Modifying Time Constants
In Synthesizer Modules
Changing time constants in synthesizer modules and audio processing equipment seems to be the first place that most people begin in customizing their equipment. Modifying the time varying circuitry can change the speed of the 2720-9 Glide module, vary the range on Envelope Generators, change clock speeds in Sequencers and Synthespin units.
Generally, in any time varying circuit there will be an R-C (resistance and capacitance) network whose time constant is used for timing the operation of the overall circuit. For our purposes, it will suffice to say that a smaller resistance and/or capacitance will make a smaller time constant. This will result in faster changes or higher frequencies. Logically, a larger resistance and/or capacitance will make a longer time constant, slower changes or lower frequencies.
A circuit design analysis is published in PAIA instruction manuals and along with information for studying or repairing the circuit, timing circuits are usually pointed out. Once the timing elements are found a decision will need to be made as to how the circuit is to be changed.
Let's take a look at a fairly complex situation as an example. In the 4740 Envelope Generator there are three time varying parameters: Attack, Decay and Release. Studying the schematic shows that there is a common timing capacitance (C3) for all of these functions. Each parameter uses a separate resistance which is comprised of a variable resistance (to change the timing for that function) and a fixed resistor which is used as a current limiter to "set" the minimum time constant when the variable resistance is 0 ohms. In the 4740, the Attack resistance is R22 and R13, Decay is R23 and R19 and Release is R25 and R7.
Now comes the decision of how the timing is to be changed. If you want all parameters changed proportionally, the easiest way would be to change the timing capacitor (C3) which is common to all three timing circuits. To make the times longer increase the value of C3 and vice versa. This change will not only change the the maximum settings but the minimum settings as well. If, on the other hand, only one function needs to be changed, it would be best to change only the resistance for that function. If you want the capability of a longer final release, replacing the release potentiometer (R25) with a larger value will do the trick. Changing the potentiometer value for this function will allow the same "minimum" timing which you had before because the current limiter (R7) remains the same. Only the maximum resistance has been changed making longer release times available. Likewise, the attack or Decay potentiometers can be changed for longer timing.
Keep in mind, whenever you want to change a circuit's design, that you should always start with correctly operating original circuitry. Also, be prepared for something to go wrong. Many times, parts in a circuit will serve several different purposes. When making extreme part value changes there is a chance that there will be a limit to the amount of change which can be made. For example; in the previously discussed 4740 modifications, if one of the timing potentiometers is replaced with a large value (2.2 meg or 5 meg) there is a chance that leakage current paths may override the timing circuit and render the 4740 permanently on or off, or make the entire circuit operate in an erratic maimer. If this happens, try a less drastic change in value until the circuit resumes normal operation. In timing circuitry it is generally a better choice to vary the timing capacitance rather than the resistances.
By: Marvin Jones, Director of Technical Service, PAIA
Feature by Marvin Jones
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