THAT2252 RMS Detector Replacement Using A THAT300 Array

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mediatechnology
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THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Mon Feb 06, 2017 7:45 am

THAT2252 RMS Detector Replacement Using A THAT300 Transistor Array

I've been kicking around some ideas for a compressor/limiter project and needed a replacement circuit for the discontinued THAT2252 RMS detector IC.

Although the THAT2252 is still widely available (and I have some available for sale) I didn't want to use that stash for a new project.

The THAT4301 Analog Engine® has an RMS detector, VCA and op amps which can be used to replace the 2252.
Its an elegant solution when all the pieces are needed.
The 4301 could be used, but a "discrete" RMS detector, made from a transistor array, would have a lower bill of materials.

The THAT4316 in surface mount has an RMS detector as well.
This is a DIY project so that rules out using surface mount.

There are lots of prior examples of log detectors available from Blackmer, Buff and others.

Most examples used 5 transistor arrays such as the CA3046 and CA3083.
The TI LM3046 is only available now in surface mount leaving the selection of five transistor arrays slim.
The THAT300-series, available in DIP, have four transistors per array.

At one time MicMix was going to become a dBx licensee and we obtained information on how to build a dBx 303.
I still have the 1970's-era hand-drawn copy we obtained from dBx.
The simplified schematic is shown below:

Image
dBx 303 RMS Detector Section Simplified Schematic

Note: The actual dBx 303A Schematic including the discrete transistor VCA can be found here: viewtopic.php?f=12&t=860

The RMS detector begins at OA2 and uses five matched diode-connected transistors.

Is it really necessary to have all five transistors matched?
Can we use a four transistor array such as the THAT300?

Matching and thermal tracking are critical to maintain symmetry of the RMS detector.

A close examination of the circuit shows that for positive inputs, which produce a negative output at OA2, there is only one logging transistor. (QA2 pins 9,10, and 11.)
In order to obtain 2X log for positive inputs OA3 is used.
2X log for negative inputs is obtained by using two forward-biased junctions. (QA2 pins 1, 16, and 15 along with QA2 pins 13, 14 and 12.)
We only need one junction to obtain the log.

If an additional non-inverting gain of two stage were added for negative input polarity one matched transistor can be eliminated. (QA2 pins 1, 16 and 15.)

Image
THAT2252 RMS Detector Replacement Using a THAT300 Transistor Array. Updated 2/07/17.

By sampling the log using one junction (QA2 pins 12, 13 and 14) and multiplying the result by 2 one transistor can become a 1N4148 diode.
Only 4 matched transistors are required.
(Looking at it from a 1970's perspective by using an "extra" diode-connected transistor we can eliminate an op amp. Now, op amps are cheaper than matched transistors.)

(Edit 2/07/17: I removed that diode to reduce the output swing requirement of the op amp. The figure has been updated. Now it looks very similar to the log Buff Dynamite detector: viewtopic.php?f=6&t=111#p6557 except for the X2 required for RMS.)

The circuit above does not include an output level-shifter or 0 dB trim.

Note that the peak currents in the charging capacitor are quite high and limited by the 4R7.
Be mindful of this current when laying out the timing cap return trace.

I used LME49720 for op amps A, B and C.
For op amp "A" bandwidth, low offset voltage and low bias current are important.

In terms of response characteristics this circuit appears to emulate the THAT2252 quite well.

The classic dBX sound can be obtained without using the discontinued THAT2252 RMS Detector, surface mount 4316 or the more expensive THAT4301.

Edit: 2/20/17. I'm posting later schematics below with links to circuit descriptions.

The second set of schematics show a conventional diode-based absolute value rectifier followed by a 2252-style log/RMS converter.
This circuit uses a single THAT300 transistor array per channel.


Image
A Diode-based fullwave rectifier with current output.

The absolute value circuit shown above has current outputs which feed the log/RMS converter shown below.

Image
A THAT2252-style RMS detector made from op amps and a THAT300 transistor array.

For higher performance a current rectifier can be used instead of a diode-based absolute value.
This current rectifier is an adaptation of the Roberts/Loft circuit used in the ubiquitous Loftec TS-1.
The output of the current rectifier feeds the log/RMS stage above.
The current rectifier uses two matched transistors in its current mirror (half a THAT300) and a THAT300 in the log/RMS converter.


Image
A current rectifier based on the Loftec TS-1 and THAT2252.

The circuits above are discussed here: viewtopic.php?f=6&t=856&start=16

The following detector uses a current rectifier that is a hybrid featuring the TS-1's common base stage combined with the THAT2252's class A/B bias scheme.
This rectifier and RMS stage requires two THAT300.
It has the added advantage of being optimized for True Power Summing.


Image
THAT2252 replacement using THAT300 and current rectification with class A-B bias optimized for True Power Summing.

Discussed here: viewtopic.php?f=6&t=856&start=25

Related reading:
Level Detectors, Absolute Value, Peak and RMS viewtopic.php?f=6&t=111
dBx 303 A Schematic viewtopic.php?f=12&t=860
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. » Wed Feb 08, 2017 10:50 am

Image

My log detector (middle of page on left side) Uses 5 array transistors, because I had them and waste is a sin. :lol:

To perform that conversion with a 4 transistor array, the two on the bottom, that form a current mirror for one polarity of the full wave rectifier would benefit from being matched, the top transistor (of those three) does not benefit from being matched. Likewise the transistor in the output of that op amp is just to juice the open loop gain for less errors at very low level and high frequency. So those transistors could be common single devices. Another popular trick is to bias the devices slightly on to reduce the dead back slewing at 0V. (CR1 in your schematic does the same thing).

The top two array transistors perform the actual log conversion and would benefit from matching for 0dB accuracy without a trim (while I had a trim in the TS-1, to also facilitate alternate 0VU standards.)

In fact the full wave rectifier could be done without matched parts, depending on the dynamic range and frequency response. The TS-1 was -3dB at 20kHz at -50dBu or something like that. Probably more than adequate for side chain use. As I recall the bandwidth dropped off quickly for lower levels (i.e. -3db @ 10kHz for -60dBu, 5kHz @ -70dBu, etc). when I first prototyped this I was pleased by a noise floor below -100dBu until I measured the bandwidth and found the frequency response was seriously compromised down there.

The approach of performing log on both polarities of the signal, reduces the rectification loss due to gain bandwidth, but introduces a tracking error between the two polarities (no free lunch, pick your poison.) I've seen Paul Buff RIP use the two polarity log paths in some of his designs.

JR

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Wed Feb 08, 2017 12:50 pm

Thanks John. Your TS-1 schematic is a very popular download here.
I need to look at the convertor in greater detail.

Gary Hebert sent me his suggestion which is very similar to the 2252 in which absolute value is performed in the linear domain followed by a unipolar log.
His approach frees up one transistor for level-shift making it more functionally-similar to the 2252.
I built it yesterday and it works great.

The original post's circuit has a -3 dB point of 28 kHz at at -40 dBu in.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Wed Feb 08, 2017 1:50 pm

John - I'm going to build that rectifier (now that I have a copy I can zoom into :lol: ).

I like the fact that it's current output.
It looks like that the rectifier's current mirror transistors don't have to match the logger's and could also be on a separate array.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. » Wed Feb 08, 2017 3:11 pm

mediatechnology wrote:John - I'm going to build that rectifier (now that I have a copy I can zoom into :lol: ).
I linked to an image on your forum.
I like the fact that it's current output.
Yes, transistors like to work that way... only downside is both sections can't be referenced to 0V, there needs to be a few volts between them to operate correctly.
It looks like that the rectifier's current mirror transistors don't have to match the logger's and could also be on a separate array.
yup... Only thing that matters for is one polarity of the rectifier, and you could add resistors in series with each emitter to force current tracking in the mirror even better. I was chasing bigger errors elsewhere... As I think I already mentioned I was able to measure errors caused by the logging transistors Rbb at +20dB (around +0.5 dB IIRC). The THAT 300 with lower Rbb should have lower errors in that regard too.

For my circuit a 28kHz -3dB point at -40dB would translate to 14kHz -3dB point at -50 dB... On my bench bandwidth dropped in half for every 10dB lower level, due to op amp gain bandwidth roll off, while slope may be different for different topologies.

I could resolve crazy low noise floors if I didn't care about bandwidth (but test equipment cares). :oops: One idea I never got around to executing on was adding a 2:1 compressor in front of the rectifier. That way my -50dB 20kHz -3dB point would double to -100 dB (in theory) 8-) Note: this is routinely done in companding NR and feedback type compressors where the rectifier only sees compressed audio. Using a premium VCA my practical noise floor would still be above -100 dBu.

JR

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Wed Feb 08, 2017 3:22 pm

I couldn't read the thumbnail version.
Glad I had a link to a "big" version of the TS-1: viewtopic.php?f=12&t=404&p=4531

I built it without the speed-up circuit and the low-level tracking is excellent.
I then grafted it onto the 2252-style log converter, log-domain filter and level-shifter.

The absolute value is very clever.
It's got at least 10 dB more useable range than a conventional absolute value.
I'm going to add the 2N3904 stage and the pre-bias to get the speed up.

THAT have an app note for a compressor-based wide range meter.
I think you sent me a copy of the Valley People one I should scan and post and at one time MicMix had a dBX meter.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Wed Feb 08, 2017 8:09 pm

The current rectifier works really well.
Until I really stared at them I didn't see the similarity between the TS-1's current rectifier and the THAT2252 rectifier.

Image
THAT2252 Simplified Internal Diagram

Image
Loft TS-1 Detector

Q3 (top figure) is CA3046 pins 14, 12, and 13.
Q1 is CA3046 pins 5, 4, and 3.
Q2 is CA3046 pins 1, 2, and 3.
In the TS-1 "V1" is about +0.5V but appears between the "OA1" output and base of "Q3."

The base of the 2252's Q2 is brought out to adjust symmetry and is normally grounded through a 20R.
The TS-1's inverting input is not at virtual ground but is at -3.75V.
The 2252's current mirror bases are both effectively tied together (at 0V) other than the external symmetry offset injected into Q2's base.

What I tested was the TS-1's detector with the current output feeding the OA2 inverting input of the cloned 2252.
The error at -60 dBu was less than a dB which is about 10 dB better than a conventional absolute value stage.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology » Thu Feb 09, 2017 8:38 am

I wanted to get your thoughts JR on an alternative current rectifier that eliminates the requirement for a matched pair.

For positive inputs in the TS-1 rectifer the diode-connected transistor portion of the current mirror is within the feedback loop.
The output of the current mirror, the CA3046 transistor at pins 1,2 and 3, runs outside the feedback loop.
That's why the current mirror requires a matched pair.
(The top transistor - as JR has pointed out - is within the feedback loop and does not need to be matched.)

Graeme in "Designing with Operation Amplifiers, Applications Alternatives" shows a current rectifier using complimentary transistors both of which are in the feedback loop.
It looks suspiciously like a Howland current pump.

Image
Graeme, "Designing With Op Amps Applications Alternatives," Figure 5-3, Current Rectifier.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. » Thu Feb 09, 2017 10:46 am

mediatechnology wrote:The current rectifier works really well.
Until I really stared at them I didn't see the similarity between the TS-1's current rectifier and the THAT2252 rectifier.

Image
THAT2252 Simplified Internal Diagram

Image
Loft TS-1 Detector

Q3 (top figure) is CA3046 pins 14, 12, and 13.
Q1 is CA3046 pins 5, 4, and 3.
Q2 is CA3046 pins 1, 2, and 3.
In the TS-1 "V1" is about +0.5V but appears between the "OA1" output and base of "Q3."

The base of the 2252's Q2 is brought out to adjust symmetry and is normally grounded through a 20R.
The TS-1's inverting input is not at virtual ground but is at -3.75V.
The 2252's current mirror bases are both effectively tied together (at 0V) other than the external symmetry offset injected into Q2's base.

What I tested was the TS-1's detector with the current output feeding the OA2 inverting input of the cloned 2252.
The error at -60 dBu was less than a dB which is about 10 dB better than a conventional absolute value stage.
Not to quibble but the 2252 is a RMS (cough) convertor so there is a 2xVbe term to perform the X^2 operation, minus 1Vbe for square root. (there is also an integration operation not shown. )

The TS-1 is just a simple LOG conversion to extract relative dB levels. I've done plenty RMS conversions with that array but it would have more going on (integration and square root involving all the transistors). In those the full wave rectifier is done more conventionally before the RMS operation.


JR

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. » Thu Feb 09, 2017 11:02 am

mediatechnology wrote:I wanted to get your thoughts JR on an alternative current rectifier that eliminates the requirement for a matched pair.

For positive inputs in the TS-1 rectifer the diode-connected transistor portion of the current mirror is within the feedback loop.
The output of the current mirror, the CA3046 transistor at pins 1,2 and 3, runs outside the feedback loop.
That's why the current mirror requires a matched pair.
(The top transistor - as JR has pointed out - is within the feedback loop and does not need to be matched.)
As I suggested earlier, adding resistors in series with each emitter can force current sharing far better than device matching. Unfortunately those two emitters are brought out on a single pin in that cheap array. I mainly used those array devices because they were already there and I am thrifty with my designs. :lol:
Graeme in "Designing with Operation Amplifiers, Applications Alternatives" shows a current rectifier using complimentary transistors both of which are in the feedback loop.
It looks suspiciously like a Howland current pump.

Image
Graeme, "Designing With Op Amps Applications Alternatives," Figure 5-3, Current Rectifier.
It hurts my head just to look at that design. :D

The four Rs need to be precision or matched, and who knows what the zero crossing dead band will look like. It is also biased up at V/2 with an inversion in one polarity of the feedback loop. Input caps and stability might be a concern... I'd pass....

JR

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