Seventh Circle Audio

[Patrick from Davis]

Nice looking pre's.  I warn you Trevor, stay away from the 500 series or 51X series of toys...  Those things are ways to much fun and way to easy to want more.  But, back to upgrading things, another way to really add to the quality of the home studio is to check out Black Lion Audio's upgrades on DAW/IO equipment. 

With respect to the C12, Chunger just got in the donor bodies for the C12, and Ela M251 over at Studio 939.  The bodies for the U87 and U47/M49 are still in development.  Good time to be be a DIY enthusiast.
Patrick

 

[mayfly]

Nice looking pre's.  I warn you Trevor, stay away from the 500 series or 51X series of toys...  Those things are ways to much fun and way to easy to want more.  But, back to upgrading things, another way to really add to the quality of the home studio is to check out Black Lion Audio's upgrades on DAW/IO equipment. 

With respect to the C12, Chunger just got in the donor bodies for the C12, and Ela M251 over at Studio 939.  The bodies for the U87 and U47/M49 are still in development.  Good time to be be a DIY enthusiast.
Patrick

Thanks for the info Patrick re the DAW upgrades - will check it out.  You know about Chunger and the boys?  I've been lurking over there (and drooling) for awhile.  I'm patiently awaiting those U87 bodies.

 

[Patrick from Davis]

Oh yes, I know about Chunger, Igor, Matador, Poctop, and of course Jeff at CAPI...  I want to build the Ela M251, the M49, and the U87, cause well, three's a magic number.  Jeff's 51X rack kit is also just one of things I'd really love to put together.  So many possibilities.  But I have to get the Profire 2626 upgraded first.  No point in getting excellent mic's, pre's, eq's, and comp's if your AD/DA conversion is crappo.
Patrick

 

[mayfly]

More info

The box only came with very skimpy documentation - just the parts list and schematics.  You're expected to pull stuff down from on-line I guess:

some nice looking transformers:

Various components are nicely organized:

This is neat: all of the resistors were arranged on a custom reel.  They were even in order from lowest resistance to highest.  Pretty neat actually and it made for easy finding of components.  I checked each one with my DMM anyway mind you:

I've got all the resistors and most of the caps on there.  I plan to do transistors over the weekend.  More later

 

[Cagey]

What?!? Transistors!! WTF? No 38 year old vacuum tubes??? Those things are gonna be all accurate and have low noise and wide frequency response and outrageous dynamic range and almost no measurable distortion! Did you know all that on the going-in side?

Y'know, it's guys like you who make music all intelligible and enjoyable. I hope you're happy. You should be ashamed.

 

[Re-Pete]

I find this a very intriguing Thread which I will be following with some interest. (Though I certainly don't have the required technical skills or soldering abilities to do this DIY preamp stuff)  :icon_thumright:

 

[mayfly]

What?!? Transistors!! WTF? No 38 year old vacuum tubes??? Those things are gonna be all accurate and have low noise and wide frequency response and outrageous dynamic range and almost no measurable distortion! Did you know all that on the going-in side?

Y'know, it's guys like you who make music all intelligible and enjoyable. I hope you're happy. You should be ashamed.

You really made me laugh out loud with all that.  Here's to you Kevin!  ccasion14:

 

[mayfly]

ok an update.

I've got one of them together:

But I must say it was a PITA.  The regular module was fine; the fun started when I had to make changes required for the one shot module.  The one shot, in addition to being a single box to put in one module, allows you to add in a switchable DI into the mix.  So far so good, but the addition of the DI means that you need to interrupt the input signal somewhere in the signal path to add it.  The instructions are very very vague on how to do this.  Paraphrasing it says, "consult the schematic and figure it out for yourself".  Not so great- especially when the schematic for the one shot boards are not published.  Eventually I determined that I could interrupt and inject the input signal right before the input transformer.  Wasn't so bad once I figured that out.

The other thing that was very annoying was the power input jack fell off the one shot module board.  Didn't take much either - the wires connecting the jack to the PCB gave up right at the PCB - probably because they were nicked when the wires were stripped.  Again since the schematic for the one shot was not available I had to figure out how to re-solder the assembly on my own.  Not so great.

Finally the mechanical design requires you to partially disassemble the work on the module - specifically the nuts and bolts holding down the output transformer.  This, and the fact that the design requires TWO insulated standoffs per screw made it very annoying to get together.

But now it's together.  Lisa and I shared a bottle of wine at dinner, so I think I'll wait until I'm sober before powering it up.  There is a bit of biasing to do on the output transistor to do, and the SCA forum is filled with threads on people screwing this up and toasting one or more resistors in the process - not something I want to do.

More later,

Trevor

 

[Cagey]

Very nice. I wish you well in your testing.

 

[mayfly]

Thanks Kevin,

Well I did power it on.  I have a current limiting power supply so I figured I could not get into too much trouble.

Rails went to where they should go, and I managed to get the output transistor biased.  Currents stayed where they should and bias was achieved.  I think I'll test the phantom power a little later, then see if it passes audio.

 

[Cagey]

You have much more patience than I do. I wouldn't be able to put it down.

 

[mayfly]

So I hooked up the signal generator and pumped some, er, signals through it:

The small wave is the input, the big one is the output.  Scales are not the same - this thing has some gain!  You can't see it on the photo, but the output is just at clipping - some 50VPP.  I adjusted the bias of the output transistor until I got the clipping symmetrical:

Bias point ended up being very close to SCA's published figure of 22.2VDC:

More testing after the kids are in bed.

 

[mayfly]

More updates.  Put it through some more tests.  Tested out every feature and it all seems to work as advertized.  So I buttoned it up and tested it again.

Tomorrow I'll put a mic through it and see what's what.

 

[Cagey]

Congratulations! Looks like it works right out of the gate. That's beating the odds for a home bench. Hell, it's pretty good for a factory part. I notice the output is inverted - is that normal or switchable?

 

[mayfly]

It's switchable - one of the features that I confirmed is working 

 

[Patrick from Davis]

That's beating the odds for a home bench.

It depends a lot on the bench being referred to.
Patrick

 

[Cagey]

Of course. And Trevor's been through the mill, so I'm sure he has a lot of finely-honed habits. But, that's not typical for home-builders. Look how many people can't even get a guitar to work right first jump out of the gate, and they're nowhere near as complex as what he just built.

I worked for a manufacturer about 100 years ago that built all their PC boards from scratch, right from cutting clad boards and etching/drilling them, through stuffing, soldering, etc. and we probably had about a 20% fail rate off the line. That's with automated stuffing machines, checklists, wave soldering and multi-point inspections. There are just too many things that can go wrong with something that complex.

 

[bagman67]

Speaking of this sort of thing, I ran across this in my BoingBoing RSS feed this morning:


http://boingboing.net/2013/09/10/autodesk-announces-free-electr.html


Free circuit design software from Autodesk, which suggests that it will be feature-limited, but worth exploring.  Autodesk aint exactly a bunch of amateurs.


http://www.123dapp.com/sandbox


All kinds of fun stuff here, actually:


http://www.123dapp.com/create


3D capture, 3D modeling, and so on.

 

[mayfly]

One negative thing that I have noticed:  the 24VAC wall warts provided with the one shot kits noticeably buzz when plugged in.  Since I plan to use a couple of these things I see a home made power supply in my future.  Then I can make it quiet and fuse it properly at the same time!  :headbang:

Now where did I put my hammond catalog....

 

[Patrick from Davis]

That issue is a major plus of the API VPR format.  Well that and that you can put 6, 8, or 11 of what ever monsters into the rack.  Those are some very nice looking pre's.  Now we need to hear a sample.
Patrick