On 10/29/2018 11:19 PM, Phil Allison wrote:
Paul wrote:
Phil Allison wrote:
** Look at the TR8 schem carefully - the LM3886s have additional feedback paths around them aside from the usual gain setting networks.
U1 has a filter path with the same component values used for HF crossover U3A while U4 has one the same as LF crossover U3B.
As the filters are in series, the ultimate roll of slope is double that of each - or 24dB / oct.
I see. I checked the roll-off of the HF crossover U3A,
and indeed it is about -12dB/oct, or -40dB/decade. The cutoff
frequency is about 2,729 Hz, which matches roughly what I heard
on the bench.
So crossover U3A is a 2nd order filter.
Can you safely assume the order of this type of active filter will
be the same as the number of reactive elements (capacitors, or
inductors), which in this case is two caps?
Just like you can simply count reactive elements in a totally
passive filter, to get the order of the filter?
** Normally yes.
If the changing impedance of a cap with frequency is reflected in the stage's output, then it is likely adding 6dB / oct to the final slope.
18db/oct active stages normally have 3 caps.
Ok, excellent. There is order in the universe!
Thanks for sharing your knowledge, Phil, it's been a great help!
