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Help! Seventh Circle Audio N72 ( Neve Preamp )
Old 18th January 2020
  #1
Gear Head
 

Help! Seventh Circle Audio N72 ( Neve Preamp )

I have recently built 4 SCA N72 preamps and I have a few questions before I plug them in for the first time. The only instructions for these linked to the SCA site are for version 1 of the circuit board. The board I have built is version Rev. 3.4. Most of the changes were obvious but there are a couple of things I'm unsure about.

1. On the smaller Carnhill transformer they have added a small circuit board to solve the problem of creating small jumpers within the transformer itself. This board then solders to the larger circuit board. My question is, this small board has 4 pins coming off of it. They are labeled J5 and J6. The pins are individually labeled p2+ to p1- and right below them is p1+ to p2-. In the kit there are small 2 pin plastic jumper connectors that fit these. Am I right to assume the jumper should connect the pins horizontally ( p2+ to p1- ) and ( p1+ to p2- )?

2. In another area labeled j2 there is another similar pin like the ones I have described.They are not labeled + or - but I assume that my last plastic pin jumper is meant to go there. Am I correct?

3. There is a part on my circuit board labeled conn3. It is next to the six pin connector going to the power supply. I did not have a part that fit this area in all 4 of my preamp kits. This leads me to believe that this connector is not needed. Am I correct in my thoughts on this?

4. What should I be testing before plugging them in?

Thanks
Any Information would be great.
Attached Thumbnails
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Old 20th January 2020
  #2
Gear Head
 
Johnny T's Avatar
 

I have a N72 microphone preamp (Revision 3.4) but I haven't built it yet.
Our best bet is to have a look at the assembly manual (PDF) and schematics (PDF).
Most of what you're interested in is on the first page of the schematics.

1. The assembly manual states:
"J5 and J6 allow input impedance to be switched from 300 ohms to 1200 ohms. Use a single jumper for 1200 ohms."
The photos show an older revision, but we should be able to figure it out. Maybe someone can comment on what the most suitable input impedance would be.

2.The assembly manual states:
"J2 connects a 604-ohm load resistor across the output. Unless you’re connecting the N72 to a piece of older gear with 600 ohm input impedance, install a jumper at J2."
The schematics confirm this.

3. I'm pretty sure you don't need to install CONN3.

4. The last few pages of the assembly manual deal with "Initial Power-Up and Testing."
Old 3 weeks ago
  #3
Here for the gear
 

I was searching for these issues as well, as i'm about to (finally) get to my N72 builds.
The 300ohm vs 1200ohms (j5/j6 jumper) depends on use:
Here's a decent thread that deals with the input impedance issue: impedance setting on neve 1073dpa
Old 2 weeks ago
  #4

. . 1200 ohm

| | 300 ohm

One jumper across top two pins for 1200 - this is the standard setting.

One jumper from top to bottom
On each side for 300 ohm

You can wire a dpdt on/off switch too. There will be 6 pins. Jumper the top two and connect the bottom two pairs across the 4 headers in a 1-1 fashion. As long as the top/bottom pairs remain on the same side you’re good.

I’m on my phone or I’d be more clear. Pm me if you need more clarification.

Last edited by Category 5; 2 weeks ago at 11:15 PM..
Old 2 weeks ago
  #5
Btw, jumper j2 and ignore the extra conn point.

Are these old kits? None available on the site anymore so let me know if you have one to sell.
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