Preamp acting as a microphone

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Steven Sesselmann
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Preamp acting as a microphone

Post by Steven Sesselmann » Thu Feb 04, 2010 11:14 am

Hi Guys,

I recently aquired a NIM crate and some gear, and now I am spending a bit of time trying to get my electronic neutron detection system working properly, but it's trickier than I first thought, Looks as if I am stuck on the first step.

I have a Reuter Stokes He3 tube RS-P4-0404, coupled to a Cremat CR-110 preamp, on the original circuit board, then 1400V bias voltage from a NIM to the preamp, and a BNC to jack cable feeding into the mike input on my laptop, which is running SignalScope (oscilloscope), see screen shot below.

Now the problem is the whole setup works like a good microphone , it is picking up the slightest sound in the room, but when everything is quiet it picks up nothing. The preamp is in a cast aluminum case.

Richard asked me in a previous post what the bias resistors were...

There are a series of three resistors ...

HV------[10M]-------C-------[100M]-[100M]------D-----[0.01uF]-----PreAmp

Where C is a 0.01 uF filter capacitor and D is the connection to the detector.

Are there some more tricks here that I need to know?

All help welcome..

Steven
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Doug Coulter
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Re: Preamp acting as a microphone

Post by Doug Coulter » Thu Feb 04, 2010 2:42 pm

Any variable capacitor with DC volts on it will act like an electrostatic microphone. Almost any two things near to one another at HV can satisfy that condition, including coax, loose wire inside the detector tube, and so on. You might try tapping one thing after another (which is tricky if they're mechanically coupled, too) and see where it is the most sensitive. Sometimes if you listen to the sound of tapping something on both an audio amp and "raw in the room" you can figure out which thing is doing it.

The effect used to drive us nuts in the recording studio business, and we wound up buying some special cable (contained graphite between the coax layers) to help get rid of that.
Why guess when you can know? Measure!

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Re: Preamp acting as a microphone

Post by Richard Hull » Thu Feb 04, 2010 2:57 pm

Microphonics can about in a high gain preamp system.

I worry that your microphonics are coming from the 3He tube. I assume it is hooked up when this condition exists. Tap it very gently and see. A microphonic input FET could also be at fault. Tap stuff gently to locate the culprit. One item or component will be found that far out sounds the others.

Admittedly, your setup should be frozen in 3 space, but true microponics in total stasis conditions shows a big issue problem.

Finally, what is the voltage rating of the .01 coupler cap? 2 or 3 kv, I hope.

Richard Hull
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Re: Preamp acting as a microphone

Post by John Futter » Thu Feb 04, 2010 7:02 pm

Steven
Replace all the caps with film type especially the blocking /coupling cap
RS has these foil caps unfortunately they bigger than the ceramic types.
All biased ceramics are microphonic
wound foil types many orders of magnitude less microphonic

All our portable radiation gauges use foil types on the detectors for filtering and signal coupling
Ceramic caps also work in reverse as speakers, all the noise that comes out of a glassman HV psu during HV breakdown comes from the multiplier caps

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Re: Preamp acting as a microphone

Post by myID » Thu Feb 04, 2010 9:49 pm

Hi Steven-

sorry but sometimes one overlooks the obvious (at least I sometimes do)..a microphone usually is the best mircrophone- for sure!
Did you check if you selected the right input for your scope SW and do not just see what the in- built microphone form your Laptop records?
If it picks up the slightest sound I would not expect it to be a cap or similar even with a sensitive CSA- but I am no expert for sure!

Btw.: I also had microphonic effects but just when I was "beating up" cables to force this effect;-)

Greets and good luck
Roman

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Re: Preamp acting as a microphone

Post by Doug Coulter » Thu Feb 04, 2010 10:03 pm

I do too, Roman -- that's a good one!

You know, with the FIR filter in most soundcard a/d's, they won't even see the pulses of this width at all sometimes, and that might also be a problem. The AC97 spec most of them meet is pretty bogus, and some that claim higher sample rates don't really do it and merely interpolate...and when using lower sample rates just pick the nearest sample instead of resampling in software, which is cycle-expensive to do correctly. I was a pro in the PC recording studio design business at one time....AC 97 seemed designed to prevent good quality "rips" if anything.
Why guess when you can know? Measure!

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Re: Preamp acting as a microphone

Post by Steven Sesselmann » Thu Feb 04, 2010 11:25 pm

Roman,

Yes, that's right, one can select either the built in mike, or the external mike input. I already figured this out myself (doh..), but as it turns out my pre amp is almost as good a microphone as the built in one.

Now the interesting part is that the preamp is sensitive to sound even without the HV bias connected. This eliminates the Tube as the main source. Of course this would not be a problem if the real pulses were significantly bigger, but at the moment I can't see any real pulses at all.

I am using an iMic USB interphase ($29.95) with Signalscope ($99.00), and it works like a dream when I plug in my CDV survey meter, with a scintillator detector. I can easily pick up the different energy levels of the clicks. I also have some PC software that works like an MCA with a signal from the audio input. Basically anyone can have an MCA for $29.95. The iMic is a terrific Analog to Digital converter.

http://www.griffintechnology.com/products/imic/

Steven
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Re: Preamp acting as a microphone

Post by John Futter » Fri Feb 05, 2010 12:22 am

Steven
Its time to go tapping with a plastic knitting needle or similar to find the errant component, or dry joint

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Re: Preamp acting as a microphone

Post by DaveC » Fri Feb 05, 2010 12:28 am

Steven -

About the only thing I can add to all the wise advice and suggestions, is to check things with a BNC (or equivalent) shorting (grounding) cap on the input cable, and then work outward to the preamp. Alternatively, but I don't think it matters here, use a 50 or 75 ohm cap. This way you can separate amp noise from cable microphonics.

You might just have a cold soldered joint in the preamp.

Dave Cooper

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Steven Sesselmann
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Re: Preamp acting as a microphone

Post by Steven Sesselmann » Fri Feb 05, 2010 10:57 am

Thanks guys for all the good advise, I found the main source of microphonics, but don't really understand why it caused the problem ???

I moved the two 9 volt batteries that power the preamp, out of the aluminum box, and the microphonics virtually stopped. I still get a signal if I tap the box sharply, but not if I clap my hands, which is what happened before.

One down, one to go...still not getting a signal from the 3He.

Do my bias resistors look right (200 MR) with 0.1 uF cap ?

Steven
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