[thesystem]

I'm planning on building 2 4-channel Access-312 preamps and decided I'd start a thread for it. This because there are many questions I need answers to and because it's nice to show the building proces along the way. I hope you guys are willing to help me with this journey since I'm still a layman in DIY.

Firstly, here are the docs from 3nity:
layout: http://twin-x.com/groupdiy/albums/userpics/312-Layout_.pdf
schematic: http://twin-x.com/groupdiy/albums/userpics/312-schematic.pdf
BOM: http://twin-x.com/groupdiy/albums/userpics/Access-bom.pdf

Next, here's my list of transformer and DOA choices:

input transformer: EA2622 sold by calssicapi (http://classicapi.com/catalog/product_info.php?cPath=21_38&products_id=80)
output transformer: EA2503 sold by classicapi (http://classicapi.com/catalog/product_info.php?cPath=21_37&products_id=79)
Op-amp: gar2520 and/or ML2520 sold by classicapi (http://classicapi.com/catalog/product_info.php?cPath=71_72&products_id=136) and whistlerock (http://www.whistlerockaudio.com/cart/index.php?main_page=product_info&cPath=35_38&products_id=94)

So far so good, but now come the questions.. Please forgive me if they sound ignorant, I guess it's better to ask than no to know right 

- For the 10K rev log I found these pots: http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=28. I also read someone mentioning these: http://nl.mouser.com:80/ProductDetail/Alpha-Taiwan/RV16AF-20-15K-C10K/?qs=sGAEpiMZZMtxdMMi52izylTGUjbYbj7qLnRWOCslzhQ%3d.
These's quite a price difference between these two. Is there also a quality difference? If so, which one would be of better quality?

- PSU: 3nity recommended the PSuniversal from diypartssupply (http://diypartssupply.com/index.php?main_page=product_info&cPath=2&products_id=34). How do you set this up so I can use it with 4 access boards? My mains are 230V. I understand you have to adjust the voltage, but I don't know how and to which value (16V?)

- This brings me to the power transformer: any recommendations for 230V mains?

- Since I'm using the EA2622 input tx I understand that some values need to be changed in the BOM. Which ones (R6-7, RC and C8?) and to which value?

- pushbuttons: are these http://classicapi.com/catalog/product_info.php?cPath=51_52&products_id=88 good choices for the 48V/pad/polarity switches? Will they fit?

- I want to use LEDs for 48V and maybe power on/off. How do you solder these to the pushbuttons and is there a specific kind of LED you need to use?

- What are sockets used for mentioned in the BOM? How many would I need for 8 channels?

- On the Access PCB there's something called 'Gain 1-2' and 'JP2'. What are these for/what goed there?

- How do you mount the 2502 output tx on the PCB? Do I need screws?


If someone can help me out with these questions, that would be really helpful for me!


I'm planning on updating this thread with photo's and build info as I go along. But before I can do that I'll need to know the answers to the above questions   

[3nity]

Hey.
This build is a pretty straightforward build if you follow the BOM.

Heres the updated 2013 BOM:http://twin-x.com/groupdiy/albums/userpics/Access-312bom-2013~0.pdf

Actually this apply to all revisions but its updated with corrections from previous errors.

THIS BOM IS UPDATED

Thesystem i suggest you take a look at the Meta, for some info before starting. 
Something that might help you a lot its looking at pics from previous 312 builds.
But heres some light to your questions:

1- 10K -50K  reverse pot 2.50$ at Smallbear good quality.
2- psu is adjustable or go with fixed voltages: read the meta and do some research.
3- There are too many power transformers so you better look something at RS-Farnell.
4- EA-2622: you need a 150K on RC, 10K at R7-R6 and 220pf at C8.
5- F2UEE are the pushbuttons to go on the access-312...
6- LED, any LED will do the job if it fits you front panel...you solder it to the lugz on the top of the switch.
7- Sockets are there so the op-amps sits on it ..it makes things easier.
8- Gain 1-2 are there so you solder the wires for the pot..you can use molex headers.
9- yes. you need screws to mount the output transformer

[3nity]

Updated BOM for everybody...this should make things easier.
http://twin-x.com/groupdiy/albums/userpics/BOMv2.pdf

[thesystem]

Thanks for the new BOM 3nity, that indeed makes things easier.

there are still some things not clear though:

On the pot: you just gave me another option for the pot. That doesn't really help me. I'm trying to learn from the whole process so just another option on an item doesn't tell me anything. I'd like to know what makes a good pot, why would the one from smallbear be better than another one? Or a different one be better than the other? What to look for? These are questions that would make me understand this better if someone can answer them.

 What is the difference between a 'push-push' switch (F2UEE) and 'alternate push-push' (F2UEE TB)?

I don't understand the ratio/jumper thing. Where is it for? I've also seen Access builds where these are left untouched.

Thanks

[hitchhiker]

I don't understand the ratio/jumper thing. Where is it for? I've also seen Access builds where these are left untouched.

This option allows you to select a higher input impedance to match a mic that has a higher output impedance. I use this with my ribbon mic that is 600 ohm output,

The lower output impedance of 150 ohm is more common and thus 150 ohm input works best with most mics. Some people may have opted to not bother with the switch because all their mics are 150 or 200 ohm output.

[Harpo]

What is the difference between a 'push-push' switch (F2UEE) and 'alternate push-push' (F2UEE TB)?

Except the missing word 'alternate' there is no difference in the switching function. For usual you don't need the TB (terminals sealed top and bottom) option.
For all avail. options see F series pushbutton switches datasheet

[thesystem]

Harpo, thanks.

This option allows you to select a higher input impedance to match a mic that has a higher output impedance. I use this with my ribbon mic that is 600 ohm output,

The lower output impedance of 150 ohm is more common and thus 150 ohm input works best with most mics. Some people may have opted to not bother with the switch because all their mics are 150 or 200 ohm output.

Hitchhiker: So say I'd like to be able to switch between input impedances. How do you get it done? I guess it would be practical to have a toggle switch on the front panel. But from the jp1-4 pad on the PCB to the toggle switch, what do you do in between?
Also, I understand that you need about 4-5 times the output impedance of a microphone for a ribbon to work best. What will be the two impedances in this build if I want to use it or can you decide for yourself?


Lastly, anyone care to explain what to look for in a good quality pot?

[3nity]

Thesystem...if you let the ratio unstuffed you will have no sound...if you dont want to bother just solder a wire leg from pin 2 to pin 3 and thats it..but yes its helpfull to have a ratio switch on the front..you never know.

For the pot..Small bear its all i ever used on my access-312 builds and they are good.

Keep the good work

[hitchhiker]

wire a DPDT (double pole double throw) toggle like below. The numbers in the pic connect to the numbered holes on the pcb.

[thesystem]

wire a DPDT (double pole double throw) toggle like below. The numbers in the pic connect to the numbered holes on the pcb.

great!! thanks hitchhiker I guess A and B are the front of the switch and A is interconnected with 3 with a bare wire right?

[Jordan]

Hi,

Where can we buy the PCB and cinemags transfo? I don't find the F2UEE ... Sorry but i'm not an expert...

Jordan (from france)

[Harpo]

I don't find the F2UEE ... Sorry but i'm not an expert...

Jordan (from france)

http://lmgtfy.com/?q=F2UEE

[hitchhiker]

I guess A and B are the front of the switch and A is interconnected with 3 with a bare wire right?

No. All six posts are on the back , I just marked the A and B to isolate the numbered  posts that connect to the numbers on the pcb. The back of the switch looks as I've drawn. Wire a jumper from
3 to A  ,  this way when you flip the switch it connects pin 2 to 3 while also breaking  the 1-2 and 3-4 connections.

[thesystem]

I guess A and B are the front of the switch and A is interconnected with 3 with a bare wire right?

No. All six posts are on the back , I just marked the A and B to isolate the numbered  posts that connect to the numbers on the pcb. The back of the switch looks as I've drawn. Wire a jumper from
3 to A  ,  this way when you flip the switch it connects pin 2 to 3 while also breaking  the 1-2 and 3-4 connections.

yeah sorry said it wrong, I meant to ask whether or not 1-3 is closest to the case or A and B (or maybe it doesn't matter). Thanks for the explanation

[toneboner]

i'm glad you started this thread, system.  many of the answers you'll get here are likely posted elsewhere, but it's nice once in a while to collect everything in one place. i'm sure others will benefit from following your progress.  i'm also starting this project (without much diy experience...)

Here's a pic where you can see clearly the output xformer mounted to the board.  xformers will sometimes come with a "channel frame" for mounting onto the chassis box. But the access compatible xformers will have 2 holes for mounting onto the board as shown.  pictured is a cinemag which i'll be using as per 3nity's suggestion.  The ea's are similar

[toneboner]

Here's the mouser cart i'm using. I know ur not in the states. but, you know, for the benefit of the next guy...

http://www.mouser.com:80/ProjectManager/ProjectDetail.aspx?AccessID=80AA125D0A

[MicDaddy]

hey toneboner (great name  )

How many cards will your mouser-cart populate?

[sr1200]

So i started wiring up my 4ch unit tonight. Got the ps calibrated nicely. I hook up a channel and couldnt resist hooking a mic up. Pop in a condenser mic. Flip the phantom switch... The little light comes on, and all i get is a really low level hum. Grounds are good. I tried moving the jp2 around to different setting (btw. What is the correct setting there?) i tried jumping the "RC" pad on the board which just makes the unit dead silent. All voltages are correct at the xlr jack for phantom pwr.  Its late now so maybe tomorrow ill get to hooking up one of the other channels... Maybe i have a bad DOA. Any thoughts?

Ps im using all cinemag tx's

[sr1200]

Making a little bit of headway. Upon rechecking my voltages all of a sudden i have 87v on my 48v rail and it cannot be lowered via the trim pot. (gonna have to go over the ps board  tonight) My +/- 16v are good as is my 12v rail. Something cooked the phantom rail. 

I plugged in a dynamic mic and although i do get some signal through, its not much and the gain pot pretty much does nothing until the last 1/16th of the turn.  The jp2 i have between 2-3.

Any thoughts on places to start looking?  Its not a big board, all solder contacts are good.

[3nity]

Can you post a high resolution picture showing the potentiometer?
For JP2 all settings are good.
If 48V shows more than that lm317 is fried.
Thanks

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