This article addresses a number of questions people have about the expanders for our Tannhäuser Gates Quad Discrete VCA/Eurorack Mixer System.
For those who are in a hurry to get cracking, the first part of the post expands on (sorry) the expansion headers. Here’s a quick overview of this article:
Apart from that, you will likely need the following resources:
- Here is the schematic in PDF and EAGLE .SCH formats (sorry, they're a bit untidy).
- Manual
- Introductory Blog Post
While we are continuously working on ideas for it, we don’t have any immediate plans for an expander at the moment. For one, there aren’t that many Tannhäuser modules in circulation (yet!), and we’re focusing on the Godeater+ and Ozymandias pedal releases, as well as a couple of surprises for Superbooth 2025.
This one is for the DIYers who want to mess with the TG before we do! PLEASE read the instructions very carefully; we take no responsibility for inadvertent damage caused, proceed at your own risk! Modifications will void your warranty!
1. Tannhäuser Gates Expanders – PCB Headers
1.1. Expansion Outputs
- These outputs are extremely Low-Z. Inverting stages in series can use resistors as low as 100Ω-1kΩ to keep noise super low.
- Post-EQ Knob, Post-Pan Knob
- Taken directly from the output of the Op-Amps that follow the PAN pot. Nominal impedance is 0Ω!
- DO NOT SHORT TO GROUND UNDER ANY CIRCUMSTANCES!!! A minimum resistance of 100Ω to ground is advised here. Accidental shorts to ground will kill this output.
- RETURN L/R are pre-attenuation. We assumed most users would have an output volume on their external FX unit, and didn’t think it necessary to route this post-attenuator. We might do this differently in the future.
1.2. Expansion Inputs
- It goes without saying that the pinout for the Expansion Input headers is the same as that of the Expansion Output, just upside down.
- Each input goes to a 10k input resistor and is summed to the main L/R buss.
- Assuming that each input sees 1V peak-to-peak signal, the summed output of all 4 inputs is 2V. The gain factor of 0.5x has been maintained to keep volume levels sane at the summing stage, in case there are loud signals at the individual inputs.
1.3. XP INSERT Header
- The four channel headers link the VCA output to the scaling/EQ/Panning functions.
- These headers are jumpered by default. Removing the jumper breaks the link, effectively muting that channel.
- The purpose of these headers was to provide channels with insert effects or aux send/return.
- In addition to these pins, we have included additional CV control pins for the MASTER and RETURN volumes – these will accept 0-5V CV control.
1.4. Soft Limit Header
This is a slightly weird one – it adds AC-coupled Zener limiting to the channels. The effect is pretty subtle and reduces the output slightly to about 5V p-p (about 9V post-boost). Removing the jumper takes the Zener diodes out of the circuit and gives you a little more headroom.
To me, this I’m not sure how you’d use this well, but perhaps you’re more imaginative than I am. It could be an interesting feedback source, or you could experiment with other clipping diodes?
1.5. Lament Output Header
Over the last few years, we’ve made a couple of attempts to build a module called "Lament Configuration". This was intended to be an end-of-chain parallel effects module for Tannhäuser Gates. So far, we've been unhappy with the end result, and continue to work on it.
This expander is a redundancy to allow patch-cable-less integration with the hypothetical Lament Configuration, if it ever sees the light of day. Essentially, this is a send/return loop on a 10-pin power header.
2. Brief History
As you probably know and I’ve spoken about at length, Animal Factory was born out of the simple fact that a lot of music electronics were either inaccessible or stupidly overpriced in India. I started building things based on the schematics I could find and easy local access to parts. I haven’t heard or played most classic synths to have a frame of reference of what they sound like.
At some point, I discovered Buchla and the low-pass gate. In the process of reading up on the LPG, I stumbled upon the Buchla 100 series and 110 Gate module.
I was attracted to the idea of building the 110 circuit as a quad VCA, with a master CV control input and mixdown to a single output. This prototype was fun, but peaked at 6V p-p output, making it unusable for “proper” Eurorack use. I got some good feedback on it from a few beta testers – eventually, we built it as a product with performance mixer functionality.
Again, we have made make no particular efforts to be 100% faithful to the Buchla original, aside from the topology. We use modern SMD parts and SMD matched arrays throughout. Further, I’m not one for clean signal paths. We love distortion and character, and this circuit did that very well.
The beautiful artwork by Aniruddh Mehta was a serendiptiious accident. I asked him to watch the original Blade Runner movie to understand the reference. He didn't, and came up with this with no context. I realized that he had unknowingly created patterns similar to DNA sequencing, which was a crazy coincidence. In the original prototype, each channel was named after one of the four replicants (an idea I scrapped at the last minute for the production version). Without knowing what the movie was about, Ani had created patterns that tied in with one of the key themes of Blade Runner.
I still think of Tannhäuser Gates as the most elegant and accomplished design we’ve built to date. Let’s be honest though – we did a terrible job of marketing it. The development and production happened during the pandemic, and was a particularly painful one. From soldering DFN and QFN packages by hand to disassembling and redoing all faceplates, this was one of the most difficult modules we’ve brought to life, and one we’re particularly proud of.
3. The Deep Dive
3.1. Channel Architecture
3.2. Discrete VCAs
Each VCA is practically identical with the old Buchla design, except for some part values that have been modified, primarily to work on a unipolar +12V supply. The signal goes directly into the INPUT GAIN potentiometer, which will attenuate the input (passively) and reduce clipping (if so desired…). We eliminated the trimpots by using BCM847DS matched arrays.
The fact that the Buchla 100 series was designed around line levels means that the VCA will start to clip (very tastily) with unattenuated modular levels. We observe clipping at around 5Vpp (peak-to-peak), this maxes out at around 6Vpp at the VCA output.
Pulling the CV input to ground effectively disables the VCA, muting the channel at the input itself. This was the method that gave us the closest thing to a pop-free mute that we could get.
3.3. POST-VCA
3.3.1. Level Boost
The fact that the Buchla 100 series was designed around line levels means that the VCA will start to clip (very tastily) with unattenuated modular levels. You should observe clipping at output signals in the ballpark of 5Vpp (peak-to-peak) and max out at around 6Vpp. A simple full-headroom op-amp booster at the output of each discrete VCA boosts this to 10Vpp.
3.3.2. Tilt EQ
The Tilt EQ is very simple and effective – CCW boosts lows and attenuates highs, CW does the opposite. The EQ is applied after the VCA output on a separate path. This means that you can hear its effects only on the summed output, not on the channel output.
3.3.3. Panning
The panning system is a very simple passive crossfader setup – unfortunately, there is no way to control this via CV. This is a trade-off that we made to not clutter up the user interface any more than we already have.
The other way would be to send the L-R channel outputs from the SEND expander to a more involved panner module with its own set of outputs.
4. Expansion Ideas
By no means an exhaustive list, a number of features that we have shortlisted, or have received suggestions for, include:
- Channel Insert Effects
- Channel Aux Send-Return, since the Send-Return currently only works on the master
- PFL / Cue buttons
- Quadrophonic Expansion
- CV-controlled Panning
- Mid/Side processing
- Feedback loops for no-input horrors
- Higher headroom/vacuum tube end-of-chain mixing stage
I’d like to think we’ll build something in 2025 or 2026, but with the new pedal line, among other things, dominating our to-do list, maybe you’ll beat us to it.
Hopefully this will spark some discussion on the ModWiggler thread – do feel free to chime in with your thoughts. If you have questions about anything else in the circuit, do post them there as well!
Happy Holidays from Bombay,
Aditya