JC's 1272 Pre-amp Design (COMPLETED + PICS !!!)

[Jazz]

I have not seen any disscussion regarding the variable resistor on the Ba283. I know if you have it set to high you will burn out some parts. I have done that one time. How are you folks setting yours?

I'm using JLM output trannies, "love his stuff" I believe Joe told me to check the voltage at the 1Watt resistor and not to exceed 1.5V I'm not sure exactly about the voltage...I lost the email ...damn computer crashed last week after doing an upgrade at Microsoft.

Here you go to the Microsoft web site with the intention of improving your OS and the upgrade halted my computer from ever booting again. The upgrade even fucked with my bios !

Gotta say those folks at MIcrosoft. "BORDERLINE GENIUS'S"
Love that company. **********NOT********** Ok I'm finished bitching.

:green:

:thumb:

 

[khstudio]

I know if you have it set to high you will burn out some parts. I have done that one time. How are you folks setting yours?

Never heard that one but I'd like to know if there is any truth to it :?

I guess this question could be grouped with mine about how the upper & lower halves of the waves clip differently.

I tracked heavy guitars today, some Fender stuff with a Uni-Vibe I just built & a LOT of vocals (U87)... the pre has a great high end... makes me feel like I'm looking right at the source in the speakers. You guys were right about cutting thru a dense mix. I don't get the NEVE warmth thing. My PM2000 pre has the most warmth I've heard yet & smooth highs. The Neve just cuts & is cleaner with a cool character... I couldn't ask for a better compliment.

Kevin

 

[Jazz]

Truth to it? You think I'm posting that to see my name on the screen?
It;s a fact buddy. Turn you varible resistor all the way to the left. Put your thumb on the heatsink .... you can cook an egg on it. It won't take long to blow.

:green:

 

[matta]

Hey Guys,

Is anyone having problems downloading the files I posted above? Please let me know if you are.

Cheers

Matt

 

[Nele]

just checked and they work fine over here (Mac -> OS X)..

Corneel

 

[Jazz]

10-4 Worked fine

 

[rascalseven]

The process I use for calibrating the output symmetry for a class-A Neve is basically as follows:

• *Load the output 600 ohms (put a 600-ohm resistor between pins 2 and 3 of the output XLR, assuming pin 2 is hot and pin 3 is cold). Be aware that if you have already attached a 600(-ish) ohm resistor to the hot and cold pins of your output transformer when building this, then you do not need to add another resistor for this step as your output is already properly loaded
  
  *Connect a sine wave generator to the input
  
  *Connect the leads of your O-scope to pins 2 and 3 of the output XLR
  
  *With the trim/attentuation knob all the way wide open (or output fader, whichever you have) adjust the gain switch just until your waveform clips on either the upper or lower portion
  
  *Pull the trim knob/fader back to the position of the onset of clipping
  
  *Adjust the trimpot either direction until the clipping disappears
  
  *Continue increasing the trim incrementally, adjusting the trimpot as you go to eliminate clipping (hint: you will be turning this pot only one direction as this process continues)
  
  *When you finally reach the point where both the upper and lower portions of the waveform begin to clip simultaneously, you're done.

That's pretty much it. If your circuit doesn't respond properly to this (you're getting pretty major clipping on either end and cannot adjust it out) then you most likely have a short on your PCB somewhere or you need to replace a transistor(s) with a fresh one.

I hope this helps.

And yes, anyone who wants to use the documents Matt posted above, have at it. I made those in Illustrator and didn't really dress them up for circulation (they were really just for myself), but they are correct, so go for it.

Also, I do not have cards for sale for this project, nor do I have plans to make any. This is a simple board to etch, and compared to many other projects it's actually a pretty simple build, which is often not the case with Neve-based preamps as the gain switches particularly tend to be rather confusing to many people. Hopefully this simplified version will encourage even DIY novices to take it on. I'm of the opinion that every studio can benifit from at least a couple of Neve channels.

Much peace,

JC

 

[khstudio]

NEVE 1272 - CLIPPING
I don't feel my unit is clipping evenly :? Pics + info:
http://www.groupdiy.com/index.php?topic=238335

--------------------------------------------------------
Neve - Test tone RINGING in CHASSIS ???
http://www.groupdiy.com/index.php?topic=20253

The ringing in the chassis seemed to settle down after I found I had a 620K LOAD on the OT instead of a 600 ohm


Kevin

 

[khstudio]

While I'm on the subject of dumb questions...

How do you remove the coating on transformer wires :?:

I tried to shorten the leads of my toroidal & had a hell of a time trying to scrape that crap off... there has to be a better way...CJ?


Kevin

 

[Lowfreq]

Wow, this a great thread!! Awesome info :thumb:

I've got a blank pcb sitting at home, quite a few of the required parts, and my L31267 arrived today.
I'm gonna make me a Neve!!!

This def looks like the cheapest and easiest way to do it.

This may be a dumb question, but are the part numbers on the overlay of your board relating to the standard BA283? If I use Gregs BA283 BOM I'm heading in the correct direction??

Now I need to decide on which output transformer I'm gonna use.......
mmmmmmmmmmmmmmmmm

 

[khstudio]

:thumb:

Advise:
Don't mess with the extra XLR input (pins 7 & 9) use +8 & -10
The line is OK but still useful

Follow the Grounding Rules!!!

DO NOT Ground the center tap (if you use one) of the Power Tranny
The alternate ACDC hookup got rid of ALKL my Hum & Noise

The small toroidal in the back corner is not bad at all (concerning HUM)
BUT - Making sure your chassis has continuity thru-out (like the bottom, top, back & FACEPLATE... I ran a wire from my ground lug & attatched it to the back of one of the front pannel pots & dropped my 60hz by 15db or more.

I'll help ypu when I can... I'm still needing help myself but it sounds KILLER :green:

Good Luck,
Kevin

 

[rascalseven]

Lowfreq,

Yes, if you're using the PDF of the traces that Kevin used (I think Matta posted them above), then the letters on the pins correspond to the letters on a 283 card. Also, the ancillary circuitry is on the board.

Other than the components for the gain switch that you want to use the BOM is just the BA283 components (check a 283 schematic) plus:

(per channel):

18k 1/4-watt resistor
270R 1/4-watt resistor
12R 1-watt resistor
4700uF/25v electro
1000uF/25v electro
PSU of your choice (needs +24vdc for the circuit and +48vdc if you want phantom power)

BTW, my board layout isn't proprietary to my JC1272 design. It is really just a pair of BA283 circuits with their supporting circuitry on the board with them. No gain components are on the board, so you can choose the gain staging of your choice (JC1272, JLM SimpleMod and HotRodMod, etc...). Since the letters correspond to BA283 pins, it's simple to just build out the gain switch of your choice and hook it all together.

The biggest difference on this PCB from a 'real' 283 is that I spaced the traces so the 2N3055 output transistor can lay flat against the PCB with plenty of room for a heatsink. 283's (and 183's as well) have this bugger mounted with standoffs and have components under the heatsink, which just cook when the thing is in use.

Hope this all helps.

Peace,

JC

 

[Lowfreq]

Thanks Joel!! . :thumb: :thumb:
I'm gathering parts as we speak.

I'm still weighing up my options for the output transformer too.
The JLM will probably be the cheapest for me here in NZ(less shipping),
but I've read a few threads around here about how a lot of the Neve sound in these baby's is in the output tranno. I've got a feeling the JLM is gonna be pretty transparent.
The sowter looks good too, and they give me the option of being able to fit it in a 1ru case, as does the JLM.

mmmmm. desicions, decisions :green:

 

[Svart]

:thumb:

 

[mick]

Hi I was trying to work out the wiring for the rotary switch for JC's boards, so for all the other's like me that hadn't done a rotary switch before here's a diagram, shout out if there's anything wrong.


http://www.twin-x.com/groupdiy/albums/userpics/JCswitch.bmp

 

[khstudio]

[quote author="mick"]Hi I was trying to work out the wiring for the rotary switch for JC's boards, so for all the other's like me that hadn't done a rotary switch before here's a diagram, shout out if there's anything wrong.


http://www.twin-x.com/groupdiy/albums/userpics/JCswitch.bmp[/quote]

Good Job :thumb:

I dug up my files & found a drawing I made of the switch... it matches.

Sorry I didn't get back sooner to your email but it seems, with a little work, you figured it out for yourself.

 

[mick]

Hi Kevin
I was trying to work it out from your pics but I couldn't quite see enough, but I checked out what I wasn't sure about with JC, I figured you must have been busy.

 

[clintrubber]

[quote author="rascalseven"]The process I use for calibrating the output symmetry for a class-A Neve is basically as follows:[/quote]

Sounds more than healthy, clear procedure :thumb:


... but there's this nagging question, would we always want 'perfect' output symmetry ?

Without doubt people have toyed around with this, but I haven't seen many front panel misadjust-a-bit-on-purpose knobs here (say an internal coarse and an external fine-control).
Not much to gain here w.r.t. additional colouring ? I'll be trying, but also curious what others have found here.

To notice eventual benefits from asym. clipping it would obviously require full swing signals, so a post-output-TX attenuator would come in handy.

Regards,

Peter