[sdiy] PCB layout question

[Seb Francis]

Paul Perry wrote:
> Incidentally, while it is true that the bypass caps might not appear to make
> much difference at audio frequencies - sometimes if you leave them out, you
> will find RF oscillations! Unsurprising, with transistors having 200Mhz
> cutoffs.

In my experience I haven't had much luck curing hi-frequency ringing or 
oscillations with bypass caps.  What is normally needed is to limit the 
bandwidth of the amp in question.

I thought bypass caps are more needed to stop switching transients 
spreading through the power supply.  And actually limiting the bandwidth 
helps this too.

For example, try a simple inverting summer with a fast opamp (e.g. 
OPA2132).  Bang a square wave through it and you will see it ring (on 
scope, you won't hear it!).  No amount of bypassing caps on the supply 
will change this.  But put a small cap in parallel with the feedback 
resistor and then ringing will disappear.   (The cap needs to be small 
enough to still keep the bandwidth well above audio range).

Seb