Valve Wizard U-Boat question

[bushidov]

I was looking at making a Valvewizard U-Boat on a breadboard, just for kicks using this schematic:
http://www.valvewizard.co.uk/uboat.html

I was looking over the section that U4 is in, which is where he uses a dual low-noise op-amp as a pair of comparators. I was wondering what it would take to make that section more compatible with an actual dual comparator chip, like an LM393.

I see that his notes at the very bottom state that:

Note that a real comparator is not suitable as it will switch too fast and cause many glitches.

Just for educational purposes, is there an easy way to resolve this issue?

[merlinb]

Just for educational purposes, is there an easy way to resolve this issue?

You could try hanging a capacitor off the output of the comparator to slow down the slew rate. Experimentation will be needed. Not sure why you want to use real comparators though?

[anotherjim]

The opamp outputs will swing High when +in is more +ve than -in. A comparator like the LM393 turns its output BJT on in that condition which will pull the output Low and also needs a pull-up resistor (10k?) to take it High when the BJT is off.
So the comparator chip will be inverting - easily fixed by swapping the +in and -in pins over.

[iainpunk]

i thought that a comparator only activates the transistor when its in the ''off'' state, which is counter intuitive, i guess...[i was wrong]

this is the schematic of the LM393/LM339
immagine that the voltage on (+) is higher that the (-) (which would dictate an ON state)
current through the mirror goes higher
the output of the Long Tailed Pair goes low because the input of the next transistor is pulled down below the 0.6v
the current from the rightmost current source can only go through the output transistor, making it pull down.

in conclusion, when using a comparator as an opamp, ignore the original input markings and reverse them.

cheers, Iain

[anotherjim]

Yeh, the positive comparison result turns the output on - which is positive logic in a way, but it results in a low! I'm told there are comp's with TTL style output that do output a High -  never had one to play with.