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<DIV><FONT color=#000000 size=2>Hi all,</FONT></DIV>
<DIV><FONT color=#000000 size=2></FONT> </DIV>
<DIV><FONT color=#000000 size=2>I have made a LM13700 based filter this weekend
and I had a few questions about it's functioning.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT color=#000000 size=2>I have built the a filter that consists of a 2
pole high pass stage and a two pole low pass stage.</FONT></DIV>
<DIV><FONT size=2>The schematics I have used for the filters comes from the
national semi datasheets for the LM13700.</FONT></DIV>
<DIV><FONT size=2></FONT><FONT size=2>The ones I have used are the single high
pass and single low pass filters and I've put two of each in
series.</FONT></DIV>
<DIV><FONT size=2>The filter works fine and I sweep the control voltage to the
OTA's from +15 v to -15v (I am not really sure if I </FONT></DIV>
<DIV><FONT size=2>can sweep the CV up to 15v but it works ok.)</FONT></DIV>
<DIV><FONT size=2>The thing is that the lowpass filter works fine but the high
pass gives some cracks when I turn the CV almost to</FONT></DIV>
<DIV><FONT size=2>-15V (the bias current is almost zero uA then). Has anybody
got ideas how this comes.</FONT></DIV>
<DIV><FONT size=2>And when I turn the CV even lower both filters stop working
because the outputs of the OTA's run into one of the</FONT></DIV>
<DIV><FONT size=2>supply rails. I have checked if this is because of a small
voltage difference betweent the inputs of the OTA and</FONT></DIV>
<DIV><FONT size=2>this indeed is the case. </FONT></DIV>
<DIV><FONT size=2>I wonder if this has something to do with the fact that I
don't use 1% resistors.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>And I was sondering what the expected tuning range is for such
a filter. The datasheets say it's possible to tune over a</FONT></DIV>
<DIV><FONT size=2></FONT><FONT size=2>few decades and I have designed the filter
to go from 20 to 20000 Hz. But it doesn't sound like it is reaching the 20000
Hz.</FONT></DIV>
<DIV><FONT size=2>When I have a 808 snaredrum across the filter the snaredrum
stays a little muddy.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>If anybody can give me some hints I would be very
happy.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Bye,</FONT></DIV>
<DIV><FONT size=2>Heiko</FONT></DIV></BODY></HTML>