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Topic: A nice article on pot tapers |
Ken Fox
From:
Nashville GA USA
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Posted 5 Jun 2012 6:23 am |
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Found this on the net and wanted to share it:
Volume controls are different. The human ear does not respond linearly to loudness. It responds to the logarithm of loudness. That means that for a sound to seem twice as loud, it has to be almost ten times the actual change in air pressure. For us to have a control pot that seems to make a linear change in loudness per unit of rotation, the control must compensate for the human ear's oddity and supply ever-increasing amounts of signal per unit rotation. This compensating resistance taper is accurately called a "left hand logarithmic taper" but for historical reasons has been called an audio or log pot. In these pots, the wiper traverses resistance very slowly at first, then faster as the rotation increases. The actual curve looks exponential if you plot resistance or voltage division ratios per unit of rotation.
If you used an audio/log taper pot for the control of the power supply we mentioned, the output voltage would increase very slowly at first, creeping up to maybe 10% of the final output at 50% of the pot rotation. It would then blast the other 90% in the last half of the rotation - very hard to control. Likewise, if we used a linear pot for volume control, the volume would come up dramatically in the first half of pot rotation, and then do very little change in the last half.
The dark horse taper is reverse audio, or more strictly "right hand logarithmic" taper. This taper traverses resistance very quickly at first, then more slowly as it is turned further. It's the inverse of the audio taper. This is used in some bias circuits and in controlling the speed of certain RC oscillators, which is where the audio tinkerer runs into it most.
The following diagram shows the three main kinds of pot tapers, along with one common approximation to an audio taper. Curve 1 is linear taper. If we clip one lead of our Ohmmeter (Hey! There he is again!) onto the leftmost lug, and the other lead on the center lug, then the resistance we read as we rotate the pot clockwise will fall on the curve that goes diagonally upwards. The proportion of the total pot resistance we traverse as we turn the pot is linearly proportional to the amount of rotational travel we turn.
Curve 2 shows what happens with an audio or logarithmic taper. As we turn the shaft, the proportion of resistance we traverse increases slowly at first, more slowly than the percentage of rotation. As we get past half the available rotation, the rate of resistance traversed speeds up as we get closer to the furthest rotation. This compensates for the human ear by increasing sound levels very slowly at first, then faster as the ear's sensitivity falls off at higher sound levels.
When we buy "audio taper" pots, we usually get something like Curve 3. For less expensive pots, manufacturers use a two or three-segment approximation to Curve 2. It's not perfect, but it usually works OK. Curve 4 is the typical resistance versus rotation curve for reverse log pots. In real life - that is, if you ever found one of these in real life - it is usually a two or three segment approximation, too.
If you have an unknown pot, you can figure out what taper it is. You measure the resistance from end to end, then turn the pot exactly to half its rotation and measure the resistance from the counterclockwise lug. The crosses on curves 1, 2 , and 4 show the most probable values. If the resistance is 50% of the total resistance, then the pot is linear. If you measure only 10% to 20% of the total resistance, the pot is an audio taper. If you measure 80%-90% of the total resistance, the pot is a reverse log taper.
At this point I should probably explain what a counterclockwise lug is. Of the three contacts on the pot, the wiper is easiest to pick out. If you turn the shaft fully counterclockwise, the wiper lug will show very small resistance to one of the other contact lugs. This is the counterclockwise or "cold" terminal. Turning the shaft fully clockwise, the wiper will show very small resistance to the most clockwise lug, also called the "hot" lug.
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Bud Angelotti
From:
Larryville, NJ, USA
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Posted 5 Jun 2012 9:29 am
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| So Ken, which of these tapers corresponds to a common pot pedal? Do you think the amp settings/tone settings fiqure in? You know how a "twangy" sound seems to cut thru? Also, since I switched to a tube amp from SS, the volume seems to cut thru further into the volume pedal travel. Not louder, but at a different spot in the pedal travel. Or are my ears playing tricks on me? |
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Ken Fox
From:
Nashville GA USA
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Posted 5 Jun 2012 9:46 am
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| Volume pedals use the audio or log taper pot. I use them on the front end of my tube amps. A lot of transistor amps use linear pots and make you think that the amp is very powerful at low settings! A great trick for selling amps, but not what I want in an amp. I prefer to be able to get a smooth taper on the vloume of the amp, not volume all at once. On a pot pedal or even a guitar volume pot there is a change in tone as you sweep across the pot. Wide open will yield the most treble. Some folks use a treble bypass cap to overcome that on the pot. That is exactly what a bright switch on a Fender amp does, a treble bypass cap is introduced on the volume control when the switch is engaged. That allows the highs to pass around the volume control's internal resistance. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 5 Jun 2012 11:00 am
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We're all after curve #2 for our volume pedals. Sadly, they dont really make them like that with a truly smooth curve. Back in the day they'd create a near curve by connecting 5 or so segments of the carbon track. That gave a pretty smooth curve like we find in the older pedal pots. The newer pots have more of a kink, like curve #3. That's why so many of us get frustrated with newer "good" pots. They hit that point where suddenly they begin to get louder very quickly instead of a gradual and smooth onset. Some newer pots have 3 sections to help round off that curve, but there's still a bit of kink. I plotted the curves of pots last year. I'll try to dig up the graphs that show the comparisons among the more common choices.
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Brad Sarno
From:
St. Louis, MO USA
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Posted 5 Jun 2012 11:07 am
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Brad Sarno
From:
St. Louis, MO USA
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Posted 5 Jun 2012 11:12 am
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The newer Dunlop has that distinct kink at 6 on the dial.
The PEC has a pretty nice and rounded curve, but a slower onset compared to others. Too bad the PEC's are notorious for getting scratchy prematurely.
The gold line of the classic Allen Bradley Type-J pots we all love clearly has the best curve with at least 6 sections of carbon track to give a nice rounded feel and the most "musical" logarithmic curve.
The newer Mexican made Clarostat type EJ pots have a really bad curve. Looks like 3 sections with kinks.
The BI Tech type has the worst curve. This is a less common pot, but some have tried it.
I'll take a vintage Allen Bradley J-Type 500k Audio Taper any day!!! |
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Bud Angelotti
From:
Larryville, NJ, USA
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Posted 5 Jun 2012 11:46 am
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| Ah-ha! That explains some of the SS vs tube sound. Is the new million mile pot a dunlop? I put one in a few months ago, don't remember what was the brand but it's a million mile pot. Also, because it's a standard goodrich pedal, the pot does not open all the way up, so of course, the amp is not "seeing" the top end. Just trying to get a handle on how these "tools" really work. Of course the bottom line is what my ears tell me. Brad, I'm using your SMS classic. So thats the Allen bradley pot on volume, correct? |
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Alan Tanner
From:
Near Dayton, Ohio
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Posted 6 Jun 2012 3:42 am
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| Ken Fox wrote: |
| Volume pedals use the audio or log taper pot. I use them on the front end of my tube amps. A lot of transistor amps use linear pots and make you think that the amp is very powerful at low settings! A great trick for selling amps, but not what I want in an amp. I prefer to be able to get a smooth taper on the vloume of the amp, not volume all at once. On a pot pedal or even a guitar volume pot there is a change in tone as you sweep across the pot. Wide open will yield the most treble. Some folks use a treble bypass cap to overcome that on the pot. That is exactly what a bright switch on a Fender amp does, a treble bypass cap is introduced on the volume control when the switch is engaged. That allows the highs to pass around the volume control's internal resistance. |
A good example of this is the Fender Hot Rod Deluxe. For me it was very hard to find a good middle of the road volume setting because it got so loud by the time you reached "3". Very touchy to get it set right. I know guys who still have them and they DO think that because the amp is so loud at "3" that they have a potent amp with lots of head room. |
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Earnest Bovine
From:
Los Angeles CA USA
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Posted 6 Jun 2012 8:19 am
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| Ken Fox wrote: |
| .A lot of transistor amps use linear pots and make you think that the amp is very powerful at low settings! A great trick for selling amps, but not what I want in an amp. |
Didn't old Fender tube amps use this shady marketing technique too?
"I can't be too loud. I'm only on 3." |
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Ken Fox
From:
Nashville GA USA
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Posted 6 Jun 2012 9:28 am
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| They used 1 meg audio pots on the older tweed, blackface and silverface amps. |
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Joseph Meditz
From:
Sierra Vista, AZ
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Posted 6 Jun 2012 7:12 pm
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| Brad Sarno wrote: |
| The newer Dunlop has that distinct kink at 6 on the dial. |
It looks as though you took 10 measurements per pot. Hence, all the kinks are on the x-axis major divisions. If you took 100 measurements per pot I don't think you would see kinks. I think these kinks are graphing resolution artifacts. |
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Will Cowell
From:
Cambridgeshire, UK
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Posted 7 Jun 2012 4:37 am
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Ken,
A small point but an important one, in an oft-neglected topic. You only get "brighter" tone with wider opening of the volume pot because the pickup has a finite impedance and the pot loads the P/U. The impedance is higher as the pitch/frequency increases (the P/U is inductive), so high frequency components of the audio are "potted down" more.
If the P/U was properly buffered first with a high-Z preamp, there would be no tonal variation with volume - a fact that is still not appreciated by most musicians.
_________________
Williams 700 series keyless U12,
Sierra keyless U14, Eezzee-Slide & BJS bars
Moth-eaten old Marshall 150 combo
Roland Cube 80XL, Peterson Strobo+HD,
EarthQuaker Despatch Master for reverb / delay |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 7 Jun 2012 6:50 am
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| Joseph Meditz wrote: |
| Brad Sarno wrote: |
| The newer Dunlop has that distinct kink at 6 on the dial. |
It looks as though you took 10 measurements per pot. Hence, all the kinks are on the x-axis major divisions. If you took 100 measurements per pot I don't think you would see kinks. I think these kinks are graphing resolution artifacts. |
Joseph. I first did some graphs with 20 measurements and it was still the same curve. However, maybe the Old AB pot was a bit more rounded. But the Dunlop was still the same with the same kink. This reveals the simple nature of how they connect a series of linear carbon tracks to emulate a rounded log curve. Pot makers admit that this is how its often done. The transition from one section of carbon track to the next is why there are kinks. With the dunlop you could take 100 samples per pot and still see that kink. There may be a slightly more rounded transition, but the straight line to straight line factor will remain.
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Will Cowell
From:
Cambridgeshire, UK
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Posted 7 Jun 2012 7:57 am
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Brad, what you say doesn't come as a surprise. I imagine it would be very difficult (i.e. expensive!), from a production process PoV, to produce infinitesimal variation in resistance gradient. I think you're right, and that the "log" track is actually a piecewise linear approximation. Obviously, the more pieces, the less abrupt the gradient transitions (kinks).
_________________
Williams 700 series keyless U12,
Sierra keyless U14, Eezzee-Slide & BJS bars
Moth-eaten old Marshall 150 combo
Roland Cube 80XL, Peterson Strobo+HD,
EarthQuaker Despatch Master for reverb / delay |
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