Gainbrain Issue

All four of the ones I built have the same issue and I have spoken to others with the same problem, so it must be design flaw. The manual knob fades between the two signals around 12 o’clock in a very narrow range , the rest of the rotation is essentially useless. The CV attenuation also has an extremely small range where it’s useful. I checked the pin on the LT1256 where CV goes and it ranges from -10 to +10 when sweeping the manual knob. The IC is only rated for 0-2.5 volts when fading between two signals which explains why only a small rotation of the pots sweeps between both signals while on the LZX Fader it works across the entire rotation of the pot.

Does anyone have a fix for this issue?
Chancing R26 from 10k to 1k should bring cv down to a more usable range, but isn’t perfect.

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Just pinging @VisibleSignals so he sees your thread.

Hi @jerry
I’ll have a look at this and come back with a fix/recommendation.
Sorry that I’ve been so quiet in the video community lately :frowning:
Thanks for the ping @rempesm <3

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Chiming in to confirm this is the same issue on my Gainbrains. I just sent you an email before I saw this thread @VisibleSignals

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Hi guys,
I’ve got a computer back up and running Eagle now (don’t ask) so let’s go.
Reducing R26 is a great suggestion - that will reduce the overall gain of the voltage going into the LT1251 chip (affecting both Manual and CV amounts by the same amount). But maybe start with more like 2.4K or so and experiment from there - smaller R26 value = less gain. Alternatively, omitting R13 would reduce the gain on just the CV input and not the Manual control.
Once the CV input gain is sorted, changing R4 to something much larger will reduce the voltage range of the Manual adjustment. 10K or more would definitely be better. My original builds of this were a bit different and I think when I changed R26 and R24 to 10K in the build guide I forgot to increase R4 to 10K to match!
Unfortunately I don’t have an accessible workshop or electronics equipment at the moment (sorry :frowning_face:) but if someone could try these changes out for me I’d be very grateful!

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Thanks @VisibleSignals, changing R4 to a 10K brings the voltage maximum down to 2.5 volts! It will still go down to -2.5 volts when turned CCW, even though this range has no effect. Seems like the Manual sweep was implemented as an attenuverter when an attenuator would have been sufficient. I think @rempesm had an idea on fixing this. I’m now noticing that the CV knob is inverted, can someone else verify the inversion after changing R4 to 10K?

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The negative offset on the Manual knob is intentional: it allows you to adjust the crossfade point to cater for arbitrary voltage offsets in your input signal (in case you’ve mixed a couple of signals together and the input to your CV exceeds 1V).

There’s an easy change to make the Manual knob only generate negative voltages (remove R6) but unfortunately getting it to only generate positive voltages would require cutting tracks and re-routing signals.

I can’t see how changing R4 could invert the polarity of the CV knob, sorry.

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So for someone building a Gainbrain, what is the TLDR for any of the resistor modifications for having a responsive Manual pot? should I just follow the BOM?

R4 should be 10K :slight_smile: .

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Thank you very much.

Thanks for solving this!
It took about a year for me to build my 5 GainBrains,
Have only used them once and was a bit perplexed by the narrow range of control but when they were dialled in they worked great.
It might take another 3 months to get around to correcting them… but at least I know it’s not just user error on my behalf.

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