Differences

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--- discrete_vca [2012/04/04 14:44] – created wadmin
+++ discrete_vca [2018/08/26 09:36] (current) – external edit 127.0.0.1
@@ Line 3: / Line 3: @@
 In my quest to design effective audio applications, I've come accross the issue of building a VCA with 5Vpp audio input while using only easy to find components (i.e. avoiding the use of any OTA).
-==== Concept ====
+=== Concept ===
 The VCA schematic is given herebelow:
@@ Line 9: / Line 9: @@
 {{ :vca_schematic.jpg? |}}
-The concept is to feed the input into a 10k + 100R network, operating as a voltage divider, and to propagate the middle point of this dividing bridge to an inverter amplifier through a transistor operated in its variable resistor part.
+The concept is reveresed to creating a control voltage attenuator base on using a transistor in its variable resistor zone. The input signal shall be provided with the maximum output level needed.
+=== how it works ===
+The idea is to feed the input into a 10k + 100R network, operating as a voltage divider, and to propagate the middle point of this dividing bridge to an inverter amplifier through a transistor operated in its variable resistor part.
   * VG1 is the audio signal source
@@ Line 18: / Line 22: @@
 R1 may also be changed if the enveloppe voltage is lower or higher than 5V.
-==== Operation ====
+=== Operation ===
 The behaviour of the VCA is shown herebelow:
 {{ :vca_amplitude.jpg?800 |}}
+The signal is slightly distorted, however the global response is transient friendly.
   * 5V triangle signal is fed into the VCA
   * Enveloppe voltage range is 5V.
-==== Pros & Cons ====
+=== Pros & Cons ===
 Pros :
-- Very low component count
+  * Very low component count
-- Large dynamic range
+  * Large dynamic range
-- Linear response
+  * Linear response
+  * Input signal can have any DC component
 Cons :
-- Sensitive to noise
+  * Sensitive to noise
-- Very sensitive to ground feedback noise
+  * Very sensitive to ground feedback noise
-- Linear response :)
+  * Slight distorsion on signal
+  * Attenuator operation, not suitable for low voltage sources
+  * Linear response :)
+==== Conclusion ====
+When building an analog synth, and looking for a simple way to control amplitude, this schematic provides a handy way to generate signal enveloppe on a 5Vpeak audio signal.
+Great care has to be taken on analog design, especially when mixing with digital components. using a dedicated analog ground for this schematic is recommended.