Wow, this is awesome! A clean looking build, too. I would love to see it in action. If you make a sick jam with it, please share! I am trying to finish off a couple of half done Music From Outer Space modules and then I plan on building one myself.
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Beautiful. Does the final version still use the same schematic that you posted on 5/3?
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yes, it does…
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I just breadboarded it - I might need to nudge the values around because I have a different pedal but it seems to work great. Well done - can’t wait to get it soldered and racked up.
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I made a Github repository:
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Nice. I put together a stripboard layout last night. I’ll solder it up this weekend and post once it’s confirmed working.
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Video
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it was interesting getting the back story on the build .
5 seconds in and I spit my beer
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Yes, red must be +12V since that’s what’s connected to the tip switches. But then the power connections on the TL072s are reversed.
(I’m surprised you didn’t use a TL074, but I suppose you had reasons.)
Yep, I threw out the old board and started a new layout, this time using a TL074, which made for a cleaner build and works much better. I want to post my layout, but I wanted to seek opinions on one small modification:
When there is no expression pedal plugged in, the input to one of the op-amp stages is floating, which seems to cause the corresponding output to behave erratically. I am considering taking the expression pedal jack’s switching ring pin and connecting it to the sleeve (ground) so that when no expression pedal is plugged in, the op-amp input will be grounded and the output will be locked in at 0 volts. My hope here is to prevent stray control voltages from going around if no expression pedal is present, but a cable is connected to the output. Does that seems sensible, or am I off in the weeds? I hardly trust myself any more.
I’m also finding that for some reason I still have about 0.3 volts output when my pedal is rolled all the way back, which I’m hard pressed to account for.
I used a 15k resistor on the LED to make it dimmer and to make (in my case) a smoother gradient from off to on as my pedal rolls forward. I also went with a 100k resistor from 12v to the CV input because I wanted to decrease the overall range. Your mileage may vary.
Once I am 100% verified on this layout I will post the TL074 dual pedal version.
Best,
-Wes
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Good catch on the floating op amp input. Your fix sounds reasonable to me but I’m far less qualified to say than some others around here.
I was puzzled by the same kind of thing until I remembered my pedal has a low limit pot and indeed it wasn’t at minimum. Having turned that down I see about 6 mv on the output with the pedal back, consistent I guess with the TL072’s offset voltage. If yours doesn’t have a low limit pot or if it does but you have it turned all the way down and you’re seeing 0.3V, that seems wonky to me.
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That should work, but if you connect the ring switched connection (RN) to the output of the first opamp, you could use this module as a 0 to 12V voltage source when nothing is connected to its inputs.
Do expression pedals really use the full range of their potentiometer? Does it go to 0 ohm between Ring and Sleeve?
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Antoine,
I think it most cases it does not, which I suppose accounts for the bit of extra voltage at the output.
Good suggestion on turning the module into a source of DC voltage. Since the output of the first op amp is already connected to the tip of the Expression jack, I suppose you can just wire together the switching tip and switching ring pins and bam - a handy little DC voltage adjustable from the range pot. I’ll give it a shot.
Edit: Was able to get rid of the extra 0.3 volts by fiddling with the switches and trimmers on my pedal.
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My comment in the other thread was about using one 100k for both CVs, which makes them depend on each other in more or less subtle ways.
Fredrik - I was concerned that might happen, but I thought I’d save a resistor. My inexperience is showing on that one.
On a lighter note, I revamped my layout and confirmed that it works beautifully, and Antoine’s suggestion works like a charm. Tames the floating output and adds extra utility to the module. I love it. Here is the layout for a dual version, which I have working happily on my bench:
Edit: Here is a link to the 3D printable cosmo panel. The description has a link to this thread and Analog Output’s github page (it’s showing an old draft of the text at the moment, it takes the site a while to show updates):
It didn’t occur to me before but the mod Antoine suggested also allows the module to serve as a good ole fashioned attenuator for incoming CV when the expression pedal is absent. This thing has a lot of utility. I’d be shocked if more people don’t build this.
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Yep, I just tested it myself. It’s a pedal interface, it’s a voltage source, it’s a CV attenuator, it’s a dessert topping. Thanks for pointing out the problem and thanks to @antoine.pasde2 for a fix that was more than a fix! I’ll go update the GitHub repository now (and I’ll throw in a link to the above dual pedal stripboard layout).
One note on that layout: I don’t see where the values of the pots are shown. 100k? Seems like you should get less than 6V max out if so…
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This seems like a total win. It’s simple as hell and it does so much. I can’t wait to see if it catches on (and I can’t wait for my second pedal to arrive). Analog Output, do you mind if I upload my own demo to youtube? I’ll of course give credit for the schematic and link to your github.
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