Amplifier can't drive 5,7 and 13 ohm loads - diyAudio
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Old 5th May 2013, 04:38 PM   #1
danny92 is offline danny92  Portugal
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Default Amplifier can't drive 5,7 and 13 ohm loads

Hi everyone,

I'm simulating an amplifier based on one of Bob Cordell's designs, but in the simulation, when I use 5, 7 and 13 ohm loads the output stays at 40V, I don't know what is causing this, for 4,6,8 and 16 ohm the output graph is ok, what's happening?
I'm using LTSpice.

Attached examples of the outputs, simulation file and Bob Cordell's models.

Best regards,
Daniel Almeida
Attached Images
File Type: png 4ohm_load_drive.png (24.8 KB, 161 views)
File Type: png 5ohm_load_drive.png (12.8 KB, 154 views)
Attached Files
File Type: asc MOSFET2_test.asc (8.7 KB, 12 views)
File Type: txt Cordell Models.txt (11.8 KB, 5 views)
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Old 5th May 2013, 04:47 PM   #2
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Well I never used Spice for simulation. I hash it out with pencil, paper, and calculator just like I was trained to do decades ago.

These results don't make sense to me. If an amplifier can drive 4, 8, and 16 ohm loads, but not 5, 7, and 13 ohm loads, then it will never be able to drive a real world speaker. There must be some kind of glitch.

Hopefully someone with Spice simulation experience can clear it up. My opinion is that Spice is wrong in this application.
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Old 5th May 2013, 04:51 PM   #3
danny92 is offline danny92  Portugal
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But the amplifier will work in the real world?
The circuit seems electrically fine.

Best regards,
Daniel Almeida
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Old 5th May 2013, 04:55 PM   #4
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Spice does funny things if inadequate accuracy settings are chosen. If you simulate for extended times, the automatically chosen timestep might be too large, causing the simulation output to be garbage.

Rundmaus

EDIT: Looked into your file. I wouldn't trust a simulation starting at time zero. Let it 'run' for 10-20s and start taking data then.

Last edited by Rundmaus; 5th May 2013 at 05:10 PM.
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Old 5th May 2013, 06:09 PM   #5
danny92 is offline danny92  Portugal
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Yes that did the trick,
The wave is fine
Thank you Rundmaus,
Best regards,
Daniel Almeida
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Old 5th May 2013, 06:25 PM   #6
jaycee is offline jaycee  United Kingdom
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you may also want to use the "startup" and "uic" options to see exactly what the circuit will do from power up. let it run for 5-10s like that and you will see any powerup oscillation and the effects of the input/feedback caps charging
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Old 5th May 2013, 06:31 PM   #7
danny92 is offline danny92  Portugal
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This efects could be harmful?
What is the directive directive for startup and uic?

Thanks for your help,
Best regards,
Daniel Almeida
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Old 5th May 2013, 07:07 PM   #8
djoffe is offline djoffe  United States
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you can invoke startup in the Edit Simulation Command dialog box, under the transient tab, click the box that says:

Start external DC supply voltages at 0V:

That puts the word startup at the end of the .tran statement


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Old 5th May 2013, 07:09 PM   #9
djoffe is offline djoffe  United States
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Here's the helpscreen (F1) on UIC;

UIC
Use Initial Conditions. Normally, a DC operating point analysis is performed before starting the transient analysis. This directive suppresses this initialization. The initial conditions of some circuit elements can be can be specified on an instance-per-instance basis. Uic is not a particularly recommended feature of SPICE. Skipping the DC operating point analysis leads to a nonphysical initial condition. For example, consider a voltage source connected in parallel to a capacitance. The node voltage is taken as zero if not specified. Then, in the first time step, an infinite current is required to charge the capacitor. The simulator cannot find a short enough time step to make the current nonsingular, and a "time step too small convergence fail" message is issued.
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Old 5th May 2013, 07:53 PM   #10
danny92 is offline danny92  Portugal
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Yes those comands after tran also works, but the problem is that I've no op.
If I use .op instead of .tran 8m startup uic it gives me wrong values, with .tran 8m stratup uic the transient is fine, but I have no operation point.
Is this normal?

Best regards,
Daniel Almeida
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