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

[khstudio]

Thanks to Joel (JC) and you guys here :thumb:
It sounds great & was a lot of work & research. I etched the PCB myself + made the faceplate from scratch using the Lazer printer/77 glue.

You may notice a few things in the pics like tranny bolt missing & some grounds cliped... this is because I'm tracking down HUMMMMMMMMMMM.

Input trannys are the L31267's, JLM ACDC PS + JLM DI on ch1, LO1166 OT, 25VA PT.
The extra jacks on the front will be connected to the extra taps on the input tranny AFTER I get rid of the HUM.

----------------------------------------------------
I've been talking with Joel over the phone for a few days & have decided to go with his "JC's 2-Stage 1272 Pre-amp Design".

Better, Cheaper, Faster...
No need for a separate BA283 card (which are sold out), EDGE connector, Hot-Rod or expensive Gain switch.

*NOTE There is no 5k gain control between the stages but can be easily added (check out the JLM Hotrod Schematic) I will be adding this to mine.

I may also design a small PCB for the gain switch (Lorlin 2x6)
& possibly a basic +24/+48 supply board. If anyone has one, or a link to one, please let me know.



Refference LINKS:

http://jlmaudio.com/neve_BA283.htm

http://jlmaudio.com/JLM1272simplemod.pdf

http://jlmaudio.com/JLM1272hotrodmod.pdf

Wiring 31267 as 10468 - JLM
http://jlmaudio.com/Neve%20transformer%20info.htm

31267 trafo in 1272 clone problems..
http://www.groupdiy.com/index.php?topic=7126

Type 1272 Preamp with JLM Hotrod Mod and BA283AV (Pics)
http://www.groupdiy.com/index.php?topic=17697

Hotrod Build Thread
http://www.groupdiy.com/index.php?topic=15373

Fabios 1272 finished
http://www.groupdiy.com/index.php?topic=18448

How Do I Wire A 31267?
http://www.groupdiy.com/index.php?topic=19107

1272 - why a step down transformer
http://www.groupdiy.com/index.php?topic=17537

1272 LINE INPUT ?
http://www.groupdiy.com/index.php?topic=14218

Any one used 31267 as a substitute for a 10468 successfully?
http://www.groupdiy.com/index.php?topic=14046

Geoff Tanner
http://www.auroraaudio.net/main.shtml

Thanks,
Kevin

 

[clintrubber]

[quote author="khstudio"]I'm starting this thread by pulling JC's design & notes from an older thread hiden in the Black Market:

http://www.groupdiy.com/index.php?topic=18746&postdays=0&postorder=asc&highlight=n72&start=15
[/quote]
Cool, nice to see this thread!

I'll link to it in that Black Market thread & change the title as well - as that one now no longer needs to be 'Labbed' & can go further for the panels & knobs.

Regards,

Peter

 

[khstudio]

OK, I got all my parts & I'm assembling now. I'll post pics soon of the progress.


Kevin

 

[mikep]

looks great!

I know you just tacked it on, but that is not a suitable DI input youve got there! I can imagine 5k is too low-Z even for some active pickups, synths and pedals to perform their best.

just a thought, since this isn't an exact clone anyway, maybe use a multipole switch, or some relays, to reverse the input transformer (and load it correstly) making it a 4:1 line input. neve actually did this on the 66 series broadcast console using digitally controlled relays (and a different input TF). a 10468 in reverse, loaded with about 1k will have 16k input Z. still quite low, but at least "out of the woods".

mike

 

[khstudio]

Yea, the instrument input that's shown will NOT be on my version & was originally just added for Synths only from what I've been told. :wink:

I have the correct outputs (LO1166) but I'll be using the St. Ives (31267) input tranny wired like JLM recommended:

http://jlmaudio.com/Neve transformer info.htm

I'll be mounting the Line input & extra Mic input on the front Panel just for the hell of it.


Kevin

 

[skipwave]

Kevin,

Did you use JC's pcb design, shown above?

I'm waiting on a 283 board that went missing in the mail. It might be a good time to just get the new pcb fabbed. Maybe JC would give his blessing for a group buy.

Thanks,

Brian

 

[khstudio]

[quote author="skipwave"]Kevin,

Did you use JC's pcb design, shown above?

I'm waiting on a 283 board that went missing in the mail. It might be a good time to just get the new pcb fabbed. Maybe JC would give his blessing for a group buy.

Thanks,

Brian[/quote]

Yes & I have the real file/artwork + the parts overlay for the PCB.

I don't think JC would mind sharing with the group. He said he was working on a web site & was going to post the PCB & info to share with you guys but I think he's been too busy to get to it. I've actually been trying to reach him for almost 2 weeks... I'm on my own right now but if all goes as planned I'll have it completed tomorrow. I'll post back ASAP with some pics & details.

BTW, I also didn't want to wait for the new batch of BA's. I almost just went with Fabio's N72 but didn't really have a grip on the project yet... know I would probably consider it more but JC's is looking good & VERY basic.


Kevin

 

[khstudio]

Well it's 5:15 am and it's done :green:

Never heard a NEVE before but this thing sounds REALLY GOOD... if I could get rid of the damn HUMMMMMMMMMMMMMMMMMMMM it would be the best pre in the studio.

I followed all the rules for grounding... even put my torroidal outside the case & it's still there. I'll have to post some pics when I wake up.


Kevin

 

[khstudio]

Added pics to 1st post

Kevin

 

[tommypiper]

Looks good.

Too bad about hum.

1272s are very sensitive to toroid radiation. Was it better with the toroid outside the box?

There've also been some issues with the 31267 and hum and discussions about it here somewhere... search around. The 31267 doesn't offer the same shielding or something... JC, Family Hoof, and others will know.

 

[rascalseven]

Well, Kevin finally caught up with me today and we had an hour-long conversation about this. He's gonna look into a few things that might be possible cuplrits and report back.

Hum/noise is the NUMBER ONE PROBLEM people have when DIYing class-A Neve's. Nine times out of ten when someone has an 'issue' with a Neve build, its a noise issue, and nine out of ten of those are caused by grounding errors. It's easy to mess up, so don't be too concerned. Once it's solved you'll be that much wiser -- that's a big benefit of DIY pro audio!! :thumb:

I doubt the input transformers are contributing much to the problem unless you operate in a high RF environment. I've built probably two dozen of these with reversed 31267's (wired per JLM) and the only one that had noise issues was one I accidentally hooked the 24-volt rail to (wow, what a fascinating noise it was too! ) Even with standard internally-mounted power transformers (not torroids) I was able to get dead-silent results. It is true, however, that hooking up the inputs according to the JLM page defeats the humbucking design of the things when wired series:series.

Also the DI shown on my above drawing is really an afterthought that I included for people who want an instrument input for active electronics (or for people who like the sound of loaded passive pickups :razz: ). True, it's not a 'proper' DI, and an active circuit like Joe's (JLM Audio) is really the way to go, or a nice transformer like the Cinemag DBX.

I had seen this type of DI circuit on a commercially available brand of Neve clones (who shall remain nameless to protect the innocent), and I'm cool with the performance when used with keyboards and samplers. It's simple to do, so I included it in the schematic. If you really want 'proper' DI performance, however, go another route.

Looks awesome Kevin. Can you link to some more detailed pics so I can have a closer look at your wiring? I might then be able to help more with figuring out the cause of the hummmmmmm....

JC

 

[rascalseven]

Kevin,

Another thing I just thought of was that it appears in your pics that you haven't ground away any of the powdercoat on the chassis panels. The powdercoat on Par-Metal chassis actually inhibits conductivity, so your transformer cores could actually be connected to nothing at the moment.

I use a grinder/sanding bit on a dremel tool and just grind away the finish around the screw holes as well as the immediately adjacent panels that touch those holes when the case is assembled. When you put the chassis together these grind marks are hidden, so it still looks good, but the whole box becomes a better shield that way, and then when you connect make a chassis connection to your AC ground pin your trafo cores are definitely grounded to Earth.

JC

Edit - I just took another look and noticed that all the audio transformers AND the psu are bolted to the same panel, so the trafo cores are probably earthed just fine...

As you were.

 

[khstudio]

[quote author="rascalseven"]Kevin,

Another thing I just thought of was that you haven't ground away the powdercoat on the panels. The powdercoat on Par-Metal chassis actually inhibits conductivity, so your transformer cores could be attached to nothing at the moment.

I use a grinder/sanding bit on a dremel tool and just grind aware the finish around the screw holes and the immediately adjacent panel that touches those holes all around the chassis. When you put the chassis together you cannot see these marks, so it still looks good, but the whole box becomes a better shield that way, and your trafo cores are definitely grounded to Earth then.

JC[/quote]

The chassis coarting was one of the first things I thought about this morning... weird, so I got out my meter & checked to see if the input case (pin 6) was reaching the other side of the case, PS/GND & it was BUT I just got my Dremel out for some griding right before reading this.. cool.

One thing I noticed, if I put my meter on the (Par-Metal) face-plate at each end I get NO reading... is that thing coated too?

#1 thing I'm doing now is removing the 10 ohm resistor.

#2 disconnecting ALL input switches/48v stuff

#3 removing the output fader

Please help me thru this man... right before my client left we listened to it & compared it to ALL my other pre's & we both felt it was just sweet & better than the rest (on vocals at least for now)


Kevin

 

[khstudio]

Joel, Pics on the way

:green:

I don't feel so alone now + this threads been here for a while... about time you showed up :razz: I kid I kid.

 

[khstudio]

Close-up Pics:

http://www.twin-x.com/groupdiy/albums/userpics/Top_A.jpg

http://www.twin-x.com/groupdiy/albums/userpics/Top_B.jpg

http://www.twin-x.com/groupdiy/albums/userpics/Switches.jpg

http://www.twin-x.com/groupdiy/albums/userpics/48___DI.jpg

http://www.twin-x.com/groupdiy/albums/userpics/Board.jpg

http://www.twin-x.com/groupdiy/albums/userpics/In_TX.jpg

http://www.twin-x.com/groupdiy/albums/userpics/ACDC.jpg

MY DIY GALLERY:
http://www.twin-x.com/groupdiy/index.php?cat=10014

Kevin

 

[khstudio]

Joel,

The hum is ALWAYS the same level at ALL volume & Gain settings.

I jumpered the 10 ohm resistor & removed the 2nd RED +24v wire.

Humm is still there, NO change.

I'm going to remove the Phase & 48v switch & DI to narrow it down.
Then try removing the output volume control.
Next is to try my 0-24 Power One (Without 48v)

Kevin

 

[Nele]

How did you connect your in- and output grounds? I haven't built a 1272 yet, but maybe you can connect all your xlr-grounds together (in- and output) with a solid (copper) wire and connect this to the ground at one point. Like here in the green pre:
http://1176neve.tripod.com/id28.html
mentioned here: thread


cheers,
corneel

 

[Greg]

I noticed that you took the GND from the IEC to the pad on the JLM PSU, but I didn't see where you took that to the chassis. Did I miss it?

Also, don't just go ripping things apart. It sounds like a typical ground loop.

 

[rascalseven]

Greg, he connected the ground from the AC ground pin to the standoff of the PSU, so it's connecting with chassis at this point, so the connection is indeed made.

Kevin, do you have an audio sample of the hum that I can hear? I'm very familiar with the typical noises made by improper grounding on the Neve stuff (comes from making lots of mistakes ), so hearing it might help determine what's wrong.

Also, by any chance are you connecting this thing to an unbalanced input on the following device? If so try lifting the ground/pin 1 connection and instead connect XLR pin 2 to the tip/hot of the following device and XLR pin 3 to the sleeve/ground pin (so no pin 1 connection at the Neve side). Otherwise any difference in ground potential between the two devices will result in a loop that affects the Neve audio path.

When I was building these early on they sounded great but they all had minor, but noticable buzz in the output. I crawled all over the circuits but couldn't find anything I'd done wrong, then it occured to me that I was using an XLR output to a TS connector (pin 1 and 3 connected to the sleeve of the TS connector) feeding a balanced TRS connection on my A/D interface. It occurred to me that this might be the problem, so I disconnected pin 1 at the XLR and the result was beautiful silence.

This is what makes DIY Neve stuff so ticklish -- not only do you need to use care when building your Neve stuff, but need to continue using care when interfacing it with other gear in your rack. I actually have a few 'special' bantam patch cables for this reason in my studio. When I connect the outputs of my Neve's to unbalanced gear I'll often use these cables so they're connected as described above to ensure silent operation.

JC

 

[khstudio]

10pm to 8:30am this morning & could not find it :sad:

I disconnected:
DI, Phase switches, 48v power & switches & took oiut the 5k output pot between stages (Jumped)

I also tried the input TX - series/series with little change to the hum BUT with this setup & also lifting the ground/sheilds apart at the PAD pot I was able to drop the hum to -60dB... still not enough but closer.

SWEEPS - all test used a 150ohm load on the XLR input
Pink: ACDC Supply inside case, ALL Volumes & GAIN OFF
Blue: ACDC Supply inside case, ALL Volumes & GAIN MAXED

Green & Red: 24v WALL Adapter - the same except I pluged the adapter into the wall in reverse to see how the AC phasing affects things.
ALL Volumes & GAIN OFF
***Notice that with the DC adapter the residue of the 120hz & Noise is below -90 ***

http://www.twin-x.com/groupdiy/albums/userpics/1272_compare_all.bmp

24v WALL Adapter, ALL Volumes & GAIN MAXED
http://www.twin-x.com/groupdiy/albums/userpics/DC_Adpt-maxV-load_plug.bmp

24v WALL Adapter, ALL Volumes & GAIN OFF
http://www.twin-x.com/groupdiy/albums/userpics/DC_Adpt-off_V-load_plug.bmp


Kevin