Hi Stewart Try Rather than directly connecting the decoupling cap to the 5v supply, try feeding it via a 100 ohm (or so) resistor. at 1ma you will only drop .1 v but you have created a low pass filter for the noise.
Rgds Andy Pevy G4XYW A future K2 owner (when the company bonus arrives). On Tuesday 30 November 2004 06:24, ext Stewart Baker wrote: > I think that I have found the problem, but have yet to work out a solution. > > To combat the lack of TX audio system gain I have a 1 transistor pre-amp > mounted on the microphone header plug on the front panel board. This gives > about 9dB of gain which provides sufficient audio for the compressor IC and > the VOX. The amplifier is supplied from the 5V pin on the header. This rail > is the 5A supply which is used by the MCU logic, plus other functions. > > The amplifier only draws about 1mA, however because of noise on the 5A > line, mainly from the LED bar graph I have had to heavily decouple the > pre-amp supply to stop the noise from being superimposed on my speech. > > What I think is happening is that this decoupling is affecting the rise > time of the 5A line which is used for the MCU reset amongst other things. > > I have tried reducing the value of decoupling. This seems to stop the > "strange" problem, but I am back with the noise. It is a shame that there > is no way to get at the +8V rail on the front panel board as that would be > an ideal mic supply. > > The way things are looking I will have to put the pre-amp outside my K2 and > power it separately. The only alternative seems to go back to shouting ! > > 73 > Stewart G3RXQ > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com