The fine wine preamp now available

This is for products and services from Joe Jarski and Doug Coulter. We've noticed others having trouble on some things, and think we can make a positive contribution to the community by making some of those things available to all at nominal pricing, so people can just get on with their own work.
Here is where these things will be described and sold.
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This is for Joe and Doug to display our joint projects for sale, and for questions and answers about them.

The fine wine preamp now available

Postby Doug Coulter » Tue Jan 31, 2012 3:15 pm

Joe Jarski has done his part of this quite nicely (as usual - hat tip Joe) so now we can offer the fine wine preamps for sale. With slight component changes, these can handle quite a wide range of requirments. These are done so things like gain and low frequency rolloff are very easy to change, bias for different supply voltage ranges and speed are also possible to change, as is input impedance.
We can offer them in kit form, but unless you do SMD things a lot - they're not that easy to build, and we'll make them available built and tested of course - with any changes you need put in from the beginning.
Here's some documentation on these:
Preamp.gif
Preamp schiz and stuff chart. Board is 1" square.


As shown, these are meant for 6v and up (tested to 15v but more is possible) power supply, positive DC. The input impedance is about 66k, and the gain is about 150 - too much for many phototubes, about right for most proportional tubes. Very easy to change that by changing one resistor, R6. The gain is the ratio between this and R5, within limits. The open loop gain is 900-something so at very high gains you'll not get the oversimplified theory number, quite.

Note, the 680pf cap on the "AC" input will NOT stand off more than maybe 100 volts. If you're coming in from something in the kV, use your own capacitor and the DC coupled input instead.

At very low gains you won't be able to get full power supply rail swings, but any gain over 10 should be fine there (why would you want a zero gain preamp?). This will drive 50 ohm coax cables very nicely, and is meant to be small enough to go right inside your sensor head for lowest input capacity and EMI. A filtered power output is provided to allow you to use that for a coax return if the sheild on the other end is floating - you get a little better power supply noise rejection that way, since our filtered positive rail is actually the input "signal ground". For those who are going to drive TTL type inputs, you can add the optional pull-up and down resistors on the output to make it want to sit at 5v, for example 100k pullup and 120k pulldown will get pretty close. This was designed to drive either an MCA or a standard CMOS input, like the extra one on our standard counter.

This is biased so the output rests right near the positive rail - it's meant for negative going pulses. If you need the other polarity, you could rebias it, but that results in pretty nasty power drain. The correct way is to flip all the transistor sexes, diode polarities, and the power supply.

And now that I have a neat schematic all nicely labeled, I can go back to the design thread and update it with actual part designators and make it more clear. Thanks Joe!

Sale price for these will be $50 + shipping, to cover NRE and building labor. Kits - inquire and we can work something out.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Re: The fine wine preamp now available - use tips

Postby Doug Coulter » Mon Nov 05, 2012 10:43 am

Notes on use and tweaking this design.

The top and bottom ground planes are tied. You can bolt this right to a metal grounded base, solder it down, whatever. You can do this by having a hole in the mounting plate to solder through, (be careful about too much heat) or a bunch of little short wires to the edges of the ground plane on top. I've also drilled a hole in a corner and bolted it down.
Ground is the negative side of your 9v battery as well. I put a switch in series with the positive side of the battery - while battery life tends to be decent, if the preamp is tuned for much drop across the output 220 ohm resistor, it will still eat batteries. Gain or bias changes can tune or retune for this - you only want a fraction of a volt (say about 1-2 ma total drain on the 9v).

The pads are on the tiny side, with small clearances to ground so they are a little hard to solder. My technique is to put a tiny blob of solder on the pad first, tin the wire (perhaps with a very short 90 degree bend on the end) and then heat it and stick it in the blob. Make sure (magnifying glass) there are no shorts to ground after this. I usually mount the board first, then put on the wires, so anything "fat" like say, the lead of the input coupling capacitor (you supply that for most situations) which is stiff, is not under stress when soldered down. Eg you do your lead bending first, not after, soldering.

To tweak gain, change the resistor that is sideways (the only one). On a stock unit, this is a 15k ohm resistor (R6). I've used up to 47k with good results on the weaker output tubes. You may want to re-bias to keep the headroom right and the battery draw low. This is done by altering the r2, and r3 resistors. Making them larger (usually both at once) reduces the current drain or drop across that output resistor, R10, which is what you want to get the biggest output signal headroom. If things are drawing too much current, you'll want to do this, it also reduces recovery time in any coupling capacitors in the output, and this has one.

Choose your input coupling capacitor according to the pulse width from your detector. For slow tubes, I use .01uf, rated at a couple of KV higher than the actual voltage it sees, to reduce corona and leakage noise. If you've got a real fast pulse situation, drop that to 1/10 or so (1000 pf or less).

This is designed for negative-going pulses in and out. You may need a pullup on your output to keep the "resting" voltage at a binary 1 for your logic. This will drive TTL or CMOS, but likes CMOS better since it draws less current on lows and therefore doesn't put much charge on our output coupling capacitor. This is pretty important when count rates get high. There is provision for pullup and pulldown resistors on the board if you can handle soldering 0802 size components, but you can do this externally to your logic supply just as well if not better.

I use the DC coupled input with an external coupling capacitor for most things. The one place you don't need a capacitor here is with a phototube input with a floating anode and negative high voltage on the cathode and dividers. This works great if the dark current is very low (and it'd better be or you have a light leak somewhere - but measure that before hooking it up). This is also the time you use that pad labled "coax rtn" which is floating up at near 9v, since this is the input "signal ground" so this return won't capacitively couple noise into the input. Only connect one end of the coax shield - the board end.

For the output, ground is ground and it doesn't matter so much after all that gain.

This isn't purely DC coupled due to R4 and C3 - it's not meant to pass huge many-millisecond pulses, those components provide a bit of a high-pass action, which also reduces the 1/f noise in the transistors.

I try to put the battery right inside the enclosure with this and the sensor. This is just a noise pickup situation, though we do have a bit of a filter on the input power.
Beware corona noise from your HV detector supply and related parts, it can be as large as the desired signal! Rounded edges and glyptal or HV "dope" are the rule there.

These are so sensitive, the way I test them is to hang a scope on the output, about 1v/div, and simply hook a 1" wire to the input. touching or merely bringing your hand near it should produce very visible output. If there's a CCFL or LED lamp nearby, you should get 7-8 volt signal out from the noise it generates at the switching frequency of the bulb electronics. Obviously, if you're using a CCFL inverter for your detector HV, it'd better be in a separate box or a very well shielded compartment in the same box. I've used soldered copper flashing to make boxes here - works a charm if you solder all the overlapping joints tightly.
Small aluminum boxes work well if the power supply has its own box and is a few inches away (use coax to move the HV to the detector, and put 1k ohms or so in each wire and a bypass at the detector end) - this will eliminate ground loop noise and at the HV, where current is minimal, you can get away with large series resistors in these connections so your output coax to the counter provides the real system signal ground.

Choose your input coupling cap based on pulse width from the detector. For the Russian tubes I use .01 uf, rated at least 1 kv higher than what it really sees. This is to reduce noisy leakage across the capacitor. If you're got a really fast (narrow) pulse, cut that to 1000 pf or less (like from a phototube and fast scintillator where you might have only 10 ns pulse width).

As noted above in red, the tiny 680pf cap on the ac coupled input will not stand off HV - 50v or so is the limit. You really can't stand off much voltage at those tiny PCB spacings even if you could find a capacitor rated for that.

Clean is the rule - clean all insulators with alcohol or something similar, and don't touch them after that. This would include the coupling cap you use, any shielding around the HV series resistor and so on.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Location: Floyd county, VA, USA


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