Unfamiliar (to me) opamp configuration

[nocentelli]

I came across a schematic for a PLL guitar synth circuit, part of which is shown below. There is a standard single transistor input stage before IC1a, but IC1b looks odd: It looks like an inverting stage with a gain of 10 (100k/10k) but with the +/- input labels reversed. Is it likely to work as drawn, or more likely a mistake in drawing up the schematic? The output goes to a 4046 for the synth sound, or direct to out for a straight fuzz.

[TejfolvonDanone]

It is called a Schmitt trigger or hysteresis comparator. It has a positive feedback so the op-amp will saturate depending on the voltage on the input. It has different thresholds for rising and falling edge. Here's a little article about it: http://howtomechatronics.com/how-it-works/electrical-engineering/schmitt-trigger/

So in an effect pedal it is a really harsh fuzz sound.

[TejfolvonDanone]

The thershold points are +/-V+*(R13/R14) + Vref = 5.3V and 3.7V for 9V supply and 7.1V and 4.9V for 12V supply.
EDIT: I really messed up the calculations. And i calculated without taking the saturation voltage into account.
So with 9V supply your op-amps can output from 1 to 8 V (and about 1.5V to 10.5V for 12V supply). So to calculate the real thresholds you have to put 3.5V (and 4.5V for 12V supply) to V+ and 4.5V (or 6V for 12V supply) to Vref. (You can think of a 9V single supply as a +/-4.5V supply and for a 12V supply think as a +/-6V supply.)
The real values are:
9V supply: 4.81V and 4.18V
12V supply: 6.41V and 5.59V.

[antonis]

So in an effect pedal it is a really harsh fuzz sound.

More likely an ON-OFF switch fuzz...

[TejfolvonDanone]

So in an effect pedal it is a really harsh fuzz sound.

More likely an ON-OFF switch fuzz...

To be precise it would be a square wave or (surprisingly) a Schmitt trigger fuzz.

[antonis]

To be precise it would be a square wave or (surprisingly) a Schmitt trigger fuzz.

Hmmm..

Now you put me in throuble of testing a voltage comparator as base driver of a BJT amplifier for a staccato fuzz..

[TejfolvonDanone]

A Schmitt trigger is a better idea because noise won't trigger it's input unlike a simple voltage comparator. The only drawback of a fuzz like that is that it's really gated: if the input isn't high enough you can't get any sound out of it and as the note fades out the sound stops suddenly instead of dying off. Maybe an envelope controlled gain stage would help. Seems like an interesting idea for another pedal but i fear we've already gone really offtopic with this little conversation. 
Let me know if you have any results though i had an idea to create a fuzz like i mentioned before but i have so many ideas like this that i don't have the time to actually breadboard them all.

[anotherjim]

Any non-inverting amp can be a Schmitt trigger. Any non-inverting logic gate too. You only need to know that the smaller the input resistor compared to the feedback one is, the more sensitive it becomes. If those resistors were equal value, the input would have to exceed the output voltage swing of the amplifier to switch the state.
In theory, it should be better than a comparator at reliably turning the guitar signal into a square wave. In practice, guitar can swell slightly during decay and the trigger toggles intermittently in the final moments before silence.

[nocentelli]

Thanks guys. I now recognise the schmitt trigger with the positive feedback from the many LFOs I have built but obviously didn't recognise it in the context of an audio circuit. The extreme gating fuzz seems about right to approximate the on/off, triggered nature of a synth: I'm pretty sure it's not designed for playing a smooth responsive, tastefully overdriven jazz-rock solo.