hmm.. input pulldown resistor effecting spice readings..

[ulysses]

just a quick question for those in the know..

when you are simulating your circuits in spice - do you leave off the input pulldown resistor?

im designing a circuit that has an opamp distortion as the first block and when i put the 1M input pulldown resistor on there in spice it basically removes all all clipping from the first distortion block..

is it me or does spice simulation not like pulldown resistors?

cheers
ulysses

[bioroids]

I think you probably have a mistake in the simulation circuit you are running. SPICE does not like or dislikes anything in particular.

Also, what are you using as the input source? If you are using the usual Voltage Independent Source then circuit will not behave as expected in real life, because the guitar pickup is not a voltage independent source.

Regards!

Miguel

[ulysses]

hmm.. im using a simulated 1v sine wave to test the circuit..

so does everyone leave pulldown resistors off their schems when testing with spice? or does it make no difference for everyone else?

cheers
ulysses

[Sir H C]

Use what is really there.  The simulator should be able to handle it, either there is a bug in the simulator or something is wrong with the circuit.

Remember 1M often is one milliohm and you need to use 1 meg.

[bioroids]

hmm.. im using a simulated 1v sine wave to test the circuit..

Then you are missing the output impedance of the pickup, and all the wacky stuff that can happen because of its inductance
I'm using this subcircuit in place of the Sinewave, I'm sure it is not perfect, but it will be a lot closer to the real thing.

(this works in AimSpice, I don't know about other simulators)
.SUBCKT GTRPICKUP 5 0
Vin 1 0 SIN(0 100mv 1500hz 0 0) AC 200mv
Lzout 1 2 4H
Rzout 2 3 5k6
*Control de volumen
Rvol1 3 5 1
Rvol2 5 0 500k
*Control de tono
Rtone 5 4 500k
Ctone 4 0 100nf
.ENDS GTRPICKUP

I'm sure the pulldown resistor should be there for precise simulations, it should cause any problems at all.

Remember 1M often is one milliohm and you need to use 1 meg.

In Aimspice I use 1000k to get a 1 Meg resistor. Can't remember why, but I think the M suffix caused me some problems when I started messing with this.

Regards

Miguel

[ulysses]

setting the pulldown resistor to 1000k fixed the problem right up.

thanks for those replies guys. i appreciate it.

cheers
ulysses