[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).
- (A) Re'an F311/ F313 series (11/ 13 mm diameter) -- 0.40 €/ 0.55 €:
Boring: 9.0 mm D-style hole + 0.0 mm nut cover section = 9.0 mm
- (B) Re'an P670 series (12 mm diameter/ 16 mm at bottom) (alt. Davies Molding 1101) -- 0.78 €:
Boring: 7.5 mm D-style hole + 4.5 mm nut cover section = 12.0 mm
Nut cover: 12.9 mm diameter (M7 nut is 12.5 mm corner to corner)
- (C) Re'an P300 series (11 mm diameter/ 15 mm at bottom):
Boring: 9.0 mm D-style hole + 3.0 mm nut cover section = 12.0 mm
Nut cover: ? mm diameter (M7 nut is 12.5 mm corner to corner)
- ~~(D) Cliff K87MAR series/ RS Pro 777-73xx: 7.5 mm hole depth + 4.5 mm skirt = 12 mm, inner size of skirt 12 mm (measured)~~
~~Notes: D-style boring has wrong size (5 mm instead of 4.5 mm flatted)~~
Potentiometers from Alps Alpine seem to have a decent quality in a small package size and quite affordable price.
From their lineup, the RK09K/D, RK11/12/14, RK09L and RK097 series are left considering our requirements (mouser prices without VAT, shortlist marked bold):
- Alps Alpine Series RK09K/D (6 mm D-style (4.5 mm) shaft):
- [Bopla ATPH 1865-0250](https://www.bopla.de/gehaeusetechnik/product/alu-topline/gehaeuse-18/atph-1865-0250.html) (front might not have enough space for the connectors)
The electrical design consists of three identical channel strips, a line I/O board and a power supply.
Each channel strip is separated in two boards, one for the top plate with the VU meter and main controls and one for the front panel with the headset jacks and headphone volume knob.
The distance between top plate and PCB is 7 mm (determined by the potentiometers and on-air switch), which should also be suitable for standard 5 mm LEDs.
Connectors and larger components will be mounted on the back side, so enough space behind the PCB is needed.
The top plate should not exceed 2 mm thickness because of the potentiometer knobs, 1 to 1.5 mm would be ideal.
The I/O boards (headphone and line signals) will just have components on the top side and might be steady enough by the XLR and 6.3 mm jack 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).
