If you have a module that accepts only positive input voltages, but your source supplies an AC voltage, a bridge rectifier is not always a good solution as it will seriously distort you input signal. A better solution might be to raise the AC input signal to a certain DC level and maybe attenuate or amplify it a bit. I built a module for this called Offset-O-Matic.
See also this thread:
Such a module is a universal utility.
It can be used as
- an attenuator with variable gain
- an inverter with variable gain
- a buffer circuit or impedance converter
- an adder allowing you to add or subtract a DC offset to a signal
- combinations of the above
I invariably use it in combination with an AR, ADSR or envelope follower. These devices produce a timed sequence of voltages but they often are not at the right DC or AC level to drive the next module.
So I choose the timing using the AR’s, ADSR’s or envelope follower’s controls and determine the gain (or attenuation) and DC offset using Offset-O-Matic and connect the outout of that to the next module.
Simple things like controlling a CV-input that is based on a VACTROL get much more versatile. The LED in the VACTROL will not light up unless you input a certain ‘rather high’ voltage of maybe 2.5 Volts and only then the variation of the signal will influence the module it is in. So if you have a low input signal the VACTROL will probably not allow you to control the module it is in.
If however you use a circuit like Offset-O-Matic, you can start giving the vactrol a DC offset of e.g. 2,5 Volts so that the LED starts giving some light and then you will see that the low level signal will have an effect. To make this effect bigger or inverse the effect, Offset-O-Matic has a gain from -2 to 2 and can add whatever voltage between -5 V and 5 V. For people familiar with tube or transistor circuits, what Offset-O-Matic in essence is used for in this example is biasing the vactrol.
Note: in the schematic U2.1 and U2.2 were added to get some visual feedback through some LEDs. That part of the circuit is not essential, but to me it makes it much more clear what the circuit is doing to the input signal which makes it more easy to understand how to control the module it is plugged into. The leds show what the attenuation or gain does to the signal and whether the adder moves the signal to a positive or negative DC level.