Power supply decoupling capacitors (beginner question)

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I'm trying to learn a bit more about audio eletronics and have found a good book which i'm working my way through. It is primarily based around op-amp circuits powered from 9-volt batteries. In virtually every circuit, the +9V supply rails have two decoupling capacitors: one 100uF electrolytic and one 0.1uF bipolar. I understand that the electrolytic acts as a smoothing cap and would really only be necessary in a transformer derived power supply (am I correct?). But what does the additional 0.1uF bipolar do and why is it necessary?

(If anyone is wondering, the book is "Practical Audio Amplifier Circuit Projects", written by Andrew Singmin and published by Newnes, 2000)
Hello bm_mode ,

The 0.1 uF caps are non polarised film caps, chosen for low ESR.
Typical electrolytic capacitors, exhibit significant internal series inductance and series resistance, which affects high frequency charge/discharge performance, and also actual uF value at high frequencies.
The bypass film caps help to reduce this total effective ESR.
Always use modern switch mode power supply electrolytics, for series connection and bypass.
These capacitors are designed and constructed to give low internal inductance.
Higher voltage rating will also give lower ESR - for example 100uF/63v will give less ESR than 100uF/16v.
You are in New Zealand, Yeah ?
http://www.wescomponents.com/ has just about every cap and semiconductor that you could ever want.

Hope this helps,
Regards, Eric.
Thanks for your help Eric and djk.

Actually, I just got the WES components catalogue sent to me about two weeks ago. Haven't ordered anything yet but I will be using them - it's hard to source parts within NZ at realistic prices.

djk - when you talk about using "anything faster than 20V/uS" are you refering to fast transient inputs for the amp to amplify? What does the 10 ohm resistor do? I'm not certain if you mean for the resistor to be in series on the power supply rail between the battery and the amp or in series with the cap between the supply rail and ground?

When I said 20V/µS I mean an op-amp like an OPA2604. Really fast stuff, say a 400V/µS AD842, need very careful layout. The 10 ohm resistor was to go in series with the 0.01µF cap, and the combination in parallel with the +/- supplies at the load. The reason for this is the 0.01µF may resonate with the inductance of the traces and the resistor helps kill the Q. Audio isn't that fast, 2V/µS per peak volt of output is plenty fast. Damm fast op-amps can induce all sorts of noise from overshoot and ringing that doesn't show up otherwise.
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