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

[madriaanse]

NICE WORK!!! I love that frontpanel as well!! Where did you get that?

These neve thingies love to hum. It looks like all your signal wires are shielded (shield tied to ground), correct? I'd definitely stick that transformer in a separate supply case along with the ACDC and move it a good 6ft away! :grin:

Great work!!

Martin

 

[khstudio]

Joel,
I'm going to try one of those PM2000 input trannys... do I need to hook up the Zobel?

Kevin

 

[khstudio]

GONE :green:

I hooked up a Power One supply & ALL the hum went away.

I DO NOT think the NEVE circuits like a grounded center tapped power tranny... I think I even read that before but I asked Joe from JLM about using the ACDC for this & he sent me the hook-up info. I will say that he did send 2 sets of instruction - one for the center tapped & one with just the 2 rails off the PS tranny (Like Matta did for his 1272's) This is my next step. Matts seems to be fine. I'll just have to mod the ACDC a little but damn I ripped ALL of my pre-amp apart... Damn you Greg :razz: ... you were right.
Joel too... he was questioning many thing but the ACDC was at the top of his list "live & Learn"


Kevin

 

[khstudio]

IF YOUR GOING TO USE THE JLM ACDC SUPPLY FOR A NEVE MIC-PRE
YOU MUST WIRE IT LIKE THIS - NO CENTER TAP POWER TRANSFORMER TO GROUND (0v).

I learned the hard way

After fixing the power supply
My noise/hum floor @ MAX GAIN (+65dB) is -80dB or better

Kevin

 

[khstudio]

PAR-METAL CHASSIS GROUNDING:

If you use these for your projects, do yourself a favor & check to see if your faceplate has contact to the rest of the chassis. The Anodized faceplates DO NOT make a good connection - which in turn doesn't sheild the "metal shell" of any control pots very well or at ALL :!:
I soldered a wire to the back of one of the pots (on the faceplate) to my chassis ground & it droped my noise & especially 60Hz by 15dB :shock:

Kevin

 

[Greg]

Good point Kevin... one of the first things I do when I get a chassis is take my dremel and scrape off some of the anodizing to make sure each piece of the chassis is showing electrical continuity. It's kind of a pain in the @$$ and takes a little time but I think it's worth the effort.

 

[JLM Audio]

IF YOUR GOING TO USE THE JLM ACDC SUPPLY FOR A NEVE MIC-PRE
YOU MUST WIRE IT LIKE THIS - NO CENTER TAP POWER TRANSFORMER TO GROUND (0v).

NOT TRUE. :?

The Mod is only needed if you want to use a single 24vac plug pack or transformer. As the mod gives full rectification instead of the normal half rectification you would normally get so gives a smoother unregulated DC to the LM317 regulator.

If you have a transformer with 2 x 22vac to 25vac windings no mod is needed as you are always getting full rectification because you are feeding both AC in's.

 

[Nele]

Any chance of an 'official' release of these boards (pdf's, films, pcb's..)?

 

[Greg]

I didn't comment on the center tap thing, but I often use center tap toroids (secondaries) when using +24VDC power supplies.

 

[khstudio]

[quote author="JLM Audio"]

IF YOUR GOING TO USE THE JLM ACDC SUPPLY FOR A NEVE MIC-PRE
YOU MUST WIRE IT LIKE THIS - NO CENTER TAP POWER TRANSFORMER TO GROUND (0v).

NOT TRUE. :?

The Mod is only needed if you want to use a single 24vac plug pack or transformer. As the mod gives full rectification instead of the normal half rectification you would normally get so gives a smoother unregulated DC to the LM317 regulator.

If you have a transformer with 2 x 22vac to 25vac windings no mod is needed as you are always getting full rectification because you are feeding both AC in's.[/quote]

Yes it did "work" but it hummed like crazy (-40dB) :?

What I was saying is I don't think the neve pre's like the CT grounded... maybe it was just mine... I don't think so. Works great now - I paralleled the secondary of my PT & made the ACDC changes & ALL the hum went away.

Kevin

 

[khstudio]

[quote author="Nele"]Any chance of an 'official' release of these boards (pdf's, films, pcb's..)?[/quote]

Show me where to post a PDF & I will share all the files :wink:

Kevin

 

[matta]

Hey Kevin,

Send me all the files and I'll host them, I have plenty of space. My e-mail address is matt(@)matt-allison(.)com (Remove the brackets).

Cheers

Matt

 

[khstudio]

I'm not home right now but I'll send them to you guys when I get in.
It's 6pm now... be home around 1am.

Can I post a PDF on the TWIN X site :?:


Kevin

 

[3nity]

Hey KJ looking good those Neve 1272'ish!
i really like those knobs. where did you get them?? i'm looking for a pair!

 

[khstudio]

[quote author="3nity"]Hey KJ looking good those Neve 1272'ish!
i really like those knobs. where did you get them?? i'm looking for a pair![/quote]

Thanks man :green:

The knobs - I have no ides where you can get those. I have a small stock of those that my tech friend gave to me. He had more but he past away last year. I could part with a FEW but not many. They're very nice, black anodized aluminum. They're also plain, No DOT, I drill the dots myself.

Kevin

 

[khstudio]

FRONT PANEL JACKS:

(hooked up like JLM's instrictions)

The line inputs sound very nice with my Triton & doesn't seem to effect the main (back) XLR's... which I definitly want to keep clean.

I tried the front XLR on the top channel only & it sounded sick :roll:
Not sure if those taps are bad or if it doesn't like the 20ft snake going to my big room, off the back jacks (MAIN jacks) at the same time - nothing pluged in.
:EDIT: The extra mic input (pins 7 & 9) just sounds dull compared to pins 8 & 10.

CHANNEL DIFFERENCES:

The bottom channel is killer.

The top channel is missing a little headroom/high end & a little dynamics compared to the tbottom channel. It does go thru the JLM DI.
:EDIT: I checked resistance from the input XLR to the input TX & all looks good.

The top channel goes to a phase switch then straight to the input TX using sheilded wire the WHOLE way. The bottom uses sheilded up to the phase switch, then single wire to the DI - then to the input TX.

Could this cause any issues with the high end or performance :?:

Kevin

 

[khstudio]

Test tone RINGING in CHASSIS ???
http://www.groupdiy.com/index.php?topic=237359

 

[khstudio]

The channels aren't too far off from each other... probably just normal circuit/component differences that I don't have control over.

CALIBRATION/OSCILLOSCOPE:

I'm a little confussed at what I'm seeing on the scope :?

I adjusted the wave until it "JUST" clipped each half... but when I turn the volume up to see what happens the bottom half just keeps flattening while the top half continues to rise with minimal change to the clip compared to the bottom :?:

Thanks,
Kevin

 

[matta]

Hi Kevin,

I've hosted the Joel's Files here:

DIY1272 PCB Layout - 2Mb
DIY1272 Component Overlay - 207Kb

Cheers

Matt

 

[Nele]

Thanks for the files!
Time for some etchin'!

Adios,
Corneel