Why not wire wound potentiometers for volume? - diyAudio
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Old 10th January 2009, 08:28 AM   #1
tem00 is offline tem00  United States
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Default Why not wire wound potentiometers for volume?

Hey everybody,

I'm new here and eager to build my first chip amp and related stuff. I thought about using a stepped resistor setup for my volume control to keep it real clean, but why not a wire wound pot? It's just one wire so i thought it may be a more pure path for the signal. They have a nice detent feel to them too.

I apologize if this is stupid question for all. I come from a laser background and totally new to fine audio. I tried doing a search, but couldn't much info about it.

Thanks
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Old 10th January 2009, 08:50 AM   #2
Mooly is online now Mooly  United Kingdom
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Hi,
Wirewounds tend to have very specific uses, usually high power rating etc and are usually only available in low ohmage values.
They would almost certainly be inductive, and I bet they would be noisy too in use. The wire would also be prone to pick up, or rather have stray hum currents induced into it.
For a chip amp I would recommend a good quality film type. The days of the traditional carbon pot are long gone, certainly for high quality audio. Don't spend a fortune either, go for a 10 or 47 K log dual pot, depending on partnering equipment.
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Old 10th January 2009, 12:33 PM   #3
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Quote:
Originally posted by Mooly
They would almost certainly be inductive, and I bet they would be noisy too in use. The wire would also be prone to pick up, or rather have stray hum currents induced into it.
...and due to their high power rating they are also bigger than usual potentiometers.
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Old 10th January 2009, 01:14 PM   #4
tem00 is offline tem00  United States
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Thanks Mooly!
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Old 10th January 2009, 02:13 PM   #5
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Quote:
Originally posted by Mooly
Hi,
Wirewounds tend to have very specific uses, usually high power rating etc and are usually only available in low ohmage values.
They would almost certainly be inductive, and I bet they would be noisy too in use. The wire would also be prone to pick up, or rather have stray hum currents induced into it.
A Helipot is a thing of beauty -- 0.25% linearity -- if you play in the piles of electronic detritus they can be had for a song -- new they cost about $40 or more:

Click the image to open in full size.

Inductance a problem -- perhaps mutual coupling is even worse.
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Old 11th January 2009, 01:27 PM   #6
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would really inductance be such a problem?

is not the inductance proporional to the length of the helicoil.
that way you would have both a resistive and inductive voltage divider, proportional to the turn of the pot.

Noise would be the same for any resistor of a certain resistance, and the wiper would probably have much less contact noise because you have much higher contact pressure than a carbon/conductive plastic pot as the metal wire is much harder. However, as you would have an oxide layer there could be difficulties with low voltages and currents.

helipots are cool! I always try to salvage them whenever I can.
The feel of turning that knob...
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Old 11th January 2009, 02:01 PM   #7
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For a volume pot you need a log taper and arn't most wire wound
pots linear?
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Old 11th January 2009, 02:33 PM   #8
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I have a couple of wire wound 10 turn potentiometers in the resistance range of 10 k but these r single channel to try...these r high precision removed from elec instruments.
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Old 11th January 2009, 02:39 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by woody
For a volume pot you need a log taper and arn't most wire wound
pots linear?
they can be wound from staggered wire diameters. Three diameters would give a nice law, but it'll be neither log nor lin.
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Old 11th January 2009, 04:44 PM   #10
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History lesson coming- be patient! Back in the days of primitive radios, service persons used a thing called a "tuning wand". This was a rod made of iron or maybe ferrite on one end, and brass on the other. They'd place an end near or inside a radio coil to determine which way to adjust it, for higher or lower inductance. Iron or ferrite increases the inductance, brass (or other materials) will reduce it.

When we want higher inductance, we use iron or ferrite cores. We hardly ever want lower inductance, but if you take apart a big precision wire wound pot, you'll find the resistance wire is typically wound on a heavy piece of enameled copper wire as a former/core. The diameter of the resistance coil is quite small, and combined with the copper core, the inductance is reduced to a very low level. My guess is they also pay attention to the direction of the wind, vs the direction of the entire coil around the inside of the housing, to lower the inductance slightly more.

Bottom line is, don't dismiss these out of hand. They work quite well in the audio band. OTOH, don't ever put one in a feedback loop!

More history- General Radio Corp. made a lot of very nice wire wound pots. They wound the wire on a flat piece of plastic (not sure exactly what) so the coil was flat. This keeps the inductance low. Then they bent the whole thing in a circle and ran the wiper around one edge. The neat thing is that by using a card with a taper, the wire length changes down the length of the card, and you get a tapered pot. That's how they got direct dB scales on their chart recorders- the feedback element was a tapered straight line pot. I don't know how low the inductance of this design really is, and it might be marginal for audio, depending on the value and how much of what gage wire is needed. Though it's sort of a project, the diy'er with a small lathe can make these quite easily.
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