Weird VCA bleed

I have a oscillator out of a Loquelic Iteritas into my Er-301 in a sepatate case powered by a TipTop micro Zeuss. IN1 is fed by the Loquelic Iteritas in the same case as an Ornament and Crime (which along with the LI is in a TipTop Mantis). Pitch from the O&C sequence is patched to pitch in of the LI and I have the OUT1 on the ER-301 running out to a EXT audio in of a Mother 32.

I have only one chain on the ER-301 which is on channel 1 with in1 bringing LI in followed by a Linear VCA. I have B1 mapped to the level of the Linear VCA. As soon as I plug a patch cable into B1 i get a significant bleed of the sequenced tones of the LI (or at least the pitch information), whilst the other end of the B1 is not patched anywhere. If I plug that free end into an output of an envelope generator it diminishes (although not entirely) the noise of the LI and it’s output is heard as expected through OUT1.

Is this some kind of RF noise? It does not happen if the patch cable is plugged into any other input, and if I change the mapping to another in, the same thing occurs.

…ok so I typed all that and then discovered VCA bleed?, inserted a negative half rectifier and lo and behold the bleed stopped…I’m assuming that the free patch cable somehow allowing a small negative value that didn’t register on the scope…but then my envelope signal mapped to B1 not making it through the rectifier (despite it being a positive signal)

…so I’m a little confounded. Can anyone assist me please?

I know it can be considered rude to answer a question with another question, so please forgive me when I ask: Why did you insert a negative-half rectifier?

In the meantime, here are some factoids to help fill in some gaps:

  • The scope display is 64 pixels high. So this means that signals with an amplitude of about -30dB will only be 1 pixel high. That is still very audible.
  • On the ER-301, all of the inputs are normaled to ground (0V). Once you plugin in a cable, you break the connection to ground and the input is now floating until you plug in the other end of the cable. A floating voltage can and will fluctuate (sometimes wildly) depending on your electrical environment. This is unavoidable because eurorack requires that you use high impedance inputs (100k) which means that patch cables that are plugged in one end only become wonderful capacitive sensors. :nerd_face:
  • The difference between a linear VCA and an exponential VCA is that a small control voltage on an exponential VCA corresponds to a much much smaller gain then the same voltage for a linear VCA. This means that the small positive offset on your envelope that you never noticed when using exponential VCAs now shows noticeable bleed on a linear VCA.
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Generally causes friction between a colleague and I :grin:, but in this case i reckon it’s well and truly justified…based on the abovementioned thread I did that first and it worked, but I tried the other two as well thereafter and the negative one stopped the bleed…

Ok, great, thanks. What was confounding me was that I was considering the VCA control in the digital domain, and so not like the other MI module in the other thread, and I couldn’t work out where the voltaage would come from. I went to great pains to describe as clearly as possible the setup (as you can see) but now I can see that I did not need so much detail! I thought there may be some grounding issue across cases in the first instance. I did do a lot of patching and repatching and tracing etc.before posting.

I did imagine that a hot, arid environment would make for significantly more static charge around…

Is the moral of the story that exponential VCA’s are the best first choice?

Ah, I see. It was the result of a trial-and-error process. No problem with that strategy but helps me understand how to help you.

Also, I think it is really important to understand the difference between a linear VCA and an exponential VCA. Here is a great explanation:

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Not necessarily. It all depends on what kind of envelopes you have and what you want as the end result. They are both very useful. I tend to work with linear VCAs because they are better at being generic building-blocks.

You can still remove all the bleed with a linear VCA: Add a small negative offset to your incoming CV (this can be done with the Offset unit) and follow that with a positive-half rectifier (to ignore negative signals).

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That’s great, thank you. That video fascinating and very informative…and yes a small negative offset did all but remove that bleed and then allow the envelope to function.