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
>
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