Loop pedal which controls volume of loop by volume into loop.

[Mr Noo]

I haven't seen anything that does this:  Adjust the volume of a loop as a function of the volume going into the looper pedal.

So for example, you would have a heavy fuzz in a loop, but the volume of the loop would be dependent on the volume of the guitar going in, so if you play softly then the fuzz sound would be soft (wheras it would usually be loud even when you play soft).

In other words, it would be a way of getting your dynamics back when using an effect which usually kills dynamics.

I have been playing withe the zvex loop gate and it is cool, but it is very on/off.

How would this be made?  Maybe some kind of simple photo-coupling?

[earthtonesaudio]

That's called a "sidechain" and is common in pro audio gear like compressors, where you want one source to modulate the volume of another.

Take a look at the Engineer's Thumb and notice how the envelope section (top half, U1b and U2a) takes no feedback from the compressed output (bottom half, U1a and U3a). To me that separation between envelope and audio makes it a side-chain type of affair.

Now the current control and LM13700 might not be the best option for general purpose audio effects; probably photoresistors would be better (they can be totally decoupled, passive, and don't even have to have a common ground with the rest of the circuit). But it's a start.

[Mr Noo]

I like the look of that compressor of yours!  I'm pretty new to this (not done electrics since school).  So I think I will go with the photoresister idea if it is a simpler thing to make.  And you say it could be passive?  It might need some mak-up gain though?

[earthtonesaudio]

That compressor is not mine, it's by a chap called merlinb.

The side with the envelope detection and driving the LED part of the photoresistor would require power, but yes, the photoresistor itself can be passive. You might want make-up gain as it's essentially a voltage divider, but depending on what you put in the loop you may already have enough gain available.

[TheWinterSnow]

That's called a "sidechain" and is common in pro audio gear like compressors, where you want one source to modulate the volume of another.

Take a look at the Engineer's Thumb and notice how the envelope section (top half, U1b and U2a) takes no feedback from the compressed output (bottom half, U1a and U3a). To me that separation between envelope and audio makes it a side-chain type of affair.

That is not a sidechain.  Sidechain is were a second audio source is routed through the compressor/expander/limiter to control the gain of the first audio source.  The envelope itself is not a sidechained signal.  What you are thinking of regarding the compressed signal having no connection to the envelope is called feed-forward the opposite of feedback, where feedback means the compressed signal would be used to create the envelope.

What you are looking for OP is what is called an expander, yes they do make them, they are just the opposite of a compressor, where increased input signal increases the gain of an amplifier.

[earthtonesaudio]

Things that perform this function fall under the general category of automatic gain control (AGC), which covers a spectrum that includes both compressors AND expanders. Commercial effects with sidechains often include compression, expansion, and gating (another point on the AGC spectrum).

In perhaps 100% of sidechains (I haven't seen them all), one signal's envelope (with optional control over ADSR params) is the independent variable, and the gain of the sidechain (with optional control of compression, expansion, gate, etc.) is the dependent variable.

For illustration purposes (ignore component values):


An envelope signal drives an LED, which shines on either rSeries or rShunt, which are photoresistors.
If more signal makes the LED brighter, then shining it on rSeries will make the sidechain output louder. If instead the LED shines on rShunt then you get the opposite effect. You could make a little window to let the light shine on: (rSeries, neither, rShunt) and adjust your way through the whole spectrum.

It's true that some use the term "feed-forward" to describe AGC devices that do not have feedback. But it's a special case of sidechain in which the same signal provides both the envelope and sidechain input.

[Lurco]

Discontinued: http://www.soundpure.com/p/toadworks-enveloope/1299