Re: Bitx40 very loud, high-pitched, low frequency noise

John Norris

My BITX40 shrieks if I get my cellphone too close to it.

On Saturday, August 24, 2019, 03:20:36 AM UTC, Ashhar Farhan <farhanbox@...> wrote:

I think it could even be an issue with the way the LM386 is wired up. The power supply to the audio amp is via a 100 ohms resistor. So, when something loud gets into the audio input, it would draw more current and the voltage across that resistor will increase, in turn decreasing the dc voltage to the LM386. The dip in the dc voltage would dip the gain of the LM386, decreasing the current... The cycle would repeat causing oscillations. But that would lead to motor boating : butbutbutbutbrrrrrr.. kind of a sound.

On Sat 24 Aug, 2019, 1:42 AM Ian Reeve, <ian.radioworkshop@...> wrote:
I think that your comment about even on battery there is faint noise and it is near your PC is significant.Some pc's give out lots of hash or wine and I wonder if moving the uBITX  further away may silence that last bit of noise.I am surprised that the capacitor combination on the output does not help,that has always worked for me as I proved by my test last evening on a randomly picked switch mode wall wart.  Anyways it's progress in the right direction,keep the leads short and you will minimise pickup.A ferrite ring or two on the cables won't do any harm,the best quality power supplies usually have fitted on the DC lead to keep switching noise away from the DC output.

From: <> on behalf of Woody <woody@...>
Sent: Friday, August 23, 2019 8:58:25 PM
To: <>
Subject: Re: [BITX20] Bitx40 very loud, high-pitched, low frequency noise
On 8/23/2019 19:39, Gordon Gibby wrote:
> You may decide that you want to build a real low pass filter, with an
> inductor etc., similar to what people had to use in the past to get
> rid of alternator whine
...Or a new / different power supply.
It would be interesting to examine the supply output with an oscilloscope.



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