Attenuverter Error

Cynosure · Posted: Wed Oct 07, 2020 5:14 pm Post subject: Attenuverter Error

Please take a look at the attached schematic. A CV comes in through the jack on the left and goes through what should be an attenuverter. However, it doesn't invert the voltage, even when trying to calibrate it with the trimpot.

It worked fine on breadboard :/

Any idea how to fix it? This is on a PCB, so hopefully I can cut some traces and run some thin wires.

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JovianPyx · Posted: Wed Oct 07, 2020 5:52 pm Post subject:

What I see is one non-inverting amplifier, followed by two inverting amplifiers. The two inverting amps for a non-inverting amp, so all you have is non-inverting amps. Hence no inversion.

PHOBoS · Posted: Wed Oct 07, 2020 6:13 pm Post subject:

I think R8 should not be connected to the wiper of the pot
but to the side that is connected to the input.

JovianPyx · Posted: Wed Oct 07, 2020 6:28 pm Post subject:

I've never seen an attenuverter like that.

What I've seen has a pot with the two ends connected to the + and - amp inputs with the input on the wiper. When you rotate the pot, if it's at the + input, it's non-invert, if it's in the middle - it's zero volts and if it's toward the - input, it starts to increase the inverted output.

Grumble · Posted: Wed Oct 07, 2020 10:42 pm Post subject:

This is the “standard” attenuverter schematic.

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JovianPyx · Posted: Wed Oct 07, 2020 11:46 pm Post subject:

I see. I like the "precision" circuit better with it's warped curve to make it easier to find the center point.

Grumble · Posted: Thu Oct 08, 2020 10:44 am Post subject:

Me2, in fact the precision version is what I use in my synth if there is a demand for an attennuverter Mr. Green

Cynosure · Posted: Sat Oct 10, 2020 5:50 pm Post subject:

Thanks everyone for the info. Is the schematic that Grumble linked to the precision one that you guys are talking about? I think mine will be close to that if I make the change that Phobos recommended. Maybe I recorded the schematic wrong compared to what I had wired up on breadboard.

Grumble · Posted: Sat Oct 10, 2020 11:32 pm Post subject:

This is the precision attenuverter. You can read about it at the blogspot of Kassutronics: https://kassu2000.blogspot.com/2018/04/precision-attenuverter-mixer.html

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Cynosure · Posted: Sun Oct 11, 2020 3:41 am Post subject:

I wonder if maybe I mixed up the inverting and non-inverting inputs in my schematic Confused

AlanP · Posted: Mon Oct 12, 2020 4:43 pm Post subject:

Grumble, by precision, you mean like the W-taper in this graph?

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Grumble · Posted: Tue Oct 13, 2020 9:48 am Post subject:

This is what Kasutronics calls the precision attenuverter and I just adopted this name, it is the one with the S curve.

PHOBoS · Posted: Tue Oct 13, 2020 12:13 pm Post subject:

interesting, don't think I'd seen that one.
I have used this one, which looks like what Jake was going for.

Cynosure · Posted: Wed Oct 14, 2020 12:45 pm Post subject:

The one I used was based on this one by Fonik: http://www.modular.fonik.de/pix/F_mixer.jpg

After reviewing his schematic again, I see that mine takes the voltage from after the pot to the inverting input, but his takes it from before that pot.

I will start there and see if I can get it to work Confused