decoupling power supply rails

[mveitch]

This one's been bugging me a bit.  I understand how passive low pass and high pass resitor/capacitor combinations work,  but i dont understand how this applies to power rails. Or indeed, does it apply?  Specifically, I'm asking about the decoupling capacitors across +9V and 0V thats common to most circuits.  Is the idea that your removing ac signals/noise from the 9V rail through the capcitor? if so, how can you calculate the 3db point of the 'filter' if there is no resitor??

i ask cause ive built a tremolo with an opamp oscillator and i can hear a small pop on each of the oscillations - i think its going into the +9V rail, and is therefore heard at the output too.  The pop is obviously low frequency and im wondering if the decoupling capacitor acts like a low pass filter, and hence makes it real difficult to get rid of low frequencies...

has this been covered somewhere or can anyone throw light on it??

[Paul Perry (Frostwave)]

Even if there is no resistor, the noise source has an inbuilt impedance of its own. But, in practive, where there is a pop coming thru the power line from an LFO, it often helps to put a 220 ohm resistor in series with the power line going to the LFO section, and a 220uF cap on the other side to ground.
Oh, I hope you didn't use a dual op amp, wiht the LFO in one half and the rest of the circuit in the other!! because, then you're screwed :cry:

[brad]

The "Widgey Box" distortion published in Silicon Chip magazine has one of the most excessively filtered power supply I've ever seen in a 9v effect.  It's got a cap array, and protection diodes up the wazoo.  I'll see if I can post it later, because it would probably be useful in a circuit with LFO noise goin'  on.

[mveitch]

I did use a dual opamp, but both amps were used to make the oscillator. the rest of the circuit is elsewhere.  Paul your solution sounds pretty good so i'll try that tonight and let you know how i got on.  

brad, that article sounds pretty interesting, look forward to seeing it.
thanks!

[rubberlips]

The "Widgey Box" distortion published in Silicon Chip magazine has one of the most excessively filtered power supply I've ever seen in a 9v effect.  It's got a cap array, and protection diodes up the wazoo.  I'll see if I can post it later, because it would probably be useful in a circuit with LFO noise goin'  on.

brad if you could post it, that woudl be great. They put some good stuff in the mag occassionaly. I'm just going through building some stuff post in EA and ET from quite a few years ago. Glad to see not much changes with analogue gear over the years

Cheers

Pete