[WIP] SumVertDub - my first go at a doepfer standard module
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#1 — homegrown · 2024-07-26
Hey y’all,
Been wanting to dip my toes into designing video modular for awhile now. So I have been working on this design with a considerable amount of help from Bastien of Syntonie, so mega shouts out to him as always. In interest of being conscious of his time and in the spirit of this post, I thought I should start tracking my progress on my modular builds/designs here.
This is a 2-input summing amp, with active attenuversion as well as exposed outs for an inverted output of the sum or a doubled output of the sum. The design borrows elements from Syntonie’s VU002, VU004, and VU010, as well as attenuversion design from the LZX Cadet VII Processor.
To quote Bastien, these were his suggested changes that I implemented compared to the original designs:
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for the offset, I would add it after the attenuverter, else it will have effect only when the attenuverter is in the fully inverted position. So keeping RV2 pin 3 tied to fixed +500mV, summing the output of U5B with the offset generated by RV1, which is now tied to +1V/-1V. Adding a /2 divider on the positive input of the op-amp, this way when the offset from RV1 is at 0V, the output stays at unity gain. And the +1V/-1V from RV1 will allow to shift your sum all the way above 1V and all way under 0V.>>- > for the rectifier, the one I used in VU002 is in fact an inverting rectifier, so the output is inverted in relation to the input, also the way it is done generate a more noticeable artifact (like the rounded diamond peaks we’ve talked about earlier). So I’d recommend the one from Cadet Ramp, as it is done it also invert, but you can swap the connection from the diodes to the output differential amplifier in order to have it non-inverted. Also in VU002, since the rectifier input is connected to the outside world through the input jack, I’ve added the buffer which does x2 gain as well as offset around 0V. Here since you’re taking the signal from the sum output, which is already buffered, you don’t need this extra stage, simply connect it to the rectifier through a 499R resistor, this will give a gain of 2 with the two 1k resistor in the feedback loop, and then add the -1V ref through a 1k to center it around 0V. And add a +1V offset at the diff amp stage to bring it back into 0V/+1V range. Also by doing so, we can use the extra op-amp to do the attenuverter + offset summing as mentioned earlier.>>>>
Here is a link to the sim that Bastien created to explain as well.
1st rev PCBs and panels should be arriving next week
will keep y’all posted
#2 — homegrown · 2024-08-02
Hole-in-one!!! Super stoked that the module works entirely as it should on first go, though I credit Bastien mostly with that haha
Only things I really need to change is fix the front panel silk screen as it gets covered a bit by the pot washers/knobs and I also need to move the pot footprints a bit further from the edge of the PCB. I don’t think this warrants another prototype, though so production panels next!
I only had 3 LM6172s left so I used an AD8034 in lieu of U4. Lars recommends that AD8034s should mostly be used for unity gain buffers but it doesn’t seem to present an issue in this case.
Here is the module summing two DSG3 shapes and passing the sum to the green channel, the inverse sum to the blue channel, and the doubled sum to the red channel. Apologies for poor caps and moire, working on a better cap solution for my modular rig.
And then here it is summing a shape and a ~60hz sine
#3 — homegrown · 2024-08-19
Now available for purchase on my site
https://www.homegrownvhs.com/eurorack/p/sumvertdub4hp video-rate summing amplifier, inverter, frequency doubler with active attenuversion
Price: USD 15.00