a little XLR microphone mixer idea

[bool]

Take a look at this ESP project:

http://sound.westhost.com/project122.htm

The simplicity of the topology used here will jump at you - if you redraw it in your LTspice. You will also notice that you can take the balanced output with it (at the first opamp - pin.1 on the schem).

[Mario44]

Only PC so far, next testing will be on both.
That ESP project looks very simple too. As soon as I get 5532 will go for testing it. 

[tubegeek]

I think I was able to get the elusive 10K reverse-log pots at Hoffman Amps.

[mth5044]

Small bear has 10k reverse audio 16mm pots with solder lugs, and probably in other packages too. Tayda has them in 9mm and 16mm PC mount. Not too elusive! Unless I'm missing a property of the pot?

[tubegeek]

Small bear has 10k reverse audio 16mm pots with solder lugs, and probably in other packages too. Tayda has them in 9mm and 16mm PC mount. Not too elusive! Unless I'm missing a property of the pot?

Cool - I may not have checked Smallbear when I was looking - it was a year or so ago.

[Mario44]

Ok guys. The first topology as PRR said is very noisy, but only for PA system and basicly it's unusable with that amount of hiss. For recording it does the job and after changing the input filters everything is fine. Idea of that circuit is quite simple and quite noisy too, no idea why  .
I decided to test second schematic from ESP and I'm nicely surprised with it's simplicity and low noise even with max gain (about 50dB). All test were made with ne5532's this time now. It has a little bit more of low frequency response and lower noise according to datasheets(couldn't hear any noise coming out from PA system).
Also I think dropping the balanced signal since this preamp is going to work only for dynamic microphones and there was no difference between bal. and unbal. going through preamp.

[bool]

Do tell ...

[Brisance]

Reverse log pots elusive? Cook your own!(<-clickable link)

[PRR]

> I think dropping the balanced signal since this preamp is going to work only for dynamic microphones and there was no difference between bal. and unbal.

In a "good" room, little electrical interference, unbalanced dynamic works well.

I worked unbalanced dynamics for many years. On stage and in recording.

Its simplicity is a virtue.

But near fluorescent lamps, lamp-dimmers, or 900-Watt bass-amp power cords, un-balanced will pick-up hum and buzz.

In balanced, the microphone signal is equal-but-opposite on two wires. Stray buzz-crap tends to come in the same on both wires. The balanced (differential) input can tell the difference, and amplify the desired signal much more than the crap.

So it can be a case of: you don't need balanced until the day you DO need it.

The other thing is-- while I agree that "everything" can be done with SM-57/58 or copycat, or similar dynamics, today there are amazing bargains on "condenser mikes". These often use Phantom power, which kinda demands a balanced line.

[Mario44]

I agree about the bal/unbal. signals. Unbal one will be totally enough since this preamp is going to be used mainly in garage studio by me and my friend. I really want to thank you guys for helping me understand things and helping with this preamp. There will be soon a final schematic that is going to be used, but first it'll be tested.

[Mario44]

So here is the final schem. It does not include any mixing anymore but splitted output. There is also a LED indicator which glows when the signal comes through. What do you think about it? Should LED opamp have separate power supply and other than symetrical +-15V for better result?