Feedback, by its nature, does not have the same detail as using a full Operator as a modifying source. Feedback is simply the output of the source being fed back on itself. It does not have its own separate/independent AEG - like using another Operator would have. An Operator is a sine wave tone source + an amp and its AEG (amplitude envelope generator). So using an Operator to create an envelope for modifying another Operator is possible, but with the Feedback loop it is simply going to generate a more complex tone based on the AEG of its own Operator.

The energy derived from Feedback is channeled towards timbre.
To input energy with its own customizable envelope, it would need to originate in another Operator.

An Operator's pitch can be either Fixed (to a specific frequency, great for applying rates), or to Ratio (where the frequency it produces is proportional to the key that is struck). You can use a Fixed Frequency Operator to modify another Operator. The target Operator can be a Modulator or you can target a Carrier. A Fixed Frequency Operator can also be used as a Carrier.

"Fixed" frequency Operators can output frequencies down to 1 cycle per second, and on the reface DX as fast as 9772 cycles per second. But in creating a control source like an LFO, naturally you will concentrate on frequencies below the range of human Pitch detection (less than 20 cycles per second), so called "Low Frequency" oscillations.

"Ratio" Operators output variable pitches based on the ratio between the Key pressed and our musical scale. If Ratio is set to 1.00 and you play the "A" above middle "C" the pitch will be 440. If you play that same note, with the Ratio set to 2.00 the pitch will be 2x440 or A880, and so on.

It would be awesome to control the feedback with the individual Envelopes. Or, since I'm new to FM-synthesis, maybe my approach is wrong? Can you get the same results another way (not based on external CCs)?

The Feedback energy in this implementation of FM is channel toward generating All overtones (harmonics) toward the Sawtooth, or just Odd harmonics toward the Pulse family of waveforms. As mentioned above it has no detailed EG of its own - it's simply feedback. It is used here to generate complex wave shapes from a single Operator.

In order to generate a sawtooth wave in basic FM (where Sine waves are the source) it takes a minimum of two Operators in a stack. The Modulator (M) on top of a Carrier (C). A Ratio of 1:1 where both are set to 1.00 will produce waveforms that contain all harmonics.
A Ratio M:C of 2:1 will produce waveforms with every other harmonic (only the Odd ones) and can produce a square wave. The higher the whole integer a Ratio the narrower the Pulse 3:1, 4:1, etc. they all produce only the odd harmonics but the wave shape is narrower and narrower. The Modualtor is not heard, you hear its influence on the Carrier. So creating an AEG shape for the Modulator will have a timbre change effect on the Carrier.

So you can create a change in the application of the modifier by using multiple Operators.

That was probably the most complete answer ever, thank you.

This is directed directly to the moderator , for the second time ..caus it seems like last time you deleted my post

I have to comment on this , since it's not completely correct .

An envelope = a n a.c. signal ( audio) is multiplied b a d.C. signal ( direct current )which is the envelope .
We call this amplitude modulation .
Every operator in the dx series has that .
We agree on that
NOw ..feedback .
Signal going back to the operator .

On the orignal dx series , there was only 1 operator per algoritm that was capable of feedback .
Agree ?
Sometimes this feedback enabled operator was a modulator , and in some algoritms it is a carrier , straight to the ouput.
Soloing an operator with self feedback directly to the output gives a saw .
You don't have to agree , but it's a fact ..and the examples are given .
The reface takes this to higher level , every operator is feedback enabled .yamaha called this positive and negative feedback .
When the feedback signal changes polarity , it just becomes an upward saw ..
Yamaha choose this term' negative feedback ' just not to confuse people .
In fact ...the square wave is achieved by rectifying the feedback signal ( rectifie means absolute value )
It's a nice added bonus , because you don't need other operators to add harmonics , you can do it with just one .
Good .
NOw ....let's get back to our envelope .

The gain output of the operator is multiplied by it's envelope .
This envelope reduces the gain of the signal , it is that signal that on the origingal dx series is also routed back to itself when feedback is enabled .
I call this post -envelope feedback ..

Take the reface dx , dial in full feedback ( saw ) and apply a decaying envelope ( no modulators , just a parallel algoritm )
The result will remain a saw wave , the operator retains all it's full harmonics ..none are taken away ..
I call this 'pre envelope' feedback ..because the feedback block is not altered by the envelope , the gain reduction of the Output SIGNAL ( shaped by our envelope ) is not a part of the feedback diagram
The feedback loop is happening before the envelope output stage

Do this on any other dx synth , again one single feedback operator directly to output , the result will be a saw wave that will lose harmonics ..because the decaying envelope is providing LESS gain in the feedback loop

Honestly, I suspect that the feedback on the Reface's operators doesn't work anything like the older DX synths. Comparing it to a DX7 and TX81Z that I've got handy, it seems more like a variable wave shape than the self-modulating feedback thing.

Hooked up to an o-scope, I don't see the higher end harmonics or glitchy noise show up in the Reface's waveform like I do on the two older synths with the feedback and operator output all the way up. Makes it harder to do like a snare drum or hi-hat sound on the Reface, but it can crank out a lot of other noises that are impossible or really difficult to dial in on the others, (especiall the TX81Z missing the pitch envelope)

Adding some minus/squarewave feedback on the Reface's carriers does a better distortion or overdrive than the built-in FX if you ask me. 🙂