SCA A12 Mic pre

[pritsinghs3]

I'm kind of stuck in a weird position. I bought some DIY preamps from Seventh Circle Audio. This post of for people who are familiar with their products and have built or at least tried to build these things. I just completed my first A12 preamp with transformer and sc25 opamp. When I power it up, the LED light on the module doesn't turn on. I'm sure I did everything correctly. My soldering also looks fine. The only thing I can recall doing is sometimes (maybe happened 4 times) I had soldered the wrong thing, and then I had to un-solder it and solder on the right component. Other than that, I KNOW I have the right components on the right part. What can I do?

PS: The ONE and only thing that didn't make sense to me wat item 18, which is the 2 way pin header. For some reason, the bill of materials say J1, J2, J5, J6 but the quantity is ONLY 1!!! Doesn't it mean install it in "either" one of those? I then looked at the high def photos of the module, and there are actually NO J1 and J2 reference points and the J5, and J6 reference points are EMPTY, so I left it empty as well. PLEASE HELP!

[a zombie]

Post a high res photo of the top and bottom of the board.

Here's what mine looks iike. It works great.

[a zombie]

from assembly manual

J1 and J2 - When using an input transformer with split primaries, these headers select the
input impedance by allowing the two halves of the primary to be connected either in series
or in parallel. For the standard A12, J1 and J2 are not needed! Do not install headers
at J1 and J2.

J5 connects a 604-ohm load resistor across the output. Unless you’ll be connecting the
A12 to a piece of older gear with 600 ohm input impedance, connect a shorting jumper
across J5

J6 - A separate, isolated output signal is available at this header. The amplitude at J6 is
half the amplitude at CONN2.

J1 and J2 are only used if you are using a different input transformer.

J5 puts a 604-ohm load accross the output. This header should be installed and the jumper should be installed as well.

J6 gives you an extra unbalanced output.

None of should cause your led not to light up.

Does you A12 pass audio?

[Analok]

Check the polarity of the LED. Outside of that it might have gotten heat stressed from the soldering iron being held too long at the joint. Diodes in general don't like heat.

[pritsinghs3]

a zombie, thanks for the reply. i'm attaching 2 photos, the front and back. the back obviously doesn't look as good as yours because i've never really soldered in my life before this, just watched youtube videos. i used LEADED solder, and the guy at radio shack told me that the color of lead solder looks darker when dry than lead-free. please let me know if you can see anything wrong.

Analok, i think you might be right about the polarity on the LED light. for some reason, i totally forgot that LED's have a positive and negative while soldering it.

PS: my gain rotary switch doesn't turn (?), and my trim switch does.

[pritsinghs3]

Analok, you were right! i pulled out the LED light, flipped it, soldered it again, and the light was working!

[Analok]

Originally Posted by pritsinghs3

Analok, you were right! i pulled out the LED light, flipped it, soldered it again, and the light was working!

Glad to help!!

[a zombie]

Right off the bat, c15 looks like it is installed backwards.

PS: my gain rotary switch doesn't turn (?), and my trim switch does.

What do you mean it doesn't turn? You installed just one stop pin right?

the back obviously doesn't look as good as yours because i've never really soldered in my life

Please don't take this the wrong way, but alot of those solder joints look really bad. I would be surprised if it worked at all.

It can be frustrating. But you really need to neaten those solder joints up. Some of the leads are sticking way out and need to be trimmed. re heat and add a little bit of solder to the joints that look like they don't have enough solder in them and trim the leads to the top of the 'solder volcano'.

your solder joints should look like this /\ not like this _|_ and not like blobs

neatness is not optional...

[pritsinghs3]

a zombie, thanks for your input, really appreciate it. so do you think i should get the soldering wick and remove all the solder from the back or just add more solder to the joints? and yes, i only installed 1 pin on the rotary switch and it doesn't turn. i installed it exactly in the area that the directions pointed at. i obviously wouldn't be able to test it out until the gain knob works.

MUCH APPRECIATED!

[Analok]

You can probably get away with re-flowing what solder there is at each joint, however some joints look to be lacking enough solder to bond well, so simply add a bit more at these 'sparse' areas as you re-flow. Remember that you need to heat both the component leads AND the circuit board pads (joints) to achieve a solid bond. 3 to 5 seconds at each joint should suffice. Now would be a good time to ask you "What size/wattage is your soldering iron?" 30 watts is a happy medium, but I'd avoid anything higher that might risk component lives due to heat stress. 15 watts will work too but might take a tad longer to flow solder properly. As for the wicking material, I'd buy a roll or two just to keep handy. Try to find wick that has a "no clean" feature. It won't require additional rosin (it's already incorpoated within the copper weaving) and will wick the solder much quicker/easier. Avoid anything wider than 1/4", cause the wider the wick is the more heat it will need to work simply due to its' mass.

Have paitence & fun!!

[pritsinghs3]

thank you a zombie and analok for helping me out with this. i reflowed and re-soldered the entire unit, i replaced the capacitor that was soldered wrong, got rid of some solder bridges, and fired this baby up. upon completion, i followed the directions to calibrate it, and the multimeter showed the exact measurements that were supposed to be shown, so it passed. it sounds GREAT! i tried this on both vocals and guitar, i was impressed. i can't wait to use this on other projects. i've already started on my n72 kit!!!

[a zombie]

That's awesome! Congratulations.

Glad we could help out. It is a really nice preamp, super low noise too. Post some pics of your love-children when your done

[pritsinghs3]

I just finished the N72 with a breeze. I've got to say, this is my FAVORITE of them all!!!! It's probably my "go to" pre for vocals. The A12 sounds amazing as well and does in fact beats the N72 in certain things, but it does lack the warmth and "bigness" the N72 creates. The A12 sounds great in vocals as well, but I would probably use it for guitars and snares. So far, my verdict is that they both are like my children, they're good in their own ways, and I really cannot choose between the two. Next one to build is the c84, I'll post photos when I'm done. Next one to buy is the J99. Has anyone ever used one or built one? How does it compare to the others?

PS: God, I love this stuff, I'm so glad I sold my Avalon 737 to get three of the SCA preamps. For anyone who thinks that the Avalon 737 is great and sounds warm, my reply to them is: Just try the N72 once.