THAT2252 RMS Detector Replacement Using A THAT300 Array

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mediatechnology
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

A picture is worth a thousand words...

For future reference I found this linear domain versus log domain filter comparison in Fred Floru's AES paper.

Image
Linear vs. log-domain filter

The diodes in the final stage of the RMS detector Q3/4 in my drawing or D3 in the above look like rectifiers or peak detectors at first glance to some but they're not.
D3, with it's exponential response, is the log-domain "analog" (pun intended) of a resistor.
D4/OA2 level-shifts.
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mediatechnology
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

Here's a replacement for the THAT2252 or THAT4301 RMS detector using a CA3046/LM3046/AS3046 5 transistor array.

Image

The THAT2252 has an output that is elevated one Vbe when the input reference current equals the timing current.
The THAT4301 and rest of the 43XX-series use detectors which have a 0V output when the input reference current equals the timing current.

The 4301 "No Vbe Up" detector is better-suited for combined True Power Sum or Quadrature summed detectors because, when they're operating at their nominal level with 0V output, have lower errors due to temperature mis-matching between CA3046 arrays.

J1 selects either mode and can be eliminated depending on the RMS IC being emulated. When a 2252 is emulated the 20K on the bottom op amp can be eliminated and the 10K from the top op amp inverting input can be grounded. Pin 13, which is the CA3046 substrate, should be lightly pulled to the negative supply if the Vbe ref op amp is not used.

The timing cap jumper eases symmetry adjustment but can be eliminated and linked directly to the timing cap.

The AS3046 arrays are available on eBay or from Cabintech: https://cabintechglobal.com/semi
AS3046 Data Sheet: https://cabintechglobal.com/pdf/ALFA_RPAR_AS3046.pdf
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