THAT2252 RMS Detector Replacement Using A THAT300 Array

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
JR.
Posts: 3720
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

Yup film caps are betta...

JR
Cancel the "cancel culture", do not support mob hatred.
Hans
Posts: 73
Joined: Fri Jun 24, 2016 8:48 am

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by Hans »

Has anyone ever looked at this VU meter topology.
It uses a complete different way to achieve a log conversion.

http://www.naun.org/main/NAUN/mcs/17-130.pdf

Hans
User avatar
mediatechnology
Posts: 5511
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

Hans - Thank you for posting that link. It's a clever circuit.
User avatar
JR.
Posts: 3720
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

Hans wrote:Has anyone ever looked at this VU meter topology.
It uses a complete different way to achieve a log conversion.

http://www.naun.org/main/NAUN/mcs/17-130.pdf

Hans
I didn't read that paper closely but it looks like a variant on something I did back in the late '70s-early '80s. I made a prototype 100 segment dB meter (actually displayed peak and VU simultaneously). I used the logarithmic discharge curve of a simple RC (e^-t/RC) to convert a FW rectified DC to dBu. Instead of generating a linear current (via PWM) wrt dB output I compared the rectified DC voltage to the RC discharge curve voltage and used that comparator output to gate a clock feeding a digital counter to generate a digital 100 tick (dB) count representing dB below FS.

I did this twice, once for the peak level and again for the average (VU) level. As I recall I alternated conversion between extracting the average dB that I displayed as a solid bar, and the peak dB that I displayed as a single segment. The fluorescent display was multiplexed internally as an array of 10x10, so I multiplexed between displaying the single segment (peak), the full all-ten segment blocks, and the partial (<10) segment to finish the average display. I probably multiplexed the display 4 ways so likely had one all-off period between the three active display periods. The persistence of the fluorescent gave a solid appearing display.

The company I designed this for fell on financial problems so I never built more than that one prototype and lost that one physical unit in a bankruptcy auction. I regret that no formal schematic survived or ever existed beyond my design notes.

I will not swear to the accuracy of this approach over the full 100 dB display range, but it was solid enough for the top 50-60 dB and made a pretty flashing light display down to the noise floor.

======
FWIW around the same time period I used a completely different circuit approach to make a dB to digital count. Inside the TS-1 I designed in the early '80s (schematic around here somewhere) I performed the log conversion more conventionally using transistor Vbe junctions to extract a linear +/- dB voltage, that I then converted that voltage to a current to feed a saw tooth integrator, and counted the number of times the sawtooth ramp reset over a fixed unit time. Positive dB or negative dB slewed in opposite directions.

I am not completely sure why I used two completely different approaches but I expect the TS-1 to be the more accurate of the two (it was test equipment after all). The TS-1 approach had worst errors at high dB (fraction of a dB high) due to Rbb in the logging transistor causing an error term. The RC approach was likely worst at very low level, while both required extra attention to the rectifier for low level high frequency as we've talked about before. IIRC I scratched out a first order solution for the Rbb error term but that never made it to production in the TS-1.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5511
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Size Matters

Post by mediatechnology »

Yup film caps are betta...
Well, this will sure look impressive on the board:

Image

At $4 each I'm glad only one is required for True Power Summing.
User avatar
JR.
Posts: 3720
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

I might be interesting to throw a pair of them in front of a mic preamp for the phantom blocking capacitors and then do a null test vs one using aluminum electrolytic to see what comes out as different. I suspect the capacitance would need to be matched closer that typical cap tolerance to eliminate that variable.

Or not... :lol:

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5511
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Size Matters

Post by mediatechnology »

I might be interesting to throw a pair of them in front of a mic preamp for the phantom blocking capacitors and then do a null test vs one using aluminum electrolytic to see what comes out as different. I suspect the capacitance would need to be matched closer that typical cap tolerance to eliminate that variable.

Or not... :lol:
You read my mind.
I found some even bigger speaker crossover capacitors for mic preamp phantom blocking.
The Duracell is a "D" size.
The Dayton 47 µF 250V Polypropylene crossover capacitors are only about $10 each.
They have a low measured DF.

Image
Huge, really huge, Dayton Polypropylene crossover capacitors for mic preamp phantom power blocking.[/size]
User avatar
JR.
Posts: 3720
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

Yup, passive loudspeaker crossovers are the heavy lifting of the capacitor world.

While I was working at Peavey I approved an ECN (engineering change notice) for a (golden ear) engineer friend who worked in a different engineering group (transducer). His own director of engineering refused to sign an ECN (to upgrade a passive crossover cap to polypropylene) because it increased the BOM cost something like $0.20 . I was willing to take the heat from management for the $0.20 increase to use the better part, where it actually mattered. 8-)

I was a little too willing to spend my political capital on things like that. Even though it technically wasn't my product responsibility, I was also an engineering director so nobody questioned my authority to approve the change. Apparently upper management never heard about my wanton spending, because i didn't get in trouble (that time).

When in doubt do what is right. :lol:

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5511
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by mediatechnology »

The Uno Compressor spawned an improved THAT2252 replacement which is a "0 Vbe-up" design having a 0V output when the input is at the reference current. The gain of 2 amplifiers, which square the log of the absolute value, are level-shifted down by one Vbe. If not for this level-shift, the outputs would be "one Vbe-up."

This particular implementation uses two detectors to provide a power summation of the left and right inputs.

Image
User avatar
JR.
Posts: 3720
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: THAT2252 RMS Detector Replacement Using A THAT300 Array

Post by JR. »

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
Cancel the "cancel culture", do not support mob hatred.
Post Reply