Pot jump?

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Pot jump?

Postby throckmorten » Sun Jul 11, 2010 12:29 pm

What makes a pot jump suddenly when you get to around 10 or 11 o’clock, so that last little turn dramatically affects the volume or tone?

I thought this was just cheap electronics. But the 51 RI doesn't see to have it and the cheap little Gretsch short scale with single pickup has one of the smoothest graduations on the vol and tone pots. The problem seems to be noticeable on passive basses with two pickups and vol vol/tone tone. My Lakland hollow body jumps noticeably, and I have a low end Peavey that does it in spades.

I’m aware that the pots either attenuate volume or cut highs. And I believe the pickups are wired in parallel. There is a bridging cap on the tone pots. There seems to be a relationship among the settings because the jump is not noticible all the time. I think possibly when each of the other 3 is set max you hear it most on your last full turn. Is there a way to minimize the jump, to make the transition from min to max smoother?


Thank you,
John
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Re: Pot jump?

Postby jshilgebass » Sun Jul 11, 2010 2:53 pm

I'm like you, in that I have a few V/V/T basses where the volume goes from very low to very high in one quick jump, within the last 1/5 of a turn or so. I'd like to find a solution, so I'm just piggybackin' here... [smilie=fun_84.gif]


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Re: Pot jump?

Postby bigfatbass » Sun Jul 11, 2010 3:43 pm

I hope there is room on that piggy's back for one more
"Time Is the river, eternity is the ocean, music is the water.
Play what you hear, hear what you play.
Trust your ears, serve your soul." ~Philip Toshio Sudo "Zen Guitar"



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Re: Pot jump?

Postby aussiemark » Sun Jul 11, 2010 3:54 pm

Some pots are linear, some are logarithmic. Log pots are common for volume controls.
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Re: Pot jump?

Postby harleyyy » Sun Jul 11, 2010 4:05 pm

What Mark says...

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.



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Re: Pot jump?

Postby Templar » Sun Jul 11, 2010 6:49 pm

My Lakland hollow body jumps noticeably...


This was my gripe about my Lakie hollow, too. (well that, and the grafted-on headstock) Mine was impossible to blend pickups to taste.

Knowing that pots can have different tapers, I just figured Lakland was cheapin' out on the Skyline elec components. I say this because I know M Tobias was involved in the design, and he's not one to spec out elecs that don't work right.

Too bad, it was a beautiful bass. (sigh)

Always wondered if the USA hollow had the same issue? [smilie=coz.gif]
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Re: Pot jump?

Postby throckmorten » Mon Jul 12, 2010 10:01 am

Thanks guys, I understand the basics of human hearing and of log pots, tapers and voltage drops across resistance.

But I also have basses (and low end ones at that) that have an even graduation (or a non-linear, logarithmically adjusted, aparent graduation) from low to high and others that don't.

I'm trying to figure out if the v/v/t/t/ configuration is responsible for the difference and what I might do to fix or minimize the problem.

If new pots will do the job, what should I look for? Or maybe it's just the value of the capacitor in the circuit.

Thanks again.
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Re: Pot jump?

Postby throckmorten » Fri Jul 23, 2010 6:38 am

Hope ya'll don't mind if I reply to myself.

From the PhD thesis on pots the audio taper pots would seem like the logical choice to fix the jumping problem, but logic and experinece don't match. The taper pots may contribute to the problem.

I've learned from elsewhere that audio taper pots are logical enough for the amp controls but not necessarily for the instrument. I got taper pots. I got jump. Probably because of the attempt to compensate for how our ears work. Log pots work.

I still think the v/v/t/t config is part of the issue but I can't get a handle on it.

It's a simple enough experiment with a couple pots and some gator clips. It takes me forever to get around to these projects but I'll order a couple different pots and let you know how it goes.
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Re: Pot jump?

Postby throckmorten » Mon Aug 16, 2010 6:28 am

Well, my first experiment was a failure.

The biggest jump seemed to be in the tone pot when the vol was cranked up full so I clipped in a 500k linear pot. No change.
So I put the tone pot back in (I didn't physically remove the pot, just unsoldered a lead and used gator clips to switch in the new one) and swapped the new pot in for the Vol.
Again no change. [smilie=smilie_kopf.gif]

Another attack would be to change out both pots but I ran out of time and patience.

It might be intersting to see if hooking the tone pot up with the taper reversed has any impact.
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Re: Pot jump?

Postby jshilgebass » Mon Aug 16, 2010 5:53 pm

I'm still following along with your pot experiments, so don't give up! [smilie=grinning-smiley-003.gif]

(...I meant "potentiometer" experiments, not :toke:)

[smilie=cheeky-smiley-025.gif]


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Re: Pot jump?

Postby throckmorten » Mon Sep 13, 2010 10:13 am

In your best Boris voice: I wanted my experiments to benefit mankind but alas my pots still jump.

I give up. Linear, tapered, vol, tone. Swap in, swap out. No dif. [smilie=smilie_kopf.gif]
I've tried most combinations I can think of. Could be I'm not clipping them in right (but how hard is that!) maybe the problem doesn't come from the pots alone but the pickups or the wiring.

Has anyone tackled this before?
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Re: Pot jump?

Postby Rob Francis » Mon Sep 13, 2010 5:12 pm

I'd be tempted tp ping Butch or Psyco Bass they probably have
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Re: Pot jump?

Postby Templar » Mon Sep 13, 2010 5:56 pm

Has anyone tackled this before?


Dunno if there's an answer, but if there is, it must be a secret. I recall emailing Lakland about it, and I posed the question in the Lakie forum on the old dudepit. Crickets.
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Re: Pot jump?

Postby throckmorten » Tue Sep 14, 2010 6:17 am

Thanks.
The problem is far worse in my bottom end Chinese Peavey than in the Lakland so quality of parts probably plays a role. And there may not be an easy way around it in a v/t/v/t configuration.

OK, so I'll find the sound I like and glue the pots in that position, let the amp take it from there.
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Re: Pot jump?

Postby noahvale » Tue Sep 14, 2010 7:41 am

It's caused by the inductance of the pickups. Volume pots actually short the pickup output to ground to reduce the volume. It's not a linear function even with a linear pot. Rick Turner wrote an article in Bass Player many years ago on the subject. His solution was no pots. A friend of mine built a fretless PJ and wired it with just a three way selector switch. It sounded great.
I rewired my Swampbass with a master volume and volumes for each of the two pickups. I can dial in the sound and then adjust just the volume with the master. It works well, but what I really like is the Bartolini NTMB preamp I put in my Hoyt. The B stands for buffers, one per pickup. The blend really works, no difference in volume all the way through the range. I don't use the tone controls, but I really like the buffered blend.
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