Humble Audio Quad Operator

This is a fantastic, if large, module.

It has a modulation matrix that is similar to Native Instruments' FM8, except in physical form!

Oscillators can be used as LFO's. I've generally used this as 2x 2 OP synths. Lots of patch points and permutations available.

Everything works as expected.In its original box which is a bit tatty.Stickers etc.
Will be well packaged.

This is what the manufacturers say.
4 Variable Waveshape Operators for Linear FM Synthesis
Implementing frequency modulation synthesis completely from scratch in firmware, the Quad Operator is able to provide a modern take on frequency modulation synthesis.

One knob per function design for true ease of use

Any FM algorithm is possible via modulation matrix

Virtual VCA per operator for both animating modulation and output volume control Includes a dedicated “AR FM” input with its own modulation sends, designed to bring an audio rate signal in from your rack for use as a modulator.

Try processing a phase locked operator, and feeding it back in!

Each operator can also be unlocked then tuned and patched independently, making the module a great general purpose oscillator bank

Convincing digital emulation of classic analog waveshapes—sine, triangle, square, and saw

An LFO mode, enabled the generation of phase locked, complex modulation signals