THAT2252 RMS Detector Replacement Using A THAT300 Array

[mediatechnology]

Then subtracting two constant current biased Vbe is not remotely dividing it by 2, or taking a square root.

Are you talking about the role of Q3 and Q4?
They neither divide it by two or take the square root.



The output of the detector is the mean of the sum of the squares at 3mV/dB scaling. (At Q3 emitter or Q4 emitter.)
Q4 provides a -Vbe level shift.

If the scale factor is defined as 6 mV/dB then the 3mV/dB mean of the sum of the squares is divided by two which is a log-domain square root of the mean of the sum of the squares.
Nothing electrically divides by two in the detector, certainly not Q3 or Q4.
A 3/6 change in scaling ratio divides it by two.

Q4 and associated components simply drop the final output by one Vbe. (And buffer the timing network.)

An actual THAT2252 has its log converter biased up by one Vbe at the noninverting input to offset itself from the absolute value current sinks.
(To give the rectifier current output compliance.)
The 2252's level shift then has a two Vbe offset to subtract: One from the log converter and one from the rectifier/log domain filter.
In the 2252 V2 = Vbe; V3 = 2Vbe.
See: http://www.proaudiodesignforum.com/imag ... ematic.JPG

The 2252 clone shown above has op amp A's non inverting input at ground.
It doesn't need to be biased up by one Vbe because the current rectifier ahead of it can work into a virtual ground.
Q4 (V3) only needs to offset one Vbe (Q3).
As a consequence, and to make the 2252 clone compatible with the THAT2252, only a single Vbe offset is required.
In the 2252 clone V2 = 0; V3 = Vbe.

There is no square root operation performed by Q3 or Q4.
If I look at the output with 3mV/dB scaling it's not giving me RMS.
If I use a 6mV/dB scale factor after the detector's output (either electrically or by definition) then a square root is performed on a 3mV/dB "mean of sum of squares" measurement and I get linear dB/log-scaled RMS readings.

[JR.]

Then subtracting two constant current biased Vbe is not remotely dividing it by 2, or taking a square root.

Are you talking about the role of Q3 and Q4?
They neither divide it by two or take the square root.



The output of the detector is the mean of the sum of the squares at 3mV/dB scaling. (At Q3 emitter or Q4 emitter.)
Q4 provides a -Vbe level shift.

If the scale factor is defined as 6 mV/dB then the 3mV/dB mean of the sum of the squares is divided by two which is a log-domain square root of the mean of the sum of the squares.
Nothing electrically divides by two in the detector, certainly not Q3 or Q4.
A 3/6 change in scaling ratio divides it by two.

Q4 and associated components simply drop the final output by one Vbe. (And buffer the timing network.)

An actual THAT2252 has its log converter biased up by one Vbe at the noninverting input to offset itself from the absolute value current sinks.
(To give the rectifier current output compliance.)
The 2252's level shift then has a two Vbe offset to subtract: One from the log converter and one from the rectifier/log domain filter.
In the 2252 V2 = Vbe; V3 = 2Vbe.
See: http://www.proaudiodesignforum.com/imag ... ematic.JPG

The 2252 clone shown above has op amp A's non inverting input at ground.
It doesn't need to be biased up by one Vbe because the current rectifier ahead of it can work into a virtual ground.
Q4 (V3) only needs to offset one Vbe (Q3).
As a consequence, and to make the 2252 clone compatible with the THAT2252, only a single Vbe offset is required.
In the 2252 clone V2 = 0; V3 = Vbe.

There is no square root operation performed by Q3 or Q4.
If I look at the output with 3mV/dB scaling it's not giving me RMS.
If I use a 6mV/dB scale factor after the detector's output (either electrically or by definition) then a square root is performed on a 3mV/dB "mean of sum of squares" measurement and I get linear dB/log-scaled RMS readings.

Sorry to be so pedantic about this, over the decades I have performed several RMS conversions to drive meters (using both analog and digital technology). For analog, the integration of the x^2 current is generally done by driving an op amp virtual earth input with CT across the feedback path, the square root is then extracted by pulling that integral of X^2 current through the second diode connected junction that is in series with the X^2 junction. Sorry again examples of this topology are all over the literature.

What dbx/THAT did is elegant, sounds good, and works...(in combination with their VCA, which is their business).

I learned long ago not to argue with people about what "they" hear... if it sounds good it is good.

JR

[mediatechnology]

If it makes you feel better John I can put a gain of 1/2 on the output and make the scale factor 3mV/dB in order for it to pass "RMS certification."

The bottom line is that it needs to be functionally equivalent to a THAT2252.
The TS-1's current rectifier is a keeper as an absolute value.

The first circuit I posted http://www.proaudiodesignforum.com/imag ... 800_BW.jpg isn't electrically the same as a 2252 because it has no level shift.
It really needs a fifth transistor matched to the other four.

[JR.]

If it makes you feel better John I can put a gain of 1/2 on the output and make the scale factor 3mV/dB in order for it to pass "RMS certification."

The bottom line is that it needs to be functionally equivalent to a THAT2252.
The TS-1's current rectifier is a keeper as an absolute value.

The first circuit I posted http://www.proaudiodesignforum.com/imag ... 800_BW.jpg isn't electrically the same as a 2252 because it has no level shift.
It really needs a fifth transistor matched to the other four.

No that won't but you do not need to make me feel better...

My ideas would make it less of a 2252 mimic, and not the design goal. (I still like the idea of adding degeneration resistors to current mirror emitters if you use that rectifier topology and the emitters are separately accesable.)

JR

[mediatechnology]

(I still like the idea of adding degeneration resistors to current mirror emitters if you use that rectifier topology and the emitters are separately accessible.)

I do want to try that.
It's in the 80s here today in February. Very hard to stay inside for long.

[mediatechnology]

Gary Hebert of THAT sent me a variation of the current rectifier which uses Q2 to pre-bias the rectifier in class A-B similar to the THAT2252.
Gary liked JR's common base stage, Q9.


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

Current rectification is performed by op amp A, Q1-Q4 and Q9. (See also: viewtopic.php?f=6&t=856&start=16)
Q3 rectifies positive inputs. Q4 mirrors the current in diode-connected Q3.
Q2 provides class A-B bias for transistor Q1.
Q1 rectifies negative inputs.
Q1's collector current is a nearly-identical copy of the negative input current.
The collectors of Q1 and Q4 are combined to provide an output current equal to the absolute value of the input current.

The absolute value current output feeds the log converter at the inverting input of op amp B.
This particular log stage level-shifts the input to the converter, using Q5, so that the emitter of Q8 sets around 0V.
The THAT4305, 4315 and 4320 RMS detectors have a similar level shift.
This reduces temperature sensitivity when the detector output is combined with another channel for True Power Summing.

Since the timing capacitor (Ct) can now see either polarity I added op amp C to bias the -Ct terminal to below ground.
For polar electrolytic capacitors the -300 mV may not be significant and Ct- could be grounded.
Film timing caps could also be grounded.

Tantalum capacitors, which have tighter tolerance and lower DA excel as a Ct.
Tantalums prefer that the Ct- terminal be either biased negative by op amp C or tied to the -15V rail to avoid any reverse-polarity.
(A non-polarized Ct could also be made from two back-to-back capacitors. In the case of tantalums this is more expensive than op amp C.)
If Ct- is tied to the negative supply it is subject to supply noise and should also be rated at >16V.

The return current through Ct- is quite large and limited by op amp B's output current.
When Ct is grounded the return trace should be large.

Q8's dynamic impedance isolates op amp B's output from the large capacitive load of Ct.
When Ct- is referenced to op amp C's output it is also isolated from Ct's capacitive load by Q8.

When Ct- is connected to the -1.3V reference Ct's charging current is isolated from ground and returns to the supply rails.
Another advantage is that low-voltage tantalums can be used.

In this configuration a stereo True Power Sum detector uses 4 THAT300, and 3 LME49720 vs. 3 THAT300 and 3 LME49720 in the previous example.

[JR.]

Tantalum capacitors, which have tighter tolerance and lower DA excel as a Ct.

A minor quibble, tantalum caps are actually known for relatively inferior DA. One reference I found suggest Silver mica, glass, and tantalum range from 1% to 5% DA. This is better than common (aluminum?) electrolytic caps at 10%, but polyester is more like 0.5% DA and my favorite dielectric polystyrene is as low as 0.002% DA.

It might be interesting to see if modern low impedance electrolytics are better for DA too, probably are since the mechanism is caused by distributed RxC so lower R should be better.

I've told this story before but back when I designed my CX record NR decoder kit, I noticed that Urie used a tantalum cap in an important att/release time-constant circuit. To afford similar response characteristics (DA actually matters for time-constant circuits that are charged and discharged through different impedances) I used a tantalum cap in my kit at that same circuit position.

JR

[mediatechnology]

Tantalum capacitors, which have tighter tolerance and lower DA excel as a Ct.

...compared to the commonly-used electrolytic timing cap.

My thinking is that a 22uF film (or a pair of 10 uF films) would be the ultimate timing capacitor. (And would be impressively big.)

[JR.]

Tantalum capacitors, which have tighter tolerance and lower DA excel as a Ct.

...compared to the commonly-used electrolytic timing cap.

My thinking is that a 22uF film (or a pair of 10 uF films) would be the ultimate timing capacitor. (And would be impressively big.)

Back in the '70s I used a 22uF cap in a RIAA gain leg so it was working pretty hard (360 ohm IIRC).

I substituted a tantalum for the aluminum in that preamp and measured dramatically less phase shift at 20kHz. In my judgement dominated by the lower ESL/ESR.

I apologize for giving you homework, but I couldn't find a DA spec for low Z aluminum.

I suspect a crude null test between a tantalum and low Z aluminum when charged and discharged through different impedances. In fact the perfect test would be two identical circuits (Q8, Rt and Ct) but one using tantalum and the other low Z aluminum. This circuit could be used to prove the benefit of tantalum over high Z aluminum (if they still make them).

Sorry.. again.. just me being me. The audible consequence of this is pretty subtle and more important for encode/decode tracking in say a NR circuit than for single ended dynamic processing use.

JR

[mediatechnology]

I was able to check the DF not the DA.

A Nichicon 22 uF UVR electrolytic measured 0.19 DF; a 22 uF tantalum 0.03 DF.

I was going to use a film capacitor anyway in my build.