UC1608 - 8 buss aux expander / distribution mixer based on JLM and CAPI

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Well-known member
GDIY Supporter
Sep 19, 2010

Some years back (6 to be exact, how the years fly by) I started thinking about building an AUX-expander for my yet to be built API style summing mixer (DR2402).
I posted a thread here on GDIY, but as a total beginner when it came to electronics, much needed to be understood to even know exactly what I wanted to build. Ian/ruffrecords and other friendly members chimed in, and so as the years passed and I finally finished the mixer it's supposed to expand in the first place, it was time to start the expander.

I contacted Joe from JLM, and gave him my specifications and parts intended for the unit, which he then very kindly offered to draw up PCB's for the design.
I had been working on the panel design for about a year or so, and wanted to use dual pots, both concentric and duals, as I wanted mono sends for AUX1 and 2, but stereo/dual pots for the rest. I also wanted to use an API style ACA, as to keep it somewhat in style with the DR2402 (color of panel and choice of fonts/symbols are made to resemble the design of the DR2402, as it will be mounted above).

All said, I wanted to make a flexible design that could be useful in different projects, and Joe did a terrific job on eliminating tedious wiring (which was the main reason for the pcb's in the first place!) and creating a pcb that fits well into his ecosystem of small building block pcb's. Another truly kind and helpful person in this weird fabulous world of pro audio diy.


Frank from frontpanels.de CNC'ed my panel-design, and as often stated on this forum, did a terrific job. Although I made prototypes in wood with a laser, it was very comforting to see that all my parts fit snug and nicely on the final anodized aluminium panel. I was especially worried about the toggle for AUX 1, which is very close to the pcb. Switches are On-Off-(On), to allow for spontaneous dub-style echo feed in addition to normal on/off switching. Pondered using vactrols for muting to the minimize clicks/pops that inevitably take place using hard switches, but it quickly became very expensive, so standard toggles it is.

The idea for the panel was to line up the channel slots to my 10-slot API 500-rack to make channel-strips (albeit stereo instead of mono) somewhat alike a normal mixing board.
Still some wiring left to do, and I need to fit a power-supply. Also need to find a suitable metering-circuit (replicated 8 times) to use as LED-drivers for each buss. I built up the circuit from the VU/peak led-pcb from JLM on breadboard, which works nicely, but I want to use bi-color LED's so as to also show signal present in green in addition to the red overload indicator.
I am planning to use some rails originally designed for the scribble-strip for a Redco patchbay, cut and mounted in the last slot on each buss so to remind the user what it's connected to. The LED goes inside the rail, and would light up behind the printed text for that buss. Still need to cut these rails with a proper tool to get them nice and symmetrical.

Anyways, still some stuff to do before it's ready to be fired up, but after all this time, I really wanted to share what I've been working on for such a long time.

To be continued...
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Here is the FPD / frontpanel design if anyone wants to use it as a starting point for their own design with these pcb's.
Few points:
-The red graphics shows the inner side of the NRG case. Are not produced
-The holes on the last slot are for mounting scribble-rails, so are a little small for normal 3mm LED's if you want to use them for that. They do however fit the screws that come with the rails so tightly that you make nice threads just by screwing it in.
-Panel thickness makes the use of washers on the pots a bit difficult; no chance for the concentrics, but you might be able to make it work with the dual pots.
-Pots used are BI Technology from CAPI.



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Been a while since I've had time for DIY, but picked this one out of storage to try to finish it and put it to good use. After a few solid days of soldering I've finished the power supply (AC/DCv4), the LED circuit (ended up with simple backlighting of the scribble strips instead of a fully fledged peak indicator pr channel, at least so far) and the wiring and power/ground distro. First power up is completed with no magic smoke, a relief considering the amount of opamps in this thing.

Everything is working fine, however AUX 5/6 and 7/8 seem to have some weirdness in connection to ACA or booster stage. Similar symptom on all channels, so should be a simple wiring issue.
Also somehow channel 9/10 appear on 11/12. There is also some volume inconsistencies on some channels (within 1-2dB), wondering if this is just less than ideal connection between the IDC and that pin/channel.

All in all I am confident that the project is going towards "finished status". It sounds really cool, a slightly cleaner shade compared to my api summing mixer with discrete 2520's on all inputs. The UC1608 (
) uses JLM Palpigrades on inputs, 8 JLM aux8 boards into 8 channels of Capi ACA with GAR2520's, into second half of JLM SUM8 for post summing gain before going to a JLM Tardigrade for final balancing. Used 47k bus resistors between AUX and ACA, and 10k resistors between ACA and SUM8 which lands at about unity gain overall.


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