[BUILD] All things RackNeve

[Ilya]

Thanks for your kind words. I try to support builders as much as I can.

the pcbs i got are of very high quality and the 1290 looks like og vintage design from below

Indeed, the design of the boards is based on original plug-in cards, but a lot of the external wiring is substituted by PCB traces in my design.

Regarding UTC A-20, I can repeat here what I told you in pm:

I don't feel this will be an easy solution. You may try, but I don't know how that turns out. The feed to the EQ is taken from pin P of BA283 card. You can see if it's possible to insert a transformer there. Before you do that, ensure that the modules work as intended to start with.
If you want to drive the A20 in that position, good luck)
If I recall correctly, UTC A20 is capable of handling +15dBu level. EQ send in 1073 is -14dBu (almost 30dB lower). You won't get any coloration at that position. I'm attaching a level diagram so you can see the level at different points of 1073.

Another big big plus plus would be if the utc as interstage could be used for suming the mic and line inputs in a elegant way or even open the option of 2x mono line In.

You can definitely use it as a line input transformer. You can even set up different ratios to step the signal up or down in level and see if you like how that sounds.
I'm not an expert in transformer summing, so I can't comment on how to get several inputs with a single transformer.

as output i ordered the UTM 3548

This is an exciting transformer. I'd be very curious to find out how it sounds, Looks like a LO1166 type clone.

Zobels for the original transformers are designed into the circuit. You can easily see them on schematics. I can't find any definitive recommendations for UTC A-20 Zobel network. Need to find a proper UTC catalogue (which I don't have at hand right now).

use the JLM mentioned wiring of the marinair 31267 to have 2 mic inputs + a line input

This looks like an interesting solution. JLM knows his stuff, so this recommendation will probably work just fine.

Good luck and keep posting here!

 

[ion]

Thanks Ilya. I was thinking of adding the UTC post gain, like right behind the gain stage. But i dont know enough about circuits and gain staging within a circuit to make sense in positioning. I understand that between pre and eq it doesnt make sense, but wonder what about the position before the -14db saying line + 18, mic + 28.

 

[Ilya]

line + 18, mic + 28

If I read this chart correctly, these are the maximum amount of gain available for mic and line inputs, not actual levels.

 

[ion]

Hi Ilya, i hope you are allright.

i wanted to quickly show my progess so far and ask a few questions.
reminder, i plan to power the pre/eq with some simple 24dc external And leave out 48v.
i know to leave the power related corner of the preamp empty of parts, but is the same true for the eq?
like rp1 rp2 etc.

-do you see any issues with my build so far?
-i was a bit worried about the pins of caps and resistors efffectivly touching via the solder on the backside of the eq pcb with the inductors. R1e is sharing solder point with the 2 caps that are extremly near. I think its ok, but please check.
-besides the missing film caps 100n and 400uf elcos is should be ready for cabling.

 

[Ilya]

I'm sort of all right, thanks. Can't sell or ship anything, but I'm still able to provide support (that means we still have Internet access here).

PSU: if you're going to use an external regulated PSU, you don't need to populate power sections on both modules. You then feed your 24V DC to the dedicated +24V pads on both modules. Essentially this is parallel connection of the power feed.
R1e connects to the caps. Those pads that are close to the resistor pads are connected with the corresponding leads. You can see it clearly through the solder mask.
The build looks fine so far. Don't forget to screw the output transistor to the PCB - the collector connection relies on the screw contact. Also, I'd suggest turning the square film caps on their side so they take up less space on the board (in the horizontal plane).

What was the reason for using such huge resistors?

 

[ion]

I can imagine. Or maybe i cant.
Didnt get screws with the transistor but i suppose any kind do.
i had to order lots of parts in small purchases bc i missed a lot for some reason.
should ve gone mouser i guess. The really big ones are simply the high wattage your guide wants.
but the really small ones now you mention aít also made me nervous. Those are 0,4 and 0,25 watt versions and tiny. Will those be fine?

i ve a few irregularities like my 2 smaller grayhill switches for the eq arent pin-stopable but i will survive an empty stop. Being 100% sure of the correct insert position is a bit confusing for me. Attached a pic for you to check if position is correct.

 

[Ilya]

Didnt get screws with the transistor

You don't. You have to provide your own. I use short M3 screws (6mm long), comparable imperial screws will also do fine. Make sure the stand-off thread matches the screw.

The really big ones are simply the high wattage your guide wants.
but the really small ones now you mention aít also made me nervous

Ah, ok. You can use common 1/4W resistors everywhere except the positions with the wattage stated explicitly. The smaller ones in your build should do just fine.

Attached a pic for you to check if position is correct

Looks correct. You can try to make stops yourself if you have suitable metal pins. Looks like you can insert pins into stop holes of the switch. With stops in place it's very easy to check for pin 1 orientation - you just turn the switch shaft CCW until it stops and then measure the resistance between the common and the pin inserted in the square PCB pad. You should see 0 Ohms or thereabout.
Although, if your switch doesn't have stops, the first pin position is less relevant. However, if you make a mistake here, the switch knob won't show the correct reading on the front panel (the knob mark depends on the D shaft position of the switch).

 

[ion]

I bought 16v 470uf elcos for the 1290 is that fine or is above 24v needed?

 

[Ilya]

You need at least 24V rated caps. If I understood correctly, the ones you need are rated at 35V in the BOM.

 

[ion]

Yup correct. Thought so. Thanks

 

[ion]

Here is a pic of my builds so far.

1) i forgot the heatsink for the 2n3055 wich i soldered on the pcb. I dont have the correct heatsink.
do you think it is a must given i stay 24V?


2) The UTM output transformer has no pin structure, i ve no clue how to do the zoebel. Need advice. Also @igs

3) the marinair 1452 had some zoebel and link bridges present wich differ from yours, given i use this wiring from Neve transformer info

I left it as it was for now. Any comment? Sorry its hard to see on pics. Theres some cap 7 to 10, bridges from 9 to 8 and 3 to 4, aswell as 10 to 6 (shield).

Any advice where its best to add simple resistive point to sum the 3 inputs?

 

[Ilya]

1. You will need a heatsink for 2N3055 - it gets hot.
2. I have no clue either. If this is a true replica, than it will require the same zobel network as the original (if it requires any at all).
3. I'm not sure what you mean. I don't see any zobel recommendations on that link. I've seen some how-tos regarding zobel networks, but I have to dig into my archives. And you'll need a scope for zobel tuning.

Can you recap what you need? Mic input or line input? What are your exact current transformers (with links to datasheets)? What inputs do you want to sum?

 

[ion]

I want to use the neve t 1452 backwards as to the guide at bottom of this page:
http://www.jlmaudio.com/Neve transformer info.htm
wich lets me use both mic and line through the same transformer. My testings will be with line as its 80% of my use. Oh i also got confused by shield pin 6 and grnd pin 5. so the wire (3 signal) i direct from the transformer to the 1290,pcb should have a connection to pin 5 grnd and not pin 6 shield correct?

the neve marinair datasheet is attached and the link to the utm output transformer is here:
i just dont know if i need the zoebel for it from your build guide or just connect as is.
https://utmindustry.com/utm3548

 

[Ilya]

Ok. Let’s do it step by step. Let’s get the basic line input circuit working, and then do the zobel and mic input.
Pin 5 is part of the winding and should be connected to signal ground. So your assumption is correct.
Pin 6 should also go to clean ground as it looks like there’s an internal electrostatic shield in the transformer. You need to connect it to ground in order for it to work correctly.

Zobel for the output transformer is probably the same as per the original schematic. I assume that this is a true replica of 1166. If not, than it’s often the manufacturer who provides you with the recommended zobel values.

 

[ion]

I am at it right now and kinda lost with wiring the transformers to the pcb since my customizations.

i have pin 3 and pin 5 of input transformer ready to meet the 1290 pcb, but where exactly?

a confusion for me is. Pin 3 on xformer is also wire #3 = signal!?

and how do i connect the output transformer to the pcb?

i just wanted to prepare wires according to your guide but since both formers are wired allready i need to connect them first and then do the other wirings.


the whole „exposed shield on one side of wire“ thing doesnt really make sense to me if the other side goes nowhere but i am following the guide. Cables are prepared.

 

[Ilya]

Let's start with the input transformer.
JLM site suggests several options, but gives a clear preference to a single one - use one input winding for a mic input, one for the output to the preamp, and leave all the rest floating.
Now, in my kit you have a "LoZ" switch that connects primary windings of the original input transformer either in series or in parallel. But since you don't have a proper mic input transformer, this switch makes no sense. I suppose that you're not going to use this option, although it's still possible to do so if you want. For now, I'm assuming that there will be no input impedance selector switch.

According to JLM, you can use any mic input winding. So you can wire either pins 7(+) and 9(-) or 8(+) and 10(-) to the input XLR. The output from the transformer is taken from pins 3(+) and 5(-). You need to connect them to the input pads located on the gain switch PCB. Use shielded cable and connect shield to (-) input pad/tx pin as per the guide. Wire "3" in this case coincides with pins 3 (signal) and 5 (shield) of the transformer, but this is true for your particular transformer only.

The output 3548 transformer seems to have the following connections. Red(+) and orange(-) is the primary. They correspond to pins 1 and 3 of the LO1166. Yellow (+) and green (-) is the secondary and correspond to pins 5 and 8. Wire your output tx according to the guide using this information. You will need to find a way to fit an RC zobel to the output winding of your UTM somehow.

As for your floating shield question, in the balanced connection you don't need to connect the shield to the clean earth because it captures all the dirt and interference. You need to connect it to chassis instead (and the chassis itself has to be connected to circuit ground, but in a single point only). I don't have any "dirty ground" available on the PCBs, so there's no sense to leave the conductor exposed since it may short other circuits.
Another case is avoiding ground loops (multiple connections to the clean ground).
These are two main purposes of this single ended connection.

 

[ion]

Finaly started wiring and looks like i manage. I leave everything tahts connected to the impedance pcb away, like i1,i2 cables, i use the line in on the transformer for testing when ready instead mic in and i dont wire the phantom pwr to xlr of course.

can please take a lookif theres anything missing accoring to my build w/o 48v, custom transformer wiring and plan to use simple 24 Dc from a powerbank?

 

[Ilya]

Looks fine as far as I can tell from the photo

 

[ion]

I will further follow the build guide and try to do zoebel on out tx. Could you try to educate me on powering the module with my mod?

 

[Ilya]

As I wrote earlier, you need to connect your +24V DC supply to the "24V DC" connector in the middle of the board. Observe the polarity - positive lead should go to the square pad.
In case your PSU is too noisy and you see HF spikes in the output spectrum of the preamp, you can make an LC filter, or better yet use a common mode choke instead of a simple inductor, and make a CLC (where L is a common mode choke) PI filter and connect it right after your PSU.
Let's not get ahead of ourselves though and have the preamp working first.