How to calculate power ripple, and then to add the sutable Capacitor?

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Sun, 2 Oct 2005:

I'm not sure what you want in the way of more details. Bridge rectifier directly on-line with about 0.5 ohm series to represent the supply impedance (otherwise you get infinite inrush current). Reservoir capacitor say 470 uF and a resistive load that takes 30 W or so. Of course, the SMPS switch doesn't look like a resistor, but at a fixed input voltage it doesn't matter.

I don't know how good EWB is at this game; many people don't like it. I use LTSpice, which was designed for the job, when Kevin isn't looking.(;-)

I am trying to know why the extra capacitor between VDD and GND of LM386 can solve this problem...

Boki.

What a professional comments!

It seems that my noise is tick-tick-tick at 5 to 50 ticks/second, when I play sound, the noise become louder, when I stop sound, the noise become smaller gradually.

If this is not the ripple problem ( I can't sure yet ), I don't know why capacitor solves this problem.

I have measured my DC power supply ripple, it is about 40~80mv(pp), how is this value? safe or not? or, what is the safe value for audio amplifier ? ( not Hi-Fi quality, MP3 AV output is enough )

Best regards, Boki.

Any suggested books?:)

Boki.

Thanks a lot!

Best regards, Boki.

Like Graham said: this is NOT ripple, it is a supply decoupling problem.

If it were ripple, the noise would be at exactly the same frequency as the AC supply line, e.g., 60Hz or 50Hz depending on where you live, or else at the switching frequency of your power supply if you are using a switching regulator or converter (which would probably be supersonic).

You're familiar with putting a 0.1uF capacitor across the power supply pins for your digital ICs, right? This is the same idea. The issue is that power supply lines have nonzero resistance and inductance, and that interacts with the amplifier to form an oscillator. The details vary depending on the circuit. The LM386 is, as Graham says, notorious for low-frequency oscillation, also known as "motorboating".

Want a more modern amplifier chip? Go to National Semiconductor's web site, and search for "audio power amplifier". Take a look at their "Boomer" series of chips. (Or, you could go to TI, or Zetex, etc. I just happen to be a little more familiar with National.) There are dozens of them, many aimed at exactly your requirements. You have your choice of linear or switching ("class D").

It depends on how much noise you want on the output, and the power supply rejection ratio (PSRR) of the amplifier. For instance, if you have 60dB PSRR, and you have 80mV of supply ripple, then you will have 80uV of ripple on the output. The LM386 is specified at 50dB PSRR. Most modern amps are a lot better than 60dB, so they can tolerate quite a lot of ripple. But you have to decide for yourself how much you can accept.

If you were driving headphones, with typical 95dB SPL/mW efficiency and 64 ohm impedance, then 80uV of ripple would be 25dB of sound pressure level, which is just barely audible in a quiet room. The noise output of the amplifier would probably dominate, especially since ripple is usually at low frequencies where our ears are less sensitive anyway.

I think it is because when there is resistance in the power supply line, and a big capacitor in series with the output, the LM386 is not stable at low frequencies.

Have you looked at the output of the amp with a scope, to see what the waveform looks like?

The ripple is fairly unimportant.

The *psu* deals with ripple current issues.

You have a supply decoupling problem. The LM386 is *notorious* for being a problem in this regard.

Use something more modern. I've said this from the moment you mentioned the LM386 btw.

Graham

Beginners books.

Graham

Well actually no help at all. The psu designer has already done this.

Graham

Please describe all the components you have connected to the LM386 and how they are connected, including the kind of power supply you are using. We should be able to eliminate the ticking.

And then I'm done with this thread.

Thanks a lot!

BR/ Boki.

Thanks for comment!

existing unit that I am repairing to a new unit : )

BR/

Boki.

Do we really need such a bigger capacitor for ripple problem?

/BR Boki.

I read in sci.electronics.design that John Popelish wrote (in ) about 'How to calculate power ripple, and then to add the sutable Capacitor?', on Mon, 3 Oct

When you ask that sort of question, boki changes the subject.

Thank you very much for this lesson : ) I will try them.

Best regards, Boki.

It's a capacitor. I = C x dV/dT It's an exponential decay, which is pretty much a straight line for time much less than a time constant. mike

No, you can use an inductor or have a large head room on a linear regulator instead.