XLR In & Out EQ/Reverb with Phantom Power

[M. Spencer]

My sax player buddy asked me to build him a box that would be useful for small gigs. In it he wants:

* 2 XLR ins
* 4 band EQ
* Reverb
* 1 XLR out
* Phantom Power

He wants two xlr ins in case he has two mics set up for different instruments (he doubles on trumpet), or he needs a background vocal. At least one of those XLR ins has to provide phantom power for his clip-on condenser mic. This box is basically for rooms with little more than a lunchbox PA and no audio engineer; to give sax man some control over his sound from the stage.  

I was thinking using the 2 Channel Mixer, series into the 4 Band EQ , series into the Box of Hall, then to a DI with XLR out.

My questions:
- How would I implement two XLR ins? So far I haven’t built anything much more complicated than compressors and reverb/delay with standard 1/4” jacks.
I’d rather not make the XLR ins “unbalanced” by shorting the 3rd pin - prefer to keep the XLRs shielded and balanced to shunt radio interference, etc.  

- Are there any recommended DIY phantom power box builds that supply their own +48v?

- How do I deal with the phantom power? As far as I understand I need it to power the active DI at the end, and the condenser mic at the front of the chain. Would I be able to power both the EQ/Reverb sections of the circuit and the Phantom Power from a 9v power jack?

- Are there any other issues I should be concerned with (matching impedance, etc)

Is this all feasible? I know this is a big ask and a complicated box, I appreciate your input.

[vid.sicious]

I'm not an expert by any means but from my knowledge, using circuits design for guitar pedals you'd face quite some drop in volume. You'd probably have to transform the balanced signal into line to make them work properly. Any kind of DI is capable of it. You'd also need quite a box to accommodate all of this stuff. But I guess it makes more sense having all in one big box rather than several sperate units.
This guy made a circuit which will modify your balanced xlr signal into line which you can confidently feed into any guitar circuits you want without loss in dB. http://www.paulinthelab.com/2012/04/balenced-to-line-microphone-preamp.html?m=1

For your project I think you'd need 2 of those circuits going into the mixer. But then you'd need 3 power supplies: 15v, 9v and the other 48v (for phantom power), unless you figure out a way how to step up from 15v to 48v and then 15v down to 9v or any other way. Check the comments in the link I posted, there are other people facing same issues. Its quite a complex idea due to different voltage requirements...

[M. Spencer]

Thanks for the link an the tips, that looks like a useful piece to this puzzle. Never messed with anything besides 9v (with the exception of negative ground mods for fuzzes and the like) so this is turning into a much bigger project than I anticipated.

[motterpaul]

I hate to be the "downer" guy but it sounds like he wants a mixer he can buy for far less trouble (and possibly even cost) of what you can build for him. That is just the reality, because there are so many small mixers that do all of that already out there, but for you it is four or five individual projects, so you should be charging him accordingly.

Your parts (3 XLRs, transformers, reverb brick, enclosure) are going to add up, and your time will be substantial. I am just saying it is not really a cost effective exercise for you or the buyer.

http://www.amazon.com/s/?ie=UTF8&keywords=2-channel+mixer&tag=googhydr-20&index=aps&hvadid=96659769626&hvpos=1t1&hvexid=&hvnetw=g&hvrand=6841632910748702023&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_6kh3rgrwvm_b_p12

That being said, you can still do it.

Here is the challenge, he doesn't need a guitar based effect - he needs a mixer platform that goes from low impedance mic level to line level (but nowhere does he need guitar level impedance) which is what we deal with most of the time.

You need to look into either active impedance changers to go from low impedance mic level to high impedance line level, or else transformers which you can buy. Either of these will isolate each mic input and can feed a mixing matrix where you do your EQ and reverb. Your output can be line level and it can still feed most mixers or PA amps.

Phantom power is easy to implement, your challenge will be getting 48v DC to feed it. You might need to start with 18v to get there.