How to add 48v to mic pre?

Hi!
I have tried to serach the forum for info about this, but I haven`t found something yet - so I`ll try to get som clean answers here 

I have racked 2 Midas Pro04 channels and I made my own PSU (from mnats design for the G1176, and then added the right resistors to get +/- 16v). The mic-pres doesn`t have 48v so I am wondering how to start out this project. As I see it there are two options: add an extra circuit that adds 48v or build a new psu that delivers +/-16 and 48v. Any suggestions and/or schematics?

And do you hook 48v dirctly to the XLR or is there an additional circuit in between?

Hi

Have you checked out Peterc's "green psu"  ?

It's a neat little circuit that gives +/-15v or +/-18v whatever you want ,and +48v on the same card .

Easy home etch!

cheers

nEon

I will check that out, but what about hooking the 48v up to my mic-pres - do I need any components between the psu output and the input XLR?

Your 48v DC should be switchable , spst switch will do, after switch a 100ohm r to a "node" of
1 x 47uf 63v cap to ground ( positive facing the 48v ) and 2 x matched 6.8k resistors which go to your
XLR + and - signal pins , so one to each.
Look up something like the Green pre or SSL9k schem for a drawing of such, it's quite simple.

Marty.

MArty i'd go with a 470Ohm resistor after the switch.

3nity said:

MArty i'd go with a 470Ohm resistor after the switch.

Click to expand...

OK, i spotted the 100ohm on a green -pre schem, that's all.

MM.

I am not familiar with the circuitry in a Midas Pro front end, but if it is not transformer isolated, or capacitor coupled with at least 50V of stand off capability, attaching a 48V phantom supply could damage circuitry.

There should be numerous schematics published showing typical treatment for isolating phantom voltage from mic pre electronics.

JR

depending on your needs, it may be easier to just get a box like this: http://www.sweetwater.com/store/detail/SPhantom/

but for the sake of learning, as the others said, the green pre and PSU have been a good project for me in regards to learning how phantom power works/is implemented

If the pre already has the 6k8 resistors, DC blocking caps (or transformers) etc. then you could just buy this PSU:

(usa) http://us.creative.com/products/product.asp?category=237&subcategory=258&product=17347&nav=
(europe) http://www.powersuppliesonline.co.uk/product/18w-48v-0-38a-desktop-power-supply/es18b48-p1j/default.htm

This is used with E-Mu 1616 interface, seems to be good enough for the job. Might be also good for JLM Baby Animals if you are not going to buy JLM's own supply (to run the higher voltage JLM99V DOA's).

MartyMart said:

3nity said:

MArty i'd go with a 470Ohm resistor after the switch.

Click to expand...

OK, i spotted the 100ohm on a green -pre schem, that's all.

MM.

Click to expand...

100Ω works. I've seen everything from 100 to 500Ω in that position.
Or you could go somewhere in the middle like Jensen does and use 200Ω.

Are the 100ohm r and 47uf cap for the purpose of slow ramp?

mhelin said:

If the pre already has the 6k8 resistors, DC blocking caps (or transformers) etc. then you could just buy this PSU:

(usa) http://us.creative.com/products/product.asp?category=237&subcategory=258&product=17347&nav=
(europe) http://www.powersuppliesonline.co.uk/product/18w-48v-0-38a-desktop-power-supply/es18b48-p1j/default.htm

This is used with E-Mu 1616 interface, seems to be good enough for the job. Might be also good for JLM Baby Animals if you are not going to buy JLM's own supply (to run the higher voltage JLM99V DOA's).

Click to expand...

It`s got a transformer on the mic-input but not on the line-input I think. So I should be good to go with 48v as long as it`s not turned on while hooked up in line mode, right? So if I make the mic/line switch cut off the 48v supply when switched to line, is that a way to do it? ...but it doesn`t seem too big of a job to make the circuit required to be on the safe side either...

MartyMart said:

Your 48v DC should be switchable , spst switch will do, after switch a 100ohm r to a "node" of
1 x 47uf 63v cap to ground ( positive facing the 48v ) and 2 x matched 6.8k resistors which go to your
XLR + and - signal pins , so one to each.
Look up something like the Green pre or SSL9k schem for a drawing of such, it's quite simple.

Marty.

Click to expand...

So if I do something like this (from the green pre) I should be okay as long as it`s transformer isolated?


Edited: or is it okay if it`s not transformator isolated as well?

Allright then I've used this circuit on a few pre-amps now, but another question has arised. How do I hook this up to multiple (2) pre-amps? Do I take the 48v from the PSU and add two of these circuits (switch, 100R, 10uf, 2x6k8)? Or do I get funny crossovers (power and/or audio) this way?

Depends o if you want global or channel selectable +48V, if global just add the parts post the switch, if per channel add another switch, both pulling from the the same +48V source.

Matt