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Old 31st March 2004, 05:16 AM   #1
sam9 is offline sam9  United States
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Default Pot with added feature. What & why ???

I've got a 100k dual pot which is otherwise an ordinary cheap pot. It has something I've not seen before - a fourth solder terminal. While the resistance between the wiper and either of the standard terminals varies 0-ohms to 100k-ohms, the resistance between the wiper and this extra terminal varies from 10k-100k. This must have some purported benefit but it beats me.

Anyone know why this is here and what it is good for?
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Old 31st March 2004, 06:15 AM   #2
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Default Re: Pot with added feature. What & why ???

Quote:
Originally posted by sam9
I've got a 100k dual pot which is otherwise an ordinary cheap pot. It has something I've not seen before - a fourth solder terminal. While the resistance between the wiper and either of the standard terminals varies 0-ohms to 100k-ohms, the resistance between the wiper and this extra terminal varies from 10k-100k. This must have some purported benefit but it beats me.

Anyone know why this is here and what it is good for?
It's a loudness tap which allows for a reduction in volume without having to change the position of the pot.

se
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Old 31st March 2004, 07:16 AM   #3
sreten is offline sreten  United Kingdom
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I'd suggest it has something to do with loudness buttons.

They are not for changing volume, they boost bass at low
volume settings and have less effect as volume is increased.

sreten.
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Old 31st March 2004, 02:23 PM   #4
sam9 is offline sam9  United States
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Sounds plausible. I think it is a feature I can do without although I'm a little uncertain how it would work - some kind og filter parralllel to the wiper-end stop i suppose.
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Old 31st March 2004, 03:00 PM   #5
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Those are indeed used for the loudness function. These were quite fashionable in earlier days. It is wired in a way (using caps and resistors) that low and high frequencies are boosted with low loudness settings -- in order to "compensate" for Fletcher-Munson.

Regards

Charles
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Old 31st March 2004, 03:11 PM   #6
markp is offline markp  United States
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Usually it is a 40% tap. Like others said for loudness compensation.
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Old 31st March 2004, 03:34 PM   #7
owel is offline owel  United States
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>These were quite fashionable in earlier days

I feel old. Yes, it's a tap for the loudness circuitry. I like them... the bass boost circuitry is only effective at low volumes.
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Old 31st March 2004, 05:55 PM   #8
sam9 is offline sam9  United States
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Can anyone post a schematic as an example?
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Old 31st March 2004, 06:01 PM   #9
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Hmmm. I guess it is for a loudness circuit of a sort, one which only gives an apparent bass boost at lower volumes. Since we lose sensitivity at lower volumes in both the lowest the highest frequency ranges, I couldn't see how such a tap could be used to implement both a bass and treble boost.

I did some poking around and the loudness circuit is implemented as a simple RC network to ground off the tap.

Learn sumfin' new every day.

se
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Old 31st March 2004, 06:03 PM   #10
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Quote:
Originally posted by sam9
Can anyone post a schematic as an example?
Click the image to open in full size.

From http://members.home.nl/kvdijke/Philips/B5X72A.html

se
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