# Envelope Attenuator

#### diirt

Joined Nov 1, 2018
8
Hey y'all
I'm trying to attenuate a 5v envelope in a way that I'm not sure is possible in the analog realm.
I've attached a photo to help explain my intentions.
Ultimately, when the pot is fully CW, I want the envelope to go from +5v to 0v.
When pot is center, I want the envelope to sit at 0v (as if it's fully attenuated down)
And when the pot is fully CCW, I want the envelope to go from 0v to +5v.

The closest thing to this I'm aware of is an attenuverter.
However, its my understanding this would result in the following:
Fully CW = +5v to +2.5 envelope
Center = +2.5v
Fully CCW = 0v to +2.5v

My first thought is to generate two envelopes - one from +5v to 0v and the other flipped over +2.5v that goes from 0v to +5v.
Then mixing the two signals with a pot - however I'm not sure about how to have a centered pot result in 0v.

Any insights on how to do this, or am I wasting my time?

Thanks
-Z

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#### DickCappels

Joined Aug 21, 2008
8,714
Do you mean like this?

#### diirt

Joined Nov 1, 2018
8
Ooooo thanks! I’ll give this a try! Is there a name for this configuration?

#### KL7AJ

Joined Nov 4, 2008
2,229
My first thought is to generate two envelopes - one from +5v to 0v and the other flipped over +2.5v that goes from 0v to +5v.
Then mixing the two signals with a pot - however I'm not sure about how to have a centered pot result in 0v.

Thanks
-Z
That is exactly how I'd do it.

#### KL7AJ

Joined Nov 4, 2008
2,229

#### KL7AJ

Joined Nov 4, 2008
2,229
Hey, you're even cleverer than me! I always forget you can put signals INTO a wiper.
Which reminds me....someone asked Michelangelo how he made the statue of David. He said, "I just took a big chunk of marble and chipped away everything that wasn't David." That's why you're a great designer, Mr. Cappels. Get right to the point with the bare minimum of floobydust. Keep it up!

#### DickCappels

Joined Aug 21, 2008
8,714
I call it a differential amplifier being fed by a pot. I'm sure there is a better name for it.

#### atferrari

Joined Jan 6, 2004
4,597
It is the well known Cappel's adjustable differential amplifier.

#### crutschow

Joined Mar 14, 2008
29,828
Ultimately, when the pot is fully CW, I want the envelope to go from +5v to 0v.
When pot is center, I want the envelope to sit at 0v (as if it's fully attenuated down)
And when the pot is fully CCW, I want the envelope to go from 0v to +5v.
So you want the output to jump from 0V to 5V when it goes slightly off center in either direction?
How much off center?
Do you mean like this?
Wouldn't that give a plus and minus output?
The TS wants the output to be positive only.

#### diirt

Joined Nov 1, 2018
8
I tried out the circuit posted by dickchappel, but as crutschow suggested, the signal was flipped over the 0v line resulting in a negative output when the signal was inverted. I want the out to only be between 0v and +5v.

I'm going to try and explain this a little better.

I have one envelope. When triggered, it jumps up to +5v and then slowly falls to 0v.
Next, I want to create an inverted version of this envelope. But this inverted copy will be inverted over the +2.5v instead of 0v.
This means the envelope will be sitting at +5v until it is triggered. When triggered, it jumps to 0v and slowly increases to +5v.

So! Now I have two envelopes. And I want to attenuate them. So for the original envelope signal to jump to +2.5v instead of +5v before falling to 0v. I can do this easily with a passive attenuator with the envelope into one leg, the other leg to ground and then the wiper to the output. And when turned all the way down, the output will sit at 0v.
With the 'inverted' signal, if I want the signal to sit at +2.5v instead of +5v before being triggered, I can also use the same passive attenuator to scale my output. And, again, when turned all the way down, the output will sit at 0v.

However! I don 't want to have two separate attenuators or a switch involved. I want to have one potentiometer. And I want to have the center point of the pot be 0v on the output.
I've attached another photo with more clarity.

Thanks so much y'all!

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#### crutschow

Joined Mar 14, 2008
29,828
If I understand you correctly, it sounds like you need a center-tap pot with the center-tap connected to ground.