OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

Please refer to the following GroupDIY thread for the BOM and more information about this project:

https://groupdiy.com/threads/open-source-diy-mic-project-ors-87-stripped-down-u87.86814/

Additional and Notes from the PCB Author:

• C7 Capacitor Selection: Only one C7 capacitor is required. The PCB includes various layout options to accommodate different capacitor sizes and types, ranging from large 1μF film capacitors to small electrolytic or tantalum capacitors. If using a polarized capacitor, ensure correct alignment of the terminals (+/-).
• C5X and C6X Capacitor Layouts: Multiple capacitor layouts are provided for C5X and C6X, allowing the use of one or two capacitors.
• C5X Low-Frequency Filtering Adjustment: C5X is used for low-frequency (LF) filtering. Increasing the capacitance value decreases the filtering effect. The U87 AI circuit uses a 47nF capacitor for C5A. If desired, an additional capacitor (C5B) can be added to further reduce LF filtering. Valid options for C5B include 22nF, 33nF, or 47nF, assuming a 33nF capacitor is used for C5A.
• C6X High-Frequency Filtering Adjustment: C6X is used for high-frequency (HF) filtering. Increasing the capacitance value increases the filtering effect. The U87 AI circuit uses a 160pF capacitor for C6A. If desired, an additional capacitor (C6B) can be added to enhance HF filtering. Valid options for C6B include 50pF, 100pF, or 120pF, assuming a 160pF capacitor is used for C6A.
• R* Resistor Installation*: The R* resistor is used after determining the correct biasing value for the trimpot. First, use the trimpot to bias the JFET and measure the trimpot resistance. Then, replace the trimpot with the appropriate R* resistor value.
• Jumper Options (J1, J2): The jumper options J1 and J2 can be used if the transformer has a center tap for the output terminals. In this case, connect the transformer's center tap terminal to TXC, place a jumper wire from J1 to J2, and omit R18 and R19 (2k2) as they will be unnecessary in this configuration.

Kind regards!

HL