Description

QMX LPF Bypass Mod

Use pin 59 (LPF_3, an unused LPF output) of the STM32 on your QRP Labs QMX to control bypassing of the radio’s built-in low-pass filters (LPFs). This enables seamless operation with external LPFs while preserving the QMX’s internal LPFs for normal operation. There’s no need to permanently bypass, remove, or modify the internal LPFs.

This modification allows you to continue using your QMX normally while expanding its operating range to 160m–6m when used with the appropriate external LPFs.

Installation & Compatibility

The mod fits entirely within the standard QMX enclosure. It installs above the 3.5mm PTT jack and USB-C port—the same location recommended by N6MTB for the Touch Key Mod.

Because they occupy the same space, this modification is not compatible with the Touch Key Mod.

This modification is specifically designed for the QMX and is rated for operation up to 5W.

Kit & Assembly Requirements

At this time, this modification is available only as a kit.

Assembly requires soldering several small SMD components, including:

• 0805
• 1206
• SOT-23-3
• One pad of a 0603 capacitor C512
• A direct connection to a pin on the STM32

The STM32 pins are extremely small, making this installation significantly more challenging than the soldering required to assemble the QMX itself. This kit is not recommended for beginners. You should be comfortable with fine-pitch SMD soldering and working with small components before attempting this modification.

Soldering to the STM32 may not be needed as the PTT_P pin can be used as a substitute. Please note that this would prevent the operation of an external amplifier that requires the use of this pin (the QMX PA 50W for example). In addition, this obviously can only be used by one mod. If you already use or plan to use my other mods that would normally require the use of the unused LPF pins on the STM32, you would not be able to without performing the more difficult soldering for those mods. In your band config you would then enable PTT Positive on BOTH transmit and receive. This will need to be done on each band that you plan to use an external LPF.

Important Information

• Kit is sold unassembled. Assembly is required.
• Component layout is provided in the table.
• Documentation is currently being improved and expanded.
• Components may vary from images due to availability.
• No returns on assembled kits.
• Shipping is limited to the lower 48 United States.
• Contains lead.

Designed and sold by AD8C

The Fine Print
Use at Your Own Risk: This is an independent project and is provided as-is. This kit is intended for educational, evaluation, or hobbyist assembly. By purchasing and assembling this board, you accept all risks. The creator/seller accepts no liability from the use or misuse of this product.

Trademark Notice: QMX is a registered trademark of QRP Labs. This project is entirely independent and is not affiliated with, authorized, or endorsed by QRP Labs.