- Aug 9, 2010
as for local decoupling, the 33R/100u/25v, the 33r goes between power and pins 4&8(7). The R is in series with the power source and ic pins and the C is between the R and the IC power pin. it does 2 things. first it keeps the currents on the board returning back to the power. if you have 2 inverting stages of different amps together, there is a tendency for the positive going electrons to leave the amp, and then go thru the negative flowing amp next door rather than back to the supply. oscillations can occur (not usually) but it sets up a tank circuit effect or 'climate' for oscillations a more nuanced word. so this keeps electrons "fresh". fluffy words to paint a concept. you find these resistors on all higher end consoles. then the Cap at the pins is a storage device at the amp. like multiple car batteries the trunk of a hi-end car stereo with thunderous bottom end. for kick's and other transient signals, the current for the waveform comes from the cap which is then resupplied in a less transient time line by the supply. I can see this on my distortion analyzer where I continuously monitor the signal and the distortion component on my scope. channel strips with audio caps only replaced and old power decoupling caps show a second order element from the power being pulled across the module. replace the caps noise is gone. The 4700 on each channel strip is the same medicine for the whole module. the ground plane strapping helps too. ground in audio is like diving in a lake from a dock. lean forward and push off. the doc firmly anchored allows you to dive effortlessly. Dive in from a raft and it's a belly flop for sure. because the raft moves back as you push off. so I stiffen up the (lower) resistance of ground and leave the power traces alone (anemic?) so as I get hot with signal I invite a little 2nd order as you get near clipping for color. the amp will still have transiet oomph from the cap, and the second order will be module wide (the R of the trace drops the current that creates the voltage) and become apart of the signal.