Adds a low-pass filter for PWM audio

Improves the audio quality of PWM audio with a simple low-pass filter.
This uses R3-C3 and R4-C4 (one filter for each channel).

kicad/WavetablePi.sch

@@ -308,8 +308,6 @@ Wire Wire Line
 	1200 5800 3700 5800
 Wire Wire Line
 	3600 5600 3600 5900
-Wire Wire Line
-	3600 5900 4200 5900
 $Comp
 L power:GND #PWR0101
 U 1 1 61CC6CE4
@@ -338,8 +336,6 @@ Wire Wire Line
 	3000 4000 3000 3950
 Wire Wire Line
 	3100 3950 3500 3950
-Wire Wire Line
-	3500 3950 3500 4600
 Wire Wire Line
 	4200 3450 3100 3450
 Wire Wire Line
@@ -575,4 +571,89 @@ Wire Wire Line
 	4300 6350 4400 6350
 Wire Wire Line
 	4300 6250 4400 6250
+$Comp
+L Device:R R3
+U 1 1 63CD298C
+P 3500 4400
+F 0 "R3" H 3570 4446 50  0000 L CNN
+F 1 "1K" H 3570 4355 50  0000 L CNN
+F 2 "Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal" V 3430 4400 50  0001 C CNN
+F 3 "~" H 3500 4400 50  0001 C CNN
+	1    3500 4400
+	1    0    0    -1  
+$EndComp
+Wire Wire Line
+	3500 4550 3500 4600
+$Comp
+L Device:C C3
+U 1 1 63CE57FB
+P 3350 4250
+F 0 "C3" V 3098 4250 50  0000 C CNN
+F 1 "100nF" V 3189 4250 50  0000 C CNN
+F 2 "Capacitor_THT:C_Rect_L7.0mm_W2.0mm_P5.00mm" H 3388 4100 50  0001 C CNN
+F 3 "~" H 3350 4250 50  0001 C CNN
+	1    3350 4250
+	0    1    1    0   
+$EndComp
+$Comp
+L power:GND #PWR01
+U 1 1 63CE7538
+P 3200 4250
+F 0 "#PWR01" H 3200 4000 50  0001 C CNN
+F 1 "GND" H 3205 4077 50  0000 C CNN
+F 2 "" H 3200 4250 50  0001 C CNN
+F 3 "" H 3200 4250 50  0001 C CNN
+	1    3200 4250
+	1    0    0    -1  
+$EndComp
+Wire Wire Line
+	3500 3950 3500 4250
+Connection ~ 3500 4250
+$Comp
+L Device:R R4
+U 1 1 63CEAFF8
+P 3750 5900
+F 0 "R4" V 3543 5900 50  0000 C CNN
+F 1 "1K" V 3634 5900 50  0000 C CNN
+F 2 "Resistor_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm_Horizontal" V 3680 5900 50  0001 C CNN
+F 3 "~" H 3750 5900 50  0001 C CNN
+	1    3750 5900
+	0    1    1    0   
+$EndComp
+$Comp
+L Device:C C4
+U 1 1 63CEBC9D
+P 3900 6050
+F 0 "C4" H 4015 6096 50  0000 L CNN
+F 1 "C" H 4015 6005 50  0000 L CNN
+F 2 "Capacitor_THT:C_Rect_L7.0mm_W2.0mm_P5.00mm" H 3938 5900 50  0001 C CNN
+F 3 "~" H 3900 6050 50  0001 C CNN
+	1    3900 6050
+	1    0    0    -1  
+$EndComp
+$Comp
+L power:GND #PWR02
+U 1 1 63CECB7F
+P 3900 6200
+F 0 "#PWR02" H 3900 5950 50  0001 C CNN
+F 1 "GND" H 3905 6027 50  0000 C CNN
+F 2 "" H 3900 6200 50  0001 C CNN
+F 3 "" H 3900 6200 50  0001 C CNN
+	1    3900 6200
+	1    0    0    -1  
+$EndComp
+Wire Wire Line
+	3900 5900 4200 5900
+Connection ~ 3900 5900
+$Comp
+L power:GND #PWR0121
+U 1 1 63D51184
+P 3350 2100
+F 0 "#PWR0121" H 3350 1850 50  0001 C CNN
+F 1 "GND" H 3355 1927 50  0000 C CNN
+F 2 "" H 3350 2100 50  0001 C CNN
+F 3 "" H 3350 2100 50  0001 C CNN
+	1    3350 2100
+	-1   0    0    1   
+$EndComp
 $EndSCHEMATC