Access-312 Build Thread

[Whoops]

Access 312 Layout

 

[Whoops]

ACCESS 312 Schematic Rev 3.1

 

[Whoops]

If a 2622 type transformer is used:

"Use 1000pf for decoupling before the 2622 primaries.  One on the Hi side and one on the Lo side.

Primaries should be in parallel as previously noted, 1:7 Ratio. Recommended

Zobel is 220pf and 10K R. The load R after the Zobel should be 150K.

This will pretty much get you where you want to be.

Best, Jeff"

 

[EarthTone]

I'm in the process of reading this entire thread and am looking forward to starting this project maybe next week or this weekend! More of a general question, but I wanted to ask; For 22 AWG hookup wire, does it matter much if you use stranded vs. solid core wire? I've always used stranded wire in builds but most of the hookup wire searches turn up solid core options. Do you have a preference?

Can't wait to build this! I have all the parts.

 

[3nity]

Solid wire can snap if bent too hard.
What I do is buy multi conductor 6 or 8 wires...there you go..

 

[EarthTone]

Solid wire can snap if bent too hard.
What I do is buy multi conductor 6 or 8 wires...there you go..

Thank you, and thanks for all your helpful posts I have been reading about this project. Looking forward to building! Will post an update soon maybe next week. I am using the Capi transformers. Eventually after it is working I want to redo it with the Cinemags but just want to get ‘er running first.

 

[3nity]

In my humble opinion you can't go wrong with CAPI transformers. Same ballpark as Cinemags.

 

[Whoops]

I am using the Capi transformers. Eventually after it is working I want to redo it with the Cinemags but just want to get ‘er running first.

An why would you do that?
Cinemag transformers are good but they aren't better than the CAPI transformers.

CAPI transformers are exact clones of the original API transformers

 

[EarthTone]

An why would you do that?
Cinemag transformers are good but they aren't better than the CAPI transformers.

CAPI transformers are exact clones of the original API transformers

It’s not a for sure thing, I’m going to build it with the CAPI’s first and get it running, but a friend who’s built a lot of these recommended some particular Cinemags to me which might help gain staging them without having to use the pads. (He also said they sound better but that is of course subjective.) I mostly record instruments so I don’t need a ton of gain. The 3124+ for example is so super hot. Always have to use the pads. I like recording with some saturation a lot of times so having some attenuation might be nice, he said these transformers knock the gain down a bit from stock. Or maybe I’ll just end up adding output attenuation with the CAPI transformers like my CAPI preamps have. I have read in the thread where folks have done that on the output with a resistor pot. Hope I can get to this soon. Have a lot of “real” work to get through this month unfortunately… Excited to build it though.

 

[Whoops]

It’s not a for sure thing, I’m going to build it with the CAPI’s first and get it running, but a friend who’s built a lot of these recommended some particular Cinemags to me which might help gain staging them without having to use the pads. (He also said they sound better but that is of course subjective.) I mostly record instruments so I don’t need a ton of gain. The 3124+ for example is so super hot. Always have to use the pads. I like recording with some saturation a lot of times so having some attenuation might be nice, he said these transformers knock the gain down a bit from stock. Or maybe I’ll just end up adding output attenuation with the CAPI transformers like my CAPI preamps have. I have read in the thread where folks have done that on the output with a resistor pot. Hope I can get to this soon. Have a lot of “real” work to get through this month unfortunately… Excited to build it though.

It's time to stop listening to your friend.

My 2 cents

 

[EarthTone]

It's time to stop listening to your friend.

My 2 cents

Ha. Noted. He's actually a great and fairly well known engineer/DIY person who I will refrain from naming here. Anyway, glad you guys think the transformers I have already are good, and I'm looking forward to hearing them in this build! Anyway, I'll post back with progress and if I have any questions coming up.

I will say that I have 6 channels of CAPI 500 series Heider pres (version 1 not the new v2 ones) and I love them. I use them on drum recordings every time. Don't even try anything else at this point. So I'd imagine these will be in the similar territory once completed.

 

[EarthTone]

Hey all,

I’m nearing the end of my Access 312 build. I’m really excited about this one.

I’ve read the thread and found some amazing help here. I just have a few remaining questions to make sure I’ve got a good grasp on everything before I do the final steps and turn them on.

Before I go on, I bought this as a “kit” from a member here who didn’t have time to build it. He thought most of the parts were there. They are, although a few are missing, and he bought the wrong output transformers… And some of the parts are questionable. So I want to make sure before I move on:

1. At C3 and C4, I received 0.1uf 50 V ceramic capacitors. This value is the same as the 100nf listed on the BOM, but are ceramic capacitors ok to use in these spots? Pic attached. Most people’s builds here I see have film caps like Wimas. Does it matter? Forgive my ignorance…

2. Same at C8 and a few other spots. Does capacitor type matter as long as the voltage ratings and values are ok?

3. At C6, I don’t know what to do. He included a 470uf 25V caps which are too large diameter to fit, so I have to order something else. BOM says any values between 250-1500uf 4~16V will work, and I found Harpo’s helpful post about what HPF frequency they each provide, but what is the best, most standard choice for C6 in this circuit? Which one should I choose?

4. I am using the Capi gainswitch pcb’s which create a resistance value of about 25k. I found a post from another user who said he used resistor values in R4 and R5 of 25kohms and 250ohms to matchup best with his Capi gainswitch resistance. BOM seems to indicate that is what I should used based on the scaling there (10k rev pot = 10kohm and 100 resistors in those spots; 20k rev pot = 20kohm and 200ohm resistors in those spots; etc.) Is this correct? Should I get 25kohm and 250ohm resistors for R4 and R5? Or should I stick with 20kohm and 200ohm resistors?

5. I’m using 1/4 watt resistors in this build as I usually do for low voltage preamps. Is that ok? Should I have used 1/2 or 1/3rd watt? The guy who sold me the kit bought 1/4 watt for everything.

6. This may or may not be answered in this thread which is fine, but I’m using Collective Cases’ DC Filtering PCB to accept external switching PSU power. The pinout of the PSU is in the picture below. It is; Pin 1. XLR Shell 0v, Pin 2. COM 0v, Pin 3. +16V, Pin 4. -16V, Pin 5. 48V. My question for anyone who’s built with this PCB before is; Do I have the pinout basically correct for the board? Would “COM” go to star ground, or should “Shell FG 0V” go to star ground, and what goes into the very last hole on the right on the DC Filter PCB where I have a question mark?

Also the guy I bought the “kit” from purchased the wrong output transformers… He got the ones meant for vintage API cards instead of the ones meant for these PCB’s. So they have weird mounts on them like this: Classic Audio Products, Inc.

I suppose if I mounted them somewhere else in the case they could potentially still work… Should I try that? Or should I just try and sell these ones and buy the right output transformers… It sucks because these are expensive. I wish I had known before he sold them to me. So far, no response from CAPI asking if I can trade them in on an email…

Thanks!

Best,
Matt

 

[Whoops]

1. At C3 and C4, but are ceramic capacitors ok to use in these spots? Pic attached. Most people’s builds here I see have film caps like Wimas. Does it matter? Forgive my ignorance…

The caps used are not standard ceramic capacitors, by the photos it's easy to see they are ceramic multilayer C0G/NP0 capacitors, those are really good. Keep them

"C0G (NP0) ceramics offer one of the most stable capacitor dielectrics available"

2. Same at C8 and a few other spots. Does capacitor type matter as long as the voltage ratings and values are ok?

All good, same as above

3. At C6, I don’t know what to do. He included a 470uf 25V caps which are too large diameter to fit, so I have to order something else. BOM says any values between 250-1500uf 4~16V will work, , but what is the best, most standard choice for C6 in this circuit? Which one should I choose?

I'm sure if you are a bit creative in bending the legs of the 470u cap that you will be able to fit it there.
The value is fine, I would use that one

4. I am using the Capi gainswitch pcb’s which create a resistance value of about 25k. I found a post from another user who said he used resistor values in R4 and R5 of 25kohms and 250ohms to matchup best with his Capi gainswitch resistance.

use that

5. I’m using 1/4 watt resistors in this build as I usually do for low voltage preamps. Is that ok? Should I have used 1/2 or 1/3rd watt? The guy who sold me the kit bought 1/4 watt for everything.

1/4 watt is enough

6. This may or may not be answered in this thread which is fine, but I’m using Collective Cases’ DC Filtering PCB to accept external switching PSU power. The pinout of the PSU is in the picture below. It is; Pin 1. XLR Shell 0v, Pin 2. COM 0v, Pin 3. +16V, Pin 4. -16V, Pin 5. 48V. My question for anyone who’s built with this PCB before is; Do I have the pinout basically correct for the board? Would “COM” go to star ground, or should “Shell FG 0V” go to star ground, and what goes into the very last hole on the right on the DC Filter PCB where I have a question mark?

COM goes to Ground (0v)
Shell and Chassis goes to the big pad with the "?"

Also the guy I bought the “kit” from purchased the wrong output transformers… He got the ones meant for vintage API cards instead of the ones meant for these PCB’s. So they have weird mounts on them like this: Classic Audio Products, Inc.

I suppose if I mounted them somewhere else in the case they could potentially still work… Should I try that? Or should I just try and sell these ones and buy the right output transformers… It sucks because these are expensive. I wish I had known before he sold them to me. So far, no response from CAPI asking if I can trade them in on an email…

Man this is DIY,
those are not "wrong" transformers, they just have a different mount.

Do DIY, be creative and build a mount for them to use in this project/PCB, use your skills to make them work,
problem solved

 

[EarthTone]

The caps used are not standard ceramic capacitors, by the photos it's easy to see they are ceramic multilayer C0G/NP0 capacitors, those are really good. Keep them

"C0G (NP0) ceramics offer one of the most stable capacitor dielectrics available"




All good, same as above



I'm sure if you are a bit creative in bending the legs of the 470u cap that you will be able to fit it there.
The value is fine, I would use that one



use that



1/4 watt is enough



COM goes to Ground (0v)
Shell and Chassis goes to the big pad with the "?"




Man this is DIY,
those are not "wrong" transformers, they just have a different mount.

Do DIY, be creative and build a mount for them to use in this project/PCB, use your skills to make them work,
problem solved

Man, thank you so much for all the help. I really love this place. Ok I will do all that. Really appreciate your guidance! I’ll be sure to post back when the build is complete. This has been a fun project so far and this thread is invaluable & full of information for anyone interested in making one of these boxes.

 

[Ricardus]

Also the guy I bought the “kit” from purchased the wrong output transformers… He got the ones meant for vintage API cards instead of the ones meant for these PCB’s. So they have weird mounts on them like this: Classic Audio Products, Inc.

I suppose if I mounted them somewhere else in the case they could potentially still work… Should I try that? Or should I just try and sell these ones and buy the right output transformers… It sucks because these are expensive. I wish I had known before he sold them to me. So far, no response from CAPI asking if I can trade them in on an email…

Yeah, like Whoops said, these trafos are correct in terms of function. Just solder wires to the pins and run the leads to the PCBs. Keep the trafos pretty close to the PCBs when you mount them.

I mean, hey, if Jeff will trade them in for others and you want to do that, that's fine, too.

 

[EarthTone]

Yeah, like Whoops said, these trafos are correct in terms of function. Just solder wires to the pins and run the leads to the PCBs. Keep the trafos pretty close to the PCBs when you mount them.

I mean, hey, if Jeff will trade them in for others and you want to do that, that's fine, too.

Thanks! I actually managed to figure out how they'll fit into the PCB's, and using some cardboard shims on the sides of the trafos on the top of the PCB's as spacers, there is now enough clearance on the bottom for everything to be kept clear of the case bottom using 6mm standoffs and still plenty of clearance for the case top to go onto easily without touching the top of the trafos. I'll post a picture when I'm done but it will work! Still waiting on some resistors and the opamps in the mail, but coming together.

 

[EarthTone]

Eeeek guys, so sorry but can anyone help me wire up this output transformer? I am using the CAPI 2623-4-Litz as outputs. The coding doesn't matchup with the way 3nity originally designed the board so I'm not completely positive what goes where. The board has 8 holes and the trafo only has 6 pins. I'm not that good with schematics...

I've attached the pinout for the output trafos below + the relevant datasheets for this build.

Thanks!

 

[Whoops]

The board has 8 holes and the trafo only has 6 pins.

Because the board was made up for the 2503 transformer, which has 3 secondaries, while the 2623-4 only has 2 secondaries.
In the 3 secondaries in the 2503, 2 were used for the audio output and 1 secondary was used to feed a VU meter.
So on the 2623-4 with only 2 secondaries, you use both for the audio output in the same way as the 2503 but you have no feed to connect a VU.
If you dont want a VU, like myself, then it's totally fine as is, but if you need a VU then you have to use an active electronic buffer circuit for that which is NBD.

Compare the Diagrams/layout of the 2503 to the 2623-4 so you understand this

I'm not that good with schematics...

Well, schematics are really easy to read and to follow, specially in a case like the 312 preamp which has a really really simple circuit, so starting to read this schematic it's a great way to learn.
So you should take the opportunity while you're doing this project to start learning and understanding basic schematics.

This is the color correspondence between the 2623-4 and the pads on the Access 312 PCB,
again, check the access 312 schematic and both the 2503 and 2623-4 diagrams to understand this:

2623-4	Access PCB
Orange	Brown
Red	Red
Black	Orange
Brown	Yellow
Green	Green
Blue	Blue
N/C	Violet	Not connected
N/C	Gray	Not connected

 

[EarthTone]

Because the board was made up for the 2503 transformer, which has 3 secondaries, while the 2623-4 only has 2 secondaries.
In the 3 secondaries in the 2503, 2 were used for the audio output and 1 secondary was used to feed a VU meter.
So on the 2623-4 with only 2 secondaries, you use both for the audio output in the same way as the 2503 but you have no feed to connect a VU.
If you dont want a VU, like myself, then it's totally fine as is, but if you need a VU then you have to use an active electronic buffer circuit for that which is NBD.

Compare the Diagrams/layout of the 2503 to the 2623-4 so you understand this



Well, schematics are really easy to read and to follow, specially in a case like the 312 preamp which has a really really simple circuit, so starting to read this schematic it's a great way to learn.
So you should take the opportunity while you're doing this project to start learning and understanding basic schematics.

This is the color correspondence between the 2623-4 and the pads on the Access 312 PCB,
again, check the access 312 schematic and both the 2503 and 2623-4 diagrams to understand this:

2623-4	Access PCB
Orange	Brown
Red	Red
Black	Orange
Brown	Yellow
Green	Green
Blue	Blue
N/C	Violet	Not connected
N/C	Gray	Not connected

Man, amazing. Thanks so much. Yeah, I'm definitely learning a lot as I go, about it all... I have a basic understanding of schematics and can follow them to a degree, but in a case like this, I was hoping to be able to simply matchup the leads of the CAPI trafo with the original but I realized didn't know which trafo was in the original design... And then didn't understand the diagrams well enough to know how to match them up. This is so helpful thank you.

And yeah, I also don't need a VU meter. I generally just use ears to gain stage stuff during tracking, and will use the pad if it starts to clip the converters. So this pinout you've provided is perfect for my build! I have plenty of pres with output trims if I want to saturate things a bit more. At some point I might add output trims to this unit, but just wanted to get it working first.

I will also do as you say and really try to understand this by comparing the diagrams though. Definitely want to learn more and understand how these things are working as I build them. The point of this place, after all!

 

[EarthTone]

UPDATE TO THIS POST: It appears the opamps I chose (Liebers Platinums) may be in part related. These opamps do not have pins for grounding. I thought it was strange when I looked at them. When I switched them out for API opamps (current production) which *do* have the ground pin, the problem basically went away. If I turn up the volume I can still hear a faint ringing, but it's basically gone and usable for recording. Pop the Platinum opamp back in, and it happens much louder like you hear in the recordings below, again. Does this sound like the problem?

Hey guys! Thanks especially to @Whoops for the diagrams, I've just powered up my Access 312 for the first time. It sounds really cool! Except I have a high pitched ringing. It changes frequencies depending on if phantom is engaged or not (phantom engaged makes the ringing frequency go up in pitch). Bummer!

I thought I had a handle on the best grounding scheme but now I'm at a loss. I've been playing with this and trying different things all night and day.

I need some help! I have recorded some examples, just me testing a mic and playing with the gain knob. It is just there to demonstrate the problem so you can hear it. It's the high pitched ringing, any low end rumble is just from me holding the mic while turning the gain switch.

Dynamic mic, no phantom engaged test file:

48V phantom engaged test file:

So here's my setup. I'm essentially doing the grounding like this (pictured below) except I have the PSU filtering PCB in between the power XLR and the Access 312 main boards.

Switching PSU from Collective cases 5 pin with 16v -16V 48V COM and chassis FG all coming in on a 5 pin XLR.

Then, I have the Collective Cases PSU filter board installed before any of the voltages hit the pcb's.

Chassis/FG from the PSU is headed straight to the chassis.

"COM PIN2" from the PSU is headed to the filter PCB input, and the filter PCB output "Ground" is headed to the Access pcb's.

Power 16, -16, and 48V all go through the filter pcb's input and the outputs of the filter PCB goes to the access pcb's.

I have grounded pin 1 of all the xlr inputs to the chassis frame as shown in the picture. Pin 1 of all XLR outputs is left open.

The only ground signal reaching the Access PCB's is the one coming from the filter PCB and it is plugged into the -16V "G" tab.

All voltages read correctly on opamp pins and pins on the boards and the preamps work & sound great except for this ringing.

At first I used unshielded cable coming from the XLR's but I switched to shielded Mogami console cable for the XLR inputs/outputs. It did not help.

I also tried taking the chassis ground and installing a cap and resistor between it and the chassis. Didn't help.

I also tried adding a cable between the PSU XLR Ground 0V and the chassis. Didn't help.

I'm at a loss! Hope to get this project finished soon. Thanks for any tips you can offer!

Do you guys think there is something wrong with the PSU?

All best,
Matt