Wiardgroup

Hi Andrew
my 'Omni' is a very early 'The Filter' version that i've chosen to
keep un-updated. At relatively low-Q settings, when i sweep through
bandpass mode, the lowest amplitude is consistently bandpass mode. The
 decrease is not as pronounced as in your example but there is a
distinct decrease. Your post is sort of weirdly synchonous in my life
because i just delivered a lecture on filter modes in one of my
classes on Wed. night. i use an EMU UAF and an Aries 327 for demos and
 i always point out that a bandpass function (without a lot of
resonance added in) physically passes the -least- energy because it's
lopping off the energy both above AND below the corner frequency. Many
designs use clever schemes to makeup the gain** but those venerable
old state variables just do the cutoff and let chips fall where they
may. (that's why they're good for demos!) i don't know how Grant
implemented the bandpass in the Omni nor can i tell you that your
decreased gain is 'normal' but i can tell you that in a raw filter
circuit , with all other factors being equal, bandpass is the mode
that passes the least audio energy.
As for the Borg, the slopes are way shallower than the 4-pole Omni and
the circuit is of a vastly different type. While i'm not certain, i
suspect that the Borg's slope varies as one sweeps through the modes,
in a way that makes the 'band-PASS' mode more of a 'band-emphasize'
that leaves a lot of unattenuated audio on either side of the center
freq. 

best,
-doc 

** maybe even not so clever, because a classic state variable has
separate outputs for each mode. Its a simple matter to simply boost
the gain of the bandpass output with an opamp until it's commensurate
with the other modes.Since the Omni doesn't separate the outputs this
scheme is precluded


--- In wiardgroup@yahoogroups.com, "andrew dalio" <bunnyman@...> wrote:
>
> I posted a couple examples of my OmniFilter in the Files section
(OmniFilter examples folder). 
> The filter really seems to lose a lot of output when entering the
bandpass mode (very 
> noticeable in the 24dB output file). The module is processing a
sawtooth wave. Coarse, Fine, 
> and Q are @ 12 o'clock. I'm manually turning the filter mode knob
from LP to AP and back 
> again a few times. It worries me a bit, since my Borg I filters have
a consistant output no 
> matter what mode they're in. Any comments? Or am I just not getting
it? (very often the 
> case ;-)
> 
> -andrew bunny
>