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Old 28th April 2015, 01:01 PM   #1
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Default LTSpice Simulation Problem

I have used LTSpice to simulate two phono ampsí I have built (and work!) as a means of getting familiar with the software. Both circuits have a DC servo to remove any offsets. When the simulation is run (ie DC op.), it will not complete the action and I have to stop the calculations (the lower left corner of the screen shows something is going on!) If I remove the DC Servo element of the circuits both will simulate correctly and the results as expected.

So why can't I run the simulations and why does the DC Servo have this effect?

One of the circuits is the HPS5.1 (see here)
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Old 28th April 2015, 01:24 PM   #2
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Originally Posted by fasterbyelan View Post
I have used LTSpice to simulate two phono ampsí I have built (and work!) as a means of getting familiar with the software. Both circuits have a DC servo to remove any offsets. When the simulation is run (ie DC op.), it will not complete the action and I have to stop the calculations (the lower left corner of the screen shows something is going on!) If I remove the DC Servo element of the circuits both will simulate correctly and the results as expected.

So why can't I run the simulations and why does the DC Servo have this effect?

One of the circuits is the HPS5.1 (see here)
Do you mean it takes longer than you are willing to wait? A servo introduces a feedback loop and feedback loop simulations can take a long time to complete.
How long did you wait?

Jan
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Old 28th April 2015, 01:40 PM   #3
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Do you mean it takes longer than you are willing to wait? A servo introduces a feedback loop and feedback loop simulations can take a long time to complete.
How long did you wait?

Jan
Yes, I did think that was the case and left it running for 30 mins once before ending the sim. Maybe I just need to be more patience!?
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Old 28th April 2015, 01:47 PM   #4
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Yes, I did think that was the case and left it running for 30 mins once before ending the sim. Maybe I just need to be more patience!?
I've had sims taking several hours, so maybe fire it up tonight before going to sleep
There are some things that take a long time, like very small timesteps, long sim time periods (obviously) and circuits that are close to oscillations (this forces the sim to use very small steps).

Jan
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Old 28th April 2015, 01:50 PM   #5
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I've had sims taking several hours, so maybe fire it up tonight before going to sleep
It's quicker when you use a soldering iron.
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Old 28th April 2015, 01:56 PM   #6
Mooly is offline Mooly  United Kingdom
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If you're willing to post the .asc file we could take a look. I've simulated servos in some of my power amps and they complete in just a few seconds.
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Old 28th April 2015, 02:21 PM   #7
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Originally Posted by jan.didden View Post
I've had sims taking several hours, so maybe fire it up tonight before going to sleep

Jan
Thanks, will do and let you know how I get on.
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Old 28th April 2015, 07:23 PM   #8
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I ran a sim on the HPS5.1 (DC op.) and this did complete after about 30 mins. The DC voltages are as expected with the servo in place

Now running an AC sim which is taking much longer, 2 hours and counting.......

Thanks for your advice Jan.
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Old 28th April 2015, 07:36 PM   #9
jcx is offline jcx  United States
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.AC don't usually take any noticeable time at all - likely its the operating point calculation having convergence problems

sometimes the operating point calc has to be "helped" with initial conditions, nodeset statement or even doing a .TRAN 1st and saving the settled operating point values

you really shouldn't jump to .AC until after you have wrung out the circuit sim in .TRAN

Last edited by jcx; 28th April 2015 at 07:41 PM.
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