Common Discrete Blackmer RMS Detector

This is a discrete circuit application of Blackmer RMS Detector in which resembles the chip of THAT2252 and/or the RMS detector block of THAT4305. This circuit applies the III Blackmer RMS model and will be working "almost" exactly the same to THAT2252 or THAT4305 RMS Detector block. The main purpose of this circuit design is for replacing the THAT2252 in application with Blackmer VCA such THAT2180/THAT2181 series, so the related application design can be more customizable with discrete components.

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

The circuit contains of 3 blocks, as a bipolar log amplifier, gain amplifier an inverting gain amplifier (consists of a non-inverting gain amplifier and a nonlinear-filter), and the nonlinear filter. The gain amplifier or the non-inverting gain amplifier is programmed to have AV = 2, to amplify the output of log amplifier into 2x, so each output of -X2 and +X2 can be acceptable by the nonlinear filter via Q3 and Q4.

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.