First of all, thanks for reading this, I tried to replicate this research as my project (not aim to publish or anything just for study purpose)
Reference: https://www.iasj.net/iasj/download/ceca8bad4ee05e7c

Now I tried to guess and do what I can do about this circuit and the result is this (Multiple feedback)
the magnitude of the decibel seems weird and wrong compare to the original in the reference
I couldn't find the UA748cp spice model so I downloaded UA748 instead and moreover is that inside the file UA748, the name of the model is UA776H, so I guess it has a similar property?
I don't know what I'm wrong with this because I'm new to this kind of topic. I would really appreciate it if you could guide me. Thank you


This is the graph from my simulation in LTspice XVII


This is the graph from the original one


I also wonder if anyone know what this (!) means?

 

hi Yourint,
Welcome to AAC,
Please post your UA748 model file.
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Ah my bad, here it is ericgibbs

 

hi Y,
I created my own from a the web ,made a Auto Gen model [ not very tidy, but it works]
Used AC as 1v in order to the correct plot.
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Tidied up..

 

One more thing I would like to ask is when I run the simulation in LTspice VXII the program keep processing for a very long time and it doesn't seem to stop, so normally I would click "ESC" to stop the process and the graph will show up. This won't affect the result right?

 

The reason the input needs to be 1Vac is that LTspice references all dB values to 1V (1V is 0dB).

 

hi Y,
My sim is instant, tried both Alternate and Normal.
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Eric, your attachment in post #4 gives me an error.
Does it work okay for you?

 

Why did you use a UA748 rather than one of the op amp models in LTspice?

 

Hi Carl,
It runs OK,
Here is the asc file
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Update:
I could not find the UA748 model in my LTS.???
Will look more closer..

File search in Win10 of my LTS folder for UA748 shows 'not found'

 

The resistor values are much too low for the very old uA776 opamp that has a low output current. It and the very old uA748 opamp have slew rate limiting of the high frequencies causing distortion.
Repeat the simulation with higher resistor values (reduce the capacitor values to match) and with a modern opamp.

 

It doesn't work for me, but the result is fine, so never mind. Thank you so much both of you ericgibbs&crutschow. I will continue my work on Sallen-Key if there are any problems, I may ask it later. Don't worry, I will try to solve it by myself first. (Audioguru again Thanks I will try that later)

 

hi Y,
Hope that Carl can come up withe reason for it not working outside my PC, I would like to know if there some error.
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Using Win 10 32 Bit , LTSXVII

 

Why did you use a UA748 rather than one of the op amp models in LTspice?

Ummm... honestly, I just can't find the UA748cp spice model, so I just assume that UA748 can replace UA748cp (._.)

 

Hope that Carl can come up withe reason for it not working outside my PC

Works now.

 

I just can't find the UA748cp spice model, so I just assume that UA748 can replace UA748cp

Not the intention of my question.
My question is, why use the old 748 op amp in any form?

 

Not the intention of my question.
My question is, why use the old 748 op amp in any form?

I'm quite new to this kind of thing, so I just look at the reference and I saw they use UA748cp so I just replicate them and try to get the same result.

 

hi Y,
Have you now solved the query or do you require further help.?

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Somehow, the graph from the simulation of the Sallen-Key are quite weird, do anyone know what is the problem for this graph?
This is from my simulation in LTspice XVII


This is from the reference


or maybe... do I need to swap capacitor and resistor in the 2nd stage?

 

hi Y,
Please post you calculations for the problem Sallen.

Check you component calculations with reference to this link, lets know the difference.
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https://www.electronics-tutorials.ws/filter/sallen-key-filter.html

also

http://www.emagtech.com/wiki/index....Lesson_2:_Designing_Active_Sallen-Key_Filters