Quad 303 simulation models

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26-Mar-2020 02:48 AM

JFB

JFB

Posts: 11

Hi eveyone,

In my attempt to fix a dead 303, I did a quick search online for a simulation of the circuit.
Didn't find anything so I made a model of the schematic using Circuit JS to better understand the workings of it (and teach myself electronics since I am as green as a golf course). Circuit JS is a small applet available online and in a downloadable version for Windows. There is a good amount of basic circuits available from a menu and it lets you built your own from scrach.
Maybe there is numerous models and better simulation environment available and I'm just an idiot ... but I thought I'd share that anyway. Happy if this is useful to anyone. Feel free to improve the circuit and correct mistakes I could have made.

Circuit JS page : https://www.falstad.com/circuit/

Here is a link to download two 303 models along some notes below : https://drive.google.com/file/d/17g-osgLRDQviPoHqJRhJCkUvueNGwGtz/view?usp=sharing
The files are .txt and can be opened directly from the desktop or web applet.

Model 1 - Quad_303-Realtime
Model 2 - Quad_303-1kHz_signal

General notes :
- I am running the desktop version of the app and it is using a good amount of CPU ressources when running. I'm guessing the circuit is a little heavy to run. I tried it quickly in the online applet and it seems to work fine
- WATCH OUT : You can zoom in/out the circuit workspace using the mouse scroll. The scroll can also be used to change components value when hovering over them. If not careful with the pointer, you might change a couple of resistors value without notice and then wonder why the whole thing doesn't function the way it should. I happened to me and it is kind of annoying. There doesn't seem to be a way to "lock" the circuit to prevent that.
- The model is the "upgraded version" (Dada kit). I inserted a 10 ohms resistor along with the PSU cap because the app requires that.
- I inserted switches for power to unit and to board, voltage test points (TP) at all pins and two VM on the "1 to 9" and "1 to 5".
- There is DC90V on the bridge, as this is what I'm measuring on my unit. It can be changed of course.
- RV100, 101 and 200 are regular resistor modulated to get roughly the expected readings on the two VM and a bias around 9.3mV. You can adjust these very precisely by right-clicking/EDIT. The model values are : RV100 = 2.5K / RV101 = 765 Ohms. ** Note that RV200 is set to 19K (?) which is way above the 4k7 trimpot. I had to set this value to get 67V on pin 1 to 9. There might be an error in my PSU model. Let me know if you figure that one out **
- You can set HFE for transistors. Default is 100. I adjusted only the following based on my readings : Tr1&2:50 / Tr3:70 / Tr200:175 / Tr201:50
- I appears you can use an actual audio file as the input. Haven't tried that yet.
- There are two "onboard" scopes showing the voltage readings of TR1 and 2 (usefull for model 2 only).

Model 1 : 
Named Realtime because the time step is set somewhere close to "normal". The circuit has no input or output and therefore corresponds to a "setup" condition. Real time has been useful to me when trying different malfunction scenarios (ex : bi-passing a transistor to simulate a short-circuit) as I didn't have to wait a minute or more for voltage to reach a new equilibrium ... if that makes any sense. 

Model 2 :
Includes a 1kHz/0,5V input signal and an 8 Ohms load at the output to simulate speakers. There are two scopes at the bottom showing the in/out signals and actual amplification. I modulated the time steps and visual current speed based on the 1 kHz signal to see the current flow through the circuit. Seing this schematic at work gave me a better idea of what was going on in the amp various components. If you change the input frequency, the app should ask you if you want to adapt the time steps accordingly. Any drastic component change to this circuit will take a fair amount of time to stabilise so better use Model 1 for troubleshooting.

Hope this can help someone :-)

Best,
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26-Mar-2020 07:53 AM

EJP

EJP

Posts: 1742

Very good, thank you very much!

EJP

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26-Mar-2020 04:56 PM

Ton

Ton

Posts: 69

Hi JFB, 


Thank you for your schema's, great work.


I recommend all readers to look at the Falstad simulation, it is great simulator to try out circuit diagrams and to discover the pleasure of electronics. 


I have been using it for some time and it is one of the best real-time simulations with lots of options and example circuits.


I always use the online version because it reacts snappier than the offline/desktop version.


The test points, scopes and amp/volt meters are great tools and give a good insight of what is happening.


To see all example circuits in one go, go to http://www.falstad.com/circuit/e-index.html 


One very very small drawback, the simulator uses a kind of default transistors, fet's, diodes and zeners, you can model some parameters, but you can't set all parameters of those components. 


In some cases that gives strange results.  


A tip, try a SPICE based simulator, there are a few free versions available. Look at https://en.wikipedia.org/wiki/Comparison_of_EDA_software 


Until this week, Micro-Cap 12 was available for free download, normally priced at around $ 4500, but last year it has been made available for free because Spectrum Software stopped development. 
At this moment it seems that the Spectrum Software website is down (This Account has been suspended), hopefully it will come back online but I fear it is gone for good.


But, for now, on the internet archive, the wayback machine, you can find a copy of the Spectrum Software website dated jan 4, 2020 and you can download Micro-Cap version 12 from there.


Pick the cd version and grab the manuals for as long they are available.


But kudo's for your work !


Ton
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26-Mar-2020 05:11 PM

Ton

Ton

Posts: 69

Archive of Micro-Cap website at the Wayback Machine (archived January 11, 2020)

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27-Mar-2020 03:43 AM

EJP

EJP

Posts: 1742

Linear Technology's LTSpice is free and under maintenance. I find the UI rather clunky myself compared to some others, but plenty of people use it professionally.

EJP

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27-Mar-2020 12:39 PM

JFB

JFB

Posts: 11

Hi EJP, Ton,

Thanks for the suggestions. I will definitely take a look at those. Looks like the post generated a bit of interest so I'm happy.

Best,

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