Simple Op-Amp Schematic Problems

[icydash]

Hey guys, I'm trying to make a simple op-amp gain (non-inverting) circuit with a stomp switch and LED, controlled by a single volume potentiometer. I think i created the circuit as correct as is possable (see link below) but it keeps crashing my MultiSim. I've been messing with this all day and for the life of me I can't figure out why there are issues with my circuit. Please help me! I've been modifying and playing with values and moving things around for like 10 hours and can't figure out what's wrong and I know its going to be some dumb small thing.

You can find the schematic here:
www.tech-area.com/h/techarea/vcirc.bmp

[gez]

All your resistors are a little on the low side.

You only have 500 Ohms input impedance (two 1k resistors - the divider - in parallel).  Connect a large value resistor (470k min) between the + input of your op-amp (marked 3 on your schematic) and the centre of the divider (marked 9 on your schematic) and couple the signal through to the + input.  Would be an idea to increase the values of those 1k resistors, too.  Say 22k - 47k.  Decouple the junction.

If you've already commited to PCB, or don't have the room for the above modification, just increase those 1k resistors in the divider.  If your amp is FET input, or bipolar input with low input-bias current, then try 1M resistors.  Failing that, min 200K.

Personally, I'd make the feedback resistors 10X larger (and the 10u cap 10X smaller) too.

[Dragonfly]

in addition to what Gez mentioned, it looks like your input cap is oriented backward.

[Paul Perry (Frostwave)]

All good suggestions, but nothing there that would stop multisim crashing, I would have thought.
I don't do any simulation myself - I'd like to, but I'm not really a computer kind of person & it's all a bit tricky for me.
But, i have heard that simulation software can be quite irrational and buggy at times, even the order in which the parts are numbered can bring on a crash or not.

[gez]

I tend to use 'Unisim' (a breadboard)! 

[MartyMart]

I tend to use 'Unisim' (a breadboard)! 

I'm a fan of "Vero-sim" ( stripboard )   

[Dragonfly]

I don't like "-isms" in general.

[Filament]

"A person should not believe in an ism - he should believe in himself. I quote John Lennon: "I don't believe in Beatles - I just believe in me". A good point there. Of course, he was the Walrus. I could be the Walrus - I'd still have to bum rides off of people."

Andy was that a nod to Ferris Bueller?

[Dragonfly]

Andy was that a nod to Ferris Bueller?

[Sir H C]

My only guess is that there is no DC path from the AC source at the input.  Try paralleling with a 1k resistor to ground.  What do the errors say, matrix is singular?

[PerroGrande]

I was able to replicate your problem on MultiSim.

For whatever reason, the simulator does *not* like switching the power to the op-amp while the sim is running.  It will occasionally work, but it always crashes eventually. 

When I wired the op-amp to always be on, the circuit worked as expected in the simulation.

[George Giblet]

I don't use multisim so I don't know it's quirks but I'd be putting a 1M resistor across the DC supply.

Also in bypass mode your circuit switches the input signal across the pot you should switch it to the output terminal.

[icydash]

Ok I've made some changes and it's the weirdest thing in the world...When I wire the circuit as shown in the diagram linked below, everything works perfectly. But when I attatch the 9v VCC that's powering the op-amp to the 9V DC source like i had it in the origional diagram, everything breaks... I must be missing something that's happening somehow between because multsim does NOT like it when I may that connection. Thanks so much for all your help so far everyone! I havn't committed to a PCB or anything yet so any changes can still be made.

the new schematic:
http://www.tech-area.com/h/techarea/vcirc2.bmp

[gez]

Your 10u input cap is still the wrong way round (+ pin should connect to + input of the op-amp).  Also, now that you've upped the input impedance, it no longer needs to be so large.

[icydash]

Ok i made the input capacitor 1uF-unpolarized so it shouldn't matter which way it's connected.

...Still the same issue though, when I connect to the VCC input of my op amp its own 9V source as seen in the second circuit link i posted, everything works great. But if instead i attatch the VCC op-amp input to the point above the 1Mohm voltage divider which should also be 9V, the whole circuit breaks and multisim crashes...

[gez]

I'm approaching this purely from a design perspective (I know nothing about simulators).  Why would you wish to turn power off when bypassing the booster?  When you reengage it, you'll get a nice loud clunk as the circuit biases up - unplug/reconnect power to an effect going to your amp to see what I mean.

[Paul Perry (Frostwave)]

Someone once complained to Bob Pease (chip designer at National Semiconductor) that one of Pease's chips did not behave properly in a simulator, though it worked perfectly when the actual circuit was soldered together.
Pease replied, "Well, suppose it DID work in the simulator, but not when you built it - who would you complain to THEN?"

[icydash]

well the idea was that when the circuit was in bypass mode then the op amp wouldn't draw any power and thus would save a lot of battery power then if it was basically always on....


lol now-in-days simulators are pretty accurate, most of the time if there's a problem in the sim i've found it to occur in real life as well...but i appreciate everything!

[icydash]

sorry i meant to say "a lot more battery power than if the circuit was always on"

[Sir H C]

Someone once complained to Bob Pease (chip designer at National Semiconductor) that one of Pease's chips did not behave properly in a simulator, though it worked perfectly when the actual circuit was soldered together.
Pease replied, "Well, suppose it DID work in the simulator, but not when you built it - who would you complain to THEN?"

Pease Rules.