A simple Audio Mixer

[Daniel Thompson]

Hey Guys,

I'm wanting to make a simple mixer that takes my headphones output from my computer and my headphones output from my ipad and mix them into one output that I can put into my logi-tech computer speakers. Any of you Audio buffs out the able to point me in the right direction with this?

Thanks,

Dan.

-- 
Dan Thompson	
VFX Artist | franke...@gmail.com
rising sun pictures | www.rsp.com.au
my blog | www.danthompsonsblog.blogspot.com

[Steve Roehrs]

The LM386 is a pretty good chip (cheap, bulletproof) for audio applications, but I've only used it in a high gain configuration as a headphone amplifier.  At the power you're running you won't need to heatsink it, and it's a nice DIP package so easy to work with.  Runs well on batteries or a single low voltage DC supply, and doesn't need split rails either.

 

To mix the two sources you just need to use a series resistor on each input - I'd link to a datasheet but my work internet is bog slow at lunchtimes for some reason :)

 

Steve

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[Steven Pickles]

Hey Dan!

The Nicolas Collins book "Handmade Electronic Music: The Art of Hardware Hacking" has a whole section on making passive audio mixers, which should be fine for mixing two headphone outputs to go into an amplified input.

Here is the diagram from the book of a 3 input two output mixer, which is already much more than you need: http://www.flickr.com/photos/_xiq/7464566720/

pix

[Ken]

A resistor in series with each output, joined at the input of the logtech amp should do it.
1kohm will be fine.  So, 2x3.5mm sockets, 1x3.5mm plug, 4x1kohm resistors any size down to 1/8W, some wire.

Ken.
 

On 29 June 2012 12:18, Daniel Thompson <franke...@gmail.com> wrote:

[Steven Pickles]

Also Dan if you don't actually need volume control you can buy little 2->1 adaptors that will mix two 3.5 audio inputs two one output. Probably available at "adelaide city discounts" on King William.

pix

[Robert Hart]

I use this allot, Its all about matching impedance here is the boring tech etc.

http://en.wikibooks.org/wiki/Circuit_Idea/Parallel_Voltage_Summer

[Daniel Thompson]

Thanks Pix,

That's exactly the sort of thing I was looking for. I wasn't sure if mixing two Amplified(slightly) signals needed a special circuit. Looks simple enough. I just want an alternative to wearing ear buds with one sound and headphones with another sound over the top of them :P

Cheers All,

Dan.

[Ken]

Good luck with this bit:

We can do a few remarks about the parallel voltage summing circuit in comparison with the series one.

• The output voltage of the parallel voltage summer is a weighted sum of the input voltages; the output voltage of the series voltage summer is a sum of the whole input voltages.

• The input sources and the load of the parallel voltage summer are grounded; only two devices (sources or a source and a load) of the series configuration can be grounded, the rest remain flying.

Talk about making a simple topic complicated!

You do NOT want to consider a "series summer".

However you may strike ground loop problems if some of the device' s chargers are not isolated from mains well enough. (They should be.)
You'll hear this as 50Hz (or 100Hz) hum.
If you don't hear it, all is well.
To be cautious about it, use a multimeter to see if there is any resistance between the respective grounds of the phone, computer and amp while they are off, but plugged in to the mains.  Power supplies for each should be double-insulated, ie not connected to ground.  That said, what else is connected to the computer?

-Damn, now I've made a simple topic complicated.  Sorry.

Ken.

[Kim Hawtin]

On Fri, Jun 29, 2012 at 3:37 PM, Ken <k...@waggies.net> wrote:
> However you may strike ground loop problems if some of the device' s
> chargers are not isolated from mains well enough. (They should be.)
> You'll hear this as 50Hz (or 100Hz) hum.
> If you don't hear it, all is well.
> To be cautious about it, use a multimeter to see if there is any resistance
> between the respective grounds of the phone, computer and amp while they are
> off, but plugged in to the mains.  Power supplies for each should be
> double-insulated, ie not connected to ground.  That said, what else is
> connected to the computer?

this is why i went to jaycar today and purchased all the 3k:3k
isolation audio transformers they had.
got something similar in mind for isolating the outputs of some radios
to the input on the sound card of the puter...

cheers,

kim
--
"Art without engineering is dreaming; engineering without art is
calculating."  --SKR

[Ken]

Do they give a frequency response for them, Kim?
You'd normally need to pay around $30 for decent bandwidth transformers, especially ones intended for microphone use (balanced to unbalanced).
In the past I've used miniature 600/600ohm transformers for running earphones off a car radio outputs where the earths weren't grounded, but push-pull.

Ken.

[Ken]

If they are this beast: http://www.jaycar.com.au/productView.asp?ID=MM2534
then the frequency response in the attached brief PDF is given as 300Hz-3kHz +- 2.5dB.
So don't expect too much bass or treble, Kim.

Whereas something like this: http://www.altronics.com.au/index.asp?area=item&id=M0707 will give a better result (at greater expense).

Ken.

[Mark Jessop]

They're going to be used in Amateur radios, which have 300Hz - 3KHz voice filters anyway, as is the standard for most commercial radio equipment.

- Mark

[Damien P]

On Friday, June 29, 2012 1:58:57 PM UTC+9:30, hardhack wrote:

I use this allot, Its all about matching impedance here is the boring tech etc.

http://en.wikibooks.org/wiki/Circuit_Idea/Parallel_Voltage_Summer

I measured the input resistance of my Logitech speakers  as 10k.  What value resistors would work best on the input?  Too high and the signal will be reduced too much, too low and the signals will go back into the other circuits (which probably isn't that much of a problem).  I guess it's easier to experiment!

On Friday, June 29, 2012 12:55:57 PM UTC+9:30, pix wrote:

The Nicolas Collins book "Handmade Electronic Music: The Art of Hardware Hacking" has a whole section on making passive audio mixers, which should be fine for mixing two headphone outputs to go into an amplified input.

There's a copy of this in the state library, if you want to read a few bits.

[Ken]

Anywhere between 100ohms and 1kohm would be fine.

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