Ludlum preamp design

Linear and non linear

Ludlum preamp design

Postby Doug Coulter » Sat Oct 02, 2010 3:07 pm

Here is another topology that obviously works well for low level signals from proportional tubes, I've touched and used this -- works great. Nowadays, one might use a chip instead, but this is just as good as any chip I know of for the job here. Here is the schematic from the manual.
LudlumPre.gif
Ludlum low level amplifier


HV power is fed to the terminal marked "SIGNAL" via a 100k resistor, elsewhere on the schematic, after post regulation filtering through another 100k and .01 uf cap.
As a proportional tube is really just a current source but has a fair amount of voltage compliance, our first coupling cap isn't much of a highpass filter, but obviously it gets rid of the DC HV.

Following that, we have a 10k current limiting resistor and the mandatory protection diodes to keep things inside the rails no matter what some dumb scientist does (like hot plug a tube
with a large cable capacity with zero charge on it). This is followed with an emitter follower, so as not to load the signal input too much. It has a very low effective output impedance, so with the 4.75k on the other side (which is a virtual ground) this capacitor coupling IS a good high pass filter, which my handy slipstick shows at about 35 khz or so.

The transistor matched pairs are wired so the bottom one is a darlington, and the top (pnp) are a current mirror source used as the collector load for the darlington, making a very high impedance node there (which probably has some low pass effect due to strays). This is followed by another emitter follower to drive the comparator input. a 200k resistor closes the feedback loop for a gain of 20k/4.75k or 42.1. Hey, we just made an opamp and closed the loop! The setup self biases to about 2 Vbe plus a little, which isn't critical here. We then capacitor couple, with a little HP action to the comparator input. The other side of the comparator input comes from a ten turn pot used to set threshold. Thus when their front panel says the threshold is 2 millivolts, it's truly set at about 84.2mV post preamp, which is a good way to do this. We have changed a negative going input pulse into a positive going output one along the way, so the comparator doesn't need to have common mode range below ground as well. So, a nice job, not too many parts. The comparator used isn't anything special (.2us response) but is low power, and this way, we don't have to care as much about it's fairly lousy input offset voltage spec.

This is followed by a little jiggery-pokerie with retriggerable/lockable one shots I assume to prevent either too narrow pulses or double pulsing -- I've not analyzed that yet. The rest of the box is fairly normal stuff, a uP, a counter display and basic power supply. This particular model doesn't seem to have a timed count feature, which would upset me if it were mine (instead of being Chad's). Perhaps the rs-232 output gives that.

Though many do, I'd not call this a "charge sensitive" amplifier, it's just a voltage amp with NFB to set gain. Whatever, it does work quite well in the intended application.
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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