10-step sequencer questions

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mikemoretti
Learning to Wiggle
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10-step sequencer questions

Post by mikemoretti » Thu Jul 08, 2021 10:27 am

I'm using Ray's 10-step sequencer design as a base for a sequencer I'm making. In his design the 12v output from the decade counter is used as the voltage reference for the pots which are then summed into the cv output op-amp via 200k summing resistors (and the op-amp has a 150k feedback resistor making the gain .75 as he says, to have the range 0-9v).

In my design, I'm doing something similar. Instead of a decade counter I'm using a microcontroller (an Arduino pro micro because I want to share this design and make it easy for others to build) with separate output pins for each gate, which also act as the voltage reference for the pots (at 5v). I'm using 100k for the summing and feedback resistors to give me a 0-5v range. If I measure the voltage at the pot wiper everything seems right when that particular gate output delivers 5v to that pot. However, at the summing input, I'm just seeing 0v and the output of the op-amp is always pegged at 12v. Here's my schematic. Does anyone have any ideas what might be wrong? Thanks!
seq8th-portamento-v1.2-sch.pdf
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mikemoretti
Learning to Wiggle
Posts: 4
Joined: Thu May 04, 2017 8:08 am
Location: Boston Area

Re: 10-step sequencer questions

Post by mikemoretti » Fri Jul 09, 2021 4:53 pm

So, it turns out that using ground instead of -12v as the negative rail of the TL072 makes it not work properly as a summer. I think I ran into this once before. Or am I on crack?!?

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stepvhen
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Re: 10-step sequencer questions

Post by stepvhen » Fri Jul 09, 2021 5:22 pm

Small bit regarding the schematic: typically, its easier to read if there is a signal flow all going in the same direction. It took me a second to realize that this was not 4 separate but identical parts, but that half of the CVcom and gate outputs were facing the other way. Not a big thing, but if you're going to share the schematic, I would suggest following a typical layout.

Next, TL07X op-amps usually require both a positive and a negative rail voltage, and they cannot output all the way to the rail. When connected to +/-12v, for example, they can output approx. +/-10v. Using ground as the negative rail for the op-amp will kinda goof it up as far as working a a summing amplifier.

Aaron Lanterman has a good lecture on active summing ("Active Additive Mixing"), as well as a ton of other great lectures, usually from a electronic sound synthesis point of view, I really suggest checking them out:


mikemoretti
Learning to Wiggle
Posts: 4
Joined: Thu May 04, 2017 8:08 am
Location: Boston Area

Re: 10-step sequencer questions

Post by mikemoretti » Sat Jul 10, 2021 6:15 am

Yeah, I've seen his lectures. They're really good. It's just that I stopped doing diy synths a couple years ago after doing it for 10 years, and forgot the little important things like this power rail issue when I decided all of a sudden that I needed a good simple sequencer.

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