Design documents

Our schematics and board design are made using Eagle version 6. Eagle is not free software but a demo version is available on their website and packaged for Debian jessie.

Development version

Version 1.2 (2016-08-04)

Version 1.1 (2014-08-31)

Version 1.0 (2014-02-02)

Design notes

PCB size

  • Spider: 137.0 x 72.5 mm
  • Extension: 117 x 53.0 mm

Power supply

The power supply circuit is simple but efficient. It uses a LM317 regulator which drops the input voltage from 12 V to 8 V. This important ration ensures a good noise filtering from the LM317.

Headphones

When plugged in, a mini-jack creates brief short-circuit between the tip and the ground. To protect against that:

  • The interpretation headphones (H1 to H12) are mounted in serie, and a small resistor (Rs) is added in parallel to each of them. The total impedance when no headphone (Rh) is plugged is:

    Re = (3.Rs)/4 = 11 (where Rs = 15)

    The total impedance when all the headphones are plugged is:

    Re = 3.Rh.Rs/(Rs+Rh)/4 = 5 (where Rs = 15 and Rh = 15)

  • The floor headphones (H13 and H14) are mounted in serie with a small resistor.

Operational amplifiers

Each operational amplifier adds a capacitor in parallel with the gain resistor to create a active low-pass filter. Setting the cutoff frequency around 15 kHz allows to eliminate high frequencies, for example radio frequencies that could add noise to the system.

The value of the capacitor can be deduced from the value of the gain resistor using the following formula:

C = 1/(2.π.R.f)

So to filter frequencies above 15 kHz, the following typical values can be used:

Value of R Value of C
10 kΩ 1000 pF
22 kΩ 470 pF
47 kΩ 220 pF

Microphone preamp

The preamplification circuit of the microphone uses a non-inverting amplifier and is really similar to the following one:

http://www.zen22142.zen.co.uk/Circuits/Audio/lf071_mic.htm

The gain is set by:

G = R8/R7 = R34/R33 = 47

Known issues

Before introducing the resistor R12 and the second ground GNDA, the amplification of the interpretation microphone was affected by the interpretation power amplifier at high volume. This was provoking a important distortion in the headphones. This was occurring on the Spider itself but not on the extension even though it had the same schematics. Removing R12 brings back this problem. We didn't manage to solve this otherwise but it might be able to improve upon our solution.