Access-312 Build Thread

[Whoops]

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.

Always shielded cable with 2 twisted conductors for Audio XLR cables, never unshielded

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

The Cap and Resistor is not be be installed between chassis ground and the chassis,
it's to be installed between 0V and Chassis. So in this case between COM and the bolt on the chassis.
To try that you need to remove the connection between 0V (com) and FG that is probably already made inside the PSU. Maybe the Cap and resistor are already inside the PSU, I don't know but check that.

The idea is to not connect 0V and Chassis directly, but doing that connection through the Cap and Resistor (Filter)

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

I don't know what the problem might be, but I would try with another PSU for sure.
Your grounding scheme seems correct for me, that how I would do it, I would just add the Cap and Resistor between 0V and Chassis if that was not installed inside the PSU, which probably is.

But doing a Linear PSU for that project is easy and not expensive so I would do that for sure.
Maybe before building it I would ask for a friend to try with theirs PSU before I invested in making a new one.

You can also test the preamps one by one, by disconnection all others. Just have 1 connected when you test it, and check if each board alone makes that noise or not. Might be just one PCB circuit that is inserted noise in the whole circuit... I don't know...

 

[EarthTone]

Always shielded cable with 2 twisted conductors for Audio XLR cables, never unshielded



The Cap and Resistor is not be be installed between chassis ground and the chassis,
it's to be installed between 0V and Chassis. So in this case between COM and the bolt on the chassis.
To try that you need to remove the connection between 0V (com) and FG that is probably already made inside the PSU. Maybe the Cap and resistor are already inside the PSU, I don't know but check that.

The idea is to not connect 0V and Chassis directly, but doing that connection through the Cap and Resistor (Filter)



I don't know what the problem might be, but I would try with another PSU for sure.
Your grounding scheme seems correct for me, that how I would do it, I would just add the Cap and Resistor between 0V and Chassis if that was not installed inside the PSU, which probably is.

But doing a Linear PSU for that project is easy and not expensive so I would do that for sure.
Maybe before building it I would ask for a friend to try with theirs PSU before I invested in making a new one.

You can also test the preamps one by one, by disconnection all others. Just have 1 connected when you test it, and check if each board alone makes that noise or not. Might be just one PCB circuit that is inserted noise in the whole circuit... I don't know...

Hey @Whoops thanks so much for the detailed reply. I've been testing it all day and night. Tend to get obsessed with problems like this one...

I discovered something really interesting. I did not bolt the cards down on their standoffs yet, since I am only at test phase. On the card I've been testing, it appears that there was a bit of pressure on it and I lifted it up a little and the noise went away almost completely. Now I only hear it at the highest gain settings. I wonder if I damaged the grounding of the PCB somehow? Or maybe it just doesn't like to have any bending pressure on it whatsoever, even the very slightest. It's difficult drilling holes perfectly in place, sometimes you have to apply a bit of light pressure to get everything seated correctly. Hope I didn't damage anything. I never heard a pop or put very much pressure when installing the cards...

I will test the other boards for sure. I heard that sound on the 2nd one I tested as well so I figured it was the same on all. Will test.

I'm convinced somethings up with this power supply. I haven't heard a sound like this before. I think I would rather do a linear internal PSU for it. Or external. I was banking on this setup as it was recommended to me but now I will check back through the thread for how others did it with linear PSU's. I built some for my EZ1073's which turned out good and it was not hard.

 

[Whoops]

It's difficult drilling holes perfectly in place, sometimes you have to apply a bit of light pressure to get everything seated correctly.

I know that it’s difficult to drill the holes on the case and make them fit perfectly with th PCB, never managed to get that right.

So, enlarge the holes in the PCB enough that when it’s screwed the PCB doesn’t bend or has unnecessary pressure. Bending the PCB puts pressure on the PCB traces and might break them.

Then just use washers on the bolts that are wider than the holes

I'm convinced somethings up with this power supply. I haven't heard a sound like this before. I think I would rather do a linear internal PSU for it. Or external.

Internal or external it’s fine but I would just do a Linear PSU.
For audio projects I personally only do and trust Linear PSUs.

It’s very easy to do a very clean Linear PSU while it’s very hard to do a clean switching PSU (although possible)

Also when it fails the Linear PSUs are very easy to fix while the switching PSUs are very hard to fix and troubleshoot, sometime even not economically feasible to try ti fix it,

I personally leave switching PSUs for charging my phone , internet router, TV…

Although you switching PSU might not be the problem, we don’t know.
But I would try a Linear PSU for sure in your case

 

[EarthTone]

I know that it’s difficult to drill the holes on the case and make them fit perfectly with th PCB, never managed to get that right.

So, enlarge the holes in the PCB enough that when it’s screwed the PCB doesn’t bend or has unnecessary pressure. Bending the PCB puts pressure on the PCB traces and might break them.

Then just use washers on the bolts that are wider than the holes




Internal or external it’s fine but I would just do a Linear PSU.
For audio projects I personally only do and trust Linear PSUs.

It’s very easy to do a very clean Linear PSU while it’s very hard to do a clean switching PSU (although possible)

Also when it fails the Linear PSUs are very easy to fix while the switching PSUs are very hard to fix and troubleshoot, sometime even not economically feasible to try ti fix it,

I personally leave switching PSUs for charging my phone , internet router, TV…

Although you switching PSU might not be the problem, we don’t know.
But I would try a Linear PSU for sure in your case

I'd just like to follow up and say that I tried powering the Access 312 with my CAPI floorbox external linear PSU which I realized has the same 5-pin XLR pinout as the way I wired the Access XLR input jack, and it completely fixed the noise issues. The Access is as quiet as a mouse with linear external power. So, unfortunately CAPI no longer seems to make the PSU kit, but I found another online from TAC and I'll make one of theirs! I want to do a dual output PSU so I can power 8x 312 preamps. I just love the way these sound.

I can say that this has been one of the funnest DIY projects I've done. It's easy and it's a very rewarding outcome. I have a bunch of pres, AML 1073's, CAPI Heiders, API 3124+, and I think these may be my favorites. Really nice & open on the top end, still fat sounding. Really great.

I used the Scott Liebers Platinum op amps and I love them. Crispy up high!

Will look forward to getting some good clean power made for them and then I'm going to do another unit with 4x more Access 312's.

Will post some photos when I have the unit tidied up. I plan to use a blue LED on the front to indicate power or maybe a vintage style lamp.

 

[Whoops]

Building a Linear PSU is extremely easy, there’s many PCBs around for that. Except the case I’m able to build one in 1 hour

 

[Ricardus]

I've just been using Jeff's SMPS with his filtered rack option.

 

[3nity]

I would suggest buy a PSU PCB from Volkers Silent Arts.

 

[EarthTone]

I would suggest buy a PSU PCB from Volkers Silent Arts.

Thanks! And thank you for such a great project. I really enjoyed it, mine all work and sound awesome. I'm going to build a TAC linear external PSU kit with dual outputs so I can power 2 of these. I plan to build another one this fall so I can have 8 channels of this amazing sound in my studio. Cheers, 3nity!

 

[sahib]

I have clicked on the link for the schematic in the post #1 but I got a warning that my pc might be infected by a virus. Has anybody else had such warning?

Many thanks.

 

[Whoops]

I have clicked on the link for the schematic in the post #1 but I got a warning that my pc might be infected by a virus. Has anybody else had such warning?

Many thanks.

Yes, the same here, but those links are long gone, twin-x website doesn't exist anylonger and the links to twin-x just go to a website trying to make you install an anti-virus. Don't worry your PC is not infected, I received the same message and I'm on a MAC, and in 12 years using a MAC I never had any virus neither I know any MAC user that ever had a Virus infection.

Anyway,
the schematic plus the other docs for this project are posted in other pages of this this, more recent ones. If you do a search inside this thread for "Schematic" you will have no difficulty in finding it

 

[sahib]

Yes, the same here, but those links are long gone, twin-x website doesn't exist anylonger and the links to twin-x just go to a website trying to make you install an anti-virus. Don't worry your PC is not infected, I received the same message and I'm on a MAC, and in 12 years using a MAC I never had any virus neither I know any MAC user that ever had a Virus infection....

Thank you for that.

 

[EarthTone]

Thank you for that.

It’s just a few pages back, along with the layout and the BOM! Kindly posted by @Whoops I think it’s between pages 31 and 32, could be 30 as well. Easy to find!

 

[sahib]

It’s just a few pages back, along with the layout and the BOM! Kindly posted by @Whoops I think it’s between pages 31 and 32, could be 30 as well. Easy to find!

Thank you. I have got it. I know it is a single op-amp topology but wanted to see it again.

 

[EarthTone]

Just in case anyone is interested, I just posted a switching PSU and pre-built DC Filter board for sale that would work for this project here: For Sale - Switching PSU 16/-16V & 48V + DC filter board for sale

I built this project and then decided to build a dual output linear PSU so I can power two of them with one PSU instead.

 

[EarthTone]

Here’s my build! Turned out awesome… Linear PSU from TAC in the UK. It will power 2 racks of this project.

Question: How do you hookup the 48V LED’s? I know this is a dumb question! The switch gives 48v which will blow up an LED. Should you put a resistor between the LED and the switch tabs? If so, which value?

Or, do you hookup the 48V phantom LED’s another way?

Sorry I know this is basic stuff. I didn’t see it covered in the thread…

 

[Whoops]

Here’s my build! Turned out awesome… Linear PSU from TAC in the UK. It will power 2 racks of this project.

Congratulations for your build looks great

Question: How do you hookup the 48V LED’s? I know this is a dumb question! The switch gives 48v which will blow up an LED. Should you put a resistor between the LED and the switch tabs? If so, which value?

Or, do you hookup the 48V phantom LED’s another way?

You always need a limiting resistor for a LED.

The circuit for the 48V and the LED and needed components are in the Access 312 schematic

 

[EarthTone]

Congratulations for your build looks great



You always need a limiting resistor for a LED.

The circuit for the 48V and the LED and needed components are in the Access 312 schematic

@Whoops Right you are! Thanks, my friend. Installed and working now.

Also guys- What are your favorite sources for this preamp? I must admit it sounds awesome but it sounds different than what I was expecting! There’s less top end and a little less body/mids than my API 3124+ with Red Dots or CAPI FD312’s with Red Dots, and much less mids than my AML 1073’s. I used Platinum Dot op-amps and the CAPI EA in/out transformers in this build.

It’s interesting! The word “vintage” definitely comes to mind when I hear it, and so far the thing that’s really stood out as a great match for it was on kick drum in a jazz-rock session. Incredible drummer NYC JK Kim who’s touring with a lot of great players these days was here this weekend for a session. The kick drum with an RE20 outside the kick had this stressed, punchy, killer tonality and vibe to it. Just sounded awesome on the kick drum. I’ll admit I didn’t love it on synth, but could’ve been the patch was better suited to my other pres.

In my opinion the sound quality is a bit mid-scooped and sort of dark on the top end, which is not what I was expecting. I thought it would be a little more open up top. But it’s a great sound, and I can’t wait to try it on more sources! If the kick sound we got was any sign, I think it’ll get some heavy usage on drums.

Now to save up to build another to go with the dual output PSU I made for it! 8 channels of this sound will be awesome to have in the studio.

 

[Whoops]

I haven’t used those Opamps, I built mine using the clones of the vintage 2520 from CAPI. Never felt mine were dark
I really like the sound, I like to use them in percussive sources.

 

[Whoops]

I've just checked my order from 2009, the Opamps I used were Gar2520 and those are not available anylonger in the CAPI store.
The EA2622 is anlso not available from CAPI, so only the EA2503 still exists as a product

This is what I used
gar2520 DOA Full Kit Bundle (gar2520 Bundle) = $35.00
EA 2503 Output Transformer (EA 2503)
EA 2622 Input Transformer

 

[jsteiger]

I've just checked my order from 2009, the Opamps I used were Gar2520 and those are not available anylonger in the CAPI store.
The EA2622 is anlso not available from CAPI, so only the EA2503 still exists as a product

This is what I used
gar2520 DOA Full Kit Bundle (gar2520 Bundle) = $35.00
EA 2503 Output Transformer (EA 2503)
EA 2622 Input Transformer

Sorry to barge in here but we certainly still have the gar2520 kits. We have 2 pages of items in our discrete kit category. They are on the 2nd page.
https://capi-gear.com/catalog/index.php?cPath=71_72&sort=6a&page=2
We also still carry the EA2622 but happen to be currently sold out and waiting on our next batch. They are due before Thanksgiving.
https://capi-gear.com/catalog/index.php?cPath=21_38