh down to 335uh and changed the 390 ohm damping to a much higher value of 1040 ohms to match the fast coil. By the way the original prescription CHANCE coil is unbelievably fast…. unshielded. However when shielded it loses a great deal of that speed. Perhaps that is why the author didn't spec any shielding.

Since you have a scope have you also tweaked the damping resistor value in your efforts to get down to 12 us? If not, I'd recommend BB SAILORS adjustable damping resistor technique outlined in his paper on 'Making a Fast Pulse Induction Mono Coil'. It quickly allows you to determine the fixed resistor value to damp your coil.

Regards,

Dan

[baum7154]

Attached is a schematic of the 2 stage pre-amp for the Barracuda that I have been working on. A quick run through of my thinking on this amp:

1. Two Non-Inverting amplifier stages based on the OP37 amp. The Non-Inverting amp matches the original design and the OP37 has better performance in virtually every respect compared to the original NE5534 amp. In addition the OP37 is available factory trimmed to 10uv which may be a benefit. That said the OP37 is a pin for pin substitute for the NE 5534 (with the exception of offset trim pin eight) so the original chip could be used as well.

2. First stage gain set at 28, Second stage set at 37. This pairing will give a gain of about 1036 to match/exceed the original design gain of 1000.

3. Single 'Trim' pot on the first stage should allow both stage offsets to be adjusted.

4. 1K interstage coupling resistor and 2ea 470pf in series (235pf effective) to ground create 677kHz 1st order LP filter allowing approximately 1.5us pulses or longer to pass.

5. 1st stage 38pf and 2nd stage 23pf capacitors in the feedback loops function with their respective feedback resistors to create approximately 677 kHz 1st order LP filters. These values take into consideration the added capacitance of the diodes in the same feedback loops.

6. Diodes in the feedback loops of both stages. The single 1N4148 diodes limit amplifier output of the positive signals to .7 volts. The double and triple diode strings limit amp out for the negative signals to -1.4v and -2.1v respectively, preventing stage saturation when large signals are present. -2.1 volt 2nd stage limiting was chosen to be just above the -1.4v operating level of the original/retained 2 diode limiter (D5 & D6). These limiters should allow desired small signals to be fully amplified while limiting the effects of large saturating signals.

7. Change of the original coil to 300uh from the original spec of 450uh is recommended for best small gold performance. Damping resistor would have to be set for critical damping with the coil change.

As I do not have a Barracuda detector I have not built this amplifier to see what other problems may need to be addressed and I welcome any one who builds it to provide feedback on it's performance. I hope this modification allows the Barracuda to operate at the short delay times necessary to detect small i.e. 1/2 gram gold, or better.


Best Regards,

Dan

[Silver Dollar]

Do you have a better picture? That ones a bit fuzzy...

[sinclairuser]

thanks for the schematic, the answer is yes and no, i did make a coil myself that i tuned the damping resistor, but for testing silverdogs coils that he is working on i replaced a 330 resistor, since the object of the silverdog coil is to provide a coil that works with the kit to save builders winding one.
which is what i was getting at, if hitting coils specs works in terms of a 330 resistor, then we should do the same for a 300uh coil, this would benefit andy(silverdog), as he could make one coil, since a 300uh coil will work for surf pi as well, he would not have to wind two separate coils.
while its true certain aspects of the barracuda's working action is lost with coils below 350uh(like when you retune to use surf pi coils), this is moot since the barracuda kits use irf fets instead of tip 127 darlingtons, the irf completely removes the slight difference in tones between iron and coloured metals.
so your new proposed coil specs could make a coil that works on barracuda or the surf pi regardless of the delay sample, and save silverdog time and work, also the barracuda could take your preamp upgrade, so its a win, win.