10th November 2010 - 9kHz Transmitter. If my Ofcom SRP application is successful my first transmitter for the 8.700 to 9.100kHz band is likely to be based on the 100 watt audio linear amplifier kit shown below. The specification is :- Music power/200W/4 ohm load RMS power/100W/4 ohm load Distortion/0.02%/1kHz/10W Frequency/3Hz-200kHz (-3dB) Sensitivity/0.6volts rms Signal to noise ratio/115dB
Completed amplifier module and circuit
20th November 2010 - Matching Transformer. The next stage was to construct a suitable matching transformer to allow the amplifier's 4 to 8 ohms output to operate into a 50 ohm load. I did a bit of searching a found an interesting article by John Andrews W1TAG who uses a Hafler audio amplifier on 137kHz, an extract is shown below. This helped me to create a transformer and using existing materials I was able to make a suitable device. After testing several cores and various turns ratio's I produced a similar transformer. The core used is from a PC monitor line output transformer which is wound with 8 turns primary and 12 turns secondary. This gave me a good match into a 50 ohm dummy load.
Sub 9kHz - 2
Unfortunately during testing at higher power all the transistors in the amplifier and some components were destroyed due to an unknown reason. I ordered replacements, fitted them and continued successfully to finish the transformer, one of the prototypes is shown below.