DIY QUAD-EIGHT MIC PREAMPS?
This project was born out of recording
gear envy, as my friend had some now converted CA127 cards into
CA227's and I was left with the build bug. I noticed some cheap
Quad Eight iron on ebay and decided to gamble. With a UTC O-10
as payment for my help with my buddy's setup, I fashioned a board
and was off....
...so I thought...
...because... on one night, as I was
particularly unfocused and perhaps several hours past bedtime,
I made the PCB I had intended to use. AND, I ended up having the
image on the wrong side of the transparency. But, all was not
lost, as I mirrored the original layout image in one of my computer's
paint programs and had that to work off - as long as I remembered
to read backwards. SO, if you happen to notice that everything
is switched around, with respect to the original card, that's
why.
The case cost me $3.00, and was originally
some kind of satellite tv receiver, until I "J.B. Welded"
all the holes shut and made my own. I have included a picture
of the back to show how nice and smooth you can fill holes in
metal with that stuff. There is a hole in the front because I
haven't quite grasped the concept of measuring audio with VU meters,
and was thinking about putting a switch to give some kind of function
with the meter.
The input transformer is a UTC O-10,
wired backwards, and to a Double-Pole 3-pos switch, yielding 3
impedances (however useful that might be, I don't know yet). If
I've calculated correctly (and understand what's going on), the
3 impedances should be somewhere around:
83 Ohms
333 Ohms
833 Ohms
Here's some info on the UTC,
straight from the catalog:
The output transformer I used is a
QEE3041, bought off ebay and was advertised as being able to handle
+30 at 20Hz. +30 what, I don't know, but Ken Hirsch of Orphan
Audio (who should start selling Quad Eight gear by late Feb.
2004 or something) told me that they aren't spec'd to handle
the same level as the original transformers used on the CA127
(makes sense - they're physically smaller), however, this is what
I could get my hands on for the money. One thing is for sure -
they start to saturate at very high levels. The transformer
used in the CA227 (3441) has a ratio of 1:4.47. I am using the
3041 with a 1:2 ratio. Here's the hookup for the 3041:
