Add rework to fix VU meter, muting delay, LP filters

Note: PCB files were not upated!

Changes/ required manual rework:
- Replace C314 with 100nF to reduce muting fade time
- Replace C301 with 680pF to have a 12 kHz low-pass filter on headphones
- Replace R309, R232, R335 to fix VU meter range (steps are a bit wonky
  afterwards, but top (and bottom) range are a lot more save now)
- Add 220pF in parallel to R602 to have a 10 kHz low-pass on microphone input

More rework is optional:
- Replace RV201 with a 10k (fixed) resistor (adjustment is not necessary)
- Replace R303 with 1k to reduce gain range from 45 to 34dB

README.md

@@ -32,7 +32,7 @@ General requirements:
     * Sum of all translators (XLR/ 6.3 mm balanced jack combo connector)
 
 All inputs (besides the microphones) and outputs should:
-- expect/ deliver a nominal level of +6dBu (german TV broadcast standard, 0 dBu = 0,775 Veff)
+- expect/ deliver a nominal level of +6dBu (~1.5 V RMS/ 4.3 V (p-p), german TV broadcast standard, 0 dBu = 0.775 V RMS)
 - be transformer balanced and galvanically isolated
 
 This results in a signal flow like this: ![](docs/signal-flow.jpg)
@@ -88,8 +88,7 @@ The casing should be a desk console (angled surface), ideally with space at the
 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 distance between top plate and PCB is 9 mm (determined by the potentiometers and on-air switch).
 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.

bom.md

@@ -1,6 +1,6 @@
 # Bill of Materials
 
-List of major components (resistors and ceramic caps without special requirements are left out).
+List of major components (resistors and capacitors are left out).
 
 All prices without VAT unless otherwise noted and without discount for higher order volume.
 
@@ -54,9 +54,6 @@ Electrolytic capacitors should have 25 V voltage rating, unless otherwise specif
 | 1     | NE5534                    | Low-noise bipolar Op-Amp          | PCBA  | JLC C9916
 | 1     | TL072{C,I,HI}DR           | Low-noise FET Op-Amp              | PCBA  | JLC C67473
 | 1     | TPA6111A2                 | Headphone Amp                     | 0.80  | JLC C497472
-| 3     | Cer. Cap. 100nF, lowESR   | SMD 0603                          | PCBA  | JLC C14663
-| 2     | Cer. Cap. 100nF C0G       | SMD 1206                          | PCBA  | JLC C97946
-| 2     | El. Cap. 68uF, lowESR     | SMD CP_Elec_5x5.8                 | PCBA  | JLC C178592
 | 3     | Generic SMD LED           | SMD LED 0805, green               | PCBA  | JLC C2297
 | 4     | Generic SMD LED           | SMD LED 0805, yellow              | PCBA  | JLC C2296
 | 6     | Generic SMD LED           | SMD LED 0805, red                 | PCBA  | JLC C84256

schematics/c3lingo_unit-assembly/c3lingo_unit-assembly.kicad_sch

@@ -6,10 +6,10 @@
 	(paper "A4")
 	(title_block
 		(title "c3lingo interpeter unit")
-		(date "2024-09-08")
-		(rev "v0.3.3")
+		(date "2024-11-16")
+		(rev "v0.3.4")
 		(company "Jannik Beyerstedt (jtbx)")
-		(comment 1 "Prototype 2")
+		(comment 1 "Prototype 2 + Rework")
 	)
 	(lib_symbols
 		(symbol "power:+15V"