Hello, recently I started poking around some diy stuff I had on protoboards and the first to try and test were befaco’s modules. I successfully (kinda) built Rampage on 2 protoboards sandwiched together,
almost all functions were working correctly except rise fader. After adjusting that fader above 50%
(closer to 12V) output goes near-instantly to the negative rail, and after triggering nothing happens (I checked it for a couple of minutes thinking that the rise portion of output might take some time.)
Not wanting to waste time on that little issue I started with VC ADSR, firstly nothing really worked so I desoldered the whole board and started soldering small sections and testing them, I got to the point that envelope was working (I skipped gate extractor) but similar to rampage after turning attack pot above 50% output goes negative, which contradicted with falstad simulation. After a bit of thinking and isolating the problem on adsr board I decided to test the core on another board to see what was causing the problem. Here is what came after extracting the core from the rest of schematic (link to falstad sim): https://tinyurl.com/2p6brpv5
(While explaining my problem I will be referencing components names in falstad sim, not the original schematic unless said otherwise)
and here is a link to VC ADSR schematic from befaco: https://befaco.org/docs/ADSR/ADSR_V2_0_3_Schematic.pdf
I tested the circuit above and the results were similar to adsr and rampage. Now I don’t really know where to start explaining my problem but here we go: after soldering the core of the original schematic I tested it with different voltages at inputs leading to bases of T1 and T3 transistors, mainly max and min values (-12v,0v,+12v), and different gate voltages (-/+12V) to see what happens at those configurations (For testing I have used Tl074 op-amp and 2n3904/6 transistors, for sanity check I have tested this circuit multiple times with different components).
When both cv inputs are unconnected gate signal passes through U3 without any rise and fall stages, it’s only inverted (-12 at the output for +12 gate and vice versa)
positive gate:
negative gate:
Results are similar as above for when +12v is applied to cv input of T1 but after releasing the button oscillation starts.
positive gate:
negative gate:
When -12v is present at T1 input for positive gate there is rise(or release, depending how you look at it) section and after releasing the button there is a quick positive spike and release section after the spike, for negative gate there is just normal gate output with release.
positive gate:
negative gate:
Finally here is the problematic configuration, when -12v is applied to cv input of T3 U3 output goes as close as it can to the negative rail and pressing the button doesn’t have any effect on the output.
There are also configurations when different cv is applied to both inputs, whenever there is -12v on T3 input, the output is always negative, no matter what cv is applied to T1. For +12v applied to T3 there is oscillation on the output, no matter what is applied to T1
I have also made photos of the output of U2 op-amp. Mainly I was interested if U2 in real live will behave similarly to the simulation (spoiler, it didn;t) here is output of U2 when -12v is applied to T1 and gate is positive:
After those edge cases, I decided to see what cv do I need to apply to the input of T3 for the output of U3 to go negative. In the range between 0v and about -6.78v output stays at 0v, for the range between -6.78v and about -10.63v output drops to about -1V and after -10.63v it goes to negative rail, how fast depends on cv level, for -12 it’s nearly instantaneous.
I have searched for similar circuits to see what can I apply to the core, but after trying some things (different transistors, matched transistors, 10k resistor between collectors of T3 and T4 and - input of U3) I saw that there is no change, no matter what I do. For another sanity check, I tied both inputs of op amp to the ground to see what would happen (don’t know why I decided to do that) and on all of my TL074 output goes to the negative rail. I have read somewhere that internal offset is amplified with a really high gain factor, so perhaps that’s the cause of my problem. Another thing that I have read is that at 0Hz capacitor in the integrator act as an open circuit so again internal offset voltage might be amplified but why is that at those cv levels, I don’t know.
Right now I don’t really know what to do, I don’t want to write to (for now at least) befaco mainly because it’s not bought from them, it’s just copy of their design (Maybe only I see that as a problem, idk for now). If someone has access to VC ADSR or Rampage and would like to test what would happen to output if rise/attack fader is set to maximum or help me understand the problem I would be grateful.
(Edit: For those wondering if I have genuine components, the answer that I don’t know, I’m pretty sure that most of my op amps are fake but they do work in typical applications, except this one, but I have tried this with ones that luke most genuine and still notching chamged. I have tried changing transistors around between 2n39xx from ali, bc557/47 also from ali and 847 857 matched npn and pnp pairs bought form digi key, same results )
