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How can I grow better POT?
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Chrometuna



Joined: Aug 14, 2006
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PostPosted: Fri Dec 08, 2006 10:29 pm    Post subject: How can I grow better POT?
Subject description: Why dont people design with radioshack in mind?
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So I try to build the APC.

But it wants 500K pots.
Radio Shack doesnt have those. They have 1M and 100K pots, and some in between, but no 500K pots.

I dont like to think for myself. It makes my brain cell hurt.

Is there a simple way I can shove a resistor to the pot to make it 500K from 1M? Or similar? What value of resistor?

Why am I so dumb and lazy?

Here in Tarpuka KS, we have Radio Shack, and ONE electronic supply store....but they have to order EVERYTHING! If you want an extention cord, they have to order it. If you want a X-value pot, and it isnt one of the two they have haning on the wall, they have to order it. And sadly, I can get anything cheaper off of Ebay or somewhere....which sucks cause Id much rather support the locally owned supply house.

I HATE HATE HATE having to order stuff and wait for it.....when I get the itch to build something, I want to make one trip to the store, get the goods, and start soldiering.

Has anyone figured out a good easy was to make PCBs (the etching end of it) ? I cant get the press and peel here.

Someone come over and hold my hand?

Cheers
bob c
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Blue Hell
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PostPosted: Sat Dec 09, 2006 6:18 am    Post subject: Re: How can I grow better POT?
Subject description: Why dont people design with radioshack in mind?
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Chrometuna wrote:
and some in between


If that inbetween vallue happens to be 470 k, take that. Adding resistors will change the curve. For a lot of purposes 220, 200 or even 100 k would work ok as well, but it depends on how it's used. For most practical purposes though 470k == 500 k.

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Uncle Krunkus
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PostPosted: Sat Dec 09, 2006 7:27 am    Post subject: Reply with quote  Mark this post and the followings unread

For a 1M pot with a 1M resistor in parallel, it doesn't change the curve very much. (You get about 340K at the half way point.) Well, that's what I'd do anyway. This is of course assuming that the pot is being used as a variable resistor, and not as a voltage divider.
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Chrometuna



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PostPosted: Sat Dec 09, 2006 6:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
For a 1M pot with a 1M resistor in parallel, it doesn't change the curve very much. (You get about 340K at the half way point.) Well, that's what I'd do anyway. This is of course assuming that the pot is being used as a variable resistor, and not as a voltage divider.


So is the value of the pot the CENTER value?

As in: A 500K pot would span between 100K and 1M, with 500K being the center?

Ive never understood how the values of pots are assigned.

Im not sure if I even saw any 470 pots in the Radio Shack drawers.

What I should probably do is just order via ebay or somewhere a big supply of pots in various values.
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DrJustice



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PostPosted: Sat Dec 09, 2006 7:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Chrometuna wrote:
So is the value of the pot the CENTER value?

The value is the full range of the pot. So for a pot of e.g. 470k, the value between one of the end-terminals and the wiper will vary between 0 and 470k.

Quote:
...order via ebay or somewhere...

Some other popular places to order components online are:
Digikey
Mouser
ELFA

DJ
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Chrometuna



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PostPosted: Sun Dec 10, 2006 12:16 am    Post subject: Reply with quote  Mark this post and the followings unread

DrJustice wrote:
Chrometuna wrote:
So is the value of the pot the CENTER value?

The value is the full range of the pot. So for a pot of e.g. 470k, the value between one of the end-terminals and the wiper will vary between 0 and 470k.


OK, so does that mean that any old pot will do? It would just limit or expand the useful range of the knob-twiddling?

0-------------------------------------------500K
0------------------------------------470K
0--------------100K




Quote:

Quote:
...order via ebay or somewhere...

Some other popular places to order components online are:
Digikey
Mouser
ELFA


I'll have to go look at those, thanks!
Do they take paypal?

It occured to me earlier tonight, that occassionally I come accross a junked mixing board (or occassionally Musicians Enemy will have some trashed returns, for cheap)

Maybe I should start snagging those and pillaging the pots and other useful hardware off of them?
DJ
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Uncle Krunkus
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PostPosted: Sun Dec 10, 2006 3:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Chrometuna wrote:

OK, so does that mean that any old pot will do? It would just limit or expand the useful range of the knob-twiddling?


In the case of the APC, yes. You could experiment with heaps of values on a circuit like that.

Chrometuna wrote:

It occured to me earlier tonight, that occassionally I come accross a junked mixing board (or occassionally Musicians Enemy will have some trashed returns, for cheap)
Maybe I should start snagging those and pillaging the pots and other useful hardware off of them?


Definately. I keep an eye out for stuff like that all the time. Council junk collection days are always a good bet to find something you can't find in a shop. Whether it turns out to be any use at all is another thing. There are certain members of society, ie: wives, who frown on the process of bringing large quantities of so-called "junk" into their living spaces. Crazy hey? but true! Laughing

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Chrometuna



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PostPosted: Tue Dec 12, 2006 1:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:

Chrometuna wrote:

It occured to me earlier tonight, that occassionally I come accross a junked mixing board (or occassionally Musicians Enemy will have some trashed returns, for cheap)
Maybe I should start snagging those and pillaging the pots and other useful hardware off of them?


Definately. I keep an eye out for stuff like that all the time. Council junk collection days are always a good bet to find something you can't find in a shop. Whether it turns out to be any use at all is another thing. There are certain members of society, ie: wives, who frown on the process of bringing large quantities of so-called "junk" into their living spaces. Crazy hey? but true! Laughing


I just scored a BIG old mixing board for $20 bucks! 16 ch, 8 bus. Lots of pots to pillage off of it....I hope they are of useful values! (Im sure they are!) And its got transformer ballanced XLR and 1/4" inputs on it, so those will be usefull. And I think its got 80mm faders on it, which will be cool.

Once I get it into my "lab" and start tearing into it, I want to see if I can save 2 or 4 channel strips to use as stand-alone pre-amps or other devices. But not having actually seen the inside of it, I dont know what format/how the channel strips are configured.

Another friend has an older cheapie 12 ch board he is giving me as well.

SCORE!

It has 6 meters as well.
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Uncle Krunkus
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PostPosted: Tue Dec 12, 2006 3:07 am    Post subject: Reply with quote  Mark this post and the followings unread

On something that size the knobs alone would be worth $100+ if you had to go out and buy them new. And believe me, once you start playing around with DIY electro-music putting knobs on all the controls costs money! Laughing
Keep a look out for hard to get linear/op-amp chips in mixers as well. If you're lucky there will be a virtually complete +/-15V PSU in there as well! Cool
Keep up the good work!
Let me know what you find.

P.S. Before you completely dismember both mixers, keep in mind that once you get a few different sound sources happening, you'll need a rough mixer to get them all happening at the same time. And also for sharing your effects around. I got a Fostex 350, 8ch recording mixer for $25 a little while ago, with the meter bridge included. It all "works" fine. I'm still debating whether to open it up or patch it in! Confused

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Chrometuna



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PostPosted: Tue Dec 12, 2006 11:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
On something that size the knobs alone would be worth $100+ if you had to go out and buy them new. And believe me, once you start playing around with DIY electro-music putting knobs on all the controls costs money! Laughing
Keep a look out for hard to get linear/op-amp chips in mixers as well. If you're lucky there will be a virtually complete +/-15V PSU in there as well! Cool
Keep up the good work!
Let me know what you find.

P.S. Before you completely dismember both mixers, keep in mind that once you get a few different sound sources happening, you'll need a rough mixer to get them all happening at the same time. And also for sharing your effects around. I got a Fostex 350, 8ch recording mixer for $25 a little while ago, with the meter bridge included. It all "works" fine. I'm still debating whether to open it up or patch it in! Confused


Ive got a Mackie 24 X 8 bus board. I love those things! (Dollars per donut, they cant be beat!) Its just rather large for carrying around to gigs. I need to get a smaller Mackie or Behringer 16ch rack-mounted mixer.

One of my goals is to ALSO build a PA system: I want a "backline" that LOOKS like Marshall 4X12 cabs, but are full-range, and is actually a PA system.

Once I get this board home and opened up, I'll try to remember to post some pics of the guts. That way some of you might help me identify things that are usable.

The problem with this board, and how/why I got it for next to nothing, is that there is no power supply.

Cheers
bobc
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Yorky



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PostPosted: Tue Dec 12, 2006 12:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Connect two 1 meg resistors from the ends of the pot to the wiper.
You're then ok to use in a voltage divider.
Disclaimer - I haven't tried this Wink

Code:
--------\/\/\/\/\/\/\/\/-----------
  |            ^              |
  |            |              |
   \/\/\/------+------\/\/\/--
               |
            wiper

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Uncle Krunkus
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PostPosted: Tue Dec 12, 2006 3:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've got a Behringer (does anyone know how to pronounce that?) MX1602 which I use as my main mixer. I know Behringer have a bit of a name for being cheap and nasty, but I've always found it to be reliable and quiet. I don't know if anything else really matters in a small mixer. It gets a good tweaking from my little assistant everyday, and has never missed a beat.

Yorky,
I like the idea, it does have some problems though. If you want the overall resistance across the pot to present as 500K then it would have to be a 1M pot with 2*500K resistors. This would be correct at the centre position. There would be a log effect either side of centre, and at each end the overall resistance would drop to 330K.

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Yorky



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PostPosted: Wed Dec 13, 2006 12:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry yes my original post said 2 x 470K but for some odd reason my fingers changed this to 2 x 1 meg

BTW this is a method for changing a linear pot to 'log' (which none of them actually are, of course)

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immigrantboy



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PostPosted: Thu Aug 09, 2007 8:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Heh, I too am way too lazy. I haven't multimetered/otherwise gauged anything to date.

What happens if you use an disproportional pot then? Most of the projects I'm working on use 6-9 volts dc, and I've been experimenting with a 50k ohm pot, used as a voltage divider.
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Uncle Krunkus
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PostPosted: Fri Aug 10, 2007 2:25 am    Post subject: Reply with quote  Mark this post and the followings unread

immigrantboy wrote:
What happens if you use an disproportional pot then? Most of the projects I'm working on use 6-9 volts dc, and I've been experimenting with a 50k ohm pot, used as a voltage divider.


Well, if you try any of these tricks with a log pot (They usually have an A printed on them somewhere) then it gets even more complicated. Usually, the non-linearity of the pot will be compounded into an extremely non-linear response, but there are ways you can do it which bring it back towards being linear.

I've found the best way to predict this sort of thing is to knock up a rough spreadsheet in Excell and play with the numbers.

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