[sdiy] Sequencer good (and bad) news update

[Tony Clark]

> I used a dual LM324 inverting amps in series for each of the stage outputs
> to sum the voltages coming out of the pots and get them up to 0 to 12 volt
> output.  Each op-amp pair is getting a signal from the pots, the correct
> pots, yet none of the summing amps are passing a signal out of either the
> final exit or from the output of the first series op- amp into the last.
> 
> This points to three things:  1) Either there's a wiring error, 2) the one
> LM324 is a dud (two banks, two op-amps in series per bank, four op-amps
> total = one LM324) -or- my design in this area is bad.  Highly likely.  Ever
> more so in that both op-amp pairs are behaving exactly the same.

> 1) Unit runs off of ground and +12

   More than likely this is where your problems are occurring.  Since the 
pots are putting out 0-12V and your op-amps are running off of 0-12V, did 
you remember to properly bias the op-amps to 6V (ie, put 6V into the + 
input of each op-amp) ??
   Also, LM324's and even TL08x series op-amps are NOT going to output a 
full 0-12V range.  Most likely a LM324 is only going to put out something 
like 1.2V-10.8V and an TL08x is more like 0.9V to 9.1V (or something 
close to that).
   The reason for that is that there are transistor (diode) drop voltages 
at the "rails" (your power supply voltages) that prevent the output from 
ever reaching the positive or negative supply rail.  In order to do that, 
you will need to use a special "rail-to-rail" op-amp, which will get you 
closer to the rails, but even then, not quite.
   This is typically why we all use bi-polar supplies and also run 
voltages _higher_ than we need to give the op-amps some headroom.

   Hope that helps,

   Tony

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