But I used AD620 which is an instrumentation amplifier, not opamp. I thought comparators are made with differential amplifiers (from Horowitz & Hill Sec 4.23) and since AD620 was a nice available instrumentation amplifier, I thought it would work (and it does work).

But this particular circuit that I made seems to be less general than I thought. 100 kOhm potentiometer makes it hard to fine tune the threshold. Also, I think I should have buffered the threshold voltage divider circuit because I see the threshold level changing with the incoming signal when the signal is more than the threshold. It doesn't affect my particular application, but I think that makes this a crappy TTL generator. But since my purpose has been served, I'll push optimizing and generalizing this box to some other day. Maybe my new SMD prototype boards would be handy.