Ok, it's been a while. A long while. Since my last post all of the components of the preamps arrived. Opamps, transformers, knobs. At this point it is finished and functioning and any parts that I am waiting for are purely aesthetic additions.
The caseI used my trusty new drill press to cut the holes for the xlr input and output jacks. The first hole I drilled was a 1/16" too big - which doesn't sound like much, but it's pretty gaping when you look at it up close. I did my best to align the xlr jack in the hole centered so you wouldn't be able to tell that it was an oversized hole. Not a half bad job I must say. Bet you can't even tell from the photo.
The power jack looks pretty pro but holy shit, it took some serious hack job man handling. Underneath that plastic jack/cover it looks like hell, but hey. Whatever.
Transformers
Input = Cinemag CM-75101APC Output = Cinemag CMOQ-2S
Really wasn't sure how to go about mounting the output transformers. The bolts that I bought didn't quite fit through the holes in the pcb so I drilled out the pcb a bit. Turned out ok - no big deal. They mounted fine after that.
The leads were a bit weird. I wasn't sure if they had to be left long or I should cut them short. I decided after looking at several other diy project pictures that cutting them was not a bad idea and that it would keep it neat and tidy. To measure them, I pulled the leads through the holes in the pcb to get the right length, then trimmed and stripped them. And finally I soldered them to the pcb.
The input transformers were easily soldered into place. I used a quarter to space them from the pcb.
Op AmpsJohn Hardy 990cThe opamps were easy enough. Solder in the pins and seat the op amp.
My First Test!!!!
This was a very exciting day. I got my power supply working and tested! Made sure that everything was taped down to the table so that it wouldn't move around and short something. Oh yeah, and I was extremely careful not to zap my stupid cat (that would learn him he he). Connected everything and plugged it in. Turned the power switch on. Yay, a light! After a quick woot! woot! I continued to test and adjust the trim pots. Both the + and - terminals tested ok and were easily adjusted to 18 volts. Unfortunately I found out that the +48 phantom would not adjust up past 7 volts. Something in the psu is a little messed up. I have a feeling it was one of the regulators getting fried when I soldered it together. The FiveFish psu is a double sided pcb with TIGHT spacing. Next time I will buy a much small tip for my soldering iron before putting one of these bad boys together.
The Big Test!!So I determined that because the +48 phantom section of the mic pre is isolated from the amp section I could connect the 312 without connecting the +48 phantom terminals to the psu and continue testing.
So here is my set up... test is across +V and ground and -V and ground op amp pins
Success! They work!!
After testing all of the pre amp cards for correct voltage I set up to test for sound. The big test: seating a (not very cheap) 990c in and turning on. I cut the ends off of an old mic cable to plug a mic in.
Flipped the switch.... lights on.... no smoke... no overheating. Great. Plugged in a 58 and tested. Working!! HUGE sigh of relief. Wow, it works. And it sounds amazing! No buzzing, no crackling or anything unusual. And, when the volume is turned to zero. There is NO sound. Very very cool.
Mounting In The CaseStill can't believe that they are working. Amazing!
Now to mount them in a rack case so I can use them in real life situations. Embarassingly it took quite a while to figure out how to mount the pcb's with enough height for the bottom of the transformer not to rub, but not too much that they wouldn't fit in a 1U rack case. The solution: a bold and two nuts. Here's how it looks with the holes drilled and the bolts through...
Not sure if it comes through clearly in the photo, but there are 1 1/4" bolts coming through the bottom of the case then lock washers, a nut and another nut for spacing. It's silly, but I kept thinking that I would need a proper pcb spacer nut for this job and I could not find the right height. Duh. Finally got it with this cheap and easy fix. The only thing (and it might be a good thing) is that the bolts need to be a bit loose to properly seat the preamps in, then tightened.
Here's what the case looks like with the preamp cards seated... beautiful.
I think I took this picture at about 1:30am - not even close to finished, but I had a session recording fast punk rock the next day so I decided to keep going until I was done. Stupid? Sure, don't mind if I do.
I cut 8 lengths of LAZER quality audio cable to connect the ins and outs and set off stiripping the wire. Sheesh, I swear that takes the most time of all. Like freaking hours!
And here we go.
Pretty damn close to finished, just need a front panel to mount the gain pots.
After multiple, like four or five tries, I finally had holes in the right places to peak the buttons through. It looks like total shit, but kind of cool in an uber diy way.This pic was taken at 7:07am. Birds chirping. I guess I pulled an all-nighter.
At about 8am I powered up the preamp for the first time. I got about 2 minutes through testing the first channel when I smelled a funny electrical burning. Oh fuck!! What the hell could it be? Quick! Power down.
It didn't take me longer than a few seconds to figure out that the op amp on channel two was overheating. Unfortunately it got fried. I took it out and tested it in another channel just to confirm. Yep. Done. $60 bucks down the drain.
Daringly (and possibly stupidly) I did an ultra quick test of a working opamp in channel two just to see if it was the channel that was messed up. Yep. Confirmed. The opamp started to heat up, so I powered down and removed it.
So with no phantom power and only 3 of 4 channels working I pressed on to the session for the ultimate test. A full day in the studio tracking drums. I used the 312 on kick, snare and rack tom with great results. More like "holy shit" results! The kick and snare are soo damb punchy.
I'll post audio samples soon.
