usb sound card modifications

[ChrisRadium2023]

Hello everybody,

last night after many trials and experiments on my philips XP2020 pmt and EMI THORNE 9813KB pmt i found out that both pmt works fine and none of them are damaged .
I got same quality spectrum ( very bad quality I mean) with both pmt and the same crystal... so i thought that the NaiTl crystal I have is faulty...and then I tried with my LYSO tiny crystal .
And then the spectrum is still as limited and bad as with the NaITl....
Then I realised that only the preamp and sound card cound be incriminated for the fact that only a short part of the spectrum is visible ; mainly the low energies oh the americum 241 ( for example )shows its nice peak.

I browsed through Thermino USB sound card modifications PDF file and it says that a 1uF capacitor should be replace by a 10nF but it's not explained why. It seems that this capacitor is a coupling capacitor for the mic signal toward the chip.

Could this 1uF be too high and hence the need to replace it by a 10nf?

I only had a 10uF to check its effect on the coupling to the pmt to see the results on my oscilloscope. ...and when I changed the 10nF I usually use for the 10uF I could see that 97% of the pulses werent visible anymore!!

Could that explain why the dynamic range on my spectrum are so short ? (before I found out about that i tryed many resistors chain combinations and voltages , or gain on theremino to no avail on the dynamic range extention)

Is my hypothesis correct ? should I then try to modify the coupling capacitor of my USB sound card ?

thanks for any help

Chris

[ChrisRadium2023]

Here is the link of the youtube video :

https://www.youtube.com/watch?v=JlyGCq08250

if it works.

[Sesselmann]

Chris,

Looks like you are seeing a spectrum, but you are possibly running it with too much gain, try lowering the voltage.

Typically you want the screen to display a range between 25 and 3000 keV. (unless you specifically want to zoom in on an area of interest)

Steven

[ChrisRadium2023]

i tried everything and its not a spectrum, nor a problem of gain, nor voltage: even when I unplug the mic I still record a "spectrum" wich alas is nothing else than the noise from the usb power supply .

I will make a screenshot and post it . The periodic pattern is very obvious and the noise omnipresent. In the end today I bought a proper good quality sound card 24bit/192kHz sampling rate. It also has a preamp included . I expect the noise level to be well filtered.

Also I remade my RC filter for the HV and was able to eliminate most of it down to a few mV with only 1 stage filter instead of 9!!

to be continued.

[iRad]

Hi Chris - Those cheap soundcards are notorious for noise and other problems, making them worthless to even fool with. You can read a lot of information about these issues on the Theremino web site as well as their documentation that comes with Theremino MCA. If you find a good one (luck of the draw), you may be able to modify it to perform better as illustrated and discussed in the documentation. Problem is that even when you find a good one, you may be sent one with completely different components on the inside (or a completely different sound card altogether) on the next shipment from the same seller.

[ChrisRadium2023]

Hello iRad ,

thats exactly what i realised and whats happened : the nowadays 2022 usb sound cards (the cheap ones) are different from the model Theremino refered to and you're absolutely right about them :(

I've received tonight my new good quality (supposedly) sound card but turned out its only a 48kHz sampling card but still I will try it and otherwize I can send it back and buy the 192kHz model ; but still I saw many people who were able to get good results @48kHz for a start , so I going to give it a try .

[Sesselmann]

Chris,

Even if your sound card input is sampling at 48kHz you can oversample the output at up to 384 kHz with and get better results.

Steven

[ChrisRadium2023]

you mean setting the 354kHz frequency in Theremino even if the card wont go more than 48kHz? like the software sort of extrapolate the sampling ?

My first trials with the new card are indeed much much better ; there is still some noise from the usb port but it seems possible to filtrate it via a usb device filter :) i'll post a screenshot too to show the comparison. Its way much better than with the "chinese cards" , wayyyyyyyyyyyyyyyy much better !

[ChrisRadium2023]

I have a question about sound card sampling rate and results :
It might seem obvious to some of you but i'm still asking the question as my budget isnt big :
Indeed as suggested before the results are getting better when I set Theremino @ 384kHz sampling rate even if my card is rated at 48kHz.
Will there be a big difference If I used a genuine 192kHz sound card compared to my current 48kHz?
Will there be a big difference between the 48kHz card "forced" to 192kHz with a 192kHz card?

I'm so impressed with the improvment between the cheap chinese and my new 48kHz that I wonder if the 192kHz would have been better ( as i can send it back and buy the 192kHz). But if the difference isnt that noticeable maybe it wont be worth the extra cost?