Thanks John for uploading the compander.
I'll go find that broken link in the OP and fix it.
I got out my crayons and sketched the schematics of the various test circuits.
The first schematic is the 2252-style clone sent to me by Gary Hebert.
The 2252, unlike the original 303, performs absolute value ahead of the RMS stage.
The following schematic is the RMS portion after the absolute value rectifier.
A THAT2252-style RMS detector made from op amps and a THAT300 transistor array.
The input is a current.
Q1 and Q2 provide a 2Xlog output for negative input currents.
D1 is a reverse polarity clamp.
Q3 Rt and Ct are the log-domain filter.
Q4, op amp B and the 1MΩ current source provide level shift.
All transistors should be matched and in thermal equilibrium so a THAT300 array is used.
Rt is not true current source but for practical purposes can be thought as one since the delta-V at the emitter of Q3 is quite small.
Both resistors should be equal value and set the timing current which is √(It1*It2).
The Absolute Value Circuit
The absolute value circuit ahead of the RMS detector can be done a number of ways and can be either voltage or current output.
Since the 2X logging stage uses a current input, a current output is preferred.
The first absolute value stage is a conventional fullwave rectifier:
A Diode-based fullwave rectifier with current output.
Halfwave rectification for positive inputs is performed by D2.
D1 clamps the op amp for negative inputs.
The sum of the currents at the output represent the absolute value of the input voltage.
The negative-going current output of the absolute value can connect directly to the current input of the RMS stage.
The second example shows current rectification as it was originally done in the THAT2252 and Loftec TS-1.
The actual circuit is the TS-1's rectifier. The 2252 is almost exactly the same.
A current rectifier based on the Loftec TS-1 and THAT2252.
I've redrawn the TS-1's rectifier in a "reinterpretation" that I can more easily follow.
I shifted the focus of the current mirror stage, Q3 and Q4 so that Q3 looks more like a rectifier.
Q1, a common base stage provides additional non-inverting voltage gain as a "helper" to improve bandwidth of op amp A.
Q1's collector load is 15K+470R.
The base is biased to -7.5V.
The emitter resistor is 470R making the added inside the loop gain about 30X.
The op amp inputs are held at -3.75V by virtue of the voltage divider formed by the 30K and 10K resistors.
This permits Q2 and Q4 to sink current for loads held at or near ground.
Negative inputs are rectified by the base-emitter junction of Q2.
This sets up a nearly-identical collector current in Q2 which provides the negative polarity rectified output.
The collector current for Q2 (when loaded by the following stage) flows from the output to the inverting input held at -3.75V.
The 470Ω base resistor pre-biases Q2 to reduce rectification deadband.
For positive inputs, the output of op amp A and the collector of Q1 swing more negative.
The emitter base junction of Q3, a diode-connected transistor, provides rectification for positive inputs.
The Q3/Q4 stage is where I took "artistic liberty" in the schematic layout to emphasize Q3's base-emitter junction's roll as rectifier.
Q3 and Q4 are a current mirror with Q3 also a rectifier diode.
As the current in Q3's base emitter junction increases a nearly-identical mirrored current is setup in the collector of Q4.
The action of the current mirror serves to invert polarity for positive inputs.
When the collector currents of Q2 and Q4 are summed, the final result provides the absolute value.
The output of the current rectifier is a current that can be directly connected to the RMS stage.
Q3 and Q4 require matched devices in thermal equilibrium.
For a stereo detector, a single THAT300 could be used and share between channels.
A low-cost dual NPN transistor pair such as the NST45011 could also be used.
Advantages and Disadvantages.
The diode-base absolute value circuit has about 10 dB or so less dynamic range but does not require matched transistors in the rectifier.
The current rectifier has more dynamic range but requires matched devices.
I may have missed a few subtleties but really wanted to get this posted before this session times out. LOL.
A Discussion About True Power Summing for Stereo Compressors https://www.proaudiodesignforum.com/for ... p?f=6&t=61
Compressor Attack Release Signature Comparisons https://www.proaudiodesignforum.com/for ... ?f=6&t=281