Hi

> A lot of highpass filters seem to exhibit this behavior -- my Doepfer
> multimode, my ASys multimode, the multimode on my Alesis Andromeda, even
> my Electrix Filter Factory a little bit. My only highpass that
> completely cuts things off at the top of its range is my -24 dB Doepfer
> highpass.

If you read carefully your own posting you will find some interesting
relation: Only multimode filters show this behaviour. Pure highpass filters
don't. I cannot proove the foollowing explanation this at the moment, but I
could imagine, that this is a question of the trimming:


Lets see the two characteristic lines against the cutofffrequency:

0dB ........... ,,,,,,,,,,,,,
.
,
.
,
-3dB
;
,
.
, : .
, : .
, : .
, : .
, : .
, : .
, : .
, : .
-100dB , : .
| | +--Lowpass Characteristic
| +------------Cutoff frequency
+----------------------Highpass Characteristic



As you can see, the cutoff frequency is not there, where the characteristic
of the filter bends down, but at the -3dB point.


So now to our multimode filter: it may be trimmed that way, that you hear
the lowpass filter start working, as soon as you turn down the cutoff pot
from 10 down to 9. This means the the real cutofffrequency will be at
position 9 at around 10kHz, to make the -100dB point be at a hearable
frequency (lets say, 17kHz - though only a few of us will hear this). The
position ten will produce a cutofffrequency at around 13kHz and a -100dB
point little over 20kHz.

Now If you switch to Highpass the cutofffrequency will be a 17kHz, but the
-1oodB point will be now at 14kHz. And this you will hear.

And keep in mind that the example uses values which would require extrem
steep filters.


Summaray: If you want a mulitmode filter, that extinguishes the signal with
highpass completely, then you may change the trimming. But then you should
be aware, that the lowpass filter doesn't seem to work in the higher ranges
of the Cutoff-knob.

(So as its always in life: twiddlin' the knobs has often very satisfying
results, but you have know what you do and which consequences you have to
bear ;-)).... )


Florian
--
Florian Anwander |ConSol* HP-Support
Tel. +49.89.45841-133 |Consulting&Solutions Software GmbH
Fax +49.89.45841-139 |Franziskanerstr. 38, D-81669 München
email: florian.anwander@... | http://www.consol.de