Changing between AC and DC coupling

There are two types of coupling between electronic circuits: AC coupling (alternate current) DC (direct current) AC coupling means that only the AC parts of the signal will pass. For this normally a capacitor is used that connects the two circuits. The minimum value of the capacitor depends upon the lowest frequency (f) that has to be transmitted and the input/output impedance (R) of the two circuits. The approximate formula for the minimum capacity is C ~ 1/(R*f) with f = lowest frequency, R = in/output resistance Example: minimum frequency = 50Hz, in/output resistance = 10kOhm -> C ~ 2 uF (u = micro = 1/1000000). A usual value would be 2.2uF in this example. AC coupling is normally used for audio signals. For audio signals AC coupling has the advantage that unwanted DC shares in the signal are removed. For some AC processing circuits (e.g. amplifiers, filters) DC voltages are not allowed in the input signal. Therefore very often a capacitor can be found in the input stage of such circuits. DC coupling means that both DC and AC parts of a signal are transmitted. For control voltages (normally) only DC coupling can be used as even fixed voltages (e.g. coming from a manual control) have to be transmitted. In a module patch each A-100 module can be treated as an electronic circuit that is connected to another one. Consequently one has to take into consideration the type of coupling (AC or DC) between modules as the strict differentiation between AC and DC applications os softened for some A-100 modules. E.g. a VCA can be used to process audio signals (i.e. normally AC coupled signals) as well as slowly changing CV voltages (e.g. envelope or modulation amount). Therefore one needs to know if a VCA used is AC or DC coupled. Another example is a divider (e.g. A-115 or A-163) as even these module can be used to process audio or (slow) clock/gate signals.

Luckily it is not very complicated to switch between AC and DC coupling. All one has to do is to bride (i.e. short circuit) the capacitor in case of an AC coupled in/output. The left picture shows how the switch is connected in parallel to the AC coupling capacitor (the broken line resistor symbol represents the load to GND that is always available in each circuit as reference to GND). If AC coupling is required for a DC coupled in/output simply a capacitor has to be added. From the schematics it can be seen if an in/output is AC or DC coupled. We will add this information also to the users manual for modules that may be used for both types of coupling. For some circuits resp. modules changing from AC to DC coupling is not possible. E.g. the "old" VCAs A-130 and A-131 (those with CEM3381 or CEM3382) are AC coupled as the special CEM circuits cannot be DC coupled because of the internal negative reference voltage. The "new" VCAs A-130 and A-131 (those with CA3080) are DC coupled and can be used to process CV signals too. A list with the type of coupling for all modules in question will follow soon. For most of the modules the question about the type of coupling does not arise. E.g. all filters are AC coupled and all CV generating and processing modules (e.g. ADSR, LFO, slew limiter, Theremin, Ribbon controller, random voltage) are DC coupled. But for other modules the type of coupling is not obvious (e.g. VCA, divider, waveshaper).