small line amp for car audio has problem

Archived from groups: rec.audio.tech (More info?)

Hello audiophiles and techies,

I have designed this small line-amp to roughly boost the line level
from .5V to 5 Volts.

VIEW SCHEMATIC HERE http://www.planetrca.com/amp/audio-boost.jpg

It actually sounds great until I turn on the headlights of the car....
Then I get a nice hum and what sounds like a WAWA pedal in the
background. They are not HID headlights, so no switching power supply
for the head lights. I also notice that the CD mechanism from a
changer is coupled into the line-amp as well... this leads me to
believe that it's being coupled through the power supply. To test
this theory, I bypassed the filtering inductor and to no surprise the
noise got VERY loud. The noise is the same whether or not there is
something plugged into the input. I could probably make the power
filtering bigger... but something tells me that my design is way to
sensitive to power noise.

I am wondering where I went wrong in my design. Anyone have any ideas?

I don't know enough about impedance matching but might that be where
the problem lies? If it is, I don't want to add transformers to the
design... I need it to be small.

Thank you,
Audiozombie

Archived from groups: rec.audio.tech (More info?)

El Meda wrote:
> lateralgroup@gmail.com wrote:
>
>
>>Hello audiophiles and techies,
>>
>>I have designed this small line-amp to roughly boost the line level
>
>>from .5V to 5 Volts.
>
>>VIEW SCHEMATIC HERE http://www.planetrca.com/amp/audio-boost.jpg
>>
>>It actually sounds great until I turn on the headlights of the car....
>>Then I get a nice hum and what sounds like a WAWA pedal in the
>>background. They are not HID headlights, so no switching power supply
>>for the head lights. I also notice that the CD mechanism from a
>>changer is coupled into the line-amp as well... this leads me to
>>believe that it's being coupled through the power supply. To test
>>this theory, I bypassed the filtering inductor and to no surprise the
>>noise got VERY loud. The noise is the same whether or not there is
>>something plugged into the input. I could probably make the power
>>filtering bigger... but something tells me that my design is way to
>>sensitive to power noise.
>
>
> Try making C2 20 times bigger, that is, 4700 uF. If the hum continues,
> try using a LM7809 voltage regulator IC between C2 and the rest of the
> circuit. The circuit will be working then at a very clean 9V, instead
> of a noisy 12V.

Replace R11 with a 6v zener.
Turn C4 and C12 the right way round.

--
Eiron.