Cadet Scaler Alternative builds discussion

Forum DIY
#1

So, after some discussion with @rempesm and others about the alternative builds of the Scaler, we decided it needs a topic to discuss the options.

I have layed out the ALT B schematic with the values as described in the Alt B BOM.
(newest version from Github)
This might shed some light on the question if this version scales from 0-1v to -5+5v

As far as I can tell, the voltage reference is used (R30 and R31 have specific values) to pull down the voltage to get bipolarity. U4 is placed (but for some reason C5 and C9 are not)

Scaler Alternate B schematic
Scaler Alternate B schematic2657×1704 65.7 KB

Notes:
R4, R18, R6 and R20 are not mentioned in the ALT B BOM .
I set these values to 100k, as used in the ALT A BOM and schematic.
C5 and C9 are DNP, but U4 is still used?

#2

R1/R4 are a voltage divider, so they can be used to scale down voltage ranges.
On a typical LZX cadet they’re often there, as 499R/100K, so inputs lose 0.5% of their voltage swing. In theory you might think you’d be better off without populating R4, to get 100% of your input voltage at the op-amp + input and not lose a tiny bit to ground, but then the op-amp input would be Not Connected when nothing is plugged in to the socket, which could cause unpredictable voltage fluctuations on the op-amp output. So it’s usually populated, as a trade-off to ensure empty sockets give 0V output on the buffer op-amp in this case.
I’d populate it with 100K (or greater) just as you’ve done.
C5 and C9 obviously should be populated for good practice, if you’re populating U4. But it’s only a TL072 so not all that bad.
Basically I agree with everything you’ve said and done :slight_smile:
Interestingly, as a consequence of this in a down-scaling scaler alternative that populates the R1/R4 (etc) voltage divider with significantly different voltages the impedance of the module will be different too. But I think that is only really important if you’re patching in to it with passive multiples.

#3

I’m going to build this one (just waiting on a few parts) and will confirm if it works. I have the other permutations (1V->5V, 5V->1V, +/-5V->1V) built so far.

#4

What BOM did you use for these versions?
Basic Schematic = ±5v to 1v
Alternate A = 5x gain
Alternative B (schematic above) =? 0-1v to ±5v ?

#5

For +/-5V to 1V, I used the BOM you posted here: https://drive.google.com/drive/u/0/folders/0B_gD4TorwvAdSzJzaG5Id0hOUm8

For 1V to 5V, I used LZX’s BOM: https://github.com/lzxindustries/documentation/blob/master/Cadet%20V%20Scaler/Cadet%20V%20Scaler%20BOM%205V%20to%201V.csv

And yes, for 1V to +/-5V, I used the schematic above. Should be able to finish building this and confirm by next week when parts show up.

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#6

@reverselandfill I built your schematic above this evening and it checks out fine on the scope. Thanks for puzzling that out!

@Z0NK0UT, would it be possible to update LZX’s BOM here to be relabeled as 0-1V to +/-5V instead of the other way around. The only errors are that R4, R18, R6, and R20 should be 100k resistors and C5 and C9 should be 100nf caps.

The +/-5V to 0-1V BOM here is correct on the scope as well and could be added to the site to cover most conversions folks would be after.

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