Re: TL071 Offset Voltage

[Mooly]

The 78 and 79 regs might not be loaded highly enough to regulate correctly depending on the opamp current draw. Add a 2k7 resistor across the negative rail and add reverse biased diodes across each rail.

Use only FET opamps in that circuit and not NE5532 types due to internal offset conditions making operation of the pots noisy.

 

[Fast Eddie D]

Basically using an opamp as a comparator is pretty bogus at the best of times, and many opamps are not suitable for open loop at all. If you want a cheap but reasonable comparator, LM339 is the go-to-device. I'd suggest always choosing CMOS versions of the 555, they are much better behaved (no 150mA spikes on the supply for one thing!) 7555 is an example.

I know that op amps generally don't work well as comparators. They can never match the open loop performance (especially speed) of a dedicated comparator.

I haven't used the CMOS version of the 555. I haven't had any problems with the old fashioned ones and I keep resistors below 1 meg.

 

[nigelwright7557]

I had a similar problem with a op amp on a 1meg input impedance buffer.
I was getting about a volt on the input pin.
It turned out the input current was 1uA and 1uA with 1 meg is about a volt.
I used a TLV274 instead of a TL794 and all was right again.