ARP (active ribbon preamp , is there a circuit

this from a reference on the shure sm 57 mod thread

is there a circuit anyone know

it s worth noting if no one said it that the 57 can sound different depending whether it is loaded with a transformer preamp or x-former less

this was discussed ad nauseam a while ago. the search function doesnt work well at all but let me see if i can find it for you.

ok, heres one thread:

http://www.groupdiy.com/index.php?topic=5743

but the idea has definitely been discussed at length elsewhere, too

i did try to search it so thanks
regards Greg

I find it very helpful to use google's search site thingy.

Just type "site:www.groupdiy.com active ribbon" into the google search form.

Or better yet, get the google toolbar, which allows you to enter a search and then press the "search current site". This helped alot when I was looking for info about the "What" compressor.

Cheers,
hejsan

Hi,

Here is the schematic for a project I built that turns an Oktava ML-52 into an active ribbon mic:

http://resources.edgeofmagic.org/arp.jpg

Some explanation is in order --

1) IN+ and IN- are connected to the output of the ribbon transformer. In my project, I replaced the Oktava xfmr with a Lundahl 2911-- a worthy upgrade all by itself.

2) OUT+ and OUT- go to pins 2 and 3 of the XLR connector. You might need to check polarity/phase on this (or just rotate the mic 180 degrees...). GND goes to pin 1.

3) A1A (first half of the LT6234) give a gain of 5x, and A1B inverts that signal for balanced output. The total gain is 10x (20 dB).

4) The +10V, +5V and GND rails act more or less like +-5V rails for powering the amp, with +5V being a 'virtual ground'.

5) VR1 and VR2 are 5V LM4040s.

Selection of the op amp was critical, to get a usable combination of low power and low noise. The LT623x voltage noise is 1.9 nV/rtHz, and the quiescent current draw is 1.2 mA per section, which makes it possible to use phantom power. Most ultra-low noise op amps are current hungry. Here is a link to the LT623x data sheet:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P2398,D2333

The real trick was getting all of this into the bottom cavity of the ML-52. It's a tight fit, but it works. Everything is sandwiched between 2 round PCBs--one holds the transformer and the other all of the circuitry. If anyone cares, I can post some pics and the artwork for the PCBs. The sound is nice, very clean (yeah, yeah, I know, IC op amps are the work of the devil...). It helps to use quality components, particularly the output coupling caps (C5 and C6) -- I used Nichicon Muse KZs.

I'm going to try replacing R1 with 300 Ohms and R2 with 1.2K Ohms, in a effort to reduce noise. The noise floor is OK, but any reduction would help.

Hope this is useful,
Da5id