Super simple 4 channel mixer???

[BillyJ]

I am off to geofex to dig around cause I remeber RG talking about this the other day so i am certain to get some good info but thought I would ask here too.
I need one for four single stringed things that are to be beaten by drummer (they like that sorta stuff).
Each channel only need a vol pot.
Any thoughts ideas or designs around??hanks!!

[Peter Snowberg]

This is pretty basic circuit with a number of ways of wiring it up. Keep in mind that the link below shows a unit with very low input impedance so you will need a booster on each input. Any simple buffer should work just fine. You could even replace the buffer's drain resistor with a pot, but the frequency response will vary with the volume (I think :? ).

http://www.harpamps.com/schematics/smixer.pdf

To make it simpler, you could always replace the "gain changing resistor" with a 100K pot to ground and connect the wiper to the output coupling cap, omitting the second opamp and its feedback resistor. However I would keep the second opamp so you get a low (and constant) output impedance. It's a little more polished than the simple mixers.

Oh, for an output cap, try anything from 1 to 10uF. Since your source is not typical, you may want to play with this value (even taking it lower).

Also, since the instrument(s) your speak of is/are being beaten, you might want to run the summing amp into an EQ. It's hard to get the perfect response from one-of-a-kind instruments and a regular 10 band stereo EQ makes a WONDERFUL sound shaping tool.

Good luck with it!

-Peter

[Greg Moss]

The new issue of Tape Op also ahas an article on JFET gain stages. This includes a JFET line mixer...

Greg

[jsleep]

I have a project of that simple mixer that Peter posted.

http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=123&op=page&SubMenu=

The gain changing resistor needs to come _way_ down  I think I ended up with 10k there.  The 100k is unity gain I think, you don't want that for a mixer, you want a little room to mix with the pots ;-)

The down side of this simple mixer is that you'll notice a percieved lowering of volume on each input as you add inputs and they are summed.  ie one input sounds good (not much mixing there), plug in another input for 2 inputs, the first input sounds weaker as it is being summed with the second.  I think the way to cure this is to have active volume control on each input, anyone have any comments?

JD

[Peter Snowberg]

Could you get around that problem by buffering each input, leading that to a volume pot, and then buffering that again before the 0.1uF/100K summing portion?

It looks to me like the tone loss is from a bunch of parallel 0.1uF/100K combos leaking the inputs to ground through the other volume pots. Would good opamp buffers would be worse as the impedance of a 5532 is LOW?

Last wondering.... Could you use single ended buffers with a common pull-up/current source to counter the opamps "fighting"? Sorry, I'm from digital land so open collector logic is the first thing that jumps to mind.

-Peter

[BillyJ]

I haven't gotten to geo on this yet but will. This however looks like it can do the job.
Anyone care to guess at the questions at Peter presents. I'd like to know myself...
Thanks everyone!!