C5 Scaler BOMs Missing Resistors?

cadet

#1

I’m building a Scaler at the moment (1v to +/- 5v) and I don’t see r4, 6, 18, or 20 on the BOM. I looked at the BOM for 1v to 5v as well (guessing it would a similar value here) and it’s also missing. Pic is the schematic for 1v to 5v. They’re necessary, yes? I’m planning to build both 1v to 5v and 1v to +/- 5v. Both 100k here?
22%20PM


#2

You’re correct! Those 100K to ground should be in the alternate BOMs. Without them, when a plug is inserted, there will be a momentary point where the non-inverting input to the op-amp will be floating, and that could produce an oscillation.


#3

thanks – that makes sense. 100k for both of the boost versions, yes?


#4

Yes, R1 is a high impedance termination resistor. You’ll see 100K to ground termination on just about any EuroRack module (or should!! :smiley:). R1 provides some current limiting and isolation, since we’re not sure what might be on the other end of the line. This is the same function as the R2 output resistor. When a patch is made, these two resistors (R1 and R2) would be in series and add to each other.

When you mult signals in a system, the terminating resistors of the input nodes (R4, respectively) are put into parallel with each other, essentially dividing their values down. This is why when you mult several times, you see a small drop in signal amplitude.

I went on a ramble there, sorry!


#5

no apology necessary – most of what I know about this stuff I’ve learned from you – i’m soaking it all up.


#6

Are there spots on the board for them?


#7

Yes, they are on the board, it’s just an issue with the two variant BOMs (not the main one) missing a line item.