[c3lingo.org](https://c3lingo.org) is doing the great job of translating many talks of the Chaos Communication Congress and other CCC-related events to multiple languages.
But the required hardware for simultaneous translation is quite expensive to rent, even if it's "just" a specialized analog audio mixer.
The hardware unit should deliver the native audio (stage/ hall mix) to the interpreter's headset and provide a sum of all interpreter's microphone to the input of the video streaming/ recording chain.
Mixing of the final translated audio (ducking the native audio with the translation) will be done as part of the streaming/ recording chain, so the interpreter unit should just provide the sum of all microphones.
Normally just 2 interpreters will provide one translation, but demanding talks might require 3 people.
So either 3 headphone inputs/ outputs should be provided or it must be possible to daisy-chain multiple units.
The user interface of the unit should be as simple, as possible to decrease the risk of mis-configuration.
This means, that no compressor and equalizer will be added in the input group.
General requirements:
- Line input of stage/ hall mix (native language) (XLR/ 6.3 mm balanced jack combo connector)
- 3x Microphone input (XLR) (at first just dynamic microphones)
* VU-Meter for each input channel. (Perhaps with special color scheme: too quiet, good, too loud, clipping)
For the microphone preamp, we are using the NE5534 low-noise opamp with a circuit design from [circuitlib microphone pre-amp](https://www.circuitlib.com/index.php/schematics/product/29-balanced-microphone-preamplifier).
The line input must not be amplified at all, because loudness control of the headphones is done by the headphone amplifier section.
But the differential line-level signal must be converted to a single-ended signal by the input stage.
The current design uses a LM833N opamp to convert the balanced signal into a single ended signal and has a second LM833N to provide some degree (+- 6 dB) of "factory" adjustment.
The second part of that circuit was taken from the [circuitlib audio mixer tutorial](https://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer).
Summing is needed in two places: Creating the sum of all microphones (not adjustable, fixed output gain) and for the headphone mix (one input level adjustable).
A simple summing circuit using one operational amplifier is enough for our application, like in [circuitlib audio mixer tutorial](https://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer).
After some long research to find single and dual channel, logarithmic scale potentiometers with the same dimensions, there are only a few options left:
1. Alps Alpine Series RK09K/D, 10 kOhm (log), 15 mm length, vertical mount: RK09K1130AJ3 (single) and RK09K12C0A19 (dual)
6 mm D-style (4.5 mm) shaft, 6 mm flattened + 2.2 mm (incl. 0.8 mm collar) = 8.2 mm from top
Notes: Knob might touch surface, assuming 2 mm front plate thickness
2. Alps Alpine Series RK09K/D, 10 kOhm (log), 30 mm length, vertical mount: RK09D117000C (single) and RK09K12C0A8K (dual)
6 mm D-style (4.5 mm) shaft, 12 mm flattened + 11.2 mm (incl. 5.0 mm collar) = 23.2 mm from top
Notes: Quite long shaft, matching knobs difficult to find
3.**Alps Alpine Series RK097**, 10 kOhm (log), 15 mm length, horizontal mount: RK097111080J (single) and RK09712200MC (dual)
6 mm D-style (4.5 mm) shaft, 7 mm flattened + 8 mm (incl. 5 mm threaded collar) = 15 mm from top
Notes: More expensive than RK09K; Thread should fix 2 mm front plate well
4. Alps Alpine Series RK097, 10 kOhm (log), 20 mm length, horizontal mount: RK097111080R (single) and RK09712200HA (dual)
6 mm D-style (4.5 mm) shaft, 12 mm flattened + 8 mm (incl. 7 mm threaded collar) = 20 mm from top
Notes: Quite long shaft, matching knobs difficult to find
The casing should be a desk console (angled surface), ideally with space at the front to mount the headset ports (XLR and 6.3 mm jack), like:
- [Bopla ATPH 1865-0250](https://www.bopla.de/gehaeusetechnik/product/alu-topline/alu-topline-gehaeuse/atph-1865-0250.html) (front might not have enough space for the connectors)
A dynamic microphone needs at least 50-60 dB gain in the pre-amp, because a typical signal is at about 1 - 100 uV (-118 to -78 dBu or -120 to -80 dBV).
