Suitable pots for passive attenuator ?

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Noodle Twister
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Suitable pots for passive attenuator ?

Post by Noodle Twister » Tue Jun 11, 2019 9:33 pm

Can any wigglers suggest two good values for some potentiometer's in a passive attenuator DIY build ?

It's a combination of linear and logarithmic.

Thanks for any help :sb:

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Rex Coil 7
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Post by Rex Coil 7 » Tue Jun 11, 2019 10:52 pm

... pretty much anywhere between 10k and 100k.


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fuzzbass
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Post by fuzzbass » Wed Jun 12, 2019 2:25 am

What rex said. Worry less about value than mounting style, knob compatibility and turning force. If you get a headache from ambiguity, just get 50K.
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Noodle Twister
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Post by Noodle Twister » Wed Jun 12, 2019 11:46 am

Thanks for the replies , will go with 2 x A100k and 2 x B100k Alpha pots then.

:banana:

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Re: Suitable pots for passive attenuator ?

Post by MillionJT » Fri Feb 19, 2021 12:19 pm

Hi folks,

i hope it's ok to hijack that thread, but i'm in the middle of building a passive attenuator and got stuck at the pot question myself. I went to the Doepfer DIY site and read a lot of different threads on this forum about that matter. And a lot of people agreed on 50k potentiometers.

Especially after reading the Doepfer DIY site I felt good about myself and ordered a "bunch" of linear 50k pots. Now I get a weird (from my amateur POV) reaction. I only have a range of ~10%. After that i get close to 100% of the voltage.
I hope this piece of art is of any help.
Image

Now i tested a range of other pots and found out that i get the same reaction on all of them from 5k to 100k. With 2k it seems to work fine. But now i get a new issue. I tested all of this with an lfo who gives me about 10V peak to peak. But with the 2k pot i only get about 7 to 8ish volts. The same with a 1k pot. I get the full range but only get about 6 volts at the output.

Maybe someone can solve that riddle for me, i would greatly appreciate it.

migueldetrool
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Re: Suitable pots for passive attenuator ?

Post by migueldetrool » Fri Feb 19, 2021 12:22 pm

Do you know if you got logarithmic or linear pots ?

MillionJT
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Re: Suitable pots for passive attenuator ?

Post by MillionJT » Fri Feb 19, 2021 12:39 pm

Actually i thought of that too, but i tried pots from 3 different vendors. And the numbers my multimeter is showing me look fine to me. At least if i got the basics right. With a 10k pot i get 10k with the pot fully CW, 5k at the middle. So i assume i got linear pots.

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plushterry
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Re: Suitable pots for passive attenuator ?

Post by plushterry » Fri Feb 19, 2021 2:44 pm

you can normally tell the curve of a pot by looking at the value printed on it, A for audio or log, and B for linear. there a few other, more exotic ones too.

for example, if it says B50K or 50KB then it's probably a linear curve.

also, how do you have it connected?

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Re: Suitable pots for passive attenuator ?

Post by jorg » Fri Feb 19, 2021 2:57 pm

You need to show us the schematic, and tell us what are the input and the output connected to?

MillionJT
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Re: Suitable pots for passive attenuator ?

Post by MillionJT » Fri Feb 19, 2021 6:10 pm

@plushterry @jorg
Thanks guys. It says B50K, but i wasn't sure if i can trust that. That's why i tested a couple of them, and from what i can see they look fine.

At the moment i'm feeding them with an lfo going from -5V to +5V and just for fun to a steady 5V signal. I connected the ground of both the in- and output jack to the ground (first pin?) of the pot. The right pin to the input jack and the middle pin to the output jack. But maybe i made a mistake there.

Actually, there is no thought out schematic. At least none that i made myself. I went with the doepfer diy one.

http://www.doepfer.de/DIY/attenuator.gif

Beside that, i made all my measurements with a multimeter and with an oscilloscope. They both basically give me the same results.

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Re: Suitable pots for passive attenuator ?

Post by nigel » Fri Feb 19, 2021 6:28 pm

Are you sure you have the input and output the right way around?

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Re: Suitable pots for passive attenuator ?

Post by MillionJT » Sat Feb 20, 2021 2:55 am

I would say so. As far as I know, when you look on the poti from the front, the left pin is ground, the middle one is the output and the right one is the input pin. So i guess thats correct. Isn't it?

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Re: Suitable pots for passive attenuator ?

Post by pjbulls » Sat Feb 20, 2021 4:11 am

What do the circuits feeding into and drawing from the pots look like? You mentioned that lower values cause significant voltage drop on your output. Resistors before or after this attenuator can create parallel loads, which can significantly change the apparent taper of your pot (have a look at the second half of this page).

You have wired up your pot correctly for CW rotation increasing the signal, CCW reducing it. From what you're saying my guess is that your circuit is being loaded down (or insufficiently buffered) elsewhere - most likely on the input side.

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Re: Suitable pots for passive attenuator ?

Post by nigel » Sat Feb 20, 2021 7:39 am

MillionJT wrote:
Sat Feb 20, 2021 2:55 am
I would say so. As far as I know, when you look on the poti from the front, the left pin is ground, the middle one is the output and the right one is the input pin. So i guess thats correct. Isn't it?
Right. So the LFO is going to the right pin of the pot, and you are connecting the middle pin to your multimeter / oscilloscope?

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