# Example simulation for power amplifier in LTSpice - what to measure and how?

#### pbilous

Paid Member
Hi all,

I was wondering if there is a definitive guide on what parameters to measure when simulating audio power amplifier?

I have found an example called 100W in LTSpice examples folder, but not sure how it runs and what it measures.

In particular, line by line here it is:

.tran 10m
I know that this is transient response with 10m seconds length.
It looks like the Vin amplitude is stepped by some value because when I run the simulation I see multiple sine outputs.

These lines probably have to do with this something:
.param V=1.44 ; 100W RMS
.step oct param V 1m 1.44 2

.four 1K V(out)
I know that this runs fourier analysis to calculate THD. To see report I go to View-SPICE Error Log.
But there are two values shown:
Partial Harmonic Distortion: 0.001013%
Total Harmonic Distortion: 0.014318%
Should I look at Total HD?

;.ac oct 50 1M 10Meg ; plot V(out)/V(out1) to see 72° phase margin @ 950kHz
This is confusing as I am not sure this is measured at all?

;.dc temp -55 125 1
This plots temperature vs voltage at out - basically shows how the offset depends on the temp.

;.noise V(out) Vin oct 10 10 20K
Running this command plots noise vs frequency. Noise is in nV/Hz1/2.
How to convert to dB?
What about signal to noise ratio in dB?

.options plotwinsize=0 numdgt=15 noopiter
not sure what this does...

What about measuring OLG (open loop gain)?
Or any other measurements for an amp?

Looking for help from more experienced folks here.

Thanks

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Paid Member

#### Mooly

Paid Member

You normally need to run a sim for longer than 10ms to get a good FFT result and you also often have to 'take out' time constants by temporarily making any electrolytics such as coupling and feedback caps much much larger.

A lot of info is in the LT thread. Unfortunately the images keep breaking following the move to the new platform: