On 10/21/2018 12:17 AM, Phil Allison wrote:
Paul wrote:
The subsonic filter looks to be a unity-gain, non-inverting
topology. The series cap C6 is obviously a high-pass filter, but
how are the values for R10 and R11 chosen? Where is the "RC time
constant" that determines when the low frequencies start rolling off?
** The active filters used are all "Sallen-Key" types.
http://sim.okawa-denshi.jp/en/OPstool.php
Similar S-K filter stages are connected around the two LM3886 power amps - after scaling each output (eg R7 & R8) to give unity gain. This doubles the roll off slopes to 24db/octave.
Oh, that's perfectly it, thank you!
I knew there was a reason I ask you folks!
The sub-sonic is a high-pass filter, and after
I plugged in the TR5 values (they are different from the TR8 values,
because the TR5 omits the active crossover U3A and U3B),
the cut-off was somewhere around 55 Hz.
http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm
And here is a good review of how to derive the transfer
functions of these filters (I was only able to find a video
for the low pass topology):
https://www.coursera.org/lecture/ele...function-rX3Ys
If the roll-off slope for the TR8 is -24dB/octave, and there is
-6dB/octave per element, then it looks like this can be considered a 4th
order filter, right? Where did you get the -24dB/octave?