SCAexy!!!

[djc251]

I just got my package in the mail; an N72 kit with chassis and power supply, and I am too excited! All the parts are accounted for and I can't wait to dig in. Once this Chemistry exam is over tomorrow, I am gonna start pluggin' some resistors. Tips anyone?

[Mylithra]

The best advice I could give someone building these is:
1. Take your time. You get in a rush.. you end up with bad results.
2. Measure several times, but only solder once. I ended up double and triple checking myself while I was putting everything together. but it came out correct the first time.
3. Its a lot easier to read the markings on the PCB under a Capacitor before you install it. Double check yourself on polarized parts . You wont be sorry.
4. I advice you to organize your parts ahead of time. I ended up taking a couple of sheets of paper, wrote what the parts were and their values, then laid the parts out on that sheet of paper, measuring the parts with my DMM as I placed them on the paper and as I took them off the paper. Again.. see 2.
Mylithra

[djc251]

Thanks! That is why I love this forum; people are genuinely cool and willing to help others with their projects. I haven't built one of these yet, but I've posted one other topic and have been scouring the forum for months now and its obvious that everyone is into this. It's great to see!

[Philo]

The best tip was to get a large piece of styrofoam, print out the parts list and tape it on top of the styrofoam, then measure each resistor and stick it through the parts lists into the styrofoam next to its entry on the parts list.

This takes 10-15 minutes, but greatly reduces the chance of putting a resistor in the wrong place.

Agree on double-checking all polarized parts right before soldering.

Be sure to read the sticky in the N72 forum on the new design and decide if you want to use the new jumpers. The sticky has info NOT in the directions yet.

The tricky bit is wiring the input transformer. In retrospect, I think it might have been easier to strip the wires, install the trasnfromer physically, solder the wires into the board, and THAN solder them to the transformer. If you do two level leads (one set close to the trasnformer body, one set (to different holes on the board) further away, this should work.

Phil

[TomB]

The previous posters all have really good advice. As has been said in other postings, some resistors have the same color-code sequence in both directions, and it's best to measure the resistors. I also find some of the colors on the 1% resistors are not very clear. Red and brown sometimes look very close.

I also double-check everything. I have built five SCA preamps without any problems.

Regarding styrofoam, please be careful here. Untreated styrofoam usually generates static electricity which can ruin your semiconductors (e.g. transistors, diodes, ICs). If you use styrofoam (or foam), please make sure it is of the anti-static variety (i.e. like the foam the ICs are in).

Take your time, and have fun!

[djc251]

Well, I just completed the preamp and it is working as it should be. I see what people mean when they say the N72 is thick; Wow!

As far as assembly goes, I had no problems. I just took my time and checked my work as I went. The input transformer is the most challenging part and even that wasn't too bad. I agree with the solder the leads to the board THEN to the transformer bit.

I was so relieved when I fired it up and it actually passed signal.

Thanks for all your help guys! I can't wait to do an A12!

[greg]

Question: did you calibrate with an oscillioscope per the instructions? I intend to build the N72 as my next project, but want to know if this step is necessary and what I will miss if I skip the step. Thanks.

[TomB]

djc251,

After building the N72, the A12 will be easier. Though, still take your take time!

Greg,

I did not use a scope on my N72, and it still sounds great. The instructions describe how to bias the pre without a scope which gets you most of the way there. I will probably check mine with a scope in the future.

[djc251]

No, I didn't use an oscilloscope either. I don't have one. I was just happy being that I got a signal through it!

Heads up; I just recorded my bass through it direct, and it is seriously the greatest direct bass sound I have ever heard, very dimensional and harmonically lush. This thing has surpassed my expectations in every way. I can't wait for tomorrow to try more things!

Also, after having listened to this preamp for a few hours, I listen to the radio and the playing field seems to be more level now. I can get tones now that were never available to me before. Cranking the gain and pushing this pre is what it is meant to do.

[Commodor]

Just to add in here.. I've built three N72's so far and I did use a scope.. but... in a general sense this isn't absolutely necessary and not using a scope isn't going to prevent it from sounding good, as you've already discovered.

If you read the directions and look at the photos in regards to this you should see that what is happening is that you are maximizing the headroom; squeezing out every last little bit as it were.

But... it has LOTS of headroom so it's not as if it's problem really. It's worth doing if you have a scope. If you don't, I wouldn't lose sleep over it.

And.. I LOVE my N72's! Two other engineers in my area have bought SCA racks now after hearing mine (so far!).

All the best,

Commodor

[greg]

Thanks for the replies. I have 2 T15s and 1 A12. N72 is my next build.