Console signal flow, input cards, general discussion on API styled DIY mixer

[boji]

I wanted to start a discussion on signal flow for my DIY console that I am hazily moving toward. It's going to be in the API vein.
I am almost done with the metalwork and would like some community help with design choices as I transition over into the world of signal flow and troubleshooting.

I will post more pictures when there is more to show for my work, but in the meantime, help with the details of my input card would be appriciated!
To start, a big thank you Jeff and Plumsolly for posting a schema of the 544 input card. I am in the process of transcribing it to .sch and once you untangle the mechanical-centric way API drafted it, you do get the sense of how simple a machine it is.
Since the original thread was started by plum I'll reference it here: http://www.groupdiy.com/index.php?topic=33504.0.
I thought I would start a new thread on it since I am starting from scratch.

 

[Arno]

 

[boji]

API schematic:
http://www.mediafire.com/file/dnzmj2umvnz/API-536-input-schematic-.jpg

Transcription (so far):
http://www.mediafire.com/file/o4yd3jvzmmn/API 536AE rev3.3.sch
revised 05/24/10
Revised 2/8/21 (c/o Walrus)
https://www.mediafire.com/file/elsrxfgwux0z7nw/API_536AE_rev3.4.2.sch/file

 

[Tubemooley]

Which schematic editor will open the .sch posted for download at mediafire? DW.

 

[tv]

I don't think that you have the right value (10k in your schematic) for the resistor that connects between the 48V source and 1000uF capacitor ("Page1").

 

[boji]

Hey Dana, Express PCB.  http://www.expresspcb.com/ExpressPCBHtm/Download.htm

Thanks TV. Let me take a look.

 

[boji]

Ack, I don't see a 1000uf on the schema TV! R23 shows 10k however.

 

[sodderboy]

Hazily?  Like, WOW!
Saul and the RAF are smiling!
Mike

 

[wolfgang]

Did you do the entire console frame yourself?
If so, its amazing work!!!!
And for sure it was a lot of work!
What are you planning for the backplane? pcb or wiring everything up?

I wish you lots of motivation for this project!!!!!!

regards,
Wolfgang

 

[fazer]

Great Metal Work. My complements.    I have a 1977 2488.  It does not have a line input trim. Only Mic trim with a 1k pot.
It pretty match matches the schematic but has 8 sub or bus outs per module.  The input modules  are labeled 528C. 

The mic preamp with gain trim, (in your schematic Amp1) is also the line input with no trim, when switched in line.  I think this is because they had the early Allison VCA automation.  I don't no for sure but I believe the idea was to not offset the gain of the VCA automation,( I dont have a VCA in mine).  Anyway if you look at the Switch that changes from mic to line ,  it creates a fixed gain and Active balance amp for A1 when in line position. 

When I had my 16 track,  I would feed the prefade preamp,(A1) to the tape input and bring the tape output into the Eq input so I could do an inline type of monitor.  My API is 24 input with 528 input modules and 550A Equalizers.  There was no Monitor section because a church bough it originally for a front of house console, not a recording console.   

I always felt that the Prefade preamp out was a fantastic clean clear natural sound.  When you use the sub groups built on the board for summing say toms or what ever the signal got darker and not as fresh and transparent sounding. 

The one thing to think about is the 550 EQ input on the patchbay is unbalanced.  but the Prefade output is balance out.  I had an AD 8000 with an output card that I used with Protools and It did not like the unbalanced input of the 550.  My digi mix24 interfaces was fine with driving the unbalanced input of the 550eq.  The AD8000 was happy driving the line input which is a electronic balanced input of the A1  when in line in (via the input switch).

I'm not sure you will want to duplicate this schematic exactly due to changes in recording over the years. Most people want a line trim these days which requires 2 amps for A1.  The simple nature of the console creates a very clean path for mixing to the Left right summing amps but the sub amps or bus amps go through a lot of transformers to get to the output. 

Hope that helps.  I'm at work and dont have my schematics to the API here.  There at home.

Michael

 

[boji]

Wolfgang: Thank you for the kind words. The fader metal and modutec meters are off a wheatstone console. With the exception of the meter faceplate, yes, it's all been done at home. I'll post a blog on it eventually. As for the backplane, it remains to be seen. The wheatstone has simple single layer ~44 pin x 8 cards that tie together with ribbon cable, I will eventualy want to get into a discussion about the thin rails on it; my hope is if I can "bulk up" all the traces it will work for the input cards and the ACA buses. For the pre's and EQ's again, not sure. I think It may be point to point with edge connectors.  

Fazer: Thank you for your input! You are correct about not needing a full dupe of the 544 or 528 input card. The redraw of the 544 was mostly done so as to learn the circuit. The first two tx's and the first op-amp don't look nessessary since the first cage will house various mic pre's. The mixer's input module ought to start at the HP filters but I'm not sure if I need some buffered input to compensate for different EQ's. I'm curious to see what Jeff (Classicapi) is doing here with his design. If you would be so kind as to offer up a look at your schematics I would LOVE to compare it to the 544. I don't know lots about the 550 but if the output is driven off a tx then I imagine there's a way to balance the output for PT.

 

[fazer]

boji:  Goto  http://www.danalexanderaudio.com/api.html      if you go to the bottom of the page you will see the 550 Equalizer Schematic.  This will show you the input connection and the whole circuit design.  There are other schematics for api living there. 

Also the A1 on the schematic (the 536 you posted) is a very well behaved balance input for the module.  There is a love of tx's but to many of them can cause a lot of phase shift.  I started out in a studio with an 8038 Neve.  I loved that board but engineers would come in and talk about the API's and that they were more transparent.  Some of this is because of the Active balanced input for A1.  No tx so you would go thru fewer when mixing from tape.   

I love the bloom of bass from the Neve and API's but the 2520 is a great sounding Preamp line input, and it solves some interface problems for impedance matching.  Also there are 2 Amps in the switch module  (528c,544, 536 etc) they drive everything and also since each amp is a phase inversion they correct for no phase flip when going through the audio chain.  You should be mindful of that that to keep absolute phase into the console  and out of the console.

My schematic blueprints are large and old.  I need to get them scanned.  I have been talking about doing this for years.  Let me see what I can do.    I can take pictures of the PC cards and mounting back planes on mine if that would help.  I think making card like these , (mostly the 550 eq back plane) would give you a better grasp of the whole design.  I'm just not sure how to post them to the forum.  Still learning this stuff.

 

[bluesbaz]

Hold the freakin presses a write up on how you build that frame is in order.......... Please?
-s

 

[fazer]

boji  I just dropped off 14 of the larger blueprints to have scanned.  I'll either post or PM the pdf's for you to post when I get them back.

 

[boji]

  WOW!    Seriously seriously thank you!

A round of Old Chub for you!  ;D
http://www.oskarblues.com/the-brews/old-chub

 

[fazer]

So these scanned pdf's are to big to attach.    Do you want to PM me your email address and I'll send them that way?

 

[jackies]

Maybe winrar them and email to groupdiy gmail account for the curious to see :

 

[abbey road d enfer]

I don't think that you have the right value (10k in your schematic) for the resistor that connects between the 48V source and 1000uF capacitor ("Page1").

This is how I remember the API we had at Barclay Studios in the 70's. I spent a few hours wondering why some mics and active DI's were not functioning properly. Obviously, this is wrong. I think something like 100R would be much more sensible.

 

[abbey road d enfer]

Transcription (so far):
http://www.mediafire.com/file/o4yd3jvzmmn/API 536AE rev3.3.sch

The fader is not properly connected. Input --> Top, output --> wiper.
There's something wrong with the filter; the 13k res should not go to gnd. It should go either to the output of the 2520 (most likely) or to its negative in. And the switching of Filt 1 doesn't make much sense.
There's also something wrong with the pre-fade echo feed; the 4k7 res, in conjunction with the 1k small fader introduces 16dB+ of attenuation, which, I'm sure, is not intentional. Maybe it's just the value of the resistor that should be lower, I don't know.

 

[boji]

Thank you for taking a closer look Abby! John Klett pointed out the same thing with the fader.
Forgive the hackneyed attempt at transcription. My first, and I'm not thinking like an electron yet.
And thanks for the advice (TV you too!) on lowering the phantom resistor.

The buses need to be tweaked as well, the API schema shows that when selection is off, 47k's are supposed to be grounded, not floating.
I'll revise this and the filter section over the weekend.