LTSpice signal voltage measurement discrepancy
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Max999
diyAudio Member

Join Date: Jun 2017
LTSpice signal voltage measurement discrepancy

When I measure the signal in LT spice using the Ac Analysis for the schematic below I get 41KV coming out of U4. This seems to be impossible to me with a B+ of 300V. I am feeding the circuit 1V AC.

Using the Transient Analysis feeding it 1V at 1Khz I get an output of 300V peak to peak.

Does anybody know why there is such a discrepanty in outcome?
Attached Images
 preamp lt spice.png (180.4 KB, 74 views)

Last edited by jazbo8; 11th October 2018 at 06:49 AM. Reason: typo

 11th October 2018, 12:34 AM #2 voltwide   diyAudio Member   Join Date: Jan 2010 Without your asc-file uploaded here you will hardly get an answer.
 11th October 2018, 02:13 AM #3 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA The classic .AC analysis card assumes Perfect Linearity. It does not know anything about "overload". 1V input gives the *Voltage GAIN*, not the overload. A better way to see this: 1mV in gives 40V out. Yes, three stages of 12AX7 is likely to give gain of 40,000.
 11th October 2018, 05:12 AM #4 MarcelvdG   diyAudio Member   Join Date: Mar 2003 Location: Haarlem, the Netherlands That's the same question I had when I ran my first Spice AC analysis 30 years ago! PRR is correct, .AC is a Spice small-signal analysis that linearizes the circuit around its bias point. When the signals are not small, the results are unphysical, but the ratios are still the same as for small signals (1 uV -> 40 mV, so 1 V -> 40 kV and 1 MV -> 40 GV).
 12th October 2018, 02:52 AM #5 Max999 diyAudio Member   Join Date: Jun 2017 Case closed, thank you gentlemen for your help!

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