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how do I make a volume controler for a ground wire? Answered

so I built a splitter that wil take my RCA, 1/8", and my 1/4" jacks together now to finish it i wnat to add a volume control on the ground wire (assuming that it will be too hard to volume left or right channels)
how would I go along doing this
the case i put this in came from an old toy which happins to have a 4.5v (3x AA) battery compartment
i want to use this for some kind of light design (maybe a status light, left chan, right chan, and ground  or my be RCA, 1/8", 1/4")  and dont know how  i would do it
any suggestions?
*just ran my first test and it grounds the right chanel when i use my guitar (which makes sense) but
there  still a problem the more inputs/out put there are the lower the volume what is the best way to fix this

* a new design is added

7 Replies

gmoon (author)2010-03-07

WE need to know what you're feeding (input) into this thing. Do NOT feed the output of an amplifier into this--your other signal sources would be unprotected from the high current (what Steve said.)

Since your inputs are unlikely to have similar levels, Steve is also correct--you need mixer circuitry, either passive or active. A mixer circuit will let you adjust the relative signal strength of each input.

Also--a volume control "on the ground?" You need to do a little more research on this before proceeding... The ground is the common return path for the signal. Using the ground alone is useless.

Any attenuation needs to be done on the signal (together with the ground.) Look up voltage divider, that's how volume controls are designed (Hint: a very good, adjustable two-resistor divider is a potentiometer.)

You can also make an adjustable passive mixer, too (see pic.)

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fastcar123 (author)gmoon2010-03-07

when useing this id would work the same on differant jack types obveously but my question is would I need 9 potentiometers? (3 pots for each type

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gmoon (author)fastcar1232010-03-08

Depends on the output you want. Some mixers keep the stereo channels separate. If you are indeed mixing all the channels together, then you really have only have one output. Anyway, I count only 8 inputs on your drawing...

When you connect everything together, you need to understand concepts like "output impedance" and "input impedance" (which are kind of slippery to understand.)

Resistive mixing helps to reduce interactions between the channels, but note that the pic above and the link use different resistor values (2K in the link, 47K in the pic.) Unless all your input signals have the similar output impedance, and similar signal strength, you will likely have to tailor the resistor values for each type of input (i.e., guitar is very different than a line, which is different than an mp3 player, etc.)

Regardless, passive mixers usually cause "attenuation" (signal loss), and certainly don't boost the signal--i.e., if you want to plug the output into earbuds, you'll only be able to hear those signals that were strong enough to drive the buds anyway...

Active mixers compensate for that loss, but are MUCH more complex.

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fastcar123 (author)gmoon2010-03-08

ok so i will need to have it resiting kinda like a potentiometer?   
I have a new design and im wondering if i used an old MP3 amp(like the kind that plugs into the 1/8" and plays the music outloud)in place to amp it
would that work ?
so i think i got something that resembles a mixer.
take a look at the second pic

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gmoon (author)fastcar1232010-03-09

Yes, volume control = potentiometer. I think you're getting closer with the second drawing. You're now designing an active mixer...

The mixing POTs should be before the amplifier--otherwise some inputs will overload the amp.

Generally, you don't see multiple outputs connected together. Mixers usually consolidate the output into one (mono) or two (stereo) channels. You can easily find stereo amp ICs (or use a pair of LM386 chips.) You can connect the outputs to different jacks, but don't use them together... (use only one output connection at a time.)

That's for several reasons--
Output impedances need to be low in order to drive other inputs. A low-impedance output (by definition) can provide more current, so two separate outputs shouldn't be connected together.

In your case, you're splitting the output, rather than connecting two separate outputs. In doing so, you'll have a lot of  "interaction" between them--which is what mixers are trying to prevent.

The total gain of the mixer will depend on which ICs you use, and what you want from the mixer:

Want a mixer that can also drive a speaker? (really be a stand-alone amp)--then use the LM386 chips, or the amps-on-a-chip that are used in car stereos.

Want a mixer with unity gain (output=input)?-- one you can plug into a different amplifier  -- then use an opamp like the TL082.

(radioshack should carry both the LM386 and the TL082.)

In short--mix on the front end. Choose an amplifier chip (or chips) based on the type of output you want.

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fastcar123 (author)gmoon2010-03-09

ok well I would like somethin that is the most easy to use (since im not exprianced at all with ICs) 
what im trying to do is to for example plug my tv (rca) into this thing then have my guitar amps(1/4") and my surrond sound computer speakers(1/8") and vice virsa with every thing else
i think i want to have the stand alone amp (hopefully it being the easier one to connect)
that is my main goal
what do u think 

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steveastrouk (author)2010-03-06

You have independently discovered why you use a mixing circuit, and not just connect things together. You REALLY need to isolate and buffer things with a mixer, I can't guarantee you'll damage something, but you are in danger of doing so !

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