Hi Stefan
I thought about your 9kHz PA efforts when I found this class D PWM
chip capable of ~300W up to 100kHz with very low distorsion.
Amazing performance from such a small $6 SMD chip!
http://www.ti.com/lit/ds/symlink/tpa3251.pdf
73 de
Johan SM6LKM
Den 2016-04-23 kl. 18:51, skrev DK7FC:
Hi all,
Since about 16 UTC i'm transmitting on 8270.00500 Hz again. The
antenna
current is about 200 mA. It is a first test over a longer time
with my
new PA.
I have been off the air for a few days now becuase i destroyed my
audio
amp that i used as a VLF PA. Somehow i started to like the idea of
a
clean sine wave coming from the PA. Low pass filters are hard to
design
for VLF, you know. Since i don't like buying and since i didn't
find
another one in the trash, i 'had to' (actually wanted) to
build
my own. It is a class-AB PA now running at 20V DC using 2x
IRFP064N
against ground. Opposite phase drive signals are generated by a
small
blue ferrite core with 20 quadfilar turns. On the primary side, 2
of
these 4 windings are in series. On the scope, the signal looks
quite
good. The 3rd harmonic is at about -20 dBc, certainly improvable.
But
it's just version 0.2 now ;-)
A large ferrite core is used to up-transform the voltage to about
220V
rms for this power level (i'm not spot on the resonance), also
providing galvanic decoupling.
A photo can be found at
https://dl.dropboxusercontent.com/u/19882028/VLF/20160423_184046.jpg
https://dl.dropboxusercontent.com/u/19882028/VLF/20160423_184059.jpg
The PA consumes 3.6A at 20V DC now. Improvements will follow...
The signal will stay on air for a few hours now.
73, Stefan
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