HH2 -- Take Two

[Stefano]

Sorry Carl, one more question...
I can't find the value of R22 and C16 in the schematic.
How much are they?

thank you.

[Stefano]

I've just found hints for them on the HH1 article.
I didn't know it had been updated. When I built HH1 and read that article the smt version wasn't ready yet.
Well, I'll start with 0 ohm and no C16,as I've read the filter is not strictly requested.

Stefano

[Nc-Dirty-digger]

is there a complete parts list yet? and are there any pre-progammed chips yet?

[joecoin]

is there a complete parts list yet? and are there any pre-progammed chips yet?

Most of the parts are the same as in the HH1 build article. You can get all the part numbers/values by looking closely at the schematic.

[authere]

HH2

Hi All,
Could someone take an upclose photo of their populated PCB for the HH2 as i am about to go and get the parts and it would help if i knew what they looked like as i am buying them. I am having trouble distinguishing between the resistor and a potentiometer,both have R before them, so a picture can tell a thousand words.
Regards Ron
PS:I have to start somewhere and pots come in many different shapes and sizes, at the moment i have
18, 31-100k
23-100
34-1m
35-5k
41-5k/audio taper
Ron

[Stefano]

Led purpose?

Hi all.
This is probably a stupid question, but what's the function of the led on the circuit? I thought it was a sort of battery checker or powers ok in the circuit since it is fed by the +12 and -5 rails but mine is switched off all the time.
I've tried to simulate it with CircuitMaker but it seems that isn't lighted in the simulator too (however I may not be using the simulator correctly...)

[KingJL]

C24?

Just getting around to building my HH2. As I am populating the board, I come across C24, 0.47uF feedback capacitor for IC6b. Is the 0.47uF value correct as it (in conjuction with the 33K feedback resistor) results in a bandwidth of ~10Hz for the second stage . This appears to destroy the response for fast TC targets. In previous discussions it was recommended the desired BW for the pre-amp be ideally ~1.5 MHz to preclude distorting of the decay. I am going to experiment by omitting C24 initially as I have had great results with a SMT HH where I kept the preamp BW at ~1.5 MHz. I will then install C24 later for comparison of results.

[KingJL]

Not a peep out of a small nugget(match head size). Still more work to do.

Try removing C24 (if you included it )... you might be pleasantly surprised!

[TryAgain]

The MAX412 was chosen because I've found it to be one of the best for PI preamps. I think only the MAX414 (quad version) is being discontinued, and probably because it's a 2-chip hybrid. The MAX410 & 412 are extremely popular opamps and shoud be around for a long time. Yes, you can use other opamps; any wideband in a standard dual pinout (NE5532, etc).

- Carl

Hello Carl and the forum.
I've found this op amp today looks interesting, thought I'd run it by you guys to see what you thought of it.

http://datasheets.maxim-ic.com/en/ds/MAX9632.pdf

Merry Christmas
And thanks for this great forum

TryAgain

[Tinkerer]

Just getting around to building my HH2. As I am populating the board, I come across C24, 0.47uF feedback capacitor for IC6b. Is the 0.47uF value correct as it (in conjuction with the 33K feedback resistor) results in a bandwidth of ~10Hz for the second stage . This appears to destroy the response for fast TC targets. In previous discussions it was recommended the desired BW for the pre-amp be ideally ~1.5 MHz to preclude distorting of the decay. I am going to experiment by omitting C24 initially as I have had great results with a SMT HH where I kept the preamp BW at ~1.5 MHz. I will then install C24 later for comparison of results.

Hi KingJL,

I agree about C24. It should be at most 47pF, better would be 4pF.

I also found that the high impedance of C19, R34 ads a lot of noise. This is a bit tricky, when using a large capacitor and the IC8B in inverting mode, low impedance, the noise is reduced very dramatically. However, this brings other problems like the electrical characteristics of the capacitor and other circuit changes.

What is your opinion about it?

Tinkerer

[KingJL]

I agree about C24. It should be at most 47pF, better would be 4pF. is even better.

Actually, none is even better! The MAX412 is very stable at these gains and capacitive feedback is not needed (vs the AD727 which usually needs come capacitive feedback to prevent oscillation at lower gains). Also, ANY capacitance will further reduce the effective BW. At the gains as designed (without using C24), the effective BW for stage 1 is ~850kHz and stage 2 is ~825kHz.

I also found that the high impedance of C19, R34 ads a lot of noise. This is a bit tricky, when using a large capacitor and the IC8B in inverting mode, low impedance, the noise is reduced very dramatically. However, this brings other problems like the electrical characteristics of the capacitor and other circuit changes.

What is your opinion about it?

Given the purpose of Carl’s design (to allow maximum adjustment and experimentation of operating parameters), I really can not recommend any changes to the C19/R34 components. As designed, they provide a HPF of 0.307Hz and 3.38Hz (depending on the setting of R34). Most other designs use ~470k paired with 0.47u to provide a hpf cutoff of ~0.720Hz and the HH seems to work well when R34 is adjusted to provide a total of 470k (R34a + R34). As you get above a total resistance of 470k and the cutoff freq gets closer to 0.307Hz, the setting becomes more sensitive and more susceptible to noise. I would recommend setting R34 so that total resistance is no more than 470k.

One area that I experimented with was C28. I changed it from 470n to 100n. It seems to allow a little faster swing speed of the detector while still providing good target response.

I believe that the noise susceptibility you reference comes from the limited LPF filtering provided by the integration stage prior to the HPF, but this is consistent for most PI designs. The effects of noise can be reduced by limiting the sample width. If you wish to sample early, you must use the shortest sample time possible as the resulting value sampled and integrated is based on the value seen at the end of the sample period, not the beginning. So the trick is to find as short a sample time as possible and yet have enough charge time on the integration capacitor to provide for adequate integration. In my case, I used a sample time of 7.5 usec and increased the size of R26 & R27 which provided more gain and better LPF filtering for the integration stage.

If the integrator was changed so that the input and feed back resistance path was only seen during the sample period (as in the Goldscan IV design), better filtering and lower gain of the integrator could be achieved, providing better noise performance (that sounds like a future project).

[nick_f]

Hi Carl,

I got a second hand Minelab DD coil (Super Goldsearch 11") and I thought I will try it on the Hammerhead. I could not find information about how to choose the resistor R14 (in parallel with the Rx coil). Can you please share some light here, please?

Thanks,
Nicolae

[Altra]

Hi Nick,

I used an ML GP series 11" on my Hammer Head II. I do not remember the exact value. But I seem to recall something like 1.2K. It's like choosing one for the TX. With a scope hooked to the preamp output add or subtract resistance until you reach critical damping. No ringing.

Regards

[nick_f]

Hi Nick,

I used an ML GP series 11" on my Hammer Head II. I do not remember the exact value. But I seem to recall something like 1.2K. It's like choosing one for the TX. With a scope hooked to the preamp output add or subtract resistance until you reach critical damping. No ringing.

Regards

Hi Altra,

Thanks for info. I will do some tests when I will have a bit more time.

Regards,
Nicolae

[cko47]

Mr. Carl!
Excuse me for simple question.
How connect a pots to conn2?
And how I can order your PCB for HH2 (with last mod) into Ukraine?
Thank You!
Have a nice weekend.