And was that a working circuit?
As I said, I’m not too familiar with OTAs.
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I’m not either; I just found this application report which might help remedy that. My very very limited understanding is that an OTA output is a current, not voltage, source, so presumably connecting them together just adds the currents and is okay. However, you can’t input current to an op amp — that current needs to become a voltage. It needs a load resistor to ground. And then an inverting amplifier such as what you’re evidently trying for needs an input resistor; the gain is Rf/Ri (feedback resistor over input resistor) and here essentially Ri = 0.
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Adding currents makes sense, so there might not be a short after all. But there then still would be a need for a resistor to transform the current into a voltage for the op-amps that follow it, I would assume.
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According to the topic in Muffwiggler, it seamed so
OKay I think I worked it out. Apparently an op amp can be used as a current to voltage converter. So in this instance, the op amp isnt being used as an inverting amplifier, but as a transimpedance amplifier. This vid explains it pretty well: https://youtu.be/OMnZehJNGCY
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No, all that’s needed is the feedback resistor; the incoming current has to go somewhere, so it goes through the feedback resistor which results in a voltage drop and the opamp has to pull up the output voltage to compensate. We went through this in the 1184 VCA thread after noticing that it had a pointless series resistor.
I just posted an explanation of the LM13700 VCA core over here, btw.
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This video explains OTAs in a clear way:
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Apologies - I just pasted the schematic together crudely in PhotoShop, so no .sch file…
Ah okay, no worries!
Just went to buy myself one from the shop… and it’s sold out!
@lookmumnocomputer - are there plans to re-stock? I hope so!
I can see the schematics and info on the Project Pages… but soldering up a PCB is a lot easier than trying to work this out onto veroboard!
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hey!!! they should be available now :O.
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They are!
Thanks Sam!
Purchase underway right now!!!
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You username reminds me of early 90’s UK Dance music 
… A genera where wearing a face mask was cool ( unlike CV19 culture… )
Banging!!
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This is the only module I’m having a problem with… The output level is significantly lower than the input… with or without the effect engaged. Does anyone have similar issues… or have any idea where I should be looking? I’ve reflowed everything twice 
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what do you have plugged in? it shouldn’t be crazy quiet though. double check if R42 and R43 are correct they are incharge of the output level. increasing R43 will increase level
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Thanks for the reply. I have just had a performance VCO plugged directly in to it. I’ll check out those resisters, thanks
I remember (bits of it) well… mask, whistle and the white gloves…
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Did we solve the time LED mystery?
my LED does not do much…
hey! not sure what the question is regarding time LED. but the time led is sort of more of an indicator of getting into high territory. when the time is very high the last few volts are pretty samey, its more of a clipping sort of thing for the high end of the voltage throw
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