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 »

JR. wrote: Wed Apr 17, 2024 9:48 am Wouldn't subtracting a reference 0VU current from the common emitters (Sym adj probe node) deliver a + and - voltage relative to 0VU threshold?

It may need an offset subtracted to account for two pairs in parallel.

JR
Not sure I completely follow the question but an existing reference current sink is provided by Rt at 1M8 which is -8.3 uA.
So yes a reference current is subtracted but, being at the very bottom of the picture, it may not be obvious.

"It may need an offset subtracted to account for two pairs in parallel."

It depends on whether you want it calibrated for one or both channels driven at reference current.
The added offset from both being driven in parallel is a log domain measure of the power sum.

The input trims can be reduced so that reference current driving both channels is 0V out if that's desired.
Or they can be calibrated so that a single channel produces 0V and both driven equally produce +18 mV.

I think JR is pointing out that the actual voltage at the Ct node increases when both inputs are driven.

For one thing the value of the timing capacitor is usually doubled when two detectors are in parallel and current-summed to provide the same timing of a single detector.
This is true for the original 2252 and 4301 as well as the 43XX-series because each IC has its own current sink.
In the detector I show It is set by a single, common, resistor.

Secondly, the Ct node is, in the log-domain, the square of the log of the absolute value of the input so rather than rising 9 mV (at 3mV/dB scaling) to indicate the 3 dB power summation of equal inputs, the actual measured increase is 2X that or about 18 mV.

As JR has pointed out earlier the detector provides about half of an RMS computation.
When coupled with a VCA having 6 mV/dB scaling the log-domain square root is taken relative to the 3 mV/dB response of the detector.
The exponential response of the VCAs control port converts the computation back into the linear domain.
If a log-domain result is desired, exponential conversion is not required, but the 6 mV/dB scaling of the detector output would need to be divided by 2 to take the square root to technically provide the log-domain RMS value of the input.

The main improvement of this detector over some of the earlier "303 types" is the added level-shift that it isn't one Vbe up and subject to a larger temperature mismatch between channels.
The 2252 was a one-Vbe-up design and that was corrected in the 43XX-series.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

I saw Rt but discounted it. 8.3 uA x 49.5k is down a few dB from typical 0VU (around 0.4V) discounting the parallel junctions.

Never mind...


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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

JR. wrote: Wed Apr 17, 2024 2:51 pm I saw Rt but discounted it. 8.3 uA x 49.5k is down a few dB from typical 0VU (around 0.4V) discounting the parallel junctions.

Never mind...


JR
OK, got it.
The normalized "0" is less than 0 dBu because there's a -6dB line receiver in the circuit it was taken from.
The trim needs to offer about +/-3 dB range as well to balance the two channel's contributions and trim out errors.
I need to adjust the trim range for the Uno Compressor.

On the prototype at 0V output, with one channel driven, the input to the 49K9 is about 360 mV RMS or about 7.2 uA RMS.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

I lowered the value of Rin to 33K2 to give myself some additional trim range.
It varies slightly between channels but at the wiper I'm measuring about 260 mV (-9.5 dBu) for an input current of about 7.8 uA.

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

That is a step up from the rectifier and log convertors I made with cheap 3086 transistor arrays.

In the TS-1 dB meter I used to get an error near the top of the DB range (+20dBu) from the Rbb of the logging transistor. I bet those array devices have lower Rbb and less subsequent errors related to that.

[edit I didn't notice what op amps you are using, but another error I had to deal with was very low level at high frequency. The best I could manage was about -3dB @ 20kHz for -50dBu. That's probably good enough for cheap test gear or line level dynamics side chain, but I speculated about throwing a VCA in series with the input. Even a 2:1 compressor in front of that rectifier would make that -50dBu more like -100 dBu. /edit]

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

The THAT300's rbb is 32Ω typical. I'm not sure I've ever seen the rbb spec for the CA3046 or CA3086.

I'm currently using TL072B for the op amps. I don't need great low-level performance and I think it would be challenged if it were used in a compander.

I need to find the post but I built a companded detector and it worked very well. With a THAT4305 RMS/VCA combo the low-level performance was accurate to around to the -80 dBu level with a total DR of just under 100 dB.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

mediatechnology wrote: Thu Apr 18, 2024 4:32 pm The THAT300's rbb is 32Ω typical. I'm not sure I've ever seen the rbb spec for the CA3046 or CA3086.
I might be able to make an educated guess from looking at the TS-1 schematic. IIRC the error was almost 1 dB at +20dBu so because of the R it measured high,
I'm currently using TL072B for the op amps. I don't need great low-level performance and I think it would be challenged if it were used in a compander.
I was using tl07x when I saw the -3dB at 20kHz... but that's why real test equipment is hard to design. ;)
I need to find the post but I built a companded detector and it worked very well. With a THAT4305 RMS/VCA combo the low-level performance was accurate to around to the -80 dBu level with a total DR of just under 100 dB.
Companding works nicely to deliver high dynamic range.

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

I found the companding RMS detector post: https://proaudiodesignforum.com/forum/p ... 23&#p17023

With Ct doubled (due to 2:1 companding) the timing characteristics are identical to the THAT2252.
Note that the input to the RMS detector from the VCA output has no resistor - both are operating in the current domain.
R2 is essential to properly load the VCA's output current mirror.
A series resistor at "RMS In", not shown, provides V to I conversion.

Image
A wide-range level detector using the THAT4305.

My RCA databooks are in boxes but I had an RCA 1967 Linear Applications book on the shelf and opened it almost to the page where they state their process rbb to be 40Ω. It had a few data sheets for early CA3000-series parts but they hadn't started making the CA3046 or CA3086. I downloaded a 80s-era book from bitsavers and sure enough no spec for rbb is given for the 3046 or 3086 arrays. 40Ω seems about right and comparable to the THAT300's 32Ω.
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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

My gut feeling is that CA3086 Rbb would be higher than THAT300's. Rbb generally tracks with low ein voltage but at this point not worth losing much sleep over it unless trying to make an accurate full range dB meter.

IIRC both DBX and Paul Buff made wide range dB meters back in the day. Both probably used VCAs and companding inside to deliver wide dynamic range (my speculation).

Sorry for yet another veer... ;)

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Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

JR. wrote: Fri Apr 19, 2024 10:54 am IIRC both DBX and Paul Buff made wide range dB meters back in the day. Both probably used VCAs and companding inside to deliver wide dynamic range (my speculation).

Sorry for yet another veer... ;)

JR
You sent me the data sheet for the Valley People meter I've been meaning to scan. I think I have a dBx Model 81 datasheet. The model 81 did use a companding RMS detector.
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