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).

Versatile DC DC Converter Wiring diagram Schematic

Here is a versatile power coupler that connects a device to 5V-19V DC generated from AC mains by a power adaptor. Power adaptors come in different voltage outputs like 5V (for mobile phones), 12V (for external hard drives) and 19V (for laptops). Sometimes the power adaptor may have a voltage rating higher than the required voltage. With the converter schema given here, the adaptor can be used to power any device at a lower voltage. 

For instance, by using a 19V laptop adaptor, you can power a TTL schema at 5V. There can also be other instances when one needs a 3V or 6V supply. All these and many other intermediate voltages are easily possible with this versatile converter schema when used together with any off-hand power adaptor.

Circuit diagram :

Versatile DC-DC Converter Circuit diagram

Fig. 1 shows the schema of the DC-DC converter. Smooth reduction in the voltage is achieved using the LM317 regulator IC. The complete unit can fit inside a piece of a glue stick tube.

Adjusting variable resistor VR1 gives the desired output voltage. The output voltage is read using a 0-100µA ammeter, whose series resistance R* is chosen such that the maximum desired voltage could be covered. For instance, if full-scale deflection (FSD) current of the meter is 100 µA and you need an output voltage of up to 15V, then R* = 15/0.0001 = 150 kΩ. The desired value of R* is obtained by using 150-kilo-ohm preset VR2. 

Use of a variable resistor which also has an on/off switch like the one in old radios is recommended. It will cut off the coupler from the input power supply without having to accomodate an additional switch. Also, use a heat-sink with LM317 to handle the desired amount of power.

Assemble the schema on a small general-purpose PCB and enclose in a suitable case. Fit the entire PCB inside a glue stick tube as shown in Fig. 2. Affix the female and male connectors on the opposite ends and place the ammeter in between the stick tube. You can directly read the output voltage on the ammeter after due calibration.

Note. You can use a suitable VU meter instead of 0-100µA ammeter and calibrate accordingly.

Source By Streampowers