0
\$\begingroup\$

It is a fairly simple question with similar answers, but I need help with my project.

I built a single supply 30W power amplifier and a preamplifier with tone control. Both amplifiers are powered from a simple AC-DC converter.

It all works fine when I use a bench power supply, but when I make my own AC-DC converter the problems start.

My simple AC-DC converter contains a bridge rectifier, 3x 3300uF capacitor and 1x 100nF capacitor (the picture is just for example and is not mine):

enter image description here

The problem is that the voltage ripple is transferred through the preamplifier into the output signal and produces noise. The power amplifier does not pick any noise from the power supply voltage ripple and works perfectly (I connected an audio source directly to the power amplifier and it works with no noise.)

I tried putting an LM317 voltage regulator between the AC-DC converter and preamplifier. There is a little improvement but it still contains a lot of ripple voltage noise (100mV.) Adding more capacitance to the AC-DC converter or at the output of the LM317 slightly reduced the noise. If I want to significantly reduce voltage ripple with capacitors, I would need a few 10000 uF capacitors and this is not cost effective for me.

How can I easily reduce the noise on the preamplifier power supply? Why is the TDA7377 power amplifier with a few extra komponents not sensitive to voltage ripple?

\$\endgroup\$
2
  • \$\begingroup\$ If you have 100mV ripple on the output, the most likely cause is a grounding problem with rectification current pulses flowing in the ground of your circuit. Looking at the shape of the ripple voltage on the output with a scope will give useful information. \$\endgroup\$
    – bobflux
    Jun 10 at 7:50
  • \$\begingroup\$ Can you post a picture of how everything is connected? How much supply capacitance on the amp and preamp board? \$\endgroup\$
    – bobflux
    Jun 10 at 8:26
2
\$\begingroup\$

My simple AC-DC converter contains bridge rectifier, 3x 3300uF capacitor and 1x 100nF

Total overkill. Your TL072 can under no circumstances even draw enough current to load the 3330 µF significantly. Very likely, using 1 µF with lower ESR and lower leakage would have been better. More isn't always better!

I would need few 10 000 uF capacitors

Not true. Sure, more capacitance reduces the ripple, but I'll leave the calculation of how long a TL072 will need at maximum supply current to discharge a single 3300 µF capacitor charged to 10 V by 100 mV up to you, pointing out that you should compare that time with the duration of half a net period.

I try to put LM317 voltage regulator between AC-DC converter and preamplifier and there is little improvement but still contains a lot of ripple voltage noise (100mV).

Good approach. But: The LM317 is really an ancient piece of regulator. Maybe use something that needs less voltage difference and has a better noise suppression.

(also, capacitors are not optional, and every regulator datasheet will tell you what in- and output capacitors are needed.)

The Supply voltage rejection of the TL072 is >-70 dB typically. I can think of many ways to insert more than a 10 millionth of the supply ripple into your output signal, so make sure you're not doing something like using an unregulated voltage as virtual ground or similar.

\$\endgroup\$
7
  • \$\begingroup\$ I use AC-DC converter for power supply POWER AMPLIFIER and PRE AMPLIFIER that's why I use 3x3300uF (before I use 2200uF and the ripple was bigger, especially when the power amplfier worked at greater power). I use star grounding and the start point is AC-DC converter. If I use better voltage regulator with less voltage drop, I will significantly reduce voltage ripple at preamplifier power supply side ? \$\endgroup\$
    – mrkefca
    Jun 10 at 8:00
  • \$\begingroup\$ The only problem with voltage ripple is at the preamplifier side. \$\endgroup\$
    – mrkefca
    Jun 10 at 8:02
  • \$\begingroup\$ well, you should probably add the full description of what you're doing to your question. And of course it's at the preamplifier side; that's the only stage that's as sensitive. \$\endgroup\$ Jun 10 at 8:02
  • \$\begingroup\$ I add description and picture. Please check it. The only problem is how to eliminate voltage ripple or significantly reduce it on preamplifier side. \$\endgroup\$
    – mrkefca
    Jun 10 at 8:23
  • 1
    \$\begingroup\$ yeah looks like: 1. better voltage regulator. 2. Rigorous isolation of ~antenna~ input cable against EMI, 3. low-ESR capacitors close to opamp 4. make very sure you're not using an unregulated ground, signal, reference voltage.... \$\endgroup\$ Jun 10 at 8:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.