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Need fairly simple help
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babuf77



Joined: Aug 05, 2007
Posts: 21
Location: Michigan

PostPosted: Thu Aug 09, 2007 6:41 am    Post subject: Need fairly simple help
Subject description: Noobish design questions...
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As I wait for the PCB for the Weird Sound Generator, I wish to construct a little project just to get myself together.

I essentially want to make a crossfader, along with something to adjust the balance (from left to right) of my music.

I don't know what is possible, but I'd like two audio inputs going through probably two sliding potentiometers for crossfaders, to my output, correct?

For balance, I know a stereo plug has two wires plus a ground, so how would I wire up a slide so that in the middle there is no resistance?

Does anyone understand what I'm going for? Maybe? I could figure out how to use a pot for volume control, but is it possible to use one for switching between inputs like this?
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elektro80
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Joined: Mar 25, 2003
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PostPosted: Thu Aug 09, 2007 7:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Something like this?
http://www.epanorama.net/circuits/linemixer.html

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babuf77



Joined: Aug 05, 2007
Posts: 21
Location: Michigan

PostPosted: Thu Aug 09, 2007 7:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, thank you. I couldn't find anything like that.

Is there any way to minimize the "attenuation" of the mixer? Also, how does one figure out what potentiometer values to use?

Thanks a ton,
Matt
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elektro80
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PostPosted: Fri Aug 10, 2007 1:34 am    Post subject: Reply with quote  Mark this post and the followings unread

I haven´t time to look that one over, but aren´t the pots at 10K?

There are other schematics out there too, but this one seemd like a pretty simple one. Perhaps I should move this thread into the DIY area?

Very Happy

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babuf77



Joined: Aug 05, 2007
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Location: Michigan

PostPosted: Fri Aug 10, 2007 5:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, please do move it if that will help. I'm trying to understand it, and this is the most confusing schematic I've ever tried to read.

I'll attach it so people don't have to click the link, but I'm assuming straight intersections are not connected, and circles mean a connection? Also, why must each pot be grounded out on its third prong? I thought it worked like an ordinary resistor, where you would have the input, and then straight output, not using the last pin.

I think I've pretty much got it though, especially if anyone can explain to me why R5-R8 are necessary. They seem redundant to me.

Adding a vertical cross fader would be pretty simple it seems, although having a horizontal one with "0" in the middle, I have no idea.


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Dan Lavin



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PostPosted: Fri Aug 10, 2007 5:36 am    Post subject: Reply with quote  Mark this post and the followings unread

babuf77,

The small circles are indeed connections. The 3rd prong/terminal of the pot goes to ground because the pot is being used as a voltage divider not a series current limiter. You might want to wikipedia voltage divider if you're not sure how those work.

For the inputs, the first dot input is the jack's tip connection. The second dot is the sleeve connection. So each input has 2 connections with the second one (sleeve) all connected to ground (or probably, more appropriately called common or common connection).
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etaoin



Joined: Jun 30, 2005
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PostPosted: Fri Aug 10, 2007 5:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:

I think I've pretty much got it though, especially if anyone can explain to me why R5-R8 are necessary. They seem redundant to me.


They are absolutely necessary. Just think of what happens when you turn one of the pots al the way to zero. At that point, the wiper is connected to ground and would pull down every other channel if R5-R8 weren't there.

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babuf77



Joined: Aug 05, 2007
Posts: 21
Location: Michigan

PostPosted: Fri Aug 10, 2007 6:21 am    Post subject: Reply with quote  Mark this post and the followings unread

I see, thanks to the two of you.

Is there any way to minimize the attenuation of the circuit? IE is 10K the minimum value for those resistors?

I really like this. I think I'm going to create a little pcb board (my first) along with two slides for balance and headphone jacks for each input (so I can listen to something before I fade it in, apparently that simple Y connection is all you need according to http://www.tkk.fi/Misc/Electronics/circuits/linesum.html )

I will now go to the electronics store to familiarize myself with some parts, and will then post back asking for "proofreading"

Thanks a lot folks, I really do appreciate it. I have nowhere else to go with this type of question, and it's quite frustrating not being able to figure something like this out.
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etaoin



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PostPosted: Fri Aug 10, 2007 6:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Is there any way to minimize the attenuation of the circuit?


Not as a passive circuit. Something like an opamp would be needed to get the signal back up to it's original level.

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babuf77



Joined: Aug 05, 2007
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Location: Michigan

PostPosted: Fri Aug 10, 2007 8:33 am    Post subject: Reply with quote  Mark this post and the followings unread

I have failed to find sliding potentiometers anywhere in this city. This is absurd.

What is the purpose of having a "center detent" in a potentiometer? Why would you want to stop it halfway? Or is there minimum resistance at the center point on this type?
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Dan Lavin



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PostPosted: Fri Aug 10, 2007 12:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
What is the purpose of having a "center detent" in a potentiometer? Why would you want to stop it halfway?


Generally one would use a center detent pot if the center point is "normal" and increasing the pot increases an effect and decreasing the pot from the detent lessens an effect.

A good example is a pitch bend wheel. Normal would be center and pushing the wheel up increases the pitch and pulling the wheel back lowers the pitch. After doing your best Jan Hammer impression (or Eddie Van Halen, if you're a guitar player), you'd certainly want to be able to return to normal pitch, wouldn't you? The detent helps you find it asap. Otherwise your bandmates give you evil stares for playing out of tune and start talking behind your back about replacing you......um, not that that ever happened to me, of course!
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