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