Spice & input impedance?

I just want to ask something about spice and input impedance.

I'm creating eq circuit with ltspice.It's ok.Simulating perfectly.But when i try to add "rser",everything is changing. rser is input impedance right?

For example

In this circuit everything is ok without "rser".

http://i.hizliresim.com/663pG0.png

But when rser is 10K (cause i'll use 10K:10K transformer),i'm getting gain loss about 20dB and changing freq response.

What should i do? What will happen if i don't use rser?

no, rser is not the input impedance. if you want to simulate a transformer you have to couple a pair of inductors in spice.

rser is more like the cable resistor in you circuit and 10k of course will turn your volume down half because it cause a voltage divisor with the 10k pot.

usually you don't have 10k feeding a 10k:10k transformer,  usually is far less, 50 ohms maybe, but you want to see a big 10k input impedance to not lose signal in the cable.

12afael said:

no, rser is not the input impedance. if you want to simulate a transformer you have to couple a pair of inductors in spice.

rser is more like the cable resistor in you circuit and 10k of course will turn your volume down half because it cause a voltage divisor with the 10k pot.

usually you don't have 10k feeding a 10k:10k transformer,  usually is far less, 50 ohms maybe, but you want to see a big 10k input impedance to not lose signal in the cable.

Click to expand...

Thanks! Now i can understand.So my design is OK,right?When i build this circuit with 10K:10K input transformers,i won't loss gain.And when i design circuit,i don't have to add rser,right?

http://i.hizliresim.com/663pG0.png

This type of EQ (in fact most EQs rely on being driven by very low impedance (rser).  Otherwise, frequency response will be wonky.

The better ones buffer the EQ input with a voltage follower so what you feed it doesn't change things.

I am not a spice guy, but Rser sounds like the source impedance of the preceding stage characterized as a simple series resistance.

Driving directly from an op amp could approach 0 Ohms at LF.  Accepting input from the output of a typical product could be anything from 50 ohms to a couple kOhms.

I was once bench testing a small mixer from a competing mixer maker that will remain nameless. They did not have a buffer on the input from the channel insert jack going to a Baxandall tone control so when I switched the generator source impedance between 50 ohms and 600 ohms I could measure a frequency response error, depending on the tone control EQ setting.

YMMV but 10k is too high, 10 ohms probably too low unless inside a chassis driven by a buffer.

JR 

rser is the serial equivalent resistor of the source generator. so yes, it is the source impedance. but input impedance is a different thing.

Thanks to everyone!

Now i understand everything about spice design.Impedance was problem for me with spice but now it's okay!