[sdiy] Low Pass filters and musically useful frequency range

[Andrew Simper]

On 7 May 2014 03:11, David Ingebretsen <dingebre at 3dphysics.net> wrote:
> 2. Why would Moog add such a high pass filter to the 914? I can see blocking
> DC, but why worry about 10 Hz getting through? Does it have to do with being
> musically useful? I don't think this high pass filter was in the 907, so why
> add it to the 914?
>
> Thanks
>
> David

I think there is some confusion here. The only "DC blocking" is done
near the buffers to remove the biasing.

The lowest frequency filter in the 907 / 914 is a 4 pole low pass
filter with a resonant peak at around 200 hz, there is a front panel
knob to control its input level, so it does pass DC when this level is
up (ie no DC blocking from it). Because a pair of LC low passes is
used without buffering this makes for a sloppy filter, so there is a
bit of sag before the resonant peak. When the level knob for this is
adjusted you don't get a high pass filter, you just get a different
level of the low pass filter being heard, when the knob is at 0 the
"high pass" actually comes from the rest of the signal being filtered
by bandpass or highpass filters.

There is a high pass filter in the 907 / 914, it is at the highest
frequency which is around 4 khz, and it too has a volume knob so you
can adjust how much it heard when all the parallel filters are added
together.

Andy