A little console to simplify tracking, somewhere between mixer and monitor controller

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bjosephs

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Hi All,


I'm exploring a build to solve a problem in my project studio. I historically have only recorded guitars and never had an issue with low latency monitoring but now I'm trying some vocal tracking and learning about the needs for a cue mix and effects. It's been a long time since I've used an analog board so I'm trying to work out some routing for my scenario. I typically am only recording 1 or 2 tracks at a time and want to keep this simple, small, and sonically transparent. I think I want to get to this:

- Route in 2 channels of performance material (preamps) and 2 channels of playback (either DAW mix down or the output of my summing box). These would have simple level controls and LCR Panning (if any panning, not sure). The levels would only go to unity.
- Send those signals to a cue mix for headphones (I think pre-fader).
- Send some signals to an external FX unit (also pre-fader? or maybe an aux off of an aux?) for headphone wetness and somehow get it back into the cue mix.
- The ability to route the four input channels mentioned above to a 'two track' out (going to converter inputs) and/or a 'monitor path', simultaneously or exclusively. I would route the playback return to the two track for mix down with the summing amp/outboard. And obviously I'd want to monitor input and output in the analog realm most of the time while sending performance to the two track. I think the assignments to the record out should be prefader and pan.

I made the simplest diagram I could to linearize my thoughts. The bussing will be a network of switches and resistors that assign sources to summing stages. I could also see myself using a third aux send to steer signals to the record outs rather than switches. Of course, I don't know why I'd record more than one stereo source in this scenario so maybe I just need a rotary switch on the record out that selects with channel to send. Because of the simplicity (no EQ or dynamics) I think I can stay balanced throughout the device from input to output. I built my summing amp and I'll use the same approach at each of the summing stages: four virtual ground mixers, 2 per L/R channel, 1 each for hot and cold. I used the OPA1632 on recent project and I think it's a good fit for the summing amps. Frankly I may use it for the input buffers too. Also I think I'll try to minimize active stages at the expanse of slightly higher impedances and potentially a little noise. Maybe as little as all-passive aside from the summing. I'll worry about that later.

Whether or not the headphone amp, talkback mic/amp, or monitor switching are internal to this project is very much negotiable. Right now it feels like my urgent need is a little 4 channel, 2 stereo bus console with a couple of aux sends. I've done preamps and a summing mixer along with tons of tube stuff, but this one feels like it needs more planning. Does this look sensible relative to how a typical analog board would be routed?
 

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Chrisfromthepast

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Mar 5, 2018
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I’ve been thinking about this problem lately, as I have reduced my home studio to almost nothing. Building it back up while space constrained is tough.
Have you considered the real estate and how you want the front panel to fit into your existing setup?
Do you, by chance, want these modules to be a part of a larger console project?
Im in the notebook sketch phase of a similar project and Im thinking of making modules that eventually fit into a “real” console build thats part of my future dreamality.
500 series wide per channel, possibly less tall, and 1” deep, was the form factor I was planning on.
Maybe we can help each other stay motivated.
You use kicad?
 

bjosephs

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I too am working with a small space. Trying to keep things minimized and efficient to maximize creativity when it strikes. I have this desk I built with some convenient rack spaces so rack mount is what I’m picturing.

If I could pull off a miracle and cram it all into a single space (picture an API 8200 but with a master section) I would but I’m ready to commit 2U. Obviously I’m not thinking linear faders in this situation.

I’m not trying to make it expandable (save for maybe a secondary summing section) and if anything trying to hold back on expanding the scope until I never start. If I end up with any extra space I’d add a few more channels and maybe a second record bus.

I’ve done all my boards with express pcb. I tried kicad but never really got anywhere.
 

Chrisfromthepast

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The desk is slick! Nice work.

The next step is to nail down your front panel.
The block diagram makes sense to me. A couple topology decisions will help fill in the details of your blocks.

Blocks like headphone amp, and talkback preamp can be built as modules and wired in.
Most of the layout decisions depend on the front panel design. With enough real estate, Id make everything a module so that troubleshooting and repair is more forgiving. However, if you’re careful with space, you could cram it into 1RU on a single pcb with some loose pots.

The thing you might want to reconsider is balanced throughout, as that decision makes your pots more expensive.
If you plan on buffering the inputs, why not make single ended at the same time?
How are you planning to drive the outputs? That corp 1646 work pretty well and are hard to kill.

Have you considered the power supply? A few op amps won’t draw much, I wonder if it would be worth it to use an external supply made for 500 series gear? Were you planning to run the op amps on +/- 15V?

The last consideration is where the mono signal will be coming from and does that signal have a direct path to your A/D?
Does it need to be transformer split to provide a direct out, or is the insert going to be half normalled?
Going to tape through a bus rather than the shortest route possible seems a little weird to me, but Im probably missing something.
 
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bjosephs

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Working backwards:

I agree on the record out. No need for a bus there with this many signals. I was just copying how an SSL I once used behaved where each channel of the consoled had 32 little assignment buttons. Since multiple channels could get the same assignment I presumed they actively summed. The smart thing would probably be a rotary switch (or other exclusively switching systems) to tie an input directly to the record out.

Power supply: I used a Five Fish audio toroid and regulator board on my Mic Pre and summing amp, they work fine. I'm also not opposed to SMPS either. I used a CUI DC-DC converter on project recently (fully implemented all recommended EMC control and filtering) and it was fine. I don't think I'd use that for a mic pre but it would be fine for line level device. But, yes, in the range of +/-15 to +/-18.

I've used That corp bal/unbal parts and I could hear them for sure. Didn't like them, that's actually where I started using the OPA1632. I'm trying to keep as much of this passive as possible so I was planning to buffer after the insert (Let the source device drive the insert device as if this was just a patch bay) and have this one stage drive the fader/pan and bus lines for the auxes, then have one additional stage before the outputs. I was thinking of using the scheme described here for fader/pan but with the impedances scaled up a bit so as to not strain the 1632.

Frankly I'll probably leave talkback out altogether as this is still a 1 room operation. Headphone amp can be external too or added later if I leave some room as you say. I want to have a lot of routing functionality at my finger tips but two cables to a headphone amp isn't the end of the world.

I've started some sketches of the front panel but I'm still hung up on routing - mostly around the auxes. I know I'm going to want to send dry signals of both mono and stereo sources (That's one stereo Aux) to HP but I also want to easily route in some wet effects as well. Is that just a second aux bus? Doesn't that make the dry and wet levels independent? Maybe that's not a big deal but in my head I see a pre-fader aux for dry cue and a post aux-aux on that signal to the FX... summed back the the first aux bus? Sounds crazy. Or maybe people just patch an FX box in series with the cue mix and adjust mix at the device? That sounds easier but then the backing tracks will be wet too. I need to go look at some aux return block diagrams.

Thanks for the complement on the desk. It took weeks but it was quite rewarding. It fills the role perfectly and is *exactly* what I wanted acoustically and ergonomically.
 

Chrisfromthepast

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in my head I see a pre-fader aux for dry cue and a post aux-aux on that signal to the FX... summed back the the first aux bus?
My perspective may be whack, because I do live sound, but we usually use a variable auxiliary bus (lets call it FX) to send to a reverb unit (real or dsp), and then “return” that signal to an input channel. On some cconsoles this is a castrated channel with less dynamics or eq capability.
In most cases, you have the opportunity to vary the send level globally, but you can probably omit that function since there are so few sends.
In your case, you would return the reverb signal to a special stereo input that has send levels to aux, cue and L/R but no knob to create havoc by sending back to the FX bus.
 

ruffrecords

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My first step is always to draw a block diagram. You don't need to worry about implemementation details but it sure helps highlight how, why and where the signal is going.

Cheers

Ian
 

bjosephs

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Massachusetts
My perspective may be whack, because I do live sound, but we usually use a variable auxiliary bus (lets call it FX) to send to a reverb unit (real or dsp), and then “return” that signal to an input channel. On some cconsoles this is a castrated channel with less dynamics or eq capability.
In most cases, you have the opportunity to vary the send level globally, but you can probably omit that function since there are so few sends.
In your case, you would return the reverb signal to a special stereo input that has send levels to aux, cue and L/R but no knob to create havoc by sending back to the FX bus.
That makes sense. An aux return can have a cue send on it, duh. Thanks. Also I think I’ll take your advise about unbalancing at the inputs. I see now that consoles use balanced busses even with single ended signals so there’s no need to double part costs here.
 
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bjosephs

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My first step is always to draw a block diagram. You don't need to worry about implemementation details but it sure helps highlight how, why and where the signal is going.

Cheers

Ian

Hi Ian, glad to run into you here. I’m still using that headphone amp you designed/recommended over on diyaudio.

I agree 100%. I did put a rough draft in the first post, not sure if you saw it. It’s changing anyway as I better understand my own goals. I’m now pouring over the signal flow diagram for the API “the box” since I know they take a deliberate minimalist approach that can borrow from. At least knowing where they use active stages informs a bit about what I can get away with. I also see that they use balanced summing busses with single ended signal using some terminating resistors on a (-) line to keep impedance close. Smart stuff.
 

Chrisfromthepast

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unbalancing at the inputs.
There are so many ways to do it, Im just leaning into the constraints.
If you want alot of knobs on a 1ru enclosure, I think single ended signals will give you more possible parts to choose from.
For the parts of the signal path that are balanced or stereo on one pot, you still have to consider the size of an alps blue beauty or a stepped attenuator vs lower tolerances in physically smaller devices.
This thread has me digging through the plans and parts for my abandoned small mixer project. I was planning on a bunch of class A stages, so my mind was set on single ended.
Bias.
In my case, space is a concern, to the point that Im stuck between 1646 and just impedance balancing it. You have to drive a 1646 , so its a similar conceptual block to a transformer for me. Transformers simply wont fit in the box that Im using as a design constraint.
Are you going full API on the output like your summing amp?
 

bjosephs

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This isn’t intended to impart color so I was planning on using modern monolithic devices with clean specs. Probably OPA1611 for input buffers and the afore mentioned OPA1632 for the output summing amps/buffers. The 1632 is a fully differential op amp (balanced in and balanced out with great CMRR) that acts like two inverting stages. Practically tailor made for balanced summing busses.

The more I look into this the more convinced I am that I can do it but I’m less sure I ought to. I may just need a good passive monitor controller and cheap mixer for headphone mix. Time/$$ maybe be better spent on a different project. Not sure yet. This project might turn into a poor man’s API8200 and leave monitoring separate so I can leverage it differently as needs change.

I have a dual concentric pot showing up from CAPI today that would serve the role of fader/pan in a small space, also could be a stereo send/pan. It’s hard to find knobs for the 3.5mm shaft though.
 

Chrisfromthepast

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OPA1632 looks like a cool chip I need to try out (and balanced summing honestly). I’ve been pretty happy with my experience with the 1642. Unity gain stable, low power, low distortion, it probably gets hot driving 600 ohms, but thats asking alot.

Api8200 looks really useful, and gets you most of the features you were talking about. Sounds like a fun build!
A little cheap utility mixer is always great to have around too.
 
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