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
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
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.
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.
Mooly said:
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:
An externally hosted image should be here but it was not working when we last tested it.
Inductance a problem -- perhaps mutual coupling is even worse.
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...
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...
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.
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.
jackinnj is right about the price - i recently scavenged this wire round pot.
i wasn't sure if i would ever have a use for it but it sounds like it might be worth playing around with. i am building a buffer now so i may try it out in the prototyping stage.
it is too bad i don't have another so i can use it in a stereo set up.
in regular use 10 turns seems like a lot of work!
check out that plus/minus 0.05% linearity.
i wasn't sure if i would ever have a use for it but it sounds like it might be worth playing around with. i am building a buffer now so i may try it out in the prototyping stage.
it is too bad i don't have another so i can use it in a stereo set up.
in regular use 10 turns seems like a lot of work!
check out that plus/minus 0.05% linearity.
Lots of really good info from all, Thanks!
I actually have some of the 10 turn pots with scale, but never thought of trying them. They do look cool though. I also do some light machining so the flat wound idea sounds like an interesting project.
**IDEA**
as far as the old flat type as Conrad mentions,
I was thinking, if the wire was not wound around the card, but made a 180 turn on the edge (via a peg or notch) and then crossed back to the other edge and repeated until you had an inch or so of length, (all on the same face of the card), wouldn't that reduce inductance even further since the wire would never form a loop? The wiper have to move across the card in a straight line of course like a slider.
This description may not be clear so I will try and do a quick drawing and post.
I actually have some of the 10 turn pots with scale, but never thought of trying them. They do look cool though. I also do some light machining so the flat wound idea sounds like an interesting project.
**IDEA**
as far as the old flat type as Conrad mentions,
I was thinking, if the wire was not wound around the card, but made a 180 turn on the edge (via a peg or notch) and then crossed back to the other edge and repeated until you had an inch or so of length, (all on the same face of the card), wouldn't that reduce inductance even further since the wire would never form a loop? The wiper have to move across the card in a straight line of course like a slider.
This description may not be clear so I will try and do a quick drawing and post.
My control 'pre' used a pair of 10k Bourns 10-turns for more than ten years. They never went intermittent or made an undue noise. When the box was originally wired out the broadband separation was >110 dB and limited by the internal harnesses, not the pots. Linear taper isn't a problem if the amps have high sensitivity. If you find most of your listening is at the low end of the dial you'll probably never notice the pots aren't log.
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