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I have so many questions as to how they're handling controlling knob changes! From the pictures it looks like they've yanked the pots out and hooked the boards
by ericwood 6y ago
I have so many questions as to how they're handling controlling knob changes! From the pictures it looks like they've yanked the pots out and hooked the boards up to their rig, which raises more questions.
This is something I've tried myself, and it's not trivial. Digipots won't work in the signal path and are noisy and imprecise, so companies that do this (there are very few!) like Chase Bliss use vactrols. Using those precisely is still tricky, as pot values in many circuits are all over the place and have different tapers, whereas most vactrols have a voltage/resistance curve that looks more logarithmic.
Would love more information on this, as it's a topic that's difficult to research.
- diggan 6y agoThis is the only snippet of information I could find about it. Seems indeed like noise and imprecision would be issues even if replacing the knobs and switches, the imprecision you could live with as it's a demo, but the noise certainly is not wanted. > The first step is to dismantle them, measure the potentiometers and replace them with digital potentiometers and switches. https://www.thomann.de/blog/en/stompenberg-fx-speaker-simulator-update/ https://www.thomann.de/blog/en/stompenberg-fx-speaker-simula...
- RedCapybara 6y agoMost of them are controlled with Digipots, but yeah indeed it was not easy to get it working for all the different setups.
- ericwood 6y agoDid you run into noise issues? I actually haven't tried a digipot in a signal path (they do work great for other controls like LFOs and the delay time on a PT2399), but everything I've read online has told me to avoid it. I'm not hearing any in the demos so clearly it's workable!
- RedCapybara 6y agoMany many problems (several revisions), but I cannot go into them, because fortunately I was not part of the hardware design team. Definitely nothing I would recommend as part time project.
- ericwood 6y agoNo worries, I appreciate the responses, knowing it's a digipot has satisfied most of my curiosity! :)
- unmasked_poker 6y agoThe digipotis were actually one of the hardest problems for this project. Additional to the many pot values and different tapers, you need to also cover a wide variety of voltages that can even be centered around zero (it is an audio signal after all), so you need to be able to handle negative voltage. DigiPots also have a capacitance, so when you have to replace high values like 1M-Ohm you will wind up with a low pass filter. We built a bunch of modules for common pot values and do the taper and uncommon values in software. If both sides of a poti are used, we will need to use two digipotis to simulate them.
- ericwood 6y agoSuper interesting, thanks for taking the time to chime in! I hadn't thought about the capacitance aspect. Was noise not as much of an issue as I've been led to believe?
- unmasked_poker 6y agoNoise was a hard problem as well. In the end what helped us a lot was to keep the FX device in its own case and even solder the digi potis right in place where the real potis had been before. You can see that here: https://im.static-thomann.de/pics/images/stompenberg/backstage/1.jpg https://im.static-thomann.de/pics/images/stompenberg/backsta... The relais modules are on the outside of the case, but on the left you will see a flatband cable (digital) run inside of the case, where the digipoti modules will sit. So all analog signals never leave the original case and the metal housing shields all kinds of external interference.
- deleted 6y ago[deleted]
- squarefoot 6y agoDid you consider driving the original pots in place through servos fixed to the pots shafts through shaft extenders? If yes, were there any drawbacks that made you discard the option?
- unmasked_poker 6y agoOur main concerns were a) the conversion process would be much more complicated b) hardware would be more expensive c) the motors would not live as long as the digi-potis
- bydo 6y agoSince you seem to have been involved with this in some way: thanks! You've earned at least one customer who had never heard of your company before.