Amplifier/Attenuator Synth Module
My first attempt at a homemade Eurorack-compatible module.
A continuation of My First Homemade Synth Module! in which I describe some the motivation and circuit design of this module :
KiCAD schematic & front panel templates are on GitHub.
To recap, my main motivation was to bridge between the different signal levels I encounter in my music room. So I have two attenuators that will typically be used to reduce the nominally 10v p-p signals of synth modules to the line level, say 1v p-p, used by my mixer. The two amplifiers go the other way, line/instrument to synth levels.
Another big motivation was that I wanted to get to know the Eurorack format. Although I’ve spent a lot of time around synth electronics I’m new to the party when it comes to the current quasi-standard for modular.
The attenuator/amp was perfect for this discovery as the circuit is fairly minimal but needs to fit the format when it comes to physical construction and power supply.
As noted in the previous post, I decided to design, build and assemble the whole thing before testing. I’d got the stripboard circuit built, the next bits where moving more into unfamiliar territory.
Front Panel
There are plenty of good resources on the Web about making front panels, eg. SyntherJack describes an approach similar to the one I used, specifically for Eurorack modules.
If this was to be more than a one-off, I think first of all I’d look to using one of the PCB-making services. They typically provide screen printing on the PCBs, and some allow aluminium as a substrate. Going this route, with a few tweaks, it’d involve the same design process as creating PCBs (using KiCAD or whatever). While cheap compared to using a dedicated front panel process/service, it would still work out pricey for making just one. Something for future investigation.
The last front panel I made was for the Chatterbox. There weren’t really any major constraints on that – it’s a one-off/prototype instrument and it needed a lot of front panel space to allow for user interaction.
For a case I used a wooden box, sold on Amazon for craft purposes. For the front panel I used an A4 sheet of vinyl-covered aluminium (designed for signage) with labels printed on an OHP transparency with a regular inkjet printer.
It worked well enough for the purposes, but had a couple of failings : the material was relatively expensive and I didn’t really solve the problem of attaching the transparent label sheet. It’s only held in place by the screws and nuts etc. of the controls. I could have tried some kind of spray mount glue, but it was good enough without. Whatever, this time I hoped to improve on that.
I started with sheet aluminium. The Doepfer standard for panel thickness is 2 mm so I ordered a 50x50cm sheet of that from Amazon.
I did use KiCAD to get a nice rendering of the schematic, but I’m not familiar with the design layers I’d need here. But I have used LibreOffice Draw a lot for graphics so went down that route.
Dimensions
The Eurorack standard is based on that of 19″ racks. Usual height 3U (about 128mm), width multiples of a ‘HP’. These are annoying units! There are details on the Synth DIY Wiki. To retain a bit of sanity I measured my one commercial Eurorack module, a Behringer 150, which is 128x80mm ~35mm deep. According to it’s specs, Width: 16 HP, Depth: 44 mm.
The rack I bought (a Tiptop Audio Happy Ending Kit) would allow for a fair bit more depth. Regarding width, I thought I could probably get away with something narrower that the Behringer 150, so I decided on 12 HP (61mm).
So I measured that on a bit of paper and put the pots & connectors on it. It looked a real squeeze, so I decided the 16 HP would be better.
However, a couple of days passed and when I fired up Draw I’d forgotten and began with the 12 HP dimensions.
Looking ahead, I was soo lucky! The controls & connectors would fit, just, literally a mm or two in it.
Layout
Doing the design in Draw probably wasn’t the greatest idea, I suspect the dimensioning would have been easier in KiCAD. But anyway, a lot of time later I had two designs ready to print :
The circles on the drilling template aren’t actually the hole sizes, rather the extent of the nuts/washers/knobs that would go in front. The positions of the mounting holes I eyeballed from the Behringer module and the rack.
These started as a combined drawing, then I deleted bits. In the process, I made the classic mistake of letting them go out of sync. This was resolved after several wasteful prints…
Drilling
This should have been easy, was quite a nightmare, took ages.
The aluminium sheet had a plastic film on one side, so after cutting a piece to size I glued the template to it. I punched centre holes and drilled pilot holes the size of the mounting holes. Even though I have a drill press, I know from experience that holes can easily drift. Hah! I drilled the first two holes for pots…and they drifted.
Subsequent holes I made by incrementing through the drill sizes, correcting if necessary, as best I could, as I went along. I still had to do a bit of filing.
I think the cause was mostly down to the drill bits I was using – from an ultra-cheap Chinese pack. They aren’t very sharp, so I was putting more pressure on than should have been necessary and the thin sheet was flexing. (When putting the drill bits away I found a stepped hole-drilling bit in the drawer I’d forgotten I had. Grr!).
At this point I checked by loosely attaching the controls :
This preview also allowed me to see where I could mount the circuit board.
With a tiny bit more filing it seemed good enough.
The back of the panel looked dreadful :
But that didn’t matter.
Labels
I had a few options for the overlay. The side of the aluminium sheet on which the labels were to go still had it’s protective plastic film, so I took that off, printed the labels on a transparent sheet and had a look.
It didn’t look great. There were imperfections around the edges and the finish just seemed lacking. I considered using a bit of sandpaper to get a brushed aluminium look. But the next experiment worked quite nicely.
What I ended up doing was printing the labels on photo paper and laminating. The result I cut out and glued to the aluminium.
I then cut out the holes with a scalpel, and made the holes to mount the circuit board.
This module had the unusual requirement of several 1/4″ jack sockets that run quite deep. But I’d got some plastic standoffs I stacked to give adequate clearance.
Wiring
The circuit of the attenuator sections is so trivial I haven’t written it down. (PS. liar! I did write it down, in text like this, in the previous post). The inputs each connected across a pot to ground, which would act as a resistive voltage divider. To the centre tap of the pots I connected a 1k resistor (for short circuit protection) and ran these to the outputs.
At this point I discovered I’d had some incredibly good fortune. I’d placed the circuit board mounting holes where there seems to be the most room. This happened to be over the the attenuator sections, leaving me access to the connectors of the amplifier sections, so I could screw the board in, attach those connections in situ.
The wiring was straightforward. I’d put 2/3 pin SIL sockets on the circuit board, so I soldered hookup wires between the pots & sockets on the front panel and SIL plugs.
The only bit of the circuit that might have been prone to environmental noise was the initial input of the amplifier sections, so I used screened lead for those.
I had a space Eurorack power cable so I made a little male-male adapter to attach that to the circuit board.
Testing/Fixing
All assembled, time to test. This was quite exciting so I recorded it on the phone, 1 minute ‘Shorts’ :
My estimates for first test :
- 39% – works first time
- 50% – something doesn’t work, but is easy to fix
- 10% – something doesn’t work, is difficult to fix (perhaps requiring whole new circuit design)
- 1% – BANG!
The attenuators and the x10 amplification on each section worked first time, phew!
Following the output line to the circuit board immediately showed me one problem: I’d wired the output connector to pin 8 (+ve) of the TL071s in each section rather than pin 7 (output).
2 minute fix, test again – one channel’s x100 still didn’t work. This took a little longer to figure out/fix. I’d got the capacitor on the input of the second stage connected to the wrong strip. Total maybe 10 minutes and it worked!
Kinda funny that they were out-by-one errors – I’m a computer programmer by trade.
Here’s the module racked up –
Not very pretty, but solid enough.
That’s prettier.
I then had a little play (see after). I did find some strangeness with the behaviour of the ring mod levels and the Wasp synth oscillator outputs. It seems there’s unusual connector wiring and/or signal levels around these somewhere. I couldn’t find any documentation so will have to spend a little time with a ‘scope to check around.
I think next on the Eurorack I need a MIDI -> CV+Gate converter. I’ve been playing with ESP32 microcontrollers a lot recently, should be pretty straightforward to use one for this.
Less boring, I’ve a few ideas for novel microcontroller-based modules I want to play with. Those will no doubt take a good while virtual on the computer before getting anywhere near hardware. In the meantime, time permitting, I’m tempted to knock together one or two trad analog modules : VCO, VCF, VCA. (While I really like the interaction of analog-style sequencers, it seems a bit silly to build one in the Eurorack format, better to enhance the interactivity in a big box. The other obvious module, an envelope generator, I think for utility/ease-of-build, that lends itself to a microcontroller-based circuit…for another day).
Switch on an hour before you intend to perform – stay tuned!
PS. I’d already got the impression from reading around that signal levels weren’t very standardized around Eurorack modules. After playing around with this module, the Behringer 150 and the Wasp synth it seemed apparent that this was indeed the case. I got the ‘scope and did some measurements : the levels aren’t even consistent within the same module!
Behringer 150
Approx. figures, measured with oscilloscope, DC input
LFO Output 1
- sine : +3 / -8v
- triangle : +3 / -8v
- square : -3 / +8v
- ramp : -3 / +8 v
LFO Output 2 (presumably inverted)
- sine : +3 / -8v
- triangle : +3 / -8v
- square : -2 / -12v
- ramp : -2 / +12 v
Noise output about 10v p-p.
(I’ve not had a play with the sample & hold etc. yet)
Behringer Wasp Deluxe
OSC Outputs (appeared AC coupled)
- ramp : 2.5v p-p
- square & ENH : 1.5v p-p
