Danger
Danger
The lowpass gate filter topology is a Sallen and Key filter where the two resistors are replaced with a vactrol. A vactrol is a device with an LDR and a LED. The brightness of the LED will change the resistance of the LDR. This gives a variable cutoff frequency. The lowest frequency has to be in the inaudible range. This will make the LPG quiet if there is no control voltage applied.
Figure 1: Block Diagram of the Sallen Key Low-Pass-Filter
The cutoff-frequency for this values is:
$$ \begin{array}{c} f_c=\frac{1}{2\pi \sqrt{R_1 R_2 C_1 C_2}}=\frac{1}{2\pi \sqrt{10M\Omega * 10M\Omega * 0.47nF * 0.47nF}}=7.34 Hz \end{array} $$The Vactrol resistance depends very much on the type and the ambient light. In the Xvive 5C3 datasheet the off resistance is stated with 10MΩ. But on the bench, it will raise to 1MΩ and higher when the desktop light is turned on. The off resistance is important, especially when you want to build the vactrol yourself. The LDR from Adafruit, as an example, has an off resistance of 500kΩ. This would result in a cutoff frequency of 146.8Hz. This is already in the audible range and the LPG would never turn off.
| # | reference | value | description | |
| 2 | C1 C2 | 22u | Aluminium Electrolytic Capacitors (50V, D=6.3 mm, LS=2.5 mm) | |
| 1 | C3 | 1u | Aluminium Electrolytic Capacitors (50V, D=6.3 mm, LS=2.5 mm) | |
| 1 | C4 | 220p | Film Capacitors Polypropylene (L=7.2 mm W=4.5 mm, P=5 mm ) | |
| 1 | C5 | 4.7n | Film Capacitors Polypropylene (L=7.2 mm W=5.5 mm, P=5 mm ) | |
| 1 | C6 | 1n | Film Capacitors Polypropylene (L=7.2 mm W=4.5 mm, P=5 mm ) | |
| 6 | C7 C8 C10 C11 C13 C14 | 0.1u | Multilayer Ceramic Capacitors MLCC (50V, L=4 mm, W=2.5 mm, LS=2.5 mm) | |
| 1 | C12 | 2.2n | Film Capacitors Polypropylene (L=7.2 mm W=4.5 mm, P=5 mm ) | |
| 1 | D1 | 3.9V | Diode_THT:D_DO-35_SOD27_P7.62mm_Horizontal | |
| 1 | J1 | 10 Pos | Pin Header IDC (2.54mm) | |
| 1 | J2 | 01x06 Female | Board to Board Connectors (2.54 mm) | |
| 1 | J3 | 01x10 Female | Board to Board Connectors (2.54 mm) | |
| 2 | R1 R2 | 10 | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 9 | R3 R4 R7 R11 R12 R13 R14 R17 R20 | 100k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 2 | R5 R22 | 15k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | R6 | 150k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | R8 | 4.7M | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 5 | R9 R18 R23 R25 R26 | 10k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 2 | R15 R21 | 470 | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | R16 | 470k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | R19 | 33k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | RV5 | 20k | Trimmer Resistor, Bourns 3339P | |
| 3 | U1 U2 U3 | TL072 | Package_DIP:DIP-8_W7.62mm_Socket | |
| 2 | U4 U5 | VTL5C | OptoDevice:PerkinElmer_VTL5C |
| # | reference | value | description | |
| 4 | D1 D5 D9 D10 | 1N4148W | Diode_SMD:D_SOD-123 | |
| 2 | D3 D7 | LED (Yellow) | LED 3mm low current | |
| 1 | D11 | LED (Green) | LED 3mm low current | |
| 1 | D12 | LED (Red) | LED 3mm low current | |
| 1 | J1 | CV1 | elektrophon:Jack_3.5mm_WQP-PJ398SM_Vertical | |
| 1 | J2 | CV2 | elektrophon:Jack_3.5mm_WQP-PJ398SM_Vertical | |
| 1 | J3 | IN | elektrophon:Jack_3.5mm_WQP-PJ398SM_Vertical | |
| 1 | J4 | 01x10 Male | Board to Board Connectors (2.54 mm) | |
| 1 | J5 | OUT | elektrophon:Jack_3.5mm_WQP-PJ398SM_Vertical | |
| 1 | J6 | 01x06 Male | Board to Board Connectors (2.54 mm) | |
| 4 | Q1 Q3 Q5 Q6 | MMBT3904 | Package_TO_SOT_SMD:SOT-23 | |
| 1 | R1 | 22k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 1 | R2 | 1k | Metal Film Resistors - Through Hole (L=3.6 mm, D=1.6 mm, 1%) | |
| 4 | R3 R7 R11 R12 | 100k | Thick Film Resistors - SMD (0805) | |
| 4 | R5 R9 R13 R14 | 5k | Thick Film Resistors - SMD (0805) | |
| 3 | RV1 RV3 RV4 | 100k | Alpha 9 mm Pots – Vertical | |
| 1 | RV2 | 10k | Alpha 9 mm Pots – Vertical | |
| 1 | SW1 | SW_SPDT | elektrophon:SPDT_KIT | |
| 1 | SW2 | SWITCH_3PDT | Toggle Switches 3PDT ON-OFF-ON |
the input jacks are wired to 5 volts when nothing is connected.
all channels are mixed to the out jack. when something is connected to the channel out, this channel is removed from the overall mix.