[sdiy] Resistor matching in Bergfotron complex VCO

[sdiy at haraldswerk.de]

Just replace one of teh resistors with a trimmer and look if you can trim it 
out. If not, there is an other fault.

Harald 


 Am Sonntag, 27. April 2008 10:57:47 schrieb Aaron Lanterman:
> Howdy gang,
>
> One of my students, Greg, is working on building the Bergfotron
> version of the Buchla 259 triangle core. He's using the Linear Systems
> LS358 (free samples they kindly sent) for the current mirror, and
> LM394s for the diff pair in the OTA and the expo converter (I had
> gotten a bunch of those from Futurlec a while back, IIRC).
>
> The Bergfotron uses 5.1 k resistors. Our stockroom didn't have those
> so I told him to use 4.99Ks (I figure that's what's used in the
> original Buchla anyway).
>
> After an afternoon of debugging, we got it oscillating with expo
> control. (I assist mostly by asking the students to talk me through
> the circuit, showing me things on the scope as the go, and they most
> often figure out the problem on their own while I pretend to be
> insightful... there's something about explaining your circuit (same
> thing works with computer programming) to someone else that makes it
> easier to find bigs.)
>
> At higher frequencies, it looks pretty triangular, but at low
> frequencies it's more like a 20% triangle. (Also, when twisting
> various knobs to get the lowest frequency, it seems to only get down
> to 600 Hz, but I suspect that will be fixed once we get the triangle
> more triangular.)
>
> The Bergfotron webpage emphasizes matching the resistors at the top of
> the OTA - it says "Buy a belt of 100 resistors and find two that give
> the exact same reading on a 4 1/2 digit ohmmeter. Any error here will
> give a non-symmetrical triangle wave." The resistors Greg is using
> currently measure at 4.963K and 4.972K, which are a far cry from
> measuring exactly the same to 4.5 digits. But, before I send Greg off
> to spend an afternoon at a DMM measuring a truckload of resistors, are
> there other sources of nontriangularness we should be looking at first?
>
> Is that circuit really that sensitive to the resistor matching? That
> seems OK for a one-off like the Bergfotron, i.e. a design a person
> makes to build just one or two of - but it seems to me that would have
> severely limited the manufacturability of the original Buchla 259
> (although perhaps that shouldn't surprise me.)
>
> - Aaron
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