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Dreadbox Erebus breakout box DIY
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2  Next [all]
Author Dreadbox Erebus breakout box DIY
drmarble
Here's a project I've been working on for a bit, after getting a Dreadbox Erebus v2 last fall. The basic idea was to do a breakout box for the Erebus, and to add a noise generator, oscillator sync, and a basic ring modulator as well.

From there I decided it needed a few utility bits, like a pair of attenuverters (so I could scale all those newly accessible signals) and a summing mixer (to combine them as well). Since there was some space left, I figured another LFO would be nice. To differentiate it from the Erebus' onboard LFO, I based this one on Ken Stone's Utility LFO, with a shape control that varies the pulse wave's width and varies the other waveshape between ramp/triangle/sawtooth. It will also have a significantly slower frequency range than the Erebus' LFO.

The whole thing is designed to fit inside a Hammond 1590P1 box, which matches the dimensions of the Erebus nicely. The circuits are laid out on a pair of Adafruit Perma-Proto boards, one half-board size and one quarter-board size.

Before finalizing the design, I would like to solicit feedback from other Erebus users, and anyone else who cares to speculate on the best possible ways to expand this semi-modular synth. Please keep in mind the design principle of enhancing the the Erebus. Obviously there's a thousand little bits that *could* get jammed onto an expansion unit, but which bits would be best accomplished by a dedicated breakout box like this, rather than just being added on by some auxilliary Eurorack modules?



When the design is built and tested, I'll be publishing it as open source.

A few other details on the workings:

The MIXER IN goes to the Erebus' pre-filter mix point, where its oscillators get summed with whatever signal is input here, plus whatever signal is connected to the Erebus' existing 1/4" Audio In jack.

The attenuverters will be normalled to a +5V reference, so that if nothing is plugged into their inputs, they can be used as a voltage offset source.

The GATE toggle switch is an ON-OFF-(ON) type, so that the gate can either be latched on for drones, or flicked on-and-off repeatedly as a manual trigger.

The ECHO output is a 100% wet, delay-only signal from the Erebus' onboard echo unit.

The AR output is the envelope that's normalled to the VCA.

Please share your thoughts!
windspirit
Ah, I have been talking with erebus about having access to the oscillators independently. Do you happen to know where those patch points are?

I think a ring mod and noise would be very nice in this design. Looks like a good project.
windspirit
If I were you though I would make enough space to use larger knobs to keep the vibe of the original instrument and also to have a nicer knobbing experience.
drmarble
windspirit wrote:
Ah, I have been talking with erebus about having access to the oscillators independently. Do you happen to know where those patch points are?


Yes, at this point, I've breadboarded all the circuits that will be used in the breakout box, and found all the patch points that will be used in the Erebus.

There are a bunch of spots where you could tap the oscillators independently. For soldering ease, I prefer to tack wires onto the ends of header pins. There are 3 headers that connect the front and rear circuit boards in the Erebus (each one is 2 x 10 pins). To access the oscillator signals, take the back panel off, and look for the header on the bottom right. It will look like this:



As you can see, I've labeled the pins that carry the two waveforms for Osc 1 and Osc 2. (My label for Osc 2 saw is a little off-centered... the correct pin for that is the one directly adjacent to Osc 2 tri.)
drmarble
windspirit wrote:
If I were you though I would make enough space to use larger knobs to keep the vibe of the original instrument and also to have a nicer knobbing experience.


That's an interesting thought. I had just been going with the skinny knob shafts, the type that the Erebus uses for Depth control for MOD and ENV in its patching section.

For space constraints, I probably need to keep the skinny knob shafts in the middle section of the breakout box (for the two attenuverter controls, and for the summing mixer level control).

But, it might be possible to use bigger knobs (like the Erebus' Attack and Release knobs) for the Noise Color and LFO controls. I'd have to shuffle the parts around and play with it a bit in Illustrator to see the fit.

On that note, if anyone knows where to get Alpha-style potentiometers, in the 6mm knurled shaft/9mm body size, but with panel-mount bushings, please let me know!
windspirit
drmarble wrote:

On that note, if anyone knows where to get Alpha-style potentiometers, in the 6mm knurled shaft/9mm body size, but with panel-mount bushings, please let me know!


I have never seen these before but good luck.

Thanks for the breakout diagram. I'll hit the pins with my scope before trying anything just to be sure.
PWM
Is that a normal resistor, in thermal contact with those two transistors, instead of a tempco?
drmarble
PWM wrote:
Is that a normal resistor, in thermal contact with those two transistors, instead of a tempco?


I don't know well enough to say. Its shape looks slightly different than the other resistors on the board, but it is largely covered in that paste, so it is hard to see.
drmarble
windspirit wrote:
If I were you though I would make enough space to use larger knobs to keep the vibe of the original instrument and also to have a nicer knobbing experience.


Here's an update to the layout. I didn't know if I'd have room for bigger knobs (anything bigger than the 6mm "plain" shaft style), but after shuffling some stuff around, this is feeling workable. I believe those 3 larger knobs are the same as what's used on the Erebus for Attack, Release, etc... they are 16mm diameter, the "small" size pointer knobs from Thonk.



Still having a devil of a time sourcing 9mm pots with threaded bushings for panel mounting. The Alpha RV09AF-40E1 series is what I want, but I'm not finding any US suppliers with them in stock. Might have to order through Thonk – they say they had a bunch of those Alpha types manufactured to their spec (without the anti-rotation tag).

Other than the updated layout, everything else is the same, except that I added a passive attenuator towards the bottom right (IN, OUT jacks with the LVL knob). I was out of breadboard room (and opamps) to do anything more exciting with that space, but rather than waste it, I figured I'd add a passive volume control (its pot will be log taper, whereas the two attenuverters have linear taper pots).

One aspect of the layout I was unsure of was how to best position the LFO and NOISE output jacks, being so close to their knobs. The concern being that patch cords would be in the way of grabbing the knobs. I think by locating the knobs towards the edges of the box, I've kept them reasonably grab-able, but I would appreciate feedback from others with more modular experience as to whether that still looks too tight.

Thanks!
windspirit
I think it looks good with the new knobs. Are you planning on releasing this as a kit/ board and box or are you just making this for yourself? If you want to release this I may hold off on modding my Erebus.
drmarble
windspirit wrote:
I think it looks good with the new knobs. Are you planning on releasing this as a kit/ board and box or are you just making this for yourself? If you want to release this I may hold off on modding my Erebus.


I plan on making all the info openly available, but no plans on doing a kit or even a board. I think it's just too small of a niche to justify that effort.

If someone else with PCB layout experience (of which I have little) wanted to step in and do a board for it, I would be open to collaboration. But I see this more as a one-off project for myself, replicable by others without the need for a custom PCB.

To that end, I have been doing the layout using Adafruit Perma-Proto boards. The Hammond case I have fits one half-size and one quarter-size Adafruit board, so that's the layout I have been mapping out. I could instead take a full-size board and cut it down to size, thereby gaining a few extra breadboard rows. However, I have the noise and LFO circuits on the quarter-size board, and all the other stuff on the half-size board. Keeping those elements on separate boards makes it easier to swap out the "fun" new generator circuits (noise and LFO), if another builder decides they'd rather put something different in that space.

The Perma-Proto board is cool in that the traces are connected like a traditional breadboard, but since you can flip it over, you can cut traces where you want to, and thereby fit more components onto a single row. I'm using a bunch of vertically-seated resistors in my layout, in order to squeeze the most out of the board real estate. And, by cutting a trace between two pads, I can orient a series resistor "along" a single row (instead of between two rows), which wouldn't be possible on a regular breadboard.
drmarble
Here's a peek at the breadboard layout in progress, as rendered in Fritzing. It really makes for an ugly mess, with no way to properly show wires run under the board, and poor control over how components are ordered in the Z-axis (wires always run on top of components, visually). And I had to make my own graphic for vertically-seated resistors (that's what the blue circles with the "R" are). But I don't know of a better alternative for complex breadboard layouts!

I'm thinking that when I publish the plans, I'll create a series of snapshots from the Fritzing layout, each one a step showing the order that the components/wires need to be assembled. I certainly wouldn't expect anyone to follow this mess, as it currently appears:

drmarble
And, lastly for now, a mockup of the panel, on the Hammond project box, next to the Erebus:

Shoggoth
PWM wrote:
Is that a normal resistor, in thermal contact with those two transistors, instead of a tempco?


I wouldn't be surprised since this is in the Erebus manual, but I guess that could be said of any osc design that doesnt involve a heater board:

"Suggested room temperature for best oscillator stability"

drmarble I'm curious on three things:

- do you think the osc signals could be tapped directly and mounted to jacks simply within the box itself?

- is there a PT2399 chip in there? delay sounds like it.

- Im curious about the filter design. are there what appear to be other matched transistors not related to the oscs? noise floor seems a little high for a v2164 but the filter sounds a bit like some of the things I can get out of a Shruthi 4PM.
drmarble
Shoggoth wrote:
drmarble I'm curious on three things:

- do you think the osc signals could be tapped directly and mounted to jacks simply within the box itself?

- is there a PT2399 chip in there? delay sounds like it.

- Im curious about the filter design. are there what appear to be other matched transistors not related to the oscs? noise floor seems a little high for a v2164 but the filter sounds a bit like some of the things I can get out of a Shruthi 4PM.


1. Tapped directly, definitely. Space on the top panel? That would be tight. There would be space on a side wood panel to add some jacks, but in addition to the awkward placement, those panels are 5/16" thick, so can't mount jacks directly on there, would have to use a metal sub-panel maybe? There is space along the bottom metal panel (the side that would be facing you if it was sitting on a table) for adding jacks. But again, awkward placement...

2. Yes, the PT2399 is the delay chip used.

3. Looks like the filter was done with a LM13700 chip. There are some other pairs of transistors on the board, but not that are thermally bonded with a resistor in between them like the oscillator transistors are. And I'm not actually sure if those other pairs of transistors are part of the filter, the VCA, or something else.
windspirit
I think that the filter is supposed to track 1v/o and be able to act as a 3rd oscillator in fully resonant mode. At least that is what I saw in the sonic state demo.

EDIT: did you happen to find a reset input for the internal LFO?
drmarble
windspirit wrote:
I think that the filter is supposed to track 1v/o and be able to act as a 3rd oscillator in fully resonant mode. At least that is what I saw in the sonic state demo.

EDIT: did you happen to find a reset input for the internal LFO?


Yeah, the filter does track 1 v/o pretty well, and does self-resonate.

Ooh, good idea about trying to find an LFO reset. I'll poke around.
tsimmert
Hey guys,
I got quite inspired by your mod-plans. I used my Erebus together with a Nanozwerg for a while, it provided S&H and an additional VCO+Noise or a multimode filter, unfortunately due to the signal flow only one of it at a time, the S&H of the Zwerg is also kind of limited and would have needed amplification in the long run. But I got rid of it a while ago and decided to make use of the Doepfer DIY synth to accomplish the Erebus...
Shoggoth
I couldn't seem to get all the detuned sounds I wanted from the detune knob so I messaged Dreadbox about changing it to detune a full octave, thought some might be interested in the response. HUGE props to a company who will share this kind of info willingly. I even told them I bought the unit second hand:

Thanks for contacting us!

Although I would suggest doing that by changing the octave setting of osc 2 here is how to do it:
If you open Erebus and remove the main PCB you will be able to see the trace of DETUNE knob pin3 leading to a resistor. The resistor though is on the other side, but if you solder on parallel something like 220k you should be able to do get an octave or even more. Originally the resistor there is a 470K leading to +12v supply and the knob is a 100k, making an attenuation with the other end at ground.
drmarble
Shoggoth wrote:
I couldn't seem to get all the detuned sounds I wanted from the detune knob so I messaged Dreadbox about changing it to detune a full octave, thought some might be interested in the response...


Definitely interested, thank you for sharing this! I haven't done any permanent mods to the basic Erebus yet, but this is one I probably will do. The Osc 2 detune range on the Erebus has seemed kind of arbitrary to me (what is it, about fourth down and a little over a fifth up?), so I'd like to widen that up to an octave either way for sure.

I also second your props to Dreadbox for sharing this kind of info willingly. They have been super supportive of the breakout box project I've been working on.
drmarble
Shoggoth wrote:
If you open Erebus and remove the main PCB...


Having removed the main PCB once already, I wanted to add that it's a bit tricky getting it seated back in again. There are 3 headers on the main PCB that need to be lined up directly over their pins, while at the same time wiggling the audio and MIDI jacks back into alignment (i.e. with horizontal motion). It's not impossible, but it did take me a bit of time to re-seat the main PCB without fearing I was going to smash down on a header pin the wrong way and bend them out of alignment. Proceed with caution!
Shoggoth
I ended up tacking on a 300k resistor on the other side of the 470k resistor mentioned by Dreadbox, and I prefer the range of the Detune pot to the stock values. Now, 'unity' is about 9 o clock, a 3rd down is about 7 o clock, a 3rd up is 11 o clock, a 5th is a bit past noon, and an octave up is about 2.

Since the 2nd osc has octave options typically one above the first osc, I would have preferred to have the more extensive tuning options below the first osc rather than above, but you would probably have to replace the 470k resistor with a higher value to do that, if its even possible. I tried messing with the Osc 2 Bias trimmer to get the Detune pot to be 'unity' at noon, but the values are dialed in so tight you can only slightly change the pitch with the trimmer.

If you're doing this mod, keep in mind that tuning the two oscs will be a little more difficult since the Detune pot has a wider range.
Licudi
Any examples of this modification in action?
Shoggoth
Example of the mod in action:


drmarble
Shoggoth wrote:
- do you think the osc signals could be tapped directly and mounted to jacks simply within the box itself?


To follow up on this question - yes, I can be done, and I have finally done it!

I pursued the route of an Erebus breakout box for some time, but never found a mechanical solution I was happy with. Eventually I got a Eurorack case and started getting into that world, so I no longer felt I needed to cram all those extra synth circuits into an add-on box for the Erebus, since I could just patch in Eurorack modules instead.

Eventually I looped back around to seeing if I could just add a few extra i/o points to the Erebus, and found that I could squeeze in open-frame Switchcraft jacks between the front panel and the first PCB down from that.

Here's how it's looking:



That's 7 new jacks on the front panel, each a 1/2" apart, and a sub-mini toggle switch around the corner on the top panel.

Jacks are: 4 direct osc waveform outs, a 100% wet delay out, an osc 2 sync input, and an (extra) external input jack (pre-filter).

The toggle is to apply 5V to the gate CV input, to hold the gate open when using the Erebus in a patching situation. The same thing can be accomplished by externally patching 5V to the Erebus gate input, I just was doing this so often that I thought I'd reduce clutter while I was in there modding things.
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