#1

I want to send control voltages from Eurorack to the Bela.io. I need to convert -10V/+10V bipolar voltages to 0V/+4V unipolar voltages. I can use a summing amplifier circuit to do this. The calculator linked below gives me the resistor values I need, but what I donā€™t know is how to choose an op amp for this particular circuit. Anybody here have any advice for me?
http://masteringelectronicsdesign.com/summing-amplifier-calculator-java/

#2

so youā€™ll want something with good-enough DC offset performanceā€¦ What is good enough in this case? Letā€™s take a 1V per Octave, 0->10V CV outputā€¦

1 cent tuning = 1/100 semitone = 1/1200V

so if youā€™re crazy about pitch stability, shoot for 1mV offset voltage.

Could try Texas Instruments OPO7CP for a high-performing through hole part:

https://www.digikey.com/product-detail/en/texas-instruments/OP07CP/296-12853-5-ND/475983

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#3

iā€™d use a TL072 :smiling_imp:

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#4

I guess Iā€™d prefer through-hole right now, given that Iā€™m just breadboarding at the moment.

Iā€™m also finding 3 pages of results for TL072 on digikey, so would still need to narrow it down a bit. Iā€™m just not sure what variables I should be paying attention to.

#5

if you want TH youā€™re down to 5 options, all from TI, just get the one with the most stock

https://www.digikey.com/product-detail/en/texas-instruments/TL072CP/296-1775-5-ND/277421

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#6

Awesome, thanks!..

#7

The Mutable Instruments circuits are useful design patterns for this kind of thing. Olivier uses the MCP6002 / 4 chips - powered on 3.3v or 5v to match the microcontroller - because they work rail-to-rail on inputs and outputs, and include the kind of protection you need - so if you accidentally throw 12v at an input, they should stay calm and just output 3.3v (or whatever theyā€™re configured for) Here are a couple of examples, donā€™t take my notes as gospelā€¦

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#8

I think it would depend on your power supply. If you have +/-12v rails then a TL074 or similar would work. For a 5v supply then you need something like the MI circuit with more specialised opamps that can work close to the rails on limited supplies.

#9

Like tehn says, first of all, you want through-hole DIL-8, so thatā€™ll narrow it down.

And then: the letters after ā€œTL072ā€ tend to just refer to quality/tolerances. Ie: some are something like ā€˜mil-specā€™ which means they can operate in a wider range of temperatures. The datasheet will have more, but frankly, theyā€™re all TL072s, so the cheapest one that you donā€™t have to buy a bazillion of is probably what you want.

#10

I ordered some op amps and resistors and some reference voltage ICs. Next time I post you should see some progress on the eurobela box.

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#11

passing on an old tip from my dad on through-hole protoboard hacks using op-ampsā€¦ chuck a couple of 0.1uF capacitors or similar value right on the power supply pins of each op amp in your circuit. I.e from the power rails to ground, as physically close to the chip as you can.

This is one of those things which isnā€™t needed 80% of the time, but seems to mitigate the risk of ad-hoc protoboard layouts unexpectedly oscillating.

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#12

I have a feeling there is more to this story! :slight_smile:

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#13

yeah, this is pretty much a given for most ICs - and also, the 100n decoupling caps should be physically close to the ICā€¦

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