DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

Hi Owel,

I made an idiot of myself today. I went to add my final SC-1 MK2 into my box (a fivefish case), but forget that I couldn't plug it in to the exact same connectors on the xlr's as I had for my 2 SC-1MK1's. Of course this led to motor boating as the pins weren't in the correct places.

Now I know I'm stupid and did something stupid and understand why (Well only the semantic of why, not the rationale behind the neuron flow) I feel a whole lot better.

What I would like to ask though, to avoid me doing anymore stupid things and just finish is:

What is the best way to layout the power supply and toroid to get the least amount of noise that you've found works without giving you headaches if you need to move things? Pictures, etchings in my brain concerning this feat of neatness and noise reduction would be appreciated, as I think what I want now is everything in the one box, working at once (before I only really needed two of my 4 channels working). I'm wishing that I can have all 4 channels working with similar noise levels. If anyone else has some tips I'd really appreciate them.

Stratman

Sorry, not sure what you mean. The power pins for both the SC-1 original and SC-1mk2 are the same for both (from left to right):

+48V -- V+ -- GND -- V-

They're both the same. You can see the silkscreen labels on the PCB.





The recommended layout for both the original SC-1 and mk2 version is position the power transformer as far away as possible.

Looking at the top view of a rack case, place all 4 preamp boards to the left side, and the power transformer to the upper corner right hand side. Keep the AC power switch to the right hand side of the case.

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I should have spelt this out more explicitly, sorry, my fault.

Owners of SC-2's need never think about or do what I did to get headers on SC-1's. And yes, wrong wiring of the power pins will lead to premature death of the pre. That's an experiment I don't ever intend on conducting.

Essentially, the audio pins aren't symmetrical and compatible because I've hacked up my own "special" connector, as the old SC-1's don't take standard headers without getting creative. On my set the audio pins can't correctly clip on both, as the "connector tab" is on the wrong/opposing side. That is, they clip together inversely. So 1-2-3 | --> | 3-2-1 when used on the different make. Which is wrong!

I ask about the flow of the wiring, as a friend suggested that I should just bunch all the power cables together and feed them from left to right, then to the power (I have an older 1848 supply). Then keep the audio wires as far away as possible from any power component. The old 1848 isn't so neat however, in that there's not multiple tabs to plug into.

I thought I might ask about wiring schemes, as it seems there are a few different ideas about - I think I saw some different ideas on this forum a while ago. Ultimately I guess I'm looking for some wisdom as you've been building these longer and more often than I have (and probably spent hundreds more hours building these than me too! Picking up useful building tips as you go too I imagine... ).

Stratman

1. The only rule I follow is keep small signal lines (mic signals) away as possible from the 110V AC line.

2. I don't believe in star-ground wiring. If you're building point-to-point vaccum tubes, yeah, do star-ground wiring. If not, you don't need it.

3. The XLR jacks gave a ground tab on the case. Connect this ground tab to Pin 1 of the XLR jack with a small glob of solder. You can use a piece of wire, but a piece of wire is longer than a glob of solder.

4. Screw the XLR jack to the case. Make sure there is "electrical contact" between the case and the XLR jack's case. Use a tester to confirm. If not, you may have to scrape some paint off.

5. Connect pins 1,2,3 of the XLR to the pins 1,2,3 on the preamp PCB. (yes, pin 1 need to be wired to the PCB, otherwise phantom power would not work).

6. Once everything is wired, and working, power up the transformer, turn gain up, connect to a monitor and rotate the power transformer to find the spot where noise is at minimum or disappears completely. Turn off power, and tighten the bolt holding the transformer.

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