LM13700 based filter

Harry Bissell harrybissell at prodigy.net
Tue Mar 7 05:12:44 CET 2000


I'd try the LM13600 instead of the 13700 because of the bias connection to the
darlington in the 13600. Or try using some external buffer before the darlington
like a fet, or use a fet op-amp and skip the darlington....

H^) harry

danial stocks wrote:

> >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
> >can sweep the CV up to 15v but it works ok.)
> these chips are really current controlled.. with a res on the bias pin this
> makes it vtg controlled.. the max i/p current = 2ma - so if you are using a
> 15K [or higher] res on bias pin then you could go to + 15V [it references
> the bias current to v- so +15 in a +/-15V really appears as 30 V on the bias
> resistor.
>
> >The thing is that the lowpass filter works fine but the high pass gives
> >some cracks when I turn the CV almost to
> >-15V (the bias current is almost zero uA then).
> Resonance high? these chips are not too good with high levels of audio drive
> sometimes either... they can bleed through etc.. try reducing the res, or
> the audio input lvl
>
> >And when I turn the CV even lower both filters stop working because the
> >outputs of the OTA's run into one of the
> >supply rails. I have checked if this is because of a small voltage
> >difference betweent the inputs of the OTA and
> >this indeed is the case.
> If you look at the internal cct diagram of the LM13700 you'll note that the
> bias pin attaches to a transistor base.. I think this trans may be part of a
> darlington [not sure, haven't looked at this in a while] at any rate, this
> pin is always 1 or 2 Vbe's above V- when any current flows.. as such, you
> will always encounter a dead zone if driving it from simple cv into resistor
> arrangement and lower the CV right down close to V-... To get around this
> problem if necessary, I usually drive the bias pin with a voltage controlled
> current source - that way, the CV in translates directly into a current
> without need to consider the Vbe offset problem.. otherwise, offsetting with
> an opamp is just as complex.. of course you can just live with it if you
> dont want to use the extra parts, it isn't really a prob for many
> applications.
>
>    And I was sondering what the expected tuning range is for such a filter.
> The datasheets say it's possible to tune over a
>    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.
> Yes, it is possible to get this sort of range, would mean 2ma to 2ua bias
> current not a problem @all Ive made vco's with these going from 5hz - 5K
> [same range..] bear in mind that 20K is pretty high for a filteer.. either
> everything you can hear gets thru or nothing does.. again, reduce the aud
> lvls and see if that helps..
> 5% res are usually fine for this sort of app..
> Cheers,
> Dan
>
>    When I have a 808 snaredrum across the filter the snaredrum stays a
> little muddy.
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