This is the process of getting a balanced input from an SM58 mic to record into the computer at a decent level via the unbalanced imic input. I've been round the houses checking the XLR wiring protocols, and then found that I need an impedance transformer.
It seems to be phantom powered in mic mode and too quiet in line mode. Huge click when the mic switch is opened and closed in mic mode - going to give me triggering problems in the software.
Theres also a lot of noise. An earth hum off the mic (when not touched), and processor noise, not sure if this is to do with the way that I have wired the balanced> sterio sockets, or if it is just innate to going balanced>unbalanced. (Is this what an earth lift does?) Should I experiment with different wirings? (Also been to Maplin. £14 for their wiring solution: sterio 1/4" to mono rca, rca to (one!) male xlr)
Tempted at this point just to get a very cheap mbox1 and dismember it...
Here is the XLR wiring protocol:
pin2 signal (hot). pin3 inverse (cold), pin1 ground (shield)
How do ballanced inputs actually work?
Good article on balanced inputs: http://community.avid.com/forums/t/58612.aspx
There are three types:
Impedance balanced [cold tied to grnd via resistor matching the output imp of the hot]
grnd pin3 to pin1, or leave pin3 floating, makes no difference.
Transformer balanced [actively drive primary winding of output transformer]
grnd pin3 to pin1
Active balanced [actively drive both hot and cold outputs]
leave pin3 floating (or it draws from the transformer, stressing it)
(...So I think the earth lift on a DI disconnects pin3 from grnd. )
Ive tried it with cold to grnd - so try with a floating pin3 instead... Nope.
Right. Found it. It is an impedance matching problem.
Theres stuff on the Griffin site:
The Sm58 is a balanced low impedance output and the imic is an unbalanced high impedance input configured for using with sterio gear and high impedance electret microphones.
Low impedance seems to be below ~ 300 ohms, high above 1.5 K ohms.
Sm58 is rated at 150 (actual 300(?)) ohms. Can't find any info on the imic.
I need to buy an impedance matching transformer.
Got a Maplin transformer for ~£9. Says its 500 ohms in and switchable 600 ohms/50 k ohms out.
Yup, thats a result! Got the SM58 triggering the new program well enough through the impedance transformer and imic. I'm using the high impedance setting with the mac system preferences slider on about 5/16ths. The sound floor does seem a bit high, but its definitely usable. Guess the first thing to do is to try and adjust the trigger sensitivity, see how good i can get it. (about £34 so far for this method, with so ~ £40 with another splitter to take the strain of the imic, and ~£49 to be able to do two separate mics. Not so cheap, but very small.)