Stepped Attenuator vs Log Taper Pot?

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Jon Tornblom

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Joined 2010
#1
I tried to replace my Alpha A10k dual pot with one of those DACT Type 21 step SMD Stepped Attenuators you see all over eBay. The problem is that at the center position the Stepped attenuator is about 160 ohms. I emailed the seller and says that is correct. I emailed other sellers and they all say the same thing (could all be from the same factory anyway).

What? Maybe I've been out of math class too long, but I am pretty sure that a log curve should have be at 10% of the total value at 50% turn.

Am I missing something here? The Alpha dual A10k pot is ~1k at 50% turn, but the sides don't match.

The only thing I can think of is that those Stepped Attenuators are specifically designed for volume controls and not controlling the gain of an opamp, like this application calls for.

Any insights? Thanks!
 
#3
DF96 said:
What exactly is it that "is" 160ohms? An attenuator has three or four connections, so which two are you using for the measurement? You may be confusing a potential divider (or potentiometer, whether switched or continuous) with a rheostat (variable resistor).
Click to expand...

My apologies. Both have three pins, lets call them 1, 2, and 3. 2 being the wiper and 3 being the other pin I was connecting to, since in both pots the resistance between pins 2 and 3 was:

~0 Ohms at full counter clockwise rotation
~10 kOhms at full clockwise rotation

I believe the only difference between measuring between pins 1&2 and 2&3 would be the which direction of rotation increased resistance.
 
#5
mcandmar said:
This might be of interest, i plotted the resistor values of a valab 100k ladder attenuator vs a vintage Goldpoint to see what the curve looked like.
Click to expand...

That is of interest for sure. It looks like the Goldpoint is a bit closer to what I was hoping for yet the Valab looks more like what I received.

I think that what is going on with the Valab and the stepped attenuators that I received is that the steps are chosen for a value that reflects a linear change in dB when used as an attenuator, rather than a strictly logarithmic change in resistance.

I think I will just need choose my own values and solder my own custom stepped potentiometer. Not really a big deal.

However, I think that the term "audio taper" and "log taper" have been used interchangeably for continuous pots, but the stepped volume control units are not consistent with the taper for continous pots.

Hopefully this helps someone choose the right parts for their circuits.
 
#6
Jon Tornblom said:
I tried to replace my Alpha A10k dual pot with one of those DACT Type 21 step SMD Stepped Attenuators you see all over eBay. The problem is that at the center position the Stepped attenuator is about 160 ohms. I emailed the seller and says that is correct. I emailed other sellers and they all say the same thing (could all be from the same factory anyway).

What? Maybe I've been out of math class too long, but I am pretty sure that a log curve should have be at 10% of the total value at 50% turn.

Am I missing something here? The Alpha dual A10k pot is ~1k at 50% turn, but the sides don't match.
Click to expand...

Interesting. Seems inconsistent with a A10K audio taper part's spec sheet:

Spec sheet of Tapers of Alpha pot

Taper "A" (logrithmic) is relevant to audio taper pots. 50% rotation corresponds to about 5% for many common volume controls.

The only thing I can think of is that those Stepped Attenuators are specifically designed for volume controls and not controlling the gain of an opamp, like this application calls for.

Any insights? Thanks!
Click to expand...

I think that you may have what you actually want in this application.

BTW if you want a really nice audio pot, check out Alps RK27 or for extra performance RK40.
 
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#7
arnyk said:
Interesting. Seems inconsistent with a A10K audio taper part's spec sheet:

Spec sheet of Tapers of Alpha pot

Taper "A" (logrithmic) is relevant to audio taper pots. 50% rotation corresponds to about 5% for many common volume controls.



I think that you may have what you actually want in this application.

BTW if you want a really nice audio pot, check out Alps RK27 or for extra performance RK40.
Click to expand...

Hey thanks! That chart was also helpful. The A Series taper seems to have a lot of parts that hit 10% or higher at 50% rotation. All I know is that unity gain was at 1 o'clock before, and it was at about 8.5 with the stepped attenuator and the signal was like super quiet until about 7 and then it would just skyrocket after that. But like I said, this is obviously not the right application for that part since it is controlling the gain of an opamp.

Thanks for the suggestion on the Alps pots. I'll check out the RK40. This is probably the solution I'm looking for!

Cheers
 
#8
Just wanted to add that by looking into the Alps RK27, I understand now that there are different log tapers. The one I was looking for was the 15A taper. The one I received was an 05A taper.

This was in the chart of Alpha tapers in mcandmar's reply, but I didn't quite get that those 05A, 15A, 20A etc. designations were universal. The

RK27 is a 15A taper so that settles that.
 
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