[BUILD]CAPI 2-ACA-Bo~Official Support Thread

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jsteiger

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As the title states, this is the official support thread for the 2-ACA-Bo, PCB build that can be found at here http://capi-gear.com/catalog/index.php?cPath=22_46. Per a request in the BM, I felt I should start this thread.

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September 21st, 2015 Update:
All support docs for this project can be found on the recently added
Support Docs page at www.capi-gear.com
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Anyhow, here is a brief history of the creation of this board. Like many projects, I created this for my own personal needs in the rebuild of my console. I am using this board as the 2-mix summing/booster stage of the custom discrete monitor bucket that I'm building. I wanted to match the exact process that API used to complete this task on the main or "input" side of my desk. My board, unlike some others, used the 4 channel ACA's for summing and 325 cards for the post fader booster amps. So, that is what the 2-ACA-Bo is comprised of. This is the exact same method used for summing all the group buses as well.

Per a behind the scenes discussion with Dennis (Prodigy member "API"), here are a few back-side pictures of my desk before my furniture/frame rebuild.

All the 4 channel ACA's are in the middle section. 300 series card cage is on the right.


And another...cage closed


The 4 boards to the right are ACA's for summing the 16 sub-groups. The 5th board is 2 channels for Input Cue 1 & 2 summing, and 2 channels for stereo 2-mix, Left and Right of course. These output transformers are the AP2623-4, PCB mount version of the 2623.


From here, the each summed signal goes to normalled points at the patchbay where they create an insert point that is -2db. The return signal is directly wired to the respective fader (group or 2-mix), which were originally 600ohm in my board. That's right, the returns are all unbalanced, and the insert device (if any) is directly driving the fader. The fader output is then wired back to the 300 series card cage.


In the card cage, you see an ass-load of beautiful and underrated 325 cards. These cards are the post fader, booster amps. These are not exactly the 325 circuit that you've all seen the schemo for. They have an obsolete TIS73 that was used to switch between the program source and a slate/tone signal. There is a 10uf input coupling cap and they use the inverting input of the 2520.

In the cage you also see 4-325's that were quad boosters for the control room, 4-325's that were the quad boosters for the studio amps and a single mono program booster.

The output of each sub-group 325 booster goes to the patchbay were it normals to the tape machine inputs. A mult is on this output that feeds a meter buffer as well. The input/group meter buffers are on the monitor cards, where you can switch the metering from input channel or monitor channel. The "direct" switch on each input channel determines if you are metering sub-group or input channel.  ??? Might sound a little confusing but it's really pretty simple.

Besides the 2-mix output, 2 mults are on the 2-mix booster card outputs. The first feeds the L-R meter buffer amps and the 2nd is the feed to the control room module. Of course there, you select 2 track return, CD player or "program" for the source you are listening to in the control room.

This 2-ACA-Bo board is an exact replication of 2 stereo ACA amps and 2 of the 325 amps, with three exceptions. The original ACA input coupling caps were 250uf, mine are 220uf. The output coupling caps are 470uf instead of the 250uf on the ACA's and the 400uf originally found on the 325 cards.

I am happy to help with anything I can on these.

Cheers, Jeff
 

substitute

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It seems like these could be really cool to use for the make up gain after a passive eq.  Would there be any point in using the ACA's and the 325's? 
 

jsteiger

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substitute said:
It seems like these could be really cool to use for the make up gain after a passive eq.  Would there be any point in using the ACA's and the 325's? 
I would assume the 325 side would be all you would need, although a vintage 325 card may be a more versatile choice. The way the 325 cards were designed, you could set them up for many different applications. However, if you look at the going eBay rates for 325's, they would be a lot more costly solution. My cards are only set up the inverting input an all 4 amplifiers, which should not be an issue.

Cheers, Jeff
 

bradb

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Sorry, im just not seeing the application for these for people not looking to replace this in their console.

Would it be possible to make a line mixer with level and pan using this as the output stage?

is that what your block diagram is showing on your site.

 

jsteiger

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bradb said:
Sorry, im just not seeing the application for these for people not looking to replace this in their console.

Would it be possible to make a line mixer with level and pan using this as the output stage?

is that what your block diagram is showing on your site.
This board is designed to be used a stereo summing solution for any unbalanced bus console, line mixer, summing mixer, mic mixer or whatever mixer. There are many home-brew mixers out there that people have built. If you wanna see what a true vintage API style 2-bus solution will sound like, then this may be a board to experiment with. My schemo as drawn, is based on 47K bus resistors, which is what API used in my console and many others they built. If someone wants to modify the feedback R and C on the ACA's, it can be tweaked for whatever gain requirements one may have, within reason.

For the few and brave who are in the process of building from scratch, there very own "console", this is a great solution for all sub-groups, echo sends, cue sends an like I mentioned already, the stereo 2-mix. Or, for someone who ones a vintage 1604 loaded with 528 modules, these boards can be used for the additional buses that aren't currently wired. The push-button switches are on the modules, the traces are on the motherboard but there is currently no summing for them. A very simple addition for that, for sure.

Cheers, Jeff
 

Tubemooley

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bradb said:
Would it be possible to make a line mixer with level and pan using this as the output stage?

is that what your block diagram is showing on your site.

1. Yes. This is the main guts. This is the summing amp stage. This is the heart of the line mixer.
2. Yes.

More later. DW.
 

bradb

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i just got a hold of one of those Speck X.Sum line mixers.  better than the standard summing box because it has level on each channel, therefore i can run PTools output signal thru outboard gear and BLAST them, and still turn level down at the line mixer.  I don't have to mix levels using output knobs of outboard compressors, etc. as i would with standard summing boxes.

I think this is the future of mixing.... a hybrid approach, in the box plug ins, and some outboard gear.  it SHOULD beat a console.  we'll see.


anyway, if youre leading up to some kind of api-based, level and pan line mixer, that would be very cool, i'd like to hear more.
 

jsteiger

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bradb said:
...anyway, if youre leading up to some kind of api-based, level and pan line mixer, that would be very cool, i'd like to hear more.
Well, if I said that was not rattling around in my head, I would be lying. I have successfully built all discrete line input modules that have 4 Echos, 2 Cues, HP filters, solo, phase flip, pan and of course level. These are a clone or replication of my input channels and sound so much like them it is crazy. They have the balls to easily drive up to 16 group buses along with stereo, just like they do in my board. The format of my custom channels is for the monitor bucket which is an odd 3/4" wide module. I have the desire to rework the layout for something more common, like possibly the upcoming GroupDIY 51X racks. Faders and buses could be added as desired. The circuit is very simple and very easy to build. Oh, and it sounds friggin' great.  8)

I have to speak with Volker about this because I will need to have a completely unique PCB back-plane which is no biggie. I suppose rack mount is the way and may be possible use the metalwork that is now being developed for the 51X racks.

The daisy chain method/theory that is used for the 8200, 7800 stuff would work very nicely.

So, a DIY, all modular, vintage API style mixer. This is so do-able, and ultimately my goal.

Cheers, Jeff
 

bradb

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well then, i'll be referring to my Speck x.sum as my "intermediate mixing solution"


keep us posted on what you come up with.  I'm definitely interested.

Like I said, I think a hybrid mixing solution is the future, as level and pan control OTB is better than ITB due to digital limitations (dropping resolution when lowering volume)
 

signalflow

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Jeff,

Where would one find the schematics for API's Buss routing, pan and fader functions?

-Casey
 

jsteiger

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signalflow said:
Jeff,

Where would one find the schematics for API's Buss routing, pan and fader functions?

-Casey
Casey, These I don't have in an electronic format. I tried to get them scanned at Kinko's but they didn't know how to use their large format scanner.  ??? I haven't seen them anywhere else on the net...yet.

Cheers, Jeff
 

martthie_08

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Jeff, I have your beautiful ACA-Booster pcb in the mail and plan to use it as main mix 3-4 output for my Studer console. Now I'm really unfamiliar with summing concepts, so my question is, will one of your API-a-like units be able to sum 24 channels? What is the advantage of having a hierarchical approach with buckets of 8 channels going to an ACA?

I think I will have to adjust the feedback resistor of the 2520 summing opamp to match the Studer channel modules, it seems this will be 3k3 as taken from the existing summing amp info: studer summing amp

I'm hoping to add an alternative (discrete) ouput section to the existing main 1-2 mix output, which is currently just a summing amp going straight to an output amp

Thanks and 2 thumbs up for all your research work! Marten




 

jsteiger

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martthie_08 said:
Jeff, I have your beautiful ACA-Booster pcb in the mail and plan to use it as main mix 3-4 output for my Studer console. Now I'm really unfamiliar with summing concepts, so my question is, will one of your API-a-like units be able to sum 24 channels? What is the advantage of having a hierarchical approach with buckets of 8 channels going to an ACA?
Marten,
The ACA's on my card will easily sum 40 or more channels. They were summing 36 in my desk with no problems, nice and quite. The 8 channel inverting cards are not necessary at all. I have added these so that when I complete my mods, I can dump my new, all discrete monitor section to the main 2-mix. Without the pre-summing, I would have 72+ channels being summing thru 1 ACA pair. The noise would be unacceptable for sure.

If you modify the feedback R, remember to also adjust the feedback C to limit HF and prevent weird oscillations.

Cheers, Jeff
 

RyanC

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This looks like exactly what I'm looking for-

I'm looking at building a 34x8x2 routing mixer that would be just L and R buss assignments.  So either 4 of these and passive 2 bus or just 5 of these seem to fit the bill.  The summing thing is new to me also.  I'm trying to understand, will there be any issues if I were to route one channel to multiple busses?

Also the 34 ch would be fed by an (unbal) mackie ultramix VCA auto system.  Is there any issues with that?

Thanks
 

jsteiger

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RyanC said:
...will there be any issues if I were to route one channel to multiple busses?
In speaking of my desk, I have 16 buses plus the stereo bus that are assignable for each channel. All bus resistors are 47K. No issues in assigning a channel to all 16 buses.
Also the 34 ch would be fed by an (unbal) mackie ultramix VCA auto system.  Is there any issues with that?
The channel output that feeds all the buses is unbalanced. So, the busing in my console is all unbalanced as well.

Jeff
 

Engfelt2000

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Hi Jeff. Any progress on your line input modules? I'm really eager :)  And judging by the response on the new Igor cult thread, quite a few others wants to build a mixer as well.
 

martthie_08

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ok, a little progress here, some notes: I started by building only the output amp and hooked up to my existing summing amp. It would have been nice to have all the connectors on Molex type pin spacings and there are no explicit solder pads for R15 and R16 (but no biggie, you can place them in the positions for Q1 and Q2).

Everything worked on first power up and I am impressed. In direct A/B comparison to the Studer output amp the API is really focused and defined. The Studer amp sounds more open, quite flattering and really exciting in the highs. I am sensing this will give me some really useful options when mixing upcoming projects!

There is a 0.6dB level drop on the API, but it might be because I am running it on +/- 15V and come to think of it, it may also be a +0.6 DB boost on the Studer, cause the Studer is adjustable up to 30 something dB... now the Studer amp is going to the Black Market to make some spare change for the API summing section, please have a look if you are interested!

more to come...
cheers, Marten
 

jsteiger

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martthie_08 said:
...It would have been nice to have all the connectors on Molex type pin spacings
Hi Marten,
Sounds like good news so far!  ;D  I'm not sure what you mean about the Molex connectors though? All the connectors are .156 Molex. Do you mean use all 3 pin instead of the 2 pin?

...there are no explicit solder pads for R15 and R16 (but no biggie, you can place them in the positions for Q1 and Q2).
Correct-amundo on this one. The original design was for me and my console use. I am using the N channel switches to work with my existing boosters. I thought it would be nice to make that part of the circuit optional. I did try to show this on the skiz but never really specified that you need the stand up the 24.9 ohm resistor and use the gate and source holes of Q1 and Q2.

Everything worked on first power up and I am impressed...
Always good news. Keep us posted when you get the ACA stages all populated an wired in.  ;)

Cheers, Jeff
 

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