[sdiy] CV keyboard divider resistor precision

[jays at aracnet.com]

Tom,

I build a reference with decade divider for octives and 12 divider for notes reference. The dividers were ran into switches and then to a op-amp summer with unity gain on the decade divider and 1/10 gain on the 12 step divider. I used a low drift and accurate voltage reference, good op-amp and hand matched resistors.

I took 1K resistors and matched them to get the exact same value using my best DVM which is a Fluke. Note that I didn't said match them to 1K but matched them to each other. I was more interested in the relative difference vs the absolute difference in the resistor dividers. I matched all the resistors at the same time so the batteries in the DVM and temp would be the same. Even though most resistors have a small temp co (most 1% are somewhere around 50-100 ppm per degree C) I did not handle them with my hands but used some tweasers. If you are like most of use you'll probably buy 1Ks in 100+ quanities so getting 9 and 11 matched sets is not hard.

I also matched the resistors in the op-amp summer. Unity on the decade and a 10 to 1 ratio for the 12 step.

The total resistance is 4.95K and with the 10V reference I am using that ends up being about 2 ma of current draw. If I remember correctly the reference I used could handle 10-20ma which is well within its limits.

Good soldering.
Jay S.

Tom Arnold wrote:

> While attempting to tune a couple VCO boards I got fed up and made a little
> box with a rotary switch that selects across a divider network.  I used 1%
> 10k resistors because they were what I had on hand but they dont seem to be
> quite accurate enough.  
> 
> What tolerance resistors are typically used for this?  I see the minimoog
> uses 1% but they are also much lower resistance ( 10ohm ).  A surplus place
> near me has some 4.5k .01% fairly cheap which I was thinking of picking up
> which should be more then adequate.  I'm also going to get a precision
> voltage regulator or 2 from them so my 10v is very stable ( my bench supply
> drifts a little ).
> 
> As my "build millions of filters" project finally nears completion, my next
> task is building VCOs so I figure a little investment will save me some time
> and heartache in the coming months.  Oh, and if anyone can talk Tony into
> doing one last run of Filtrix boards, I recommend it.  Finally got mine
> racked and its a beast I've actually had the thought "Maybe I should have
> gotten 2..."
> 
> -- 
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