HX3 Extension Board Installation

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Extension board connected to HX3 mainboard


The extension board for HX3 mk3/mk4 mainboards adds two additional audio outputs, Leslie(R) control and preamplifier for 9- and 11-pin Leslies, headphone jack, USB interface for parameter control to your HX3 mainboard. It also adds a MIDI OUT in case HX3 mk3 board is jumpered for two MIDI inputs.

The extension board connects to HX3 by 26-pin header PL33 and flat ribbon cable. Length of cable must not exceed 15 cm (6") as high-frequency digital signals are transmitted. For additional audio outputs, the extension board has a secondary A/D converter. Outputs may be configured by MenuPanel or HX3 Remote to any HX3 audio source, but we recommend using the two channels for plain audio and bass pedal signal as they are routed to the Leslie connector. Use HX3 Remote Application or MenuPanel to route four signals appropriate to your needs.

HX3 extension board brings its own power supply for Leslie preamp (DC/DC converter). Normally, PL30 may be left open. If noise appears on Leslie audio, connect PL30 to common +5V supply of HX3 (green header).

Potentiometer Inserts

A 16-pin header PL32 provides send/return of Leslie and headphone signals to insert potentiometers (any 10k to 47k audio taper pot will do) or effects like 3-band equalizer. If no pots or effects are inserted, pins 1-2, 3-4, 5-6 and 7-8 must be bridged by jumpers. The connector also provides symmetrical 12V power supply for audio preamp circuits (see schematics PDFs).

PL32 Connection to insert pots
Pin 1 Leslie Rotary signal left to pot
Pin 2 Headphone signal left from pot wiper
Pin 3 Leslie Bass/Stationary signal to pot
Pin 4 Headphone signal right from pot wiper
Pin 5 Audio Left (Simulation) signal to Headphone pot
Pin 6 Leslie Rotary signal from pot wiper
Pin 7 Audio Right (Simulation) signal to Headphone pot
Pin 8 Leslie Bass/Stationary signal from pot wiper
Pin 9,10 Audio Ground for pots/inserts
Pin 11,12 +12V optional opamp supply
Pin 13,14 +5V optional digital supply
Pin 15,16 -12V optional opamp supply

For rotary simulation on headphone jack and dry signal to 11-pin connector, use a crossed connection by short jumper cables: pin 1 to 6, pin 3 to 8, pin 5 to 2 and pin 7 to 4. Refer to HX3 drawbar expander user manual for a description of audio configuration settings.

Leslie Connection

PL31 carries complete 11-pin Leslie(R) control signals (power, slow/fast, audio signals organ and bass pedals). It connects to 11-pin Amphenol jack one-on-one, pins 12 to 14 of PL31 remain open. For connection of 9-pin Leslies, mind the different pinout. 6-pin Leslies (122/147) need an additional interface/relay circuit; please ask an experienced Hammond tech.

Trim pots R8 and R9 on HX3 extension board may be adjusted to required Leslie output level (set to mid position as default).

PL31 Connection to 11 pin Leslie
Pin 1 Rotary audio channel
Pin 2 Stationary audio channel
Pin 4 Audio ground
Pin 5 Common ground for control signals
Pin 6 Power ON
Pin 7 Fast
Pin 9 Slow
Pin 11 Leslie +30V supply


USB interfaces to HX3 controller for setup via HX3 Remote or HX3 Flash only. Connect a USB A/B cable to your PC and follow instructions on HX3 Remote or HX3 Flash page. USB jack is no USB MIDI interface!

MIDI OUT on extension board is always an output independant of PL10 jumper setting on HX3 mainboard.

Output Level for 147 type Leslies

Output level of leslie output pins is not sufficient for old tube Leslies. Bridge R1 and R2 on HX3 Extension board with two additional 22kOhms SMD resistors (size 0805) to obtain doubled output level. Please note that 147 Leslies need a relais for SLOW/FAST switching. Relais coil may be driven from FAST motor control output of extension board.