Output hum humdrum

[xray]

Howdy:
Looking for a little information on the increasing of the S/N. I have a portable electret microphone feeding a preamp and then to an adio output power amp, LM4860. My problem is as the power output of LM4860 goes up the voltage regulator LM7805 shows a ripple of the increased demand of the power out put. As the power goes up the ripple becomes amplified and can cause hum in the background or it gets large enough to oscilate the whole system. I've tried to place the preamps on a seperate Lm7805 from the single battery supply helped only marginally. Looking to create a silent background operation from the headphones. Any help out there would be appeciated.

Sincerely;
Cy Drollinger

 

[tipool]

Hi dear;
First you must check it whit battry opration ;if it's ok; you have to use a power filter.
BYe

 

[Egress]

Sounds like you neeed to decouple the preamp power supply from the poweramp supply. The ripple you see on the preamp
regulator is probably due to its supply dropping below its rated output voltage (or just too close to it). This drop will be due the internal resistance of the battery, when the
system load current is high the internal voltage drop in the
battery will also be high ( vis Thevanin equivelent circuit theory).

You can do this in two ways ( or both)

1) stick a diode followed by a smoothing cap ( 470uF or greater) before the preamp regulator. Thus when the battery
terminal drops the input voltage at the preamp regulatior
wont drop because the diode will be reverse biased and the
storage cap will be able to keep the regulators supply up for the duration of the brownout.

2) same as above but use a 10ohm resistor instead of the diode. This then becomes a first order low pass RC filter.
two stages of this are often good for getting rid of supply born noise, particularly in mains connected systems. You need to consider the voltage drops that thes solutions will incur but I have sometimes used rc filters post regulator to get rid of unwanted HF regulator noise (in the phantom supply of an audio console).

Hope this helps.