Surf PI gain modification

[TH Jim]

Hi Guys,

Just thought I'd share with you this very useful mod I've made to my SP. I have problems with a 212Mhz DAB transmitter right next to the beach where I detect, it plays havoc with my SP. Reading the user manual for the SP Pro I see they added a gain control that can be used to reduce sensitivity when detecting in noisy environments. Not having the SP Pro schematic here on Geotech I thought I'd experiment a bit. Turned out that a simple way to do this is to change the 1M feedback resistor on the NE5534 for a variable pot of the same value. It works really well, I can knock out most of the interference by running at about 75% of the full gain.

Regards, Jim

[6666]

Thanks Jim
interesting to know

[john696d]

Surf PI gain modification

Hi Jim,
You might find this schematic interesting, higher gains in later amplification stages, time constants as per original and gain pot(sensitivity adjustment). Not sure if this is genuine Pi Pro circuit, posted on here as barracuda_Pi.jpg by someone, somewhere that I cant remember! ..........Cant work out how to attach it in this reply ( this is my first post)
Looks sensible to me ( but I'am no expert!) Will post later when I have worked out how to attach a picture.

I have built surf pi too, from a silverdog kit.

Modifications that worked for me were:-
Increased Tx pulse 50 to 100%
Adjust transmit frequency for minimal beat frequency with 5v power supply. Not sure why synchronization was removed in this design, possibly for lower ripple on 5V due to higher frequency?? I think a much higher frequency 5v supply still synchronized to the tx pulses would work better but have not tried this yet.
Highest damping resistor value that still damps the coil
Reduced delay time improves small gold a lot.
Reduce gain in first amplifier reduces noise
Increased gain in last amplifier regains signal.
Gain adjustment very useful "in the field"

Regards, John.

[6666]

John attach the picture by using the manage attachments under additional options

[john696d]

surf pi pro circuit ?

Thanks 6666,
here is schematic. Circuit also has changes around 2n3906, and extra 100uF in parallel with 0.1uF across 5v supply.

Maybe one of the experts on this forum can explain these mods to an amateur like me.
I am guessing 5v ripple/ noise will be reduced by extra capacitance but is placement on the board also important? hence two caps shown in parallel??

I assume extra 100pf across 1st op amp input is just to reduce noise? I can understand changes to later op amp feedback circuits for added gain, but keeping time constants as original, with the added sensitivity control. What is the logic of altering the gain this way rather than adjusting the feedback resistor on the op amp?

Regards John

[ApBerg]

Hi Guys,

Just thought I'd share with you this very useful mod I've made to my SP. I have problems with a 212Mhz DAB transmitter right next to the beach where I detect, it plays havoc with my SP. Reading the user manual for the SP Pro I see they added a gain control that can be used to reduce sensitivity when detecting in noisy environments. Not having the SP Pro schematic here on Geotech I thought I'd experiment a bit. Turned out that a simple way to do this is to change the 1M feedback resistor on the NE5534 for a variable pot of the same value. It works really well, I can knock out most of the interference by running at about 75% of the full gain.

Regards, Jim

Hi Jim,

Thanks !

In the past I have used a 5K potentiometer in place for the 3k3 resistor near the 555. This way you can adjust the puls 'on time' I think.. test it out, it can give some extra depth..
Power consumption is going from ca. 50mA to ca.75mA at no sound

Best regards

Ap

[kt315]

are you using shielded coil?

[ApBerg]

are you using shielded coil?

Yes most coils have some schielding. The large one's (more than 1 m) not.

Best regards

Ap

[TH Jim]

Hi All,

Thanks for your suggestions, look forward to trying them out

I am using a shielded coil - conductive mylar space blanket for shield (cheers Don!).

Regards, Jim.

[TH Jim]

Hi John,

At a quick glance the schematic seems to only differ from a normal surf pi by just the pot between the 2 stages of the LM358. will look more closely later. Is there a larger print version that would be easier to see?

Cheers, Jim.

[john696d]

Hi Jim,
That's all I have got I am afraid, I downloaded it from this forum a while ago, so maybe the original poster can supply a clearer copy?

If you look closely you will see that the last two amplifiers have higher feedback resistors to increase the gain, and the capacitors have been increased in value to keep the RC time constants as the original design.

If you just want to add a gain control to the standard surf , use a 220K pot instead of the 100k and your time constants will be about right.

You can get a bit more gain by increasing the feedback resistor in the last amplifier from 100k to 220k.

I think someone else was claiming that whites had designed the circuit wrong and it works better back to front, I think they meant lower gain on the preamp, and higher gain on the DC amp at end. There may be a grain of truth in this as I got less noise by reducing R9 a bit
and increasing R22 a bit, as you have also found.

You could swap R22 and R9 and see what happens!!

On the other hand I could have got this totally wrong, as the poster was talking in riddles for some unknown reason, and I am sure whites will have used the gains they chose for each amp for good reason.

There are a few other small changes that I mentioned before that I don't fully understand.
I was hoping someone with a bit more Knowledge could explain them............

Regards, John.

[TH Jim]

Hi John,

Yes i see the changes to the gain in the LM358 stages and how the caps have been adjusted to keep the RC time constant the same, although in the first section it looks like the resistor has been increased and the cap decreased (working from memory here, I think the cap was 470n in the original). I wonder if this has any effect on the sweep speed tho? This is something I considered trying myself as the RF noise I get at the beach has more affect the faster the detector is moved.

As for the decoupling caps on the 5534 it would surely not make much of a difference to put 100pf in parallel with 100nf. I would just use the 100n to keep the supply to the 5534 clean.

It looks like someone has taken the SP schematic thats here on the forum and worked in the hand drawn notes, put the power supply sync to the 7660 back in and tweaked the gain on the 358, then tried passing it off as the Baracuda!

Interesting none the less, I am tempted to try the mods to the 358 section, I have a SP permantly set up on bread board just for this kind of occasion!!!

[kt315]

is there MOS-FET or you see TIP32C transistor Q2 inTX stage ? what value of R6?

[TH Jim]

Hi KT315,

The above schematic looks like it uses IRF9640 for Q2, as do I. Can't remember exactly which part R6 is, but if it's the damping resistor then the above looks like 390 ohms, the same as i use.

Regards Jim.

[plumabob]

Hey Apberg,

I tried your 5k pot for the 3K3. I does seem to give a bit more depth. I adjusted the 5K to max out sensitivity to some small gold jewelry and saw that my larger gold rings did indeed show a nice increase. Probably about an inch or so I'm guessing. Nice easy mod. Good job!

Thanks,

Bob