Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MoparWade on April 27, 2016, 07:48:30 pm
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I have been playing with the Duncan amps Tone Stack Calculator and it lists 3 types of pots. Linear is obvious. What is not obvious is the difference between a Log A and Log B pot. Any insight would be much appreciated.
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Hi,
I think the difference is all about what resistance is reached at half rotation.
One reaches 30% resistance at 50% rotation, and the other 10% resistance at 50% rotation.
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I think the difference is all about what resistance is reached at half rotation.
One reaches 30% resistance at 50% rotation, and the other 10% resistance at 50% rotation.
You could know for sure either with the Help file (I presume it says in there...), or by replacing the particular log pot with a linear.
1. Use a given log pot as a Mid pot, set the resistance half-up, and take a snapshot of the curve.
2. Replace the log pot with a linear pot, set the resistance for 10% of the total log pot resistance you used before. Set this pot full up, and compare to the previous snapshot.
3. Repeat with the other log pot type. Or repeat step 2, but with a total resistance of 30% of the original log pot used.
4. Etc.
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As said: Ratio at mid-rotation.
A Linear will tend to be 50% at half way.
But consider a radio volume knob. You may want a weak station playing loud, or a strong station played soft. This is around 100:1 range of desired gains. If you try it with a Linear, the "strong played soft" gain happens about 1% rotation, or on a 0-10 scale, about "0.1". It is incredibly twitchy for low-gain settings. It also "doesn't do much" from 10 to 7 (your ear is more log than linear).
The old standard "Audio" taper was 10% at mid-rotation. Formed from two linear segments such as 90K and 10K. This gives a good spread of gains 10-6, and 5-1, with tolerable jump 6-5 and 1-0.
("Audio" pots with more sections to get the gain change very smooth have been made but are too expensive outside major studios.)
(True part-LOG pots were made for electro-mechanical computers but way too expensive for audio use.)
Some jobs do not need the full 10%-center taper. Also a 10% taper is very hard for the pot-maker to do (at the low-low prices we like). There has been a tendency to less taper. My observation is that general "Audio" pots tend to 20% taper, but I know 30% is used in some specific designs.
Taper is purely a user problem. Linear always works. But the knob "normal" setting may be far off from center for a log ear and a lin pot, and some of the range may feel all cramped-up at the end of rotation.
In simulation you can always use Lin, sweep your curves 0-10, then hand-adjust the simulated pot to find a nice "normal" curve. If "normal" happens around 10%-30%, buy "Audio" taper so the feel is generally right.
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Many thanks for the explanation
when I've read the question the first thing that comes to my mind was that there are Linear pots, log pot and reverse pots, nothing to do with the difference on % of the taper at 1/2 rotation, this info is new to me
Many Thanks again
Franco
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Thanks everyone! I played with the TSC a bit but knowing why it was doing what it was doing was unclear. Now I can see the light.