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John Stewart
 
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Default Low Level 45 Measurements

This AM I did some low level Noise + Distortion (SINAD) measurements
on an experimental 45 SET Amp I've been messing with now & then over
the past few months. The plot is over at ABSE. One page, about 250K.

Besides the several harmonics you will see some interesting things
going on at 300, 420 & 540 Hz. These enter the signal through the heater

leads along with the 2.5 v, 60 Hz supply. There is another barely
visible
at 180 Hz. This could have been better displayed had
I set the sampling rate higher.

They are caused by distortion of the supply in the power transformer.
If you look with a scope at your heater supply you will find both top
& bottom of what was a sine wave to be bit off.
That mainly by the HT rectifier.

The other responses you see just below the 3,4 & 5H are artifacts
called 'aliasing' caused by the digital sampling of the wave form.
For those familiar with the Superhet Receiver, the mathematics of
aliasing is identical with that of signal image response.

Even if you look directly at the power line (carefully) you will see the

mains supply is far from perfect. There is a lot of concern these days
for
the purity of our power systems, worldwide. A proliferation of
non-linear
loads such as FAX machines, PC's, Copiers, Etc. has
put all systems at risk.

While in sales one of the most important pieces of test equipment I
sold allowed measurements of power system harmonics up to the 50th.
That instrument is a product of Voltech out of the UK.

Cheers, John Stewart

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