Non-Symmetrical Blackmer RMS Detector

This is yet one another Blackmer RMS Detector that purposed to resemble THAT2252 RMS detector chip, it is a non-symmetrical RMS detector. This model would be the most applicable to apply with a Blackmer VCA such as THAT2180 / THAT2181 series in a feed-forward audio signal compressor, as the THAT2252 chip production was discontinued by That Corporation.

The schematic source (in KiCad EDA format) is available for download: https://sourceforge.net/projects/analog-audio-signal-processors/files/BASICS/NON-SYMMETRICAL-DISCRETE-BLACKMER-RMS-DETECTOR.zip/download

This model applies a single direction log amplifier with two-serialized diode-connected transistor as the logarithmic negative feedback. The application of single direction log amplifier is useful to avoid the complicated bidirectional logarithmic amplifier in which requires two-matched transistor to be applied as a bidirectional logarithmic negative feedback. As it is applying a single direction log amplifier, the audio input signal must be converted into the full wave peak signal to get the absolute values, and for this purpose an absolute-value amplifier is applied to supply the full-wave rectified audio signal into the log amplifier.

This model is also designed to be adjustable. It has a low limit adjustment and a level adjustment. The low limit adjustment as RT1 is used to set the lowest output voltage limit in which usually in approximate to -230 mV. The level adjustment adjustment as RT2 is used to set the output level to have the output voltage levels as accurate as possible to the output voltage levels of THAT2252.

Notes About Transistor Type Selection

It is important to note that this discrete Blackmer RMS detector model can only applied specific transistor type for Q1 ~ Q4, it is 2SA1015 or another choise is BC556.