Rob Hordijk Active Matrix

The 4U MU or 5U MOTM Rob Hordijk Active Matrix module is a fully buffered eight by eight matrix where any one of eight input signals can be added to any one of eight outputs. By using ¼-inch tip-ring-sleeve insert jacks for the matrix nodes (equal to stereo jacks) a whole range of applications ecome possible. First the column input signal is buffered and then routed to the tips of the nodes in that column. The ring signals of the nodes are basically summing inputs and summed to the final output signals at the ends of the rows. By connecting a stereo jack where the tip and the ring are connected directly together, a connection with unity gain is made from a column input to a row output. If the tip-ring connection in the jack goes through a resistor an additional attenuation can be accomplished. E.g. a 30k resistor will attenuate by 6dB and a 91k resistor by 12dB.

When a stereo audio cable is soldered to a jack plug and on the other side of the cable a potentiometer is attached the potentiometer will act like a pot on the node, enabling to set the mix level by the pot. Basically each node is an insert, just like the inserts on a mixing desk. And can thus be used in the same way. So, using a jack with a pot means to ‘insert’ the pot into the signal path. This means that you can insert any other outside world device in the signal path by using an insert cable with a stereo jack on one side and two mono jacks on the other, provided signal levels match of course (e.g. 5V pp oscillator output signals will severely overload line level inputs on e.g. a digital effects rack or the guitar input of a stompbox). One could also connect a resistive sensor like a light dependent resistor (LDR) to a jack and make the node light sensitive.

Each column also acts like a multiple. When a mono jack is connected into a node it will pick up the column input signal from the tip. But the ring input is now short circuited to the ground through the sleeve of the mono jack and will so disable any input from this particular node to the row output. This will not interfere with any other nodes in the same column or row, because of the full buffering of both the column inputs and row outputs. So, any node that is not used to route a signal to a row output can be used as a multiple output. Meaning that the matrix is also eight multiples with one buffered input and eight buffered outputs on each multiple.

Matrices with bigger sizes can be built on demand. Input columns come in multiples of eight and any number of output rows are possible.

Mini Bay
The 2U wide MiniBay is a smaller version of the Active Matrix. It has a four input by six output fully buffered matrix plus two passive multiples with five jacks each. The matrix section works exactly like the 8x8 Active Matrix.