[sdiy] Does this look like phase reversal?

Roman Sowa modular at go2.pl
Thu Sep 7 13:18:38 CEST 2017

Maybe it's a trap to let the builder learn something, or maybe original 
design also had negative supply. Or maybe because it says 14.4V supply 
and you made it 12V, so voltage at [-] inputs is 1.55V in your case and 
1.88V in the schematics. Maybe that 0.3V is just enough to let those 
jFETs feel comfortable enough to make it work. Change that 15k resistor 
at opamp [-] input to 20k and see what happens.


W dniu 2017-09-07 o 12:41, Guy McCusker pisze:
> Thanks for the responses.
> I'm kind of annoyed with myself that the reason for the behaviour I
> was seeing only occurred to me after I left the house, and now I won't
> be back for a few days. Anyway, it's reassuring to hear that I might
> be on the right track, and very good to have learned something about
> what can happen when using these devices out of spec. In fact, if I
> get it working, it's better for me this way than if it worked first
> time.
> For clarity, I should add that Ken Stone now specifies a TL074 in that
> location -- I wasn't just wantonly using the wrong device. I wonder
> why he changed from the LM324 to that? I guess he must have had some
> TL074 that worked in there or he wouldn't have done so. Presumably the
> behaviour of ICs when used out of spec is... unspecified?
> Cheers,
> Guy.
> On Thu, Sep 7, 2017 at 10:55 AM, Roman Sowa <modular at go2.pl> wrote:
>> Youre driving TL074 out of working range, and you know it. Ignoring
>> datasheet's limits intentionaly is one way to get into troubles.
>> P-FET inputs of TL074 have no space to bias properly at such a low voltage,
>> so they simply conduct from source to gate, polarized like a diode. So this
>> is where you get 0.88V at 100k resistor.
>> LM324 is designed to work with inputs going down to negative supply, TL0x is
>> not. Use LM324 or any rail-to-rail input opamps.
>> Roman
>> W dniu 2017-09-07 o 10:23, Guy McCusker pisze:
>>> Hello,
>>> I'm looking for some help troubleshooting the simplest circuit I've
>>> built in ages, a CGS28 sequential switch.
>>> http://www.cgs.synth.net/modules/cgs28_seq_switch.html
>>> There seems to be trouble in the input stage, which is a TL074
>>> configured as four comparators. I've been poking around the clock
>>> input (pins 3, 2, 1 on the schematic.)
>>> The TL074 is powered from +12V and 0V. Pin 2 (-ve input) is biased at
>>> 1.55V and measures fine. With no input CV, pin 3 (+ve input) is at
>>> 0.88V; I can't see where that is coming from and seems way too high to
>>> be input bias current with the 100k resistor to ground. (It goes away
>>> when I remove the IC from the circuit so it's not coming from an
>>> obvious construction error elsewhere.)
>>> With those inputs, the output on pin 1 sits at about +11V. But it
>>> should be negative i.e. near 0V, so something is wrong.
>>> When I apply a 0-5V square wave to the input, I can see it clearly at
>>> pin 3, and the output goes between 11.4V and 11V, with very narrow
>>> spikes to about 0V at the transition points.
>>> Does this sound like phase reversal in action? Or something else? And
>>> would that explain the apparently excessive input bias current as well
>>> as the weird outputs?
>>> I've tried a few different TL074s from different batches, with similar
>>> results, though the actual output voltages differed a bit. i noticed
>>> that the TL074 data sheet specifies common mode input down to the
>>> negative supply + 4 V, so the inputs here are certainly out of range.
>>> Ken Stone originally specified an LM324 here which should be ok with
>>> input down to 0V. Should I try one of those?
>>> Alternatively I might have messed something else up --- if you have
>>> any ideas I'd love to hear them.
>>> Annoyingly I won't be able to tinker with this for a couple of days
>>> but the benefit of your expertise will be much appreciated.
>>> Cheers,
>>> Guy.
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>>> Synth-diy at synth-diy.org
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