cota build LF412

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Pav
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cota build LF412

Post by Pav » Fri Oct 06, 2017 12:01 pm

Hi,
similar question to before
build guide for COTA asks for qty 2 LF412A ..


On checking there is a LF412ACN @£3.52 1468876 which has spec

1mV Maximum internally trimmed offset voltage
7µV/°C Typical input offset voltage drift
50pA Low input bias current
0.01pA/√Hz Low input noise current
1.8mA/amplifier Low supply current
10¹²R High input impedance
<=0.02% Low total harmonic distortion
2µs Fast settling time to 0.01%


Problem is I ordered LF412CP from Farnell @ £1.29 2292099

Internally trimmed offset voltage
50pA Typical low input bias current
0.01pA/√Hz Typical low input noise current
4.5mA Typical low supply current

looks like offset voltage and low supply current might be deciding factor
Can I assume either are ok ?
rgds Pav

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Synthbuilder
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Post by Synthbuilder » Sun Oct 08, 2017 10:01 am

Either will do here. The A does give about half the voltage offset of the non A but in reality that means you'll get an extra mV of offset from the non A version. That's not going to make any real difference when we are dealing with audio signals in the volts.

I think the price difference between the two variants will not have been as much as it is now back when the COTA was originally designed.

I shall update the Builder's Guide to reflect this.

Tony

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Pav
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Post by Pav » Sun Oct 08, 2017 1:16 pm

Thanks Tony...
COTA is built and working...
I've got a first pass calibration in place ..I'm struggling
With cutoff frequency setting advice that basically lets me do my own thing.a recipe for poor calibration in my case.. :despair: .. In lay person terms is this about minimising the "dead" spots at either end of the sweep of the knob ..or skewing to one end.?
..I notice that a moog sub37 filter has sub audio for a quarter turn before I hear it..so should I try and match that?
rgds Pav

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Synthbuilder
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Post by Synthbuilder » Mon Oct 09, 2017 7:52 am

Pav wrote:In lay person terms is this about minimising the "dead" spots at either end of the sweep of the knob
Partly. But it's more about making every filter in your system behave over the same range.

Ultimately, you want the cut-off frequency pot to allow you to control the filter in a way that is musically useful when your usual sources of CV are connected to it. Thus one would expect to cover the whole audio band but with a bias towards the lower end. This is because many CVs are positive only (eg. velocity, EGs, aftertouch) so you do need to be able to get the filter to go low enough even when these CVs are being used. In other words do what the Moog does.

Tony

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