Power supply filter caps and output caps

Submitted by bkforge on Tue, 07/03/2012 - 22:13

Why so many filter caps on Vcc? There are C2, C8, and C10, some seem redundant.

Also, what is C3 doing on the output? Is it affected by speaker impedance or supply voltage.

Just wondering, still fiddling around, not an EE.

Thanks

Member for

11 years 5 months

John

Wed, 07/04/2012 - 01:24

Don't think of them as filter caps, necessarily. Think of them as little reserve batteries.

C10 is there because if the opamp needs a sudden surge of current, it can handily pull it from C10 instead of trekking all the way across the board to pull it out of C8.

C2 is there because the TBA820M is a power amplifier and so it needs a bigger "reserve battery" as it were. The same sudden transient that caused the opamp to surge is going to hit the power amp and now it needs to pull a lot of current, fast, and everything is going to go smoother if the amplifier can pull that from C2 instead of the battery. The battery can only spit out power at a certain rate. If you try to pull out more power straight from the battery, the voltage droops and that can lead to all kinds of problems. With this circuit, if the voltage droops too much, the amplifier cuts out in a harsh, unmusical fashion.

Now, think back to the caps as being filter caps. C2 is only good at filtering big, low frequencies. It doesn't do anything for the smaller, higher frequencies. And you need your smaller, higher frequency VCC filters to be as close to the IC as possible, because otherwise they are less effective.

Now, as for C3, well, I pulled C3 straight from the application data sheet. I am not an EE so I lack the skills to analyze exactly why it is there. Stability? >20k frequency rolloff? Not sure.