Summing CVs with passive multiples

This i more kind of a theoretical question. Let’s say that I have 3 envelopes, these all have a short attack and a medium decay. All go from 5V to 0. Let’s say I patch these into a passive (unbuffered) multiple and use the summed result to drive the cutoff of a filter.
Now let’s say at one point all these three envelopes start at the same time, what will happen? Will I get an envelope that goes from 15V to 0?

A passive summer is actually an averager, so you will get in the ideal case the sum of the 3 envelopes divided by 3.

The ideal case is when the 3 envelope modules all have the same resistor in series with their output. If this is not the case, the one with the lowest resistor will dominate. If a module has no protection resistor on its output (or if its protection resistor is in the feedback loop of the op-amp - something I do in some of my modules), then this module will dominate. Imagine three guys pulling a car with rubber bands. The car will never go faster than the speed of the fastest guy. The guy who pulls the stiffest rubber band is in control.

This is why summing through multiples is a bad idea - because it relies on a part of the circuit, the output resistor, for which there is no standard.

Thanks a lot for the detailed explanation! I think I really need a second buffered multiple :slight_smile:
edit: corrected weird sentence structure

“I think I really a second buffered multiple I think :)” --> how would that help you to mix the envelopes?

On a related issue: would it do any harm to use a passive summer (maybe with diodes, so an passive OR)?
When precision is not so important it could be a cheap and cheerful way to mix for example LFOs to create complex modulation. In this case also crosstalk (for lack of a better word … meaning interactions between the signals that you’d usually want to avoid) might even be counted as a feature.

One could even add a variable resistor to offset the protection resistor.

On the other hand, a proper dc mixer without attenuation is so simple that even i could manage to build one …

@morcego: I was referring to a second Links, and yes you’re right a normal buffered multiple would not do the trick. But the Links is not your run-of-the-mill multiple, it also has a mixing section, that can take 3 inputs and mix them with average gain to a fourth channel.

If anybody has a simple DC mixer schematic, I’d like to see it! :slight_smile:

The Mutable Links and Pittsburgh M3 (and others I would imagine) are inexpensive enough that it doesn’t seem worthwhile to build your own.

@rumpelfilter: ok, Links is a different issue.

There are plenty of schematics. The one for Links is also available (I think it is dc coupled). The simplest is probably foniks:

It uses: 1xTL072, 6x100k resistors, 3 other resistors, and 5 cheap ceramic capacitors. No reverse power protection, no nothing. But it doesn’t get more simple than that, I guess.

actually, the schematic from the Links 3->1 is almost identical (minus the LED). Unfortunately, I do not know sufficiently to understand the differences in design.

Foniks seems closer to another very simple dc mixer:

> would it do any harm to use a passive summer (maybe with diodes, so an passive OR)?

A passive summer is just some resistors going into the same junction point.

The problem with them is that the summing resistor must be at least one order of magnitude higher than the average protection resistor (1k) to prevent the module with the smallest protection resistor to take control of this mix. So let’s say a 10k summing resistor. But then you lose 10% of gain because all modules have an input impedance of 100k.