[Fastjack]

Hi folks

I have a Meta that I want to drive with a power supply to save on batteries. Unfortunately the regulated PS (13.8V) I had lying around introduced a hiss I could pick up with my Etys (Porta Pros don't care

) that wasn't there when battery powered.

Trying a trick mentioned by Tangent here some time ago I tried putting a capacitor across the PS's output. While he said to use a small cap I had the best results with a 2200µF electrolytic. Not exactly a small one I'd say. Well, with that one the hiss is almost completely gone.

To the question(s): Would I get better results with another cap? Maybe of another type. Or is there another simple way to clean up my source?

 

[Voodoochile]

If the big electro is handling most of it already, then great.

If you parallel a small film cap with the electro, such as a 0.1 or 1uF, that might very well take care of the remaining hash that is too fast for the electrolytic to handle. It's a very inexpensive thing to try.

 

[Fastjack]

Ah, thanks a lot for the tip. I thought there might be something like responsiveness in caps but didn't really know anything about it. I'll get me another few caps at work tomorrow and try it.

 

[tangent]

It was probably ppl that recommended that one, actually.

If you want to try a film as well, put it in C4.

 

[Fastjack]

OK, I tried some stuff now and I think it's fine now. No idea whatsoever WHY it's fine though.

Adding a film cap as per Voodoochile's suggestion didn't do any difference so I wanted to replace the electrolytic cap by an even bigger one. But first I put a 60 Ohm resistor in series with the amp (before the cap) to reduce high currents when the cap loads up. That left about 12 Volt for the amp so that's OK. The strange thing now is that the hiss was gone. Even without any parallel cap there was only silence. Great but why/how does it work???

A coil in series to kill high frequencies, that I would have understood but a resistor.....

Oh well, as long as it does work

 

[tangent]

You've got an RC "pi" filter. (The other leg of the "pi" is the cap in your wall wart.)

A resistor in series followed by a cap to ground is a low-pass filter. In other words, it will not pass high frequencies. Let's say you've got 1000uF on the META42 board. f=1/(2*pi*RC) says the corner frequency is under 3 Hz. Therefore, almost nothing but DC gets through that filter.

The disadvantage of this is that you now have a very high impedance power supply. This is very bad. You would be better off with an LC pi filter. But, since a pi filter fixes your noise problem, that tells me your power supply is very noisy. Rather than adding passive filtering inline with the power inputs, perhaps a better plan would be to drop in an LM317 configured for 10V output. Or, an LM1086 configured for 12V.