LM 386 + PDA+ PS = Noise

[[email protected]]

Hi
Regarding my earlier post on fade in/out
I decided to Kiv that for a while.. So I
built a quick kit of an LM 386 amp and
I had the following problem<p>Power Source= Cigaratte lighter
LM386 board is powered direct from 12++ volts
in the car. Power is smoothed using a 1000UF
and a 10UF capacitor<p>I plugged the PDA headphone output into the
circuit, voice/sound works fine<p>However when I plugged the PDA into a PDA
12 volt to 5 vdc power adaptor, all hell breaks
loose on the LM386 board..noise of every type
came out of the small speaker.<p>How do I clean up the whole circuit? I use
shielded cables from the headphone jack to
the Lm386 board.<p>One of my frieds say the reason is I am
getting all kinds of feedback and that my
12 Vdc -5vcd adaptor is noisy.. The gain
of my amp is too high and thus, I am hearing
all the horrible sounds when the PDA is plugged
into the dc-dc power supply<p>I am using a standard lm386 circuit and I can
cut the gain down, I know, but how do you put
in preventive electronic parts to chop the noise?<p>Regds
Allan

[bodgy]

Sounds like interferance from the PDA's crystal unless the PS adapter is a switchmode one.<p>What happens with the PDA switched off but its PS plugged in?<p>
bodgy

[[email protected]]

Hi
I just built a few different circuits
using different amps: lm380 and Tba820M<p>the noise is dramatically less than the
LM386...<p>Ok the Noise is a high frequency 'hum'
When the PDA power supply (12v to 5 v) is
disconnected, clean sound, no hum<p>if the PDA is switched off, no noise at all
even with the 12vto5v adaptor turned on.<p>I plugged in a SONY headphone set to hear
what happens, clean sound, with the 12vto5v adaptor plugged in.<p>So somehow the adaptor is leaking hum into the
LM380/386 circuit and it is being picked up
and amplified.. <p>How can I fix this?<p>Thanks
Allan

[[email protected]]

I just tried using a 7809 regulator on
the LM380 side, same problem..<p>It is very hard to get rid of this noise..
In fact I tried other 12->5v LM317 based
stepdown transformers same problem..
Regds
Allan

[[email protected]]

Anyway the problem has been solved..
I used audio isolation transformer
on the output from the headphone jack..<p>Regds
Allan